Biology: The Living Earth

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What two things must a testable question name?
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   An observation you could actually make, and what would count as each possible answer

2. What does a hypothesis have that a prediction does not?
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   A proposed mechanism: it says why, not only what will happen

3. What does it mean for a hypothesis to be falsifiable?
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   You can state in advance a possible result that would show it is wrong

4. Which type of study can support a claim about cause, and why?
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   An experiment, because the investigator changes one thing on purpose rather than recording what is already there

5. Rewrite "Is this soil better?" so that it is measurable.
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   Any version that fixes a criterion, for example "Does this soil produce a greater mean dry mass of bean plants after thirty days?"

6. "Some species are more important than others." Explain why biology cannot settle this, and what it can contribute.
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   Importance is a judgment about value, and no measurement returns a value. Biology can measure how many other species depend on a given one, what happens to a community when it is removed, and how much of an ecosystem process it performs, all of which are real and relevant findings. But turning those numbers into "more important" requires deciding what we are trying to protect and why, which is a question about goals rather than about organisms. Science informs the decision without making it. Value is not measurable, though the consequences that inform the judgment are

7. A student writes: "My hypothesis is that I will test whether light affects algae." Explain what is wrong with it and fix it.
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   This states an intention, not an explanation. It predicts nothing, so no result could contradict it, and a claim that cannot be contradicted teaches nothing when the data arrive. A hypothesis must propose a mechanism and commit to an outcome. A corrected version: "Increasing light intensity increases the rate of oxygen production in pond algae, because light supplies the energy for the light-dependent reactions of photosynthesis, and light is the limiting factor at low intensity." It states an intention rather than a falsifiable explanation

8. Why is it important to write down the falsifying result before collecting data?
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   Because afterwards it is very easy to reinterpret whatever happened as support. If the fertilized plants grow less, an investigator who has not committed in advance can decide the dose was wrong, the season was unusual or the effect takes longer, and keep the hypothesis regardless of the evidence. Fixing the falsifying result first means the data get a genuine chance to overturn the idea, which is the only way a test is informative. It prevents reinterpreting any outcome as support after the fact

9. A study finds that towns with more veterinarians have more cases of a cattle disease. Explain why this does not show that veterinarians cause the disease.
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   Nothing was changed on purpose, so this is an observational study and a third factor can produce both patterns at once. Towns with more cattle will have both more veterinarians and more cases of a cattle disease, and herd size is doing the work in both columns. The two measurements rise together without either causing the other. Distinguishing the possibilities requires either holding herd size constant or changing veterinarian numbers deliberately, which nobody would do. A third factor, herd size, produces both patterns

10. Give a question about photosynthesis that biology can answer and one it cannot, and say what makes the difference.
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    Answerable: "Does raising carbon dioxide concentration from 0.04 percent to 0.10 percent increase the rate of oxygen production in pond weed at constant light and temperature?" It names a change, a measurement and a comparison. Not answerable by measurement: "Should we raise carbon dioxide concentration in greenhouses?" That depends on cost, on energy use and on what else the greenhouse is for. The difference is that the first specifies an observation that could come out either way, while the second requires weighing goods against each other. The first names a possible observation; the second requires a value judgment

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. How many independent variables should an experiment have?
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   Exactly one

2. What is the difference between the control group and the controlled variables?
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   The control group is a group that goes untreated; controlled variables are conditions held the same across all groups

3. Define a confounding variable.
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   A variable that changes along with the independent variable, so two explanations fit the results equally well

4. In the worked example, name the dependent variable and its units.
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   Percentage of seeds germinating within five days, in percent

5. Why is measurement sometimes done without knowing which group is which?
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   Because expectation shifts borderline readings when the measurer knows what result is hoped for

6. A student waters one tray daily and another every three days, and also places the daily tray on a warmer shelf. Explain precisely what is wrong.
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   Two things differ between the trays: watering frequency and temperature. If the daily tray grows better, the cause could be the extra water, the extra warmth, or both acting together, and the data cannot distinguish them. This is a confound rather than sloppiness, because collecting more measurements from the same setup will not help. The fix is to put both trays on the same shelf so that watering frequency is the only deliberate difference. Two variables changed together, so the result has two equally good explanations

7. Why does an experiment need a control group at all, if the treated group is measured carefully?
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   Because a measurement on its own has nothing to be compared against. Radish seedlings grow whether or not you add fertilizer, so observing that the treated plants reached 18 cm tells you nothing until you know what untreated plants reached in the same fortnight. The control group supplies that baseline, and it must experience everything the treated group experiences except the treatment itself, so that the difference between them can be attributed to one cause. It supplies the baseline showing what would have happened anyway

8. A drug trial gives the treatment group a tablet and the control group nothing at all. Explain the flaw and how to fix it.
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   The groups differ in two ways: the drug, and the experience of being given and taking a tablet. The second can change reported outcomes on its own through expectation, so improvement cannot be assigned to the drug. The fix is to give the control group an identical tablet containing no active ingredient, so that the only difference between the groups is the substance being tested, and ideally to keep both patients and assessors unaware of who received which. Taking a tablet is itself a difference; give the control an inactive one

9. In the germination example, why report a percentage rather than the number of seeds?
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   A count depends on how many seeds were placed in the dish, so 44 germinating means something quite different out of 50 than out of 200. Converting to a percentage removes that dependence and makes dishes with different seed numbers directly comparable, both within this experiment and against anyone else's results. It also makes the comparison easier to state: 88 percent against 94 percent is immediately readable in a way that 44 against 47 is not, once the denominators differ. Percentages stay comparable when the number of seeds differs

10. Design a fair test of whether soil pH affects bean seedling height. Name each variable type.
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    Independent variable: soil pH, set at several levels such as 5, 6, 7 and 8 using buffered solutions. Dependent variable: mean stem height in centimeters after twenty one days. Controlled variables: seed variety and batch, number of seeds per pot, soil type and volume, water volume and frequency, light and temperature, and pot position rotated to even out any bench effects. The pH 7 pots serve as the comparison condition since that is ordinary soil, and every pH level needs several pots so that variation between pots can be judged. pH is independent, height is dependent, everything else is held constant across several replicate pots

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Calculate the mean of 4, 7, 7, 10 and 12.
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   The sum is 40, and 40 divided by 5 is 8.0

2. What is the range of that set?
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   12 minus 4, which is 8

3. What does replication mean in an experiment?
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   Repeating the measurement on different individuals, not remeasuring the same one

4. In the worked example, which group varied more, and by how much?
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   The fertilized group, with a range of 7 cm against 4 cm

5. Why is measuring one plant three times not replication?
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   It measures the precision of your ruler, not the variation between individuals

6. Two groups both have a mean height of 15 cm. Group A ranges from 14 to 16 cm and group B from 4 to 26 cm. Explain why these are not the same result.
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   The means are identical but the groups behave completely differently. In group A every individual is close to 15 cm, so the mean describes each plant well and a prediction about a new plant would be reliable. In group B individuals range from very small to very large, so the mean describes almost nobody and predicting a new plant from it would be nearly worthless. The spread tells you how much the mean can be trusted as a summary, which is why reporting it is not optional. The mean summarizes group A well and group B badly

7. A student concludes that a plant food works because their treated plant grew 3 cm taller than their untreated plant. Explain the flaw.
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   With one plant in each group there is no way to know how much two untreated plants would have differed from each other. Ordinary variation between individuals of the same variety can easily produce a 3 cm gap, as the worked example shows, where untreated plants ranged across 4 cm with no treatment at all. The comparison cannot separate an effect of the plant food from normal variation, so the conclusion is unsupported regardless of how carefully the two plants were measured. One plant per group gives no measure of ordinary variation

8. Explain why increasing the number of individuals per group strengthens a conclusion.
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   Each individual carries its own random departure from the group's typical value, and those departures partly cancel when averaged. With more individuals the mean of the sample sits closer to the value that would be obtained from the whole population, so the same measured difference between groups becomes less likely to have arisen from an unlucky selection of individuals. Larger samples also give a better picture of the spread itself, which is what a claim about a difference has to be judged against. Random individual differences cancel out, so the sample mean is a better estimate

9. The treated group in the worked example contained one plant shorter than three untreated plants. Does this refute the conclusion? Explain.
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   No. The claim is about the average effect on a group, not a promise that every treated individual will exceed every untreated one. Individuals vary for many reasons at once, including seed quality and position, and the treatment shifts the distribution rather than setting each plant's height. A single overlapping individual is exactly what an overlapping range describes. What the overlap does mean is that a conclusion drawn from one plant per group would have been unreliable, which is an argument for larger samples rather than against the finding. The claim concerns group means, and overlap between individuals is expected

10. Explain why biological variation should not be described as experimental error.
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    Experimental error is a failure of measurement or procedure that better technique could reduce, such as a misread ruler or an unevenly watered tray. Variation between individuals is not a failure at all: organisms of the same species genuinely differ in their genes and their development, and they would still differ under perfect technique. Treating that variation as error suggests it could be removed, when in fact it is the material natural selection acts on and one of the central facts of the subject. It is a real property of living things, not a flaw in the method

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Which axis carries the independent variable?
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   The horizontal axis

2. When is a bar chart correct rather than a line graph?
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   When the independent variable is separate categories rather than a continuous quantity

3. In the worked example, what is the minimum oxygen value and where does it occur?
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   2.4 mg/L at 2 km downstream

4. By how much does oxygen fall between 0 and 2 km?
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   From 8.2 to 2.4 mg/L, a fall of 5.8 mg/L

5. What is wrong with the sentence "the graph increases"?
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   The graph does not increase; the measured quantity does, and it must be named with units

6. Explain how starting a vertical axis at 95 rather than 0 can mislead, without any number being false.
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   Every plotted value can be perfectly accurate while the picture is misleading, because the eye judges the size of a difference from the height of the bars or the steepness of the line rather than from the numbers. If two values are 96 and 98 and the axis runs from 95 to 100, the second bar appears three times the height of the first, suggesting a large effect where the real difference is about two percent. Nothing is fabricated; the scale has simply been chosen to magnify. The eye reads relative heights, which a truncated scale exaggerates

7. Why is describing the trend before explaining it worth the extra sentence?
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   Because an explanation offered first tends to determine what you think you see. A student who begins with "bacteria used up the oxygen" will often describe a steady decline and miss the recovery entirely, since the explanation they reached for does not include one. Describing first forces the actual shape into view, including the parts that need accounting for, and it is also how these questions are marked: the description and the explanation earn separate credit. Explaining first makes you describe the data you expected rather than the data present

8. The recovery takes far longer than the fall. Suggest a biological reason.
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   The fall is driven by bacterial respiration, which is fast because the sewage supplies a large concentrated pulse of organic matter and bacterial populations can grow rapidly. The recovery depends on oxygen dissolving back into the water from the atmosphere, which happens only at the surface and at a rate set by turbulence and surface area rather than by biology. A fast biological process removes the oxygen and a slow physical process replaces it, so the curve is steep going down and gradual coming back. Fast bacterial consumption against slow physical reaeration

9. A student joins the tops of four bars showing mean masses of four different species with a line. Explain why this is wrong.
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   A line asserts that values exist between the plotted points, so joining the bars claims there are organisms partway between two species with intermediate masses. Species are separate categories, not points on a continuous scale, and the order of the bars is arbitrary, so the line's slope carries no meaning and would change if the bars were rearranged. A bar chart keeps the categories distinct, which is the honest representation of this kind of data. A line implies intermediate values, and species are discrete categories

10. What could you not conclude from this graph alone, even though the pattern is clear?
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    You could not conclude that the sewage outfall caused the oxygen sag, because nothing was changed on purpose and no comparison river was measured. The pattern is consistent with that explanation, but an unmeasured factor changing along the same stretch, such as a change in depth, flow rate or temperature, could also lower oxygen. Supporting the causal claim would need either upstream measurements, a comparable river without an outfall, or records from before the outfall existed. That the outfall caused the sag, since nothing was controlled or compared

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Name the three parts of the structure.
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   Claim, evidence, reasoning

2. Which part do students most often leave out?
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   The reasoning

3. What makes a statement evidence rather than description?
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   Specific values with units, selected because they bear on the claim

4. State the test for whether reasoning is present.
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   Could a reader accept the evidence and still reject the claim? If so, reasoning is missing

5. Why does naming a limit earn credit?
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   It distinguishes a supported conclusion from an overclaim

6. Explain why "the treated plants were taller, which shows the fertilizer worked" contains no reasoning.
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   The second clause repeats the first in different words rather than connecting it to any biology. It does not say what fertilizer contains, what a plant uses that substance for, or why a shortage of it would limit height, so a reader who accepts the heights is given no principle for preferring the fertilizer explanation over better light or better drainage. Reasoning would supply that principle: nitrogen is needed to build amino acids and proteins, and where soil nitrogen limits growth, adding it allows more tissue to be built. It restates the evidence instead of naming the mechanism

7. Write a claim, evidence and reasoning answer for the germination data in lesson 1.2 (88 percent at 20 degrees, 94 percent at 30 degrees).
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   Claim: germination was higher at the warmer temperature in this trial. Evidence: 47 of 50 seeds germinated at 30 degrees Celsius, which is 94 percent, against 44 of 50 at 20 degrees, which is 88 percent, a difference of 6 percentage points. Reasoning: germination depends on enzyme-controlled reactions that mobilize the seed's stored food, and raising the temperature toward the enzymes' optimum increases the frequency of successful collisions between enzyme and substrate, so the reactions proceed faster and more seeds reach germination within the five day window. Limit: with one dish per temperature there is no measure of variation between dishes. Claim, then figures with units, then the enzyme mechanism, then the limit

8. Why is quoting every number collected a weakness rather than thoroughness?
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   Because selecting evidence is itself part of the argument, and a reader cannot tell which figures you think matter if you supply all of them equally. An answer that lists twelve values has not shown that it knows which two carry the claim, and it buries the comparison that does the work. Choosing the starting value, the minimum and the recovery value demonstrates understanding of the shape of the result, whereas reproducing the table demonstrates only that the table exists. Selection shows which data bear on the claim; a data dump hides it

9. A student claims a pond is polluted because they found few fish. Identify what is missing and supply it.
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   The claim has almost no evidence and no reasoning. Evidence would need specific figures, such as the number of fish per sampling effort compared with a similar unpolluted pond, along with a measurement bearing directly on pollution such as dissolved oxygen or nitrate concentration. Reasoning would then have to connect them: many pollutants raise the growth of bacteria or algae whose respiration lowers dissolved oxygen below the level fish require, so a low oxygen reading alongside low fish numbers supports the claim in a way that fish numbers alone cannot. Both a comparative measurement and the oxygen mechanism linking it to fish numbers

10. Explain why a good answer says what its evidence cannot establish.
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    Because the strength of a conclusion is part of the conclusion, and claiming more than the data support is a specific error rather than enthusiasm. Observational data can show that two things occur together but cannot by themselves rule out a third factor or a reversed direction of cause, and an answer that says so is demonstrating exactly the judgment the question tests. It also directs the reader to what would settle the matter, which is the next step in any real investigation. Stating the strength of a conclusion is part of drawing it

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Name the three kinds of model and give an example of each.
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   Physical (a gelatin cell), mathematical (exponential growth), conceptual (a food web)

2. What assumption does exponential growth make?
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   That resources are unlimited

3. By what standard is a model judged?
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   Whether it predicts well enough for its purpose, not whether it is realistic

4. Give one thing the grass to rabbit to fox chain cannot represent.
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   An organism feeding at more than one trophic level, or the decomposers

5. Why is "this model is unrealistic" not an answer?
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   All models simplify; the answer must name the specific prediction the simplification gets wrong

6. Explain why a more realistic model is not automatically a better model.
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   Adding detail costs clarity and usually costs predictive power that can actually be used. A food web including every species in a meadow would be more realistic than a four-link chain and far less useful for calculating how much energy reaches the top consumer, because the arithmetic would be buried in hundreds of arrows whose values nobody has measured. A model earns its detail only when that detail changes the answer to the question being asked, which is why the purpose has to be fixed first. Detail that does not change the answer costs clarity for nothing

7. A Punnett square predicts a 3 to 1 ratio, but a cross produces 26 tall and 14 short. Explain whether the model has failed.
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   No. The square models probability per offspring, not a guaranteed tally, so the prediction is that each offspring has a three in four chance of being tall, and departures from the exact ratio are expected in small samples. Forty offspring predicts thirty tall and ten short; twenty six and fourteen is a plausible outcome of chance around that expectation, in the same way that forty coin tosses rarely give exactly twenty heads. The model would be in trouble if large samples consistently departed from three to one, which is how non-Mendelian patterns were discovered. It predicts probabilities, and small samples vary around them

8. Why must the assumptions of a model be stated when the model is used?
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   Because the assumptions define the conditions under which the model's predictions can be trusted, and a reader cannot judge a conclusion without them. Exponential growth predicts a population will double indefinitely, which is correct only while resources remain unlimited, so applying it to a population near its carrying capacity produces a confident and wildly wrong forecast. Stating the assumption tells the reader where the boundary is and signals when a different model is needed. They set the conditions under which the prediction is valid

9. Suggest a case the biological species concept handles badly, and say why that does not make it useless.
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   It defines a species by the ability to interbreed and produce fertile offspring, so it cannot be applied to bacteria, which reproduce asexually, or to organisms known only as fossils, where no breeding test is possible. That does not make it useless: for sexually reproducing living animals it captures exactly the boundary that matters, because populations that exchange genes evolve together and populations that do not diverge. A tool with a stated domain is useful inside it. It fails for asexual and fossil organisms but works where gene flow is the relevant boundary

10. You want to know how quickly a disease will spread through a school. What kind of model would you use, and what would you have to assume?
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    A mathematical model, since the question asks for a rate and a number infected over time rather than a description. It would have to assume something about how many people each infected person contacts per day, the probability that a contact transmits, how long a person stays infectious, and that mixing is reasonably even across the school. Each assumption is a place the prediction could fail, particularly even mixing, since friendship groups and year groups make contact far from random. A mathematical model, assuming contact rate, transmission probability, infectious period and even mixing

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. How many nanometers are in one micrometer?
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   1000

2. Give the approximate length of a typical bacterium.
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   About 1 to 2 micrometers

3. A cube has a side of 3 cm. Find its surface area, volume and ratio.
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   Surface area 54 square centimeters, volume 27 cubic centimeters, ratio 2

4. What happens to the surface to volume ratio as an object gets larger?
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   It decreases

5. Why is "a cell is very small" not an acceptable answer?
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   It is not a measurement; give a figure with units or a comparison with a factor

6. A typical animal cell is about 20 micrometers across and DNA is about 2 nanometers wide. How many orders of magnitude separate them?
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   Convert to the same unit first. 20 micrometers is 20000 nanometers, and DNA is 2 nanometers, so the cell is 10000 times wider. Ten thousand is ten to the fourth power, so they differ by four orders of magnitude. Doing the conversion before the division is the whole of the method here, and mixing micrometers with nanometers is the commonest way to get an answer wrong by a factor of a thousand. Four orders of magnitude, a factor of 10000

7. Explain why the small intestine is lined with millions of projections.
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   Absorption happens across a surface, and the quantity of food to be absorbed is set by the volume of the gut contents, so a smooth tube of that diameter would present far too little area for the demand. Folding the lining into villi and covering those in microvilli raises the surface area enormously without increasing the volume of the organ, which is the same trick as increasing a cube's surface without increasing its bulk. The ratio, not the absolute area, is what the problem is about. It raises surface area without raising volume, improving the exchange ratio

8. Why do very large organisms need circulatory systems while a single-celled one does not?
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   A single-celled organism has a high surface to volume ratio and a very short distance from its membrane to any point inside, so diffusion alone delivers oxygen and removes waste fast enough. As body size rises, the ratio falls and the distances grow, and diffusion becomes hopelessly slow over centimeters rather than micrometers. A circulatory system solves both problems at once by moving fluid in bulk to within diffusion distance of every cell, which is why size and transport systems appear together across the animal kingdom. Falling surface to volume ratio and longer diffusion distances make diffusion alone insufficient

9. Two cells are cubes of side 10 and 20 micrometers. Which meets its needs more easily, and by what factor do their ratios differ?
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   The smaller one. For side 10 micrometers the surface area is 600 and the volume 1000 in consistent units, giving a ratio of 0.6. For side 20 the surface is 2400 and the volume 8000, giving 0.3. The ratios differ by a factor of two, so the smaller cell has twice as much membrane serving each unit of its contents and can supply its interior more easily, which is the general reason cells divide rather than continuing to grow. The smaller cell, with twice the surface per unit volume

10. Why does this course insist on units at every step?
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    Because a number in biology is meaningless without them and because carrying them catches mistakes automatically. Writing 20 when the figure is 20 micrometers and comparing it with 2 nanometers produces an answer wrong by a factor of a thousand, and the error is invisible until the units are restored. Units also make an answer checkable by someone else, which matters when a result is going to be used, and they force you to notice when a calculation has produced something in the wrong kind of quantity altogether. Units give a number meaning and catch conversion errors automatically

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define an autotroph.
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   An organism that makes its own organic molecules from inorganic ones, using light or chemical energy

2. Through what kind of organism does energy enter almost every ecosystem?
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   A producer, which is an autotroph, usually photosynthetic

3. What energy source supports a hydrothermal vent community?
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   Chemical energy from compounds such as hydrogen sulfide, used by chemoautotrophic bacteria

4. In the worked example, what percentage of sunlight is captured by photosynthesis?
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   1 percent, since 10,000 divided by 1,000,000 is 0.01

5. How much energy is available to herbivores in the worked example, and why is it less than the amount captured?
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   6,000 kJ, because 4,000 kJ was used in the plant's own respiration

6. Explain why a plant is not "getting energy from the soil", and say what the soil does supply.
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   Soil supplies water and dissolved mineral ions such as nitrate, magnesium and phosphate, all of which the plant genuinely needs, but none of them is an energy source: the plant cannot release useful energy by taking up a nitrate ion. The energy for building sugars comes from light absorbed by chlorophyll, and the carbon comes from carbon dioxide in the air. Minerals are raw materials for particular molecules, magnesium for chlorophyll itself and nitrogen for amino acids, which is a different role from fueling the process. Soil supplies raw materials, not energy; light supplies the energy

7. Why must an ecosystem's energy supply be continuous, when its matter can be used over and over?
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   Because every energy transfer within the ecosystem loses some energy as heat, and heat disperses into the surroundings where no organism can capture it again. Matter behaves differently: a carbon atom released as carbon dioxide can be fixed again by a plant tomorrow, so the same atoms circulate indefinitely. Energy makes a one way trip from the sun through the organisms and out as heat, so unless it keeps arriving the system runs down. Energy degrades to heat and leaves; matter stays and can be reused

8. A grassland captures 1 percent of the light that falls on it. Explain why this is not evidence that plants are poorly built.
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   Most of the loss is not the plant's failure. A large share of the light misses leaves entirely and lands on soil, much of the rest is at wavelengths no pigment absorbs, and some passes through thin leaves or reflects off waxy surfaces. Of the light that is actually absorbed by chlorophyll, the conversion is far more efficient than one percent. The figure describes the whole field including its gaps and its unusable wavelengths, not the performance of the biochemical machinery. Most light never reaches a pigment, so the field figure understates the process

9. Suppose a cave community receives no light and no chemical energy source, but does receive organic debris washed in from outside. Is it an ecosystem with no producers? Explain.
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   It is a community with no producers of its own, which is possible only because it is not a closed system. The organic debris was built by producers somewhere else, usually plants on the surface, so the energy still entered the biosphere through photosynthesis and has simply been imported. Such communities exist and are entirely dependent on that import: interrupt the inflow and they starve, because nothing inside the cave can fix energy from non-living sources. Its energy is imported from producers elsewhere, so it depends on them

10. Why are producers counted as the first trophic level rather than the sun?
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    Trophic levels describe feeding relationships among organisms, and the sun is not an organism and is not eaten. The producers are the first living step, the point at which energy becomes stored in organic molecules that something else can consume, so counting begins there. The distinction also keeps the levels meaningful: every subsequent level is defined by what it eats, and there is no sense in which a plant eats the sun. Trophic levels count feeding relationships, and the sun is neither an organism nor eaten

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What does an arrow in a food web mean?
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   That energy flows in the direction of the arrow, from the food to the consumer

2. Which trophic level do producers occupy?
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   The first

3. In the worked example, name the two primary consumers.
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   The grasshopper and the rabbit

4. Give a four-link chain from the worked example.
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   Grass → grasshopper → shrew → kestrel

5. What is the difference between a carnivore and a secondary consumer?
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   Carnivore describes diet; secondary consumer describes feeding position in a particular chain

6. Explain how one bird can be both a secondary and a tertiary consumer at the same time.
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   Trophic level is a property of a feeding route rather than of a species. The kestrel eating a grasshopper is three steps from the grass, making it a secondary consumer on that route, while the kestrel eating a shrew that ate a grasshopper is four steps from the grass and a tertiary consumer on that one. Both happen in the same week, so the bird genuinely occupies both positions, and its average trophic level is somewhere between the two depending on how much of each prey it takes. Level depends on the route, and the kestrel feeds along two of them

7. Why does a food web predict the effect of losing a species better than a food chain?
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   Because a web contains the alternative routes that make a community resilient, and a chain by construction contains none. If the only model is grass → rabbit → fox, removing rabbits necessarily starves the fox, and the prediction is confident and wrong. The web shows the fox also takes shrews, so the realistic prediction is a shift in diet with knock-on effects further down, which is what actually happens in communities where most consumers have more than one prey species. Webs contain alternative routes, so they can model a shift rather than a collapse

8. Removing the shrew could increase the amount of grass eaten. Explain the chain of reasoning.
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   The shrew preys on grasshoppers, so removing it reduces the death rate of grasshoppers and their numbers should rise. Grasshoppers eat grass, so more of them means heavier grazing pressure and less grass. The effect travels two steps down the web from an animal that never touched a blade of grass, which is an indirect effect, and it is exactly the kind of prediction a food chain cannot generate because the chain does not show that the shrew and the grass are connected through anything. Fewer shrews means more grasshoppers, and grasshoppers eat grass

9. A student draws the arrow as rabbit → grass and explains that the rabbit eats the grass. Explain why this is marked wrong.
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   The convention is not arbitrary: the arrow represents the direction energy moves, and energy moves from the grass into the rabbit when the rabbit eats. Drawing it the other way states that energy flows from the rabbit into the grass, which is false and would make every trophic level in the diagram inverted. Because the whole point of the diagram is to trace energy through the community, a reversed arrow is not a labeling slip but a claim about the biology that is the opposite of the truth. The arrow tracks energy flow, and energy moves into the consumer

10. What quantity is missing from every food web diagram, and why does it matter?
    Show the full solution

    The amount of energy or biomass passing along each arrow. Without it, every feeding link looks equally important, so a species that makes up ninety percent of a predator's diet is drawn exactly like one that makes up one percent. That makes the web useless for judging the size of an effect: it can say that losing rabbits will affect foxes but not whether the effect is trivial or catastrophic. Quantifying the arrows is what pyramids and productivity measurements in the next lessons are for. The size of each transfer, without which effect sizes cannot be judged

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Roughly what fraction of energy passes to the next trophic level?
   Show the full solution

   About 10 percent

2. Name the three destinations of the energy that does not pass on.
   Show the full solution

   Heat from respiration, undigested material egested as feces, and parts that are never eaten

3. Starting from 50,000 kJ in producers, how much reaches the secondary consumers?
   Show the full solution

   500 kJ, since 10 percent of 50,000 is 5,000 and 10 percent of that is 500

4. Why are food chains rarely longer than four or five links?
   Show the full solution

   Because the energy remaining after four or five transfers cannot support a viable population

5. How much producer biomass roughly supports 1 kg of tertiary consumer?
   Show the full solution

   About 1,000 kg

6. Explain why calling the missing 90 percent "wasted" is wrong.
   Show the full solution

   Most of it is spent rather than wasted. The largest share goes on respiration, which is the process that powers movement, growth, repair and every other activity that keeps the consumer alive, so an organism that did not lose that energy would be dead. Some is genuinely unavailable rather than misused, such as cellulose an animal cannot digest or bones it does not eat. Calling it waste implies an inefficiency that could be engineered away, when it is a direct consequence of being alive. It is spent on staying alive, chiefly through respiration

7. Explain why top predators are rare and hold large territories.
   Show the full solution

   Their energy supply is what is left after three or four multiplications by about a tenth, so a very large area of producers stands behind each individual predator. A territory has to contain enough prey, which in turn requires enough of their food, and so on down to the plants, meaning the land needed per predator is roughly a thousand times the land needed per herbivore of the same mass. Low density and large ranges follow directly, and so does their vulnerability when habitat is fragmented. Compounding losses mean each predator needs a very large producer base

8. A farmer can grow grain and sell it, or feed it to cattle and sell beef. Explain the energy argument.
   Show the full solution

   Feeding the grain to cattle inserts an extra trophic transfer between the crop and the person, and about ninety percent of the energy in the grain is spent on the cattle's own respiration, egested, or held in parts not eaten. The same field therefore delivers roughly ten times more food energy to people as grain than as beef. That is an argument about energy only: it says nothing about protein quality, about land that can support grazing but not crops, or about price, all of which bear on the real decision. Adding a trophic level costs about ninety percent of the energy

9. Why do the losses multiply rather than add?
   Show the full solution

   Because each transfer takes its percentage of whatever arrives at that level, not of the original amount. The second transfer operates on the thousand kilojoules that survived the first, so it yields a hundred rather than subtracting another fixed quantity. Repeated proportional reductions are a multiplication, so three steps give 0.1 times 0.1 times 0.1, a thousandfold reduction rather than a thirty percent one. This is why chain length collapses the available energy so quickly. Each step takes a proportion of what remains, which compounds

10. An ecosystem is measured and the transfer between two levels is 18 percent. Does this refute the ten percent rule? Explain.
    Show the full solution

    No, because the rule is a working approximation rather than a constant of nature, and measured values range from roughly five to twenty percent. Transfer efficiency depends on what is being eaten and by what: consumers of other animals digest a larger share of what they swallow than herbivores chewing cellulose, and warm-blooded animals spend far more on respiration than cold-blooded ones. An eighteen percent figure sits within the expected range and invites an explanation, which is how a model should behave. The rule is an approximation with a real range, and 18 percent sits within it

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What does a pyramid of energy measure, and in what units?
   Show the full solution

   Energy passing through each trophic level over time, in kilojoules per square meter per year

2. Which pyramid can never be inverted?
   Show the full solution

   The pyramid of energy

3. Give an example of an inverted pyramid of numbers.
   Show the full solution

   One oak tree supporting thousands of insects

4. What does a pyramid of biomass measure?
   Show the full solution

   The mass of living material at each level at one moment in time

5. In the worked example, what is the ocean phytoplankton biomass, and why is the pyramid inverted?
   Show the full solution

   4 g/m², and the pyramid inverts because phytoplankton are replaced very rapidly

6. Explain, using thermodynamics, why an energy pyramid cannot invert.
   Show the full solution

   All the energy at a trophic level arrived from the level beneath it, and energy cannot be created within an organism. Every transfer also releases some energy as heat through respiration, and heat disperses where no organism can recapture it. A level therefore always passes on less than it received, so each bar must be narrower than the one below. An inverted energy pyramid would require a level to contain more energy than was supplied to it, which no biological process can achieve. Energy cannot be created, and each transfer loses some as heat

7. Explain turnover, and why it lets a biomass pyramid invert.
   Show the full solution

   Turnover is the rate at which organisms in a level are replaced. Phytoplankton may divide once a day and be grazed almost immediately, so the population present at any instant is small even though the mass produced across a year is enormous. Zooplankton live for weeks and accumulate mass. Because a biomass pyramid is a snapshot rather than a measure of production, it captures the small standing crop rather than the large output, and can show less mass at the base than above it. Rapid replacement means a small standing mass supplies a large total production

8. Why is a numbers pyramid the least useful of the three for comparing ecosystems?
   Show the full solution

   Because it treats every organism as equivalent regardless of size, so one oak counts the same as one aphid. Two ecosystems with identical energy flow can produce entirely different numbers pyramids depending only on whether their producers are large or small, which means the shape carries information about body size rather than about the ecosystem's function. Energy pyramids are comparable between ecosystems because a kilojoule is a kilojoule wherever it is measured. It ignores organism size, so its shape reflects body size rather than energy flow

9. A student says an inverted biomass pyramid shows the ecosystem is failing. Explain why this is wrong.
   Show the full solution

   Inversion here is a normal feature of open water communities rather than a sign of collapse, and the ocean ones that invert are among the most productive systems on the planet. The inversion arises from the mismatch between a snapshot measurement and a rate: the producers are supplying plenty, they are simply being consumed almost as fast as they appear. Reading it as failure mistakes a property of the measurement for a property of the ecosystem, which is the error lesson 1.6 warns about. It reflects fast turnover, and such systems are highly productive

10. You want to compare the productivity of a grassland and a forest. Which pyramid do you use and why?
    Show the full solution

    The pyramid of energy, because it measures the rate at which energy moves through each level over a stated period and so answers the question directly. A biomass pyramid would be badly misleading, since a forest holds enormous mass in wood that is not productivity but accumulated past growth, and a grassland with far less standing mass may fix comparable energy each year. A numbers pyramid would compare tree counts with grass plant counts, which bears on nothing. Energy, since it measures rate of flow rather than accumulated mass

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Name the two main groups of decomposers.
   Show the full solution

   Bacteria and fungi

2. What is extracellular digestion?
   Show the full solution

   Secreting enzymes onto material outside the body and absorbing the products

3. How does a detritivore differ from a decomposer?
   Show the full solution

   It eats dead material in pieces rather than digesting it externally

4. In the worked example, what percentage of litter is handled by bacteria and fungi?
   Show the full solution

   70 percent, since 4,200 divided by 6,000 is 0.7

5. What form must nitrogen be in before a plant can absorb it?
   Show the full solution

   An inorganic ion such as nitrate or ammonium

6. Explain why saying decomposers "return energy to the soil" is wrong.
   Show the full solution

   Decomposers respire the organic matter they consume, and respiration releases energy as heat that disperses into the surroundings where nothing can capture it. What returns to the soil is matter: ammonium, phosphate and other inorganic ions that producers can absorb. The confusion matters because it suggests energy circulates in an ecosystem, when the entire structure of this unit rests on energy making a one way trip and matter going round. They return nutrients; the energy leaves as heat from their own respiration

7. Why does breaking litter into smaller pieces speed decomposition?
   Show the full solution

   Decomposers digest externally, so the rate depends on how much surface the secreted enzymes can act on. Fragmenting a leaf into many small pieces greatly increases total surface area without changing the mass, exactly the surface to volume relationship from lesson 1.7, so more enzyme can work at once and the material is processed faster. This is why detritivores such as earthworms and woodlice accelerate decomposition even though they digest relatively little of the material themselves. Fragmentation raises surface area for extracellular enzymes

8. Predict what happens to a woodland over fifty years if its decomposers are removed but everything else continues.
   Show the full solution

   Leaf litter and dead wood accumulate instead of being processed, so the nitrogen, phosphorus and other nutrients held in that tissue are never returned to inorganic form. Soil nutrient concentrations fall as living plants continue to take up what remains, and primary productivity declines because producers become nutrient limited. Sunlight still arrives and photosynthesis still works in principle, so the ecosystem fails for want of accessible matter rather than for want of energy. Nutrients lock up in undecayed tissue and producers become nutrient limited

9. In many forests more energy flows through the detrital web than the grazing web. Suggest why.
   Show the full solution

   Most of a tree's mass is wood, bark and root, which are structural, low in nutrients and difficult or impossible for herbivores to digest, so the great majority of what a forest produces is never eaten while alive. Leaves are shed rather than grazed in most years. All of that material eventually enters the system as dead organic matter, so the detrital pathway receives the bulk of the annual production while the grazing pathway receives the small palatable fraction. Most forest production is woody or shed rather than eaten alive

10. How are decomposers connected to the existence of coal?
    Show the full solution

    Coal formed from plant material that escaped decomposition, typically because it was buried in waterlogged, oxygen-poor swamp conditions where decomposers could not respire it away. The carbon in that tissue was therefore removed from the fast biological cycle and locked into rock over geological time. Every carbon atom in coal is one that decomposers failed to return to the atmosphere, which is why burning it now releases carbon that has been out of circulation for hundreds of millions of years. Coal is carbon that decomposers never processed, buried out of circulation

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Write the relationship between GPP, NPP and respiration.
   Show the full solution

   NPP equals GPP minus respiration by the producers

2. Which of the two is available to consumers?
   Show the full solution

   Net primary productivity

3. A meadow has a GPP of 25,000 and respires 15,000 kJ/m²/yr. Find the NPP.
   Show the full solution

   10,000 kJ per square meter per year

4. Name the two factors that usually limit productivity on land.
   Show the full solution

   Temperature and water availability

5. Name the two that usually limit it in the ocean.
   Show the full solution

   Light and mineral nutrient supply

6. Explain why a mature forest can have huge biomass and only moderate NPP.
   Show the full solution

   Biomass is accumulated over a century of past growth, while NPP measures only what is added this year. A mature forest carries an enormous mass of living wood, bark and roots, all of which respire, so a large share of the energy it fixes each year is spent simply maintaining tissue that already exists. The net addition is therefore modest relative to the standing stock. A young growing forest can have far less biomass and higher NPP, which shows the two quantities are independent. Biomass is accumulated stock; NPP is this year's rate, much of which goes on maintenance

7. Why do desert plants spend a larger fraction of GPP on respiration than forest plants?
   Show the full solution

   Respiration rate rises with temperature, and deserts are hot, so the maintenance cost per unit of tissue is high. At the same time water shortage forces stomata to close for much of the day to limit transpiration, which also blocks carbon dioxide entry and suppresses photosynthesis. The plant therefore has an unusually high bill and an unusually constrained income, so the proportion of gross production consumed by respiration is larger, seventy percent against sixty in the worked figures. Heat raises respiration while closed stomata suppress photosynthesis

8. The open ocean has low productivity per square meter but contributes a large share of global production. Explain.
   Show the full solution

   Productivity per unit area and total productivity are different quantities, and the ocean wins on area. Open ocean covers the majority of the planet's surface, so even a modest 2,000 kJ per square meter per year multiplied across that area gives an enormous total. Highly productive ecosystems such as tropical forest have far higher rates but occupy a small fraction of the surface. Global contribution is rate multiplied by area, and neither factor alone determines it. Total production is rate times area, and the ocean's area is vast

9. An area of ocean near a coast is far more productive than open ocean at the same latitude. Suggest why.
   Show the full solution

   Coastal water receives mineral nutrients from rivers and from upwelling that brings deep nutrient-rich water into the lit surface layer, and it is shallow enough that nutrients released by decomposition on the bottom return to where photosynthesis happens. In the open ocean, material that sinks below the lit zone is effectively removed, so surface water becomes nutrient poor even though light is plentiful. Since nutrients rather than light are the limiting factor, the coastal supply raises productivity sharply. Coastal water gets nutrients from rivers and upwelling into the lit layer

10. Would clearing a temperate forest and planting fast-growing crops raise or lower NPP? Explain what the answer depends on.
    Show the full solution

    It could do either, and the answer depends on how much of the year the ground carries growing plants and how the maintenance cost changes. A crop removes the large respiring mass of wood, which lowers the respiration term and can raise net production during the growing season. But cropland is often bare for months, fixing nothing, while the forest fixed energy for as long as its leaves were out. Comparing them requires annual totals rather than peak rates, plus any irrigation or fertilizer that lifts the crop's limits. Lower maintenance respiration against fewer growing days, so annual totals decide it

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Which verb goes with energy, and which with matter?
   Show the full solution

   Energy flows; matter cycles

2. In what form does energy leave an ecosystem?
   Show the full solution

   As heat

3. At which process do the paths of a carbon atom and its energy separate?
   Show the full solution

   Respiration

4. Is an ecosystem open or closed with respect to energy?
   Show the full solution

   Open: energy enters as light and leaves as heat

5. What do decomposers return, and what do they not return?
   Show the full solution

   They return nutrients in inorganic form; they do not return energy

6. Explain why the sentence "energy is recycled in an ecosystem" is always wrong.
   Show the full solution

   Every transfer and every metabolic process converts some energy to heat, and heat disperses into the surroundings at a temperature no organism can exploit to do work. Recycling would require collecting that dispersed heat and rebuilding it into chemical bonds, which nothing in biology can do. The energy in an ecosystem is therefore a stream passing through rather than a pool going round, and it must be replenished continuously from outside, which is exactly why the sun cannot stop shining. Energy degrades to dispersed heat, which no organism can recapture

7. A sealed glass sphere contains water, algae, a shrimp and bacteria, and sits on a windowsill. Explain why it can persist for years.
   Show the full solution

   It is closed to matter and open to energy, which is exactly the condition an ecosystem needs. Light passes through the glass continuously, so the algae can keep fixing energy, while the carbon, nitrogen and other atoms inside are recycled indefinitely between the algae, the shrimp and the bacteria. Nothing material has to enter or leave. Move it into a dark cupboard and it dies within weeks, not because matter ran out but because the energy supply was cut. Light keeps entering while the atoms inside cycle

8. Why does the ten percent rule apply to energy but not to carbon atoms?
   Show the full solution

   The ten percent rule describes energy lost from the food chain at each transfer, principally as heat through respiration, and that heat genuinely leaves the system. Carbon atoms are not lost in the same sense: the ninety percent that does not enter the next trophic level is still present, as carbon dioxide breathed out, as feces feeding the detrital web, or in uneaten tissue that decomposers process. The energy is gone from the ecosystem; the carbon has merely taken a different route within it. Lost energy leaves as heat, while unpassed carbon stays in the system by another route

9. Why is the earth as a whole a good example of this distinction?
   Show the full solution

   Almost no matter enters or leaves the planet, so every atom in every living thing has been used many times over and will be used again, which makes the global matter cycles genuinely closed loops. Energy behaves in the opposite way: sunlight arrives continuously and an equivalent amount of heat is radiated back to space, so the planet is a throughput system for energy. The biosphere runs on that steady stream rather than on any stored supply, which is the same structure as the sealed sphere on a larger scale. The planet is closed to matter and open to a continuous energy throughput

10. Rewrite this sentence correctly: "Decomposers recycle the energy in dead organisms back to the plants."
    Show the full solution

    A correct version: "Decomposers release the energy in dead organisms as heat through their own respiration, and return the nutrients those organisms contained to inorganic forms such as ammonium and phosphate, which plants can absorb." The correction replaces energy with nutrients as the thing returned, names respiration as the fate of the energy, and specifies the inorganic forms, since dead tissue is not directly usable by a root. Each of those three changes fixes a separate error in the original. Nutrients are returned in inorganic form; the energy leaves as heat

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define a population.
   Show the full solution

   The individuals of one species in one area at one time, able to interbreed

2. Name the three dispersion patterns and say which is commonest.
   Show the full solution

   Clumped, uniform and random; clumped is commonest

3. Write the Lincoln index in words.
   Show the full solution

   Population equals number marked times size of second sample, divided by number of marked individuals recaptured

4. Forty are marked, 50 are caught later and 10 are marked. Estimate the population.
   Show the full solution

   40 times 50 divided by 10, which is 200

5. Which sampling method suits a plant, and which a mouse?
   Show the full solution

   Quadrats for the plant, mark and recapture for the mouse

6. Explain why quadrats must be placed at random.
   Show the full solution

   Because a person choosing where to put them will not choose neutrally. Quadrats get placed where the plants are interesting to count, or where the ground is easy to walk on, and either bias shifts the mean away from the true density of the whole field. Random placement, generated from coordinates rather than judgment, makes the sample representative so that scaling up is valid. It also means the spread between quadrats genuinely reflects patchiness in the field rather than patchiness in the surveyor's choices. Human placement is biased, and the scaling step assumes a representative sample

7. A student marks 50 fish, and the next day catches 50 of which 25 are marked. They report a population of 100. Explain why this may be badly wrong.
   Show the full solution

   The arithmetic is right but the assumptions are almost certainly broken. One day is unlikely to be long enough for marked fish to mix evenly back through the lake, so a second sample taken near the release point will be rich in marked fish. That inflates the recapture number, and since the recapture number is the divisor, it drives the estimate down. A population of 100 from a sample of 50 also implies catching half the lake's fish in a day, which should itself prompt suspicion. Too little time for mixing inflates recaptures and understates the population

8. Why is clumped dispersion the commonest pattern in nature?
   Show the full solution

   Because resources are almost never spread evenly. Water, shade, soil depth and food occur in patches, so organisms accumulate where conditions are tolerable and are absent where they are not. Many species also gain directly from being together, through group defense, shared warmth or easier mate finding, and many plants drop seeds close to the parent. Uniform spacing requires something actively pushing individuals apart, such as territoriality, and true randomness requires individuals to be indifferent to both resources and each other. Resources are patchy and many species benefit from grouping

9. Explain how marking that makes an animal more visible to predators biases the estimate, and in which direction.
   Show the full solution

   Marked animals would suffer higher mortality than unmarked ones, so by the time of the second sample the marked fraction of the population would be smaller than it should be. Fewer marked individuals would be recaptured, and since the recapture count sits on the bottom of the Lincoln index, a smaller divisor produces a larger answer. The estimate would therefore be too high, and it would be confidently too high, since nothing in the arithmetic signals that anything has gone wrong. Fewer recaptures means a smaller divisor, so the estimate is too high

10. Why is density often more useful than the total count?
    Show the full solution

    Because a raw count says nothing without the area it came from. Three hundred voles is a crowded meadow or an almost empty forest depending on whether the area is a hectare or a hundred, so totals from different sites cannot be compared. Density divides that out and makes the figure portable: it can be set against another site, against the same site in another year, or against a known carrying capacity. It is also the quantity that actually matters biologically, since competition and disease transmission depend on how close individuals are. Density is comparable between areas and is what competition and disease depend on

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What shape does exponential growth give on a graph?
   Show the full solution

   A J curve

2. How is per capita growth rate calculated?
   Show the full solution

   Births minus deaths, divided by the population size

3. A culture of 200 cells doubles every 30 minutes. How many after 2 hours?
   Show the full solution

   Four doublings, so 200 times 16, which is 3,200

4. What assumption does the exponential model make?
   Show the full solution

   That resources are unlimited

5. A population of 500 has 90 births and 40 deaths in a year. Find the per capita growth rate.
   Show the full solution

   50 divided by 500, which is 0.10 per year

6. Explain why "exponential" does not simply mean "fast".
   Show the full solution

   Exponential describes the shape of the relationship rather than the speed. A population growing at one percent a year is growing exponentially, because the increase each year is proportional to the number present, so the curve steepens even though it looks almost flat for decades. A population adding exactly a hundred individuals annually is growing quickly at first but linearly, and its graph is a straight line forever. The distinction is whether the growth rate depends on current size. It means growth proportional to current size, whatever the speed

7. Why does exponential growth always stop in the real world?
   Show the full solution

   Because its assumption is physically impossible to sustain. Every environment has a finite supply of food, water, space, oxygen and nesting sites, and a finite ability to dilute or remove waste. As the population multiplies, those supplies are divided among more individuals and waste accumulates, so birth rates fall and death rates rise. The population cannot go on multiplying because the conditions that permitted multiplication have been consumed by the multiplying itself. Finite resources and accumulating waste break the unlimited assumption

8. Why does a bacterial culture grow exponentially at first even though the flask is obviously finite?
   Show the full solution

   Because while the population is small relative to the supply, the resources are effectively unlimited from each cell's point of view: no cell is short of nutrients, nothing is crowded, and waste is too dilute to matter. Under those conditions the model's assumption holds well enough and the prediction is accurate. The flask's limits only begin to bite once the population is large enough to draw down the broth measurably, which is when growth departs from the J curve and the logistic model takes over. While the population is small the resources are effectively unlimited

9. A species is introduced to an island with no predators. Sketch in words what happens to its numbers and why.
   Show the full solution

   Growth is slow at first because there are few individuals reproducing, then accelerates into a J curve as each generation contributes more breeders, with no predation removing any of them. That cannot continue: food and space on the island are finite, so eventually the population either levels off near what the island can support or, commonly with introductions, overshoots the supply, strips the vegetation and crashes. Which of those happens depends on how quickly the damage to resources builds up relative to the reproduction rate. A J curve, then either leveling off or overshoot and crash as resources run out

10. Two populations both grow exponentially, one at 2 percent and one at 8 percent per year. Explain why the gap between them widens over time.
    Show the full solution

    Each population multiplies by its own factor every year, and repeated multiplication by different factors separates the totals faster and faster. The faster population is not only adding more individuals but adding them to a base that is itself growing more quickly, so next year's increase is larger again. After one year the difference is small, but the ratio between the two populations grows without limit, which is why small differences in growth rate matter enormously over decades even when they look trivial annually. Repeated multiplication by different factors compounds the difference

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What does K stand for?
   Show the full solution

   Carrying capacity, the population an environment can sustain indefinitely

2. What shape is the logistic curve?
   Show the full solution

   An S curve

3. At what population size is growth fastest?
   Show the full solution

   At about half of the carrying capacity

4. What is the growth rate when the population equals K?
   Show the full solution

   Zero

5. With K of 800 and a per capita rate of 0.5, find the growth at 400 individuals.
   Show the full solution

   0.5 times 400 times 0.5, which is 100 individuals per year

6. Explain why a population at carrying capacity has zero growth despite being the largest.
   Show the full solution

   Carrying capacity is defined as the size at which the environment's resources exactly support the individuals present, so births and deaths balance. Every new individual would need food, space and other resources that are already fully committed, and the shortfall raises mortality or suppresses reproduction until the balance is restored. The population is large precisely because it has used up the surplus that would allow it to get larger, which is why maximum size and maximum growth never occur together. At K resources exactly support the population, so births equal deaths

7. Explain why growth is slow at both very low and very high population sizes, for different reasons.
   Show the full solution

   At low numbers resources are abundant per individual, but there are very few individuals reproducing, so the total number of offspring produced is small even though each one has an easy time. At high numbers the opposite holds: there are many potential breeders but almost no spare resources, so reproduction is suppressed and mortality is high. Growth requires both plenty of breeders and plenty of resources, and only a population near half of capacity has a reasonable amount of each. Too few breeders at the bottom, too few resources at the top

8. A population overshoots K, then falls below it, then rises again in smaller swings. Explain this pattern.
   Show the full solution

   Populations respond to resource shortage with a delay, because individuals already born continue to grow and reproduce for a time after the supply has been outstripped. The population therefore climbs past K before the shortage takes effect, and the resulting mortality is severe enough to carry it below K, at which point resources recover and growth resumes. Each swing is smaller than the last because the mismatch shrinks, producing damped oscillation around the capacity rather than a smooth approach. A lag between resource depletion and population response causes damped oscillation

9. Why is carrying capacity not a fixed number for a given place?
   Show the full solution

   Because it depends on the resources actually available, and those change. A wet year grows more vegetation and raises K for herbivores; a drought lowers it within months. Habitat loss, a new disease, an introduced competitor or a change in season all shift it, and human action can raise it deliberately through irrigation and fertilizer or lower it through pollution. Treating K as a constant of the landscape leads to badly wrong predictions whenever conditions move, which is most of the time. It tracks available resources, which vary with season, weather and human action

10. A fishery wants the largest catch it can take year after year. Where should it hold the stock, and why not higher?
    Show the full solution

    At roughly half of carrying capacity, because that is where the population replaces itself fastest and therefore produces the largest surplus that can be removed without shrinking the stock. Holding it near K would mean a large standing population producing almost no net growth, so almost nothing could be taken sustainably. Fishing it far below half is worse still, since too few breeders remain. The counterintuitive result is that a deliberately reduced population yields more than a full one. Near half of K, because that is where net production is greatest

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define a density-dependent factor.
   Show the full solution

   One whose effect intensifies as population density rises

2. Give two examples of density-independent factors.
   Show the full solution

   Any two of flood, fire, drought, frost, volcanic eruption

3. Which kind of factor can regulate a population, and why?
   Show the full solution

   Density dependent, because it pushes harder when numbers rise and eases when they fall

4. State the test for classifying a factor.
   Show the full solution

   Ask whether doubling the density would change the proportion affected

5. Why is disease usually density dependent?
   Show the full solution

   Because transmission needs contact, and contact rates rise with crowding

6. Explain why the biotic and abiotic split is not the same as the density dependent and independent split.
   Show the full solution

   The two classifications answer different questions. Biotic and abiotic asks what kind of thing the factor is; density dependence asks how its effect scales with crowding. They often coincide, since competition and disease are both living and density dependent while frost is neither, but not always. Competition for a limited number of nest holes in rock is density dependent even though the holes are abiotic, and a parasite that arrives in a single contaminated water delivery can hit a population regardless of its density. One asks what the factor is, the other how its effect scales with density

7. Explain how density-dependent factors produce a carrying capacity.
   Show the full solution

   They create negative feedback. As the population rises, competition, disease and predation all intensify, which lowers births and raises deaths until growth stops. If the population then falls, those same pressures relax, births rise above deaths and the population recovers. The result is a level toward which numbers are pushed from both directions, which is exactly what carrying capacity describes. A factor that did not vary with density could never produce this, because it would push equally hard at every size. They provide negative feedback that pushes numbers toward a level from both sides

8. A hurricane destroys 80 percent of an island's birds. Explain why this does not set a carrying capacity.
   Show the full solution

   The hurricane would have destroyed a similar proportion whatever the population size, so it carries no information about what the island can support. It resets the number to a fifth of whatever it happened to be, producing a crash rather than a level, and it does not push back if the population subsequently grows too large or fall silent if it shrinks. After the storm the population simply resumes growing under the density-dependent factors, and it is those, not the hurricane, that determine where it settles. It hits the same proportion at any density, so it resets numbers without setting a level

9. Why do epidemics tend to break out in dense populations rather than sparse ones?
   Show the full solution

   Because a pathogen spreads only when an infected individual contacts a susceptible one, and the rate of such contacts climbs steeply with crowding. In a sparse population an infected individual may recover or die before meeting anyone, so the outbreak fails to establish. Above a threshold density each infection produces more than one new infection on average and the disease spreads through the population. This also explains why epidemics often end before infecting everyone: they reduce the density of susceptible individuals below that threshold. Transmission needs contact, and contact rates rise sharply with density

10. Classify "competition for a limited number of nesting holes in a cliff" and justify it.
    Show the full solution

    Density dependent. Apply the test: doubling the number of birds does not change the number of holes, so a larger share of the population fails to find one and is excluded from breeding. The effect therefore intensifies with density, which is the definition. The fact that the holes themselves are abiotic is irrelevant to this classification, and treating it as abiotic and therefore density independent is the standard mistake this lesson is built to prevent. Density dependent, since a fixed number of holes excludes a larger share as numbers rise

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What is the difference between intraspecific and interspecific competition?
   Show the full solution

   Intraspecific is within one species; interspecific is between different species

2. Define a niche.
   Show the full solution

   The full role of a species: what it eats, where and when it lives, and the conditions it tolerates

3. State the competitive exclusion principle.
   Show the full solution

   Two species cannot occupy identical niches in the same place indefinitely

4. In the worked example, in which years do hares and lynx peak?
   Show the full solution

   Hares in year 3, lynx in year 4

5. Name three ways species partition a resource.
   Show the full solution

   By time of activity, by location within the habitat, and by the size of food taken

6. Explain why the predator peak comes after the prey peak rather than with it.
   Show the full solution

   Predators convert food into offspring, and that conversion takes time: an abundance of prey must first improve adult survival and breeding condition, then gestation and rearing must follow before the extra predators actually exist. The predator population is therefore responding to conditions that applied months or a year earlier. By the time those extra predators are hunting, the prey population has often already begun to decline, which is why the two curves are offset rather than rising and falling together. Converting abundant prey into more predators takes time

7. Why is intraspecific competition usually more intense than interspecific?
   Show the full solution

   Because members of the same species have identical requirements. They need the same food of the same size, the same kind of nest site, the same conditions, and they are active at the same times, so the overlap is total. Two different species almost always differ somewhat in what they take or when they take it, so their demands overlap only partly and each has some resources the other does not use. Complete overlap means every individual is competing with every other for exactly the same things. Members of one species have completely overlapping requirements

8. Two warbler species feed in the same trees and coexist. Explain how this is possible without violating competitive exclusion.
   Show the full solution

   Their niches are not identical even though their habitat is. One may feed mainly in the outer branches near the top while the other works the inner branches lower down, or they may take insects of different sizes or breed at slightly different times. That partitioning reduces the overlap in what each actually uses, so each has a share of the resource the other does not exploit. Competitive exclusion forbids identical niches, not shared habitats, and the distinction between the two is what this question tests. They partition the resource, so their niches differ even in the same tree

9. A student writes that lynx control hare numbers. Rewrite the claim accurately.
   Show the full solution

   A better version: hare and lynx populations regulate each other through a lagged feedback loop, in which rising hare numbers allow lynx numbers to rise, the increased predation then drives hare numbers down, the resulting food shortage drives lynx numbers down, and reduced predation allows hares to recover. The original sentence describes only one arm of the loop and implies the lynx are the active party, when in fact each population is both cause and effect, which is why the pattern cycles instead of settling. Each population drives the other, so the relationship is a loop rather than one-way control

10. Predict what happens to hare numbers over several years if lynx are removed entirely, and say what limits them then.
    Show the full solution

    Hare numbers would rise steeply at first, since the main source of mortality has gone, and the cycle would flatten out because its driver has been removed. Growth would not continue indefinitely: other density-dependent factors would take over, chiefly competition for vegetation and the disease transmission that comes with high density, and the population would settle near a carrying capacity set by food supply. It might also overshoot and crash if grazing damaged the vegetation faster than the population responded. They rise until food competition and disease limit them at a new carrying capacity

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Give the sign pattern for each of the three categories.
   Show the full solution

   Mutualism plus and plus, commensalism plus and zero, parasitism plus and minus

2. What does symbiosis mean?
   Show the full solution

   A close, long-term relationship between two species, of any kind

3. Give an example of mutualism involving bacteria.
   Show the full solution

   Rhizobium in legume root nodules, or gut bacteria aiding digestion

4. Why is a parasite that rapidly kills its host at a disadvantage?
   Show the full solution

   It destroys its own habitat and its route to new hosts before reproducing fully

5. Why is commensalism hard to demonstrate?
   Show the full solution

   Proving an effect of exactly zero requires measurement, and closer study usually finds a small cost or benefit

6. Explain why calling mutualism cooperation is misleading.
   Show the full solution

   Cooperation implies intention and an agreement between partners, and neither exists. Each species is doing what increases its own survival and reproduction, and the relationship persists only because for each partner the benefit exceeds the cost. If conditions changed so that one partner gained less than it paid, selection would erode the relationship regardless of how well it had worked before. Describing it as a bargain also invites teleological explanations, which is the single most damaging habit in biology. Neither partner intends anything; each is acting in its own interest

7. Explain how a mutualism could become a parasitism.
   Show the full solution

   By a change in the balance of costs and benefits rather than a change in category. Gut bacteria that break down material the host cannot digest supply a genuine benefit at a small cost in nutrients, but if the host's immune system is weakened or the bacteria reach tissues outside the gut, the cost rises sharply while the benefit disappears. The species involved have not changed; the accounting has. This is why the three categories are best treated as regions of a continuum rather than fixed labels. The balance of costs and benefits shifts, which is all the categories describe

8. Explain why virulence is not a fixed property of a parasite.
   Show the full solution

   Virulence is shaped by selection acting on the parasite's own reproductive success, which depends on how long the host survives and how readily it transmits the parasite onward. Where transmission needs a mobile living host, selection favors restraint, and long-established parasites tend toward milder chronic infections. Where transmission happens by a vector or through water and does not need the host to move, high virulence carries less penalty. A parasite newly arrived in an unfamiliar host has had no such selection at all, and is often far more damaging. Selection tunes it to the transmission route and the length of host association

9. A bird eats insects disturbed by grazing cattle. Classify the relationship and justify it.
   Show the full solution

   Commensalism as usually described: the bird plainly gains an easier food supply, and the cattle appear unaffected, since the insects were disturbed as a side effect of grazing they would have done anyway. It is worth applying the caution from this lesson, though. If the bird also removes biting flies from the cattle, they gain and the relationship is mutualism, and if its presence disturbs grazing it becomes a small cost. Deciding requires measuring the effect on the cattle rather than assuming it is zero. Commensalism on the face of it, but the effect on the cattle should be measured

10. Why might a plant benefit from housing bacteria that cost it sugars?
    Show the full solution

    Because the nitrogen the bacteria supply is the factor actually limiting the plant's growth, while sugars are not. Plants can make sugars from light and carbon dioxide whenever conditions allow, but they cannot use atmospheric nitrogen, and soil nitrate is frequently in short supply. Paying in the abundant currency to obtain the scarce one is a net gain, which is why legumes can colonize poor soils where other plants fail. The relationship would not persist on soils already rich in nitrogen. It pays in sugars, which it can make, for nitrogen, which limits it

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define succession.
   Show the full solution

   Directional change in a community over time following disturbance or the exposure of new ground

2. What distinguishes primary from secondary succession?
   Show the full solution

   Primary starts with no soil; secondary starts where soil remains

3. Name two pioneer species types on bare rock.
   Show the full solution

   Lichens and mosses

4. Give two properties that make a good pioneer.
   Show the full solution

   Tolerance of harsh conditions and wide dispersal

5. Why is secondary succession faster?
   Show the full solution

   Soil, nutrients and a seed bank already exist

6. Explain how a stage can make conditions worse for itself.
   Show the full solution

   Grasses build soil by adding organic matter as they die back each year, and deeper soil is exactly what shrubs and trees require to establish. Once those larger plants arrive they cast shade, and the grasses are light demanding, so their growth is suppressed by the conditions their own accumulated remains made possible. The stage has not made a mistake and is not sacrificing itself; it has simply altered its surroundings in a way that suits a competitor better than it suits itself. Grasses build the soil that supports the shrubs whose shade excludes them

7. Explain why lichens can colonize bare rock when grasses cannot.
   Show the full solution

   Grasses need soil to anchor roots and to supply water and mineral ions, and bare rock offers none of these. Lichens require no soil at all: they attach directly to the rock surface, absorb water and minerals from rain and dust, and tolerate drying out completely between rainfalls. Their combination of a fungus and a photosynthetic partner means they can also make their own food in place. Those tolerances let them occupy a surface that offers nothing a flowering plant can use. Lichens need no soil and tolerate desiccation; grasses need both

8. Why is the idea of a climax community now treated with caution?
   Show the full solution

   Because it implies a single stable end state that a site reliably reaches and then holds, and real landscapes rarely behave that way. Fire, storm, disease and flood recur on timescales shorter than the time needed to arrive at such a state, so many communities are permanently somewhere in the middle of recovering from something. The same starting point can also end in different communities depending on which species happen to arrive first. The classic model describes a tendency rather than a destination. Disturbance is frequent and outcomes vary, so a fixed end state is rarely reached

9. Suggest how an ecologist can study a process that takes centuries.
   Show the full solution

   By substituting space for time, comparing sites of different known ages that are otherwise similar, which is called a chronosequence. Lava flows from eruptions in different centuries, or land exposed at different distances from a retreating glacier, can be surveyed in a single season and arranged in order of age to infer the sequence. The method assumes the sites differ only in age and followed the same path, which is its weakness, so it is strengthened by combining it with long-term records from fixed plots. A chronosequence: compare sites of different known ages instead of waiting

10. A field is abandoned and a woodland stands there sixty years later. Explain the sequence and why it is secondary succession.
    Show the full solution

    It is secondary because the soil, its nutrients and a bank of buried seeds survived the farming, so the slow work of building soil from rock was not required. Fast growing annual weeds colonize within the first season from that seed bank and from wind-blown seed, followed by perennial grasses and herbs, then shrubs and fast growing light-demanding trees such as birch, and finally slower shade-tolerant trees that can germinate beneath an existing canopy. Each stage alters light and soil conditions in ways that favor the next. Soil and seed bank survived, so weeds give way to grasses, shrubs and then trees

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What is the difference between a reservoir and a flux?
   Show the full solution

   A reservoir is a store measured as an amount; a flux is a transfer measured as an amount per unit time

2. Define transpiration.
   Show the full solution

   Loss of water vapor from plant leaves to the atmosphere

3. Write the formula for residence time.
   Show the full solution

   Reservoir size divided by flux

4. A pond holds 9,000 cubic meters with an inflow of 750 per day. Find the residence time.
   Show the full solution

   12 days

5. Name three processes that move water out of a soil reservoir.
   Show the full solution

   Transpiration by plants, evaporation from the surface, and runoff or drainage

6. Explain why a long residence time makes pollution harder to deal with.
   Show the full solution

   Residence time measures how quickly a reservoir's contents are replaced, so a long one means there is no substantial flux carrying material away. A contaminant entering such a store is diluted slowly and flushed slowly, and it stays available to organisms drawing on that reservoir for the whole period. An aquifer with a residence time of decades will still contain a pollutant long after the source has been shut off, which is why groundwater contamination is treated far more seriously than an equivalent spill into a fast flowing river. No fast flux removes it, so the contaminant persists for the length of the turnover

7. A forest is cleared and local rainfall falls. Explain the mechanism.
   Show the full solution

   Trees move large volumes of water from soil to atmosphere by transpiration, drawing it up through roots and releasing it as vapor from their leaves. That vapor is a genuine input to the local atmosphere and contributes to cloud formation and rainfall downwind. Removing the trees removes the flux, so less water enters the air over that area, and rainfall can decline as a result. The cleared ground also sheds more water as runoff rather than storing it, which reinforces the drying. Removing trees removes the transpiration flux that supplied local atmospheric water

8. Two lakes have the same residence time but one is ten times larger. What can you say about their inflows?
   Show the full solution

   The larger lake must have ten times the inflow. Residence time is reservoir divided by flux, so holding the ratio constant while multiplying the reservoir by ten requires multiplying the flux by ten as well. This is worth noticing because it shows residence time carries no information about the absolute size of either quantity: a small pond fed by a trickle and a great lake fed by a major river can behave identically in how fast their contents are replaced. The larger lake has ten times the inflow

9. Why is the water cycle described as closed while the energy supply is not?
   Show the full solution

   Water molecules are neither created nor destroyed as they move between ocean, atmosphere, soil and organisms, so the same molecules circulate indefinitely and the planetary total stays effectively constant. The energy driving that circulation behaves differently: sunlight supplies the energy to evaporate water, and that energy is released again as heat when the vapor condenses and eventually radiates to space. The matter goes round while the energy passes through, which is the distinction from lesson 2.7 applied to a specific cycle. The molecules circulate indefinitely while the driving energy passes through once

10. Suggest why the residence time of water in the atmosphere is only about ten days, despite the atmosphere being enormous.
    Show the full solution

    Because the atmosphere holds very little water relative to how fast water moves through it. Air can carry only a small mass of vapor before it saturates and precipitates, so the reservoir is small, while evaporation from the oceans and transpiration from land supply an immense flux every day. A small store divided by a very large flux gives a short residence time. The physical size of the atmosphere is irrelevant: what matters is the quantity of water it actually contains. A small water store divided by a very large daily flux

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Name four carbon reservoirs.
   Show the full solution

   Atmosphere, oceans, living biomass and soil, and rock including fossil fuels

2. Which two processes drive the fast carbon cycle?
   Show the full solution

   Photosynthesis and respiration, including decomposition

3. How long does the slow carbon cycle take to turn over?
   Show the full solution

   Millions of years

4. In the worked example, what is the net annual change to the atmosphere?
   Show the full solution

   An increase of 5 units per year

5. What happens to ocean pH as it absorbs carbon dioxide, and why?
   Show the full solution

   It falls, because dissolved carbon dioxide forms carbonic acid

6. Explain why burning fossil fuels is different from burning wood, in carbon cycle terms.
   Show the full solution

   Wood is fast-cycle carbon: the tree fixed it from the atmosphere within the last few decades, and burning it returns carbon that was recently removed, so over the life of a regrowing forest the exchange can be close to balanced. Fossil fuel is slow-cycle carbon that left the fast cycle hundreds of millions of years ago and would have stayed in rock for millions more. Burning it adds carbon the fast cycle was not previously handling, which is an addition rather than a return. Wood returns recently fixed carbon; fossil fuel injects slow-cycle carbon into the fast cycle

7. The photosynthesis flux is twelve times the fossil fuel flux. Explain why the smaller one still changes the atmosphere.
   Show the full solution

   Because size is not what matters; balance is. Photosynthesis is matched almost exactly by respiration and decomposition returning the same carbon, so the two large fluxes cancel and leave the reservoir unchanged however big they are. The fossil fuel flux has no matching removal, since nothing is burying carbon back into rock at a comparable rate, so every unit of it accumulates. A small unmatched flux changes a reservoir while an enormous balanced pair does not. Large fluxes cancel; a small unmatched one accumulates

8. Explain why planting trees slows atmospheric carbon increase but does not reverse fossil fuel emissions permanently.
   Show the full solution

   A growing forest genuinely removes carbon dioxide and stores it as wood, so while it is growing it is a real sink. But that carbon is now in the fast cycle, and it returns to the atmosphere when the trees die and decompose or if the forest burns, so the storage lasts decades to centuries rather than the millions of years the carbon had previously been out of circulation. The forest also stops accumulating once mature. It buys time and moves the problem, rather than restoring the slow-cycle burial that fossil fuels reversed. Tree carbon stays in the fast cycle and returns on death or fire

9. Why does carbon escaping decomposition matter for the slow cycle?
   Show the full solution

   Decomposers normally return the carbon in dead tissue to the atmosphere within months or years, keeping it in the fast cycle. Where conditions prevent that, typically waterlogged oxygen-poor sediment, the material accumulates and can be buried, compressed and eventually incorporated into rock. That burial is the only route by which carbon leaves the fast cycle for geological time, and it is how coal, oil and gas formed. The slow cycle is therefore built entirely from decomposition's failures. Burial of undecomposed material is the only route out of the fast cycle

10. The ocean absorbs part of the excess carbon dioxide. Explain why this is not simply good news.
    Show the full solution

    The uptake slows the rise in atmospheric concentration, which is a real benefit, but the carbon does not vanish. Dissolved carbon dioxide reacts with water to form carbonic acid, lowering ocean pH, and more acidic water makes it harder for corals, molluscs and some plankton to build calcium carbonate shells and skeletons. Since many of those organisms sit at the base of marine food webs, the cost is transferred from the atmosphere to the ocean rather than removed, which is a trade rather than a solution. It transfers the problem to ocean chemistry, harming shell-builders

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What proportion of the atmosphere is nitrogen, and why can plants not use it?
   Show the full solution

   About 78 percent, and the triple bond in N₂ is too strong for them to break

2. Name the process that converts N₂ to ammonium.
   Show the full solution

   Nitrogen fixation

3. What does nitrification produce, and why does it matter?
   Show the full solution

   Nitrate, which is the form most plants absorb

4. Which process returns nitrogen from dead tissue to the soil?
   Show the full solution

   Ammonification, by decomposers

5. Which process returns nitrogen to the atmosphere?
   Show the full solution

   Denitrification, by denitrifying bacteria in oxygen-poor soil

6. Explain why a farmer might plant clover in a field between crops.
   Show the full solution

   Clover is a legume, and its root nodules house nitrogen-fixing bacteria that convert atmospheric N₂ into ammonium. Growing clover therefore adds usable nitrogen to the soil rather than depleting it, and plowing the crop in releases that nitrogen through ammonification as the tissue decomposes. Since nitrogen is usually the nutrient limiting crop yield, this restores fertility without manufactured fertilizer, which is why the practice long predates any understanding of the bacteria involved. Its root nodule bacteria fix atmospheric nitrogen, restoring soil fertility

7. Why is waterlogging bad for soil fertility, in nitrogen terms?
   Show the full solution

   Water displaces air from the soil spaces, so oxygen becomes scarce. Denitrifying bacteria thrive in those conditions because they can respire using nitrate in place of oxygen, and doing so converts nitrate into nitrogen gas that escapes to the atmosphere. The soil therefore loses precisely the form of nitrogen plants can absorb. At the same time the low oxygen slows the nitrifying bacteria that would replace it, so losses rise while resupply falls. Low oxygen favors denitrification, which strips nitrate from the soil

8. Explain why the nitrogen cycle depends more completely on bacteria than the carbon cycle does.
   Show the full solution

   Every step of the nitrogen cycle that changes the chemical form of nitrogen is carried out by bacteria: fixation, both stages of nitrification, and denitrification, with only ammonification shared with fungi. No plant or animal can perform any of them. The carbon cycle is quite different, since plants fix carbon themselves through photosynthesis and all organisms return it through respiration, so carbon can move through its cycle without any bacterial step at all. Remove the bacteria and the nitrogen cycle stops entirely. Bacteria perform every form-changing step, which no plant or animal can do

9. A plant is grown in distilled water with all minerals except nitrate. Predict and explain the result.
   Show the full solution

   The plant grows poorly and turns yellow, particularly in its older leaves. Nitrogen is required to build amino acids and therefore all proteins, including enzymes, and it is a component of chlorophyll, so a shortage limits both growth and photosynthesis. The yellowing appears first in older leaves because the plant moves the limited nitrogen it has to the growing tips, where it is most needed. Photosynthesis is not directly blocked, since light, water and carbon dioxide are all present, but the machinery cannot be built. Stunted growth and yellowing, since nitrogen is needed for proteins and chlorophyll

10. Why does the nitrogen in a leaf not enter through the leaf?
    Show the full solution

    Leaves exchange gases with the air through stomata, but the nitrogen in air is N₂, which the plant has no means of breaking apart. The nitrogen in its proteins arrived as nitrate ions dissolved in soil water, absorbed by root hairs and carried upward in the xylem. The atoms did come from the atmosphere originally, but only after nitrogen-fixing and nitrifying bacteria converted them into an absorbable form in the soil, so the route is through the roots rather than the leaf surface. Leaves cannot use N₂; nitrate enters through the roots

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What makes the phosphorus cycle different from the carbon and nitrogen cycles?
   Show the full solution

   It has no significant atmospheric phase

2. What is the natural source of phosphate?
   Show the full solution

   Weathering of rock

3. Name three biological molecules that contain phosphorus.
   Show the full solution

   DNA, RNA, ATP and phospholipids

4. Which organisms cause the oxygen crash in eutrophication?
   Show the full solution

   Decomposing bacteria respiring the dead material

5. Put in order: oxygen falls, algal bloom, fish die, nutrient enters, light blocked, decomposers multiply.
   Show the full solution

   Nutrient enters, algal bloom, light blocked, decomposers multiply, oxygen falls, fish die

6. Explain why saying "the algae use up the oxygen" is wrong.
   Show the full solution

   Algae are photosynthetic, so during daylight they release oxygen rather than consuming it, and a healthy bloom raises daytime oxygen concentrations. They do respire, but their net contribution while alive is positive. The crash comes after they die, when bacteria decomposing the enormous mass of dead algae and shaded plants respire aerobically on a scale the water cannot supply. Naming the algae rather than the decomposers identifies the wrong organism at the critical step of the sequence. Living algae release oxygen; the bacteria decomposing them consume it

7. Why is phosphorus naturally scarce in most ecosystems?
   Show the full solution

   It has no gas phase, so unlike carbon and nitrogen it cannot be drawn from a large atmospheric reservoir available everywhere. Its only natural source is the slow weathering of phosphate-bearing rock, which releases small quantities over long periods. Once washed into the sea it settles into sediment and can be locked away for millions of years until geological uplift exposes it again. Scarce supply and slow return together keep environmental concentrations low, which is why adding it produces such a dramatic response. No atmospheric reservoir, slow release by weathering, and long burial in sediment

8. Two lakes receive the same phosphate input, but only one develops a severe bloom. Suggest reasons.
   Show the full solution

   Phosphate may not be the limiting factor in the second lake: if light, nitrate or temperature limits its algae, extra phosphate changes little. The lakes may also differ in residence time, since a lake flushed in days exports the nutrient before algae can respond while a slow-turnover lake retains it. Depth and mixing matter too, as a deep well-mixed lake dilutes the input through a large volume, and a healthy population of zooplankton grazing the algae can suppress a bloom before it forms. Phosphate may not be limiting there, or flushing, depth and grazing may differ

9. Explain why the species of fish that survive tell you something about the cause.
   Show the full solution

   Species differ in how much dissolved oxygen they require, so a low oxygen event kills them in a predictable order rather than at random. Trout and other demanding species die first, while carp and some invertebrates tolerate low concentrations and persist. Finding a community reduced to tolerant species is therefore evidence that oxygen was the killing factor, which points to decomposition following enrichment rather than to a toxin, since a poison would not sort the community by oxygen requirement. Oxygen-demanding species die first, so the surviving community identifies oxygen as the cause

10. Suggest two ways to reduce eutrophication, and say which step of the sequence each interrupts.
    Show the full solution

    Removing phosphate from sewage before discharge and applying fertilizer at rates crops can absorb both cut the input at step one, preventing the whole chain from starting, which is the most effective point to act. Planting uncultivated buffer strips of vegetation between fields and watercourses also acts at step one by intercepting runoff. Aerating a lake mechanically attacks step six instead, replacing the oxygen the decomposers consume, which treats the symptom and must be maintained indefinitely. Cut the nutrient input at the start, or aerate to offset the oxygen loss near the end

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What does negative feedback do to a change?
   Show the full solution

   It opposes it, pushing the system back toward a set point

2. What does positive feedback do?
   Show the full solution

   It reinforces the change, amplifying it

3. Does positive feedback mean a good outcome?
   Show the full solution

   No; it describes the direction of the effect, not whether it is beneficial

4. Give one biological example of each kind.
   Show the full solution

   Negative: body temperature regulation. Positive: blood clotting or childbirth contractions

5. Why is ice albedo a positive loop?
   Show the full solution

   Melting ice exposes dark water that absorbs more energy, raising temperature and melting more ice

6. Explain why negative feedback is what makes homeostasis possible.
   Show the full solution

   Homeostasis means holding an internal condition near a constant value despite changes outside, and that requires a mechanism that detects a departure and acts to reverse it. Negative feedback does exactly this: a rise triggers a response that lowers, a fall triggers one that raises, so the variable is pushed toward the set point from either direction. Positive feedback would do the opposite, driving the variable further from the set point once it moved, which is incompatible with maintaining a stable internal state. It reverses departures from the set point from either direction

7. Explain why a positive feedback loop must be stopped by something outside itself.
   Show the full solution

   By construction the loop makes the change larger, so nothing within it ever reduces the driver, and it contains no mechanism for stopping. Left alone it accelerates until some external limit intervenes: childbirth contractions intensify until the baby is delivered and the stretch stimulus is removed, and clotting accelerates until the damaged surface is sealed. In each case the terminating event comes from outside the loop. This is why positive feedbacks in the climate system are treated so seriously. The loop only amplifies, so termination must come from outside

8. Thawing permafrost releases methane, a greenhouse gas. Classify the loop and explain.
   Show the full solution

   Positive feedback. Warming thaws permafrost that has been frozen for millennia, decomposition of the newly available organic matter releases methane and carbon dioxide, those gases increase the greenhouse effect, temperatures rise further, and more permafrost thaws. The loop closes on its starting variable and each step reinforces rather than opposes the original warming. Nothing within the sequence slows it down, which is why it is treated as one of the more serious amplifying mechanisms in the climate system. Positive: warming releases gases that cause more warming

9. A student describes predator and prey cycles as positive feedback. Correct them.
   Show the full solution

   It is negative feedback with a time lag. Rising prey numbers lead to rising predator numbers, which then reduce the prey, and falling prey numbers reduce the predators, which lets the prey recover. Each half of the loop opposes the change that produced it, which is the definition of negative feedback, so the system is regulated rather than running away. The oscillation comes from the delay in the response rather than from amplification: without the lag the two populations would settle at a steady point. Negative feedback with a lag, which produces cycling rather than runaway

10. Why does the distinction between the two kinds matter for predicting climate?
    Show the full solution

    Because it determines whether a small initial change stays small. If the dominant loops are negative, the system absorbs a disturbance and returns close to where it was, and a modest forcing produces a modest response. If strong positive loops exist, the same forcing is amplified, and effects can be far larger than the original push and can continue after the push stops. Identifying which loops operate, and how strong each is, is therefore the central problem in projecting how much warming a given emission produces. Positive loops amplify a small forcing into a large response, negative loops absorb it

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Where and when did the continuous record begin?
   Show the full solution

   Mauna Loa, Hawaii, in 1958

2. What causes the annual sawtooth?
   Show the full solution

   Seasonal photosynthesis and decomposition, dominated by northern hemisphere vegetation

3. Calculate the mean annual rise between 1960 and 2020 from the table.
   Show the full solution

   97 ppm over 60 years, which is about 1.6 ppm per year

4. What was the rate between 2000 and 2020?
   Show the full solution

   45 ppm over 20 years, which is 2.25 ppm per year

5. Why can this dataset be used and republished freely?
   Show the full solution

   It is a US federal government dataset, maintained by NOAA

6. Explain why the seasonal cycle cannot account for the long-term rise.
   Show the full solution

   The seasonal cycle is a closed loop that returns to its starting point each year: carbon drawn down by northern summer growth is released again by autumn and winter decomposition, so its net effect over any whole year is approximately zero. The long-term trend is visible when comparing the same month across different years, which removes the seasonal component entirely, and it still rises. A repeating oscillation cannot generate a sustained increase, no matter how large its amplitude. The seasonal loop nets to zero annually, so it cannot produce a sustained rise

7. Why does the northern hemisphere dominate the seasonal signal?
   Show the full solution

   Because the great majority of the world's land area lies north of the equator, and land is where seasonal vegetation grows. The southern hemisphere is largely ocean, whose phytoplankton do not produce a comparable synchronized annual pulse. Northern forests and croplands therefore green and senesce together, producing a drawdown and release large enough to appear in the global average. If land were distributed evenly, the two hemispheres' opposite seasons would largely cancel and the sawtooth would be much smaller. Most land, and therefore most seasonal vegetation, is in the north

8. A student says the graph proves humans caused the rise. Explain what more is needed.
   Show the full solution

   The graph establishes that concentration is rising and accelerating, which is a description rather than an attribution, and by itself it is consistent with any source of carbon. Establishing the cause needs evidence that identifies the carbon: the isotopic composition of the added carbon dioxide matches ancient plant material rather than volcanic sources, atmospheric oxygen is declining in the ratio combustion predicts, and the measured rise is consistent with recorded fuel consumption after accounting for ocean and land uptake. Isotopic signature, falling oxygen, and consistency with recorded fuel use

9. Why is comparing the same month across years a better method than comparing consecutive months?
   Show the full solution

   Because consecutive months differ for two reasons at once, the season and the trend, and the seasonal swing is much larger over a few months than the trend is. A fall from May to September reflects northern summer growth and says nothing about the long-term direction. Comparing every May with every previous May holds the seasonal position constant so that only the trend remains, which is the standard way of separating a cycle from an underlying change in any dataset. It holds the seasonal position constant so only the trend remains

10. Using lesson 4.2, explain why the concentration keeps rising even though photosynthesis removes far more carbon each year than burning fuel adds.
    Show the full solution

    Photosynthesis is one arm of a balanced loop: almost all the carbon it removes is returned within years by respiration and decomposition, so however large the flux, its net effect on the reservoir is close to zero. Fossil fuel burning has no matching return, since nothing is burying carbon into rock at a comparable rate, so it is an unmatched addition. The atmosphere accumulates the difference between additions and removals rather than responding to the size of any single flux. Photosynthesis is matched by respiration, while fossil carbon is added unmatched

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. State the three parts of cell theory.
   Show the full solution

   All living things are made of cells; the cell is the basic unit of structure and function; all cells come from pre-existing cells

2. What was Hooke looking at, and in what year?
   Show the full solution

   Cork, in 1665; the empty walls of dead plant cells

3. What did Leeuwenhoek add?
   Show the full solution

   Observations of living single-celled organisms

4. What idea did Pasteur's experiment defeat?
   Show the full solution

   Spontaneous generation

5. What did the swan neck allow in, and what did it exclude?
   Show the full solution

   It allowed air in and trapped dust and microorganisms in the bend

6. Explain why the shape of the flask neck mattered more than the boiling.
   Show the full solution

   Boiling sealed flasks had been done before and had not convinced anyone, because defenders of spontaneous generation argued that sealing excluded some vital principle in the air that living things needed to form. The swan neck removed that objection by leaving the flask open: air moved freely in and out, so any such principle had full access, while the bend trapped settling dust. The neck therefore isolated the single variable in dispute, which is exactly what the earlier experiments had failed to do. It admitted air while excluding dust, removing the vital principle objection

7. Identify the independent variable, dependent variable and control in Pasteur's experiment.
   Show the full solution

   The independent variable is whether airborne dust can reach the broth, manipulated by leaving the swan neck intact or snapping it off. The dependent variable is whether the broth becomes cloudy with microbial growth, and how quickly. The intact-neck flask serves as the comparison condition showing what happens without dust access. Everything else is controlled: the same broth, the same boiling, the same temperature, the same flask, and free access to air in both cases. Dust access is independent, microbial growth is dependent, and the intact flask is the comparison

8. Why did Hooke's observation not by itself establish cell theory?
   Show the full solution

   He observed a repeating compartmental structure in one dead plant tissue, which supports none of the three statements on its own. It does not show that animals are similarly built, since cork is not representative of life generally; it does not show the compartments are units of function, since what he saw were empty walls with no contents; and it says nothing about where new cells come from. Establishing a general theory required observations across many organisms and, for the third statement, a controlled experiment. One dead plant tissue supports none of the three statements generally

9. Spontaneous generation fitted everyday observation. Explain why, and what that teaches.
   Show the full solution

   Meat left out grows maggots, broth turns cloudy, and grain stores fill with mice, all apparently from nothing, so the idea was an economical reading of what people could actually see. It was wrong because the relevant causes were too small or too quick to observe: flies laying eggs, airborne spores settling. The lesson is that a theory can fit all the available observations and still be false, which is why controlled experiments that exclude one specific cause are worth more than any quantity of casual observation. It fitted the visible evidence, showing that observation alone cannot settle causes

10. Why is "all cells come from pre-existing cells" the most far-reaching of the three statements?
    Show the full solution

    Because it makes every living cell part of an unbroken chain of division stretching back through time, which means all life is connected by descent rather than arising repeatedly and independently. That single claim underlies the continuity of heredity, since genetic material must be copied and passed at each division, and it underlies common ancestry, which unit 10 develops. It also implies that a sterile environment stays sterile, which is the basis of food preservation, antiseptic surgery and every technique in microbiology. It makes all life a single unbroken chain of division, grounding heredity and common ancestry

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What is the defining difference between the two cell types?
   Show the full solution

   Eukaryotic cells have a membrane-bound nucleus; prokaryotic cells do not

2. Where is prokaryotic DNA found, and in what form?
   Show the full solution

   In the nucleoid region of the cytoplasm, as a single circular chromosome

3. Name four features both cell types share.
   Show the full solution

   Cell membrane, cytoplasm, ribosomes and DNA

4. Give typical sizes for each.
   Show the full solution

   Prokaryotic 1 to 2 micrometers, eukaryotic 10 to 100 micrometers

5. What is a plasmid?
   Show the full solution

   A small circular piece of DNA separate from the main chromosome in a prokaryote

6. Explain how internal membranes allow a cell to be larger and more complex.
   Show the full solution

   Membranes let a cell run incompatible processes at once by keeping them in separate compartments, so digestive enzymes can be concentrated inside a lysosome without destroying the cytoplasm, and conditions such as pH can differ between regions. Compartments also concentrate enzymes with their substrates instead of relying on chance encounters in one large space, which keeps reaction rates workable as volume grows. Without them, every reaction would share one environment, and the compromises required would limit both size and the number of processes possible. Compartments separate incompatible chemistry and concentrate reactions locally

7. Why does an antibiotic that disrupts peptidoglycan harm bacteria but not human cells?
   Show the full solution

   Peptidoglycan is the material of the bacterial cell wall and occurs nowhere in human cells, which have a membrane but no wall at all. A drug that blocks its synthesis therefore has a target present in one organism and absent in the other, so the bacterial cell cannot maintain its wall and bursts while human cells are unaffected. This is selective toxicity, and it is why structural differences between the two cell types are of direct medical importance rather than merely classificatory. Human cells contain no peptidoglycan, so the drug has no target in them

8. Explain why "prokaryotes are simple organisms" is misleading.
   Show the full solution

   It is accurate about structure and badly wrong about capability. Prokaryotes lack compartments and are smaller, so their architecture is simpler, but their metabolic range far exceeds that of eukaryotes. Only prokaryotes fix atmospheric nitrogen, only they carry out chemosynthesis at vents, and they perform forms of anaerobic respiration no eukaryote can manage. They also occupy environments lethal to everything else. Calling them simple invites the assumption that they are primitive or unimportant, when the nitrogen cycle depends entirely on them. Structurally simpler but metabolically far more capable

9. What do the shared features of the two cell types suggest, and why?
   Show the full solution

   They suggest descent from a common ancestor. Both use a phospholipid membrane, both store information in DNA, both translate it using ribosomes, and both read essentially the same genetic code. There is no functional reason the code had to be the same in two independently arising lineages, since many alternative codes would work equally well, so the shared arbitrary details are far better explained by inheritance from a single ancestral cell than by coincidence. This is the molecular evidence of lesson 10.7 in miniature. Common ancestry, since the shared genetic code is arbitrary and need not have matched

10. Bacteria divide in minutes while eukaryotic cells take hours. Suggest why.
    Show the full solution

    A prokaryote copies a single circular chromosome sitting free in the cytoplasm and separates the two copies as the cell elongates, a short and direct process. A eukaryote must replicate several much longer linear chromosomes, break down and rebuild the nuclear envelope, assemble a spindle, align the chromosomes and separate them precisely, then divide the cytoplasm and distribute organelles. More DNA, more structures to reorganize and more checkpoints to pass all add time, and the smaller prokaryotic cell also has less material to duplicate. Less DNA and no nucleus, spindle or organelles to reorganize

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What is the function of the nucleus?
   Show the full solution

   It stores DNA and controls the cell by determining which proteins are made

2. What do ribosomes do?
   Show the full solution

   Assemble proteins from amino acids using the instructions in mRNA

3. What is the difference between rough and smooth ER?
   Show the full solution

   Rough ER has ribosomes and handles proteins; smooth ER makes lipids and detoxifies

4. What does the Golgi apparatus do?
   Show the full solution

   Modifies, sorts and packages proteins into vesicles for dispatch

5. Name the organelle that carries out aerobic respiration.
   Show the full solution

   The mitochondrion

6. Explain why saying mitochondria "produce energy" loses a mark.
   Show the full solution

   Energy cannot be created, so nothing produces it. Mitochondria transfer energy that is already present in the bonds of glucose into the more usable form of ATP, and a substantial fraction is released as heat in the process rather than captured. Writing that they produce energy contradicts the conservation principle the whole of unit 2 rests on, and it also obscures what the mitochondrion actually does, which is convert a store the cell cannot spend directly into one it can. They transfer energy from glucose to ATP; energy is never made

7. A cell is found to be packed with rough ER and Golgi. Predict its job and justify it.
   Show the full solution

   It is almost certainly a secretory cell, producing proteins for export, such as a pancreatic cell making digestive enzymes or a plasma cell making antibodies. Rough ER is where proteins destined for export are fed and folded, and the Golgi modifies and packages them, so large amounts of both indicate a high throughput along that particular route. A cell making proteins only for its own cytoplasm would rely on free ribosomes and need neither organelle in quantity. A secretory cell, since that pathway handles proteins for export

8. Why does a muscle cell contain many mitochondria?
   Show the full solution

   Contraction requires ATP continuously, both to drive the sliding of the filaments and to reset the cell afterward, and a muscle cell can demand that energy at very high rates during activity. Since cells store only small amounts of ATP, supply must match demand almost moment to moment, which requires a large capacity for aerobic respiration. Numerous mitochondria provide it, and the pattern is general: mitochondrial density across cell types tracks how much ATP each type uses. Contraction demands ATP continuously, and ATP cannot be stored in quantity

9. Why must a lysosome's enzymes be enclosed in a membrane?
   Show the full solution

   They are digestive enzymes capable of breaking down proteins, lipids and nucleic acids, which is to say the materials the cell itself is built from. Released into the cytoplasm they would destroy the cell's own structures indiscriminately. The membrane confines them to a compartment where the pH is also kept low enough for them to work, which the cytoplasm is not, so even leakage is partly self-limiting. This is the clearest single illustration of why compartmentalization matters. They would digest the cell's own structures if released

10. Explain why the protein in the worked example never travels loose through the cytoplasm.
    Show the full solution

    Keeping it inside membrane-bound compartments at every stage serves several purposes at once. It prevents a digestive enzyme from acting on the cell's own contents, it keeps the protein with the modifying enzymes it needs to meet in sequence, and it allows the cell to direct the product to a specific destination by labeling the vesicle rather than relying on diffusion. Vesicle transport also lets the cell move large quantities in one operation and control the timing of release. Enclosure protects the cell, keeps the sequence ordered, and allows targeted delivery

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What scales with area, and what with volume?
   Show the full solution

   Exchange capacity scales with surface area; demand scales with volume

2. Give the surface to volume ratios of cubes of side 1, 2 and 3 cm.
   Show the full solution

   6, 3 and 2

3. In the worked example, what fraction of the 2 cm cube is reached?
   Show the full solution

   7 of 8, which is 87.5 percent

4. How does the time for diffusion change if the distance doubles?
   Show the full solution

   It quadruples, since time rises with the square of the distance

5. Name two ways a cell can raise its surface area.
   Show the full solution

   Microvilli or other folds, and a flattened or elongated shape

6. Calculate the fraction of a 4 cm cube reached by dye penetrating 0.5 cm, and comment.
   Show the full solution

   The unreached core measures 4 minus 1, which is 3 cm on each side, so its volume is 27 cubic centimeters. The total volume is 4 cubed, which is 64, so the colored portion is 64 minus 27, which is 37, and 37 divided by 64 is about 58 percent. The sequence across the four blocks is 100, 87.5, 70 and 58 percent, falling steadily as size rises even though the dye's speed never changes. Size alone determines how much of the interior is served. About 58 percent, continuing the decline from 100, 87.5 and 70

7. Explain why a cell divides rather than continuing to grow.
   Show the full solution

   As a cell enlarges, its volume and therefore its demand for oxygen, glucose and waste removal grow with the cube of its linear size, while the membrane area that must handle all that exchange grows only with the square. The distance from the membrane to the center also increases, and diffusion time rises with the square of distance, so the interior is served ever more slowly. Division restores both a high surface to volume ratio and a short diffusion path, which is why growth proceeds by increasing cell number. Demand outgrows exchange capacity and diffusion distances lengthen

8. Red blood cells are flattened discs with a dip in the middle. Explain the advantage.
   Show the full solution

   The shape increases surface area relative to volume compared with a sphere of the same content, so more membrane is available for oxygen to cross, and it reduces the maximum distance from the membrane to any point inside, so oxygen reaches all of the interior quickly. Since the cell's entire function is rapid uptake and release of oxygen, both effects matter directly. The flexible flattened shape also allows the cell to deform and pass through capillaries narrower than itself. More membrane per unit volume and a shorter internal diffusion distance

9. Why can a very large single-celled organism exist at all, given this argument?
   Show the full solution

   Because the constraint can be evaded by shape and by internal movement rather than by size alone. A large single-celled organism is typically long and thin or highly branched rather than spherical, which keeps every point close to the membrane despite a large total volume. Some also stream their cytoplasm actively, moving material by bulk flow rather than waiting for diffusion, and some contain many nuclei so that no single nucleus has to serve an impossible volume. Elongated shapes, cytoplasmic streaming and multiple nuclei evade the limit

10. Connect this lesson to why humans have lungs and a circulatory system.
    Show the full solution

    A human body has an extremely low surface to volume ratio and interior tissues centimeters from any outer surface, so diffusion across the skin could never supply them: the distances involved would take days. Lungs solve the area problem by folding an enormous exchange surface into a small space, the same trick as microvilli on a larger scale. The circulatory system solves the distance problem by carrying blood in bulk to within diffusion range of every cell, so the final step is always short. Lungs supply the area and circulation removes the distance, since diffusion alone cannot

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Describe the two ends of a phospholipid.
   Show the full solution

   A phosphate head attracted to water, and two non-polar fatty acid tails that are not

2. Why is the model called fluid mosaic?
   Show the full solution

   Fluid because components drift laterally; mosaic because of the scattered embedded proteins

3. Name two substances that cross the bilayer freely.
   Show the full solution

   Oxygen and carbon dioxide

4. Why can a sodium ion not cross unaided?
   Show the full solution

   It is charged, so it cannot enter the non-polar interior of the bilayer

5. Name three jobs done by membrane proteins.
   Show the full solution

   Channels or carriers for transport, receptors for signals, and enzymes

6. Explain why a bilayer forms spontaneously in water.
   Show the full solution

   Phospholipids have heads that interact favorably with water and tails that cannot, so any arrangement leaving tails exposed to water is unstable. Two sheets placed tail to tail solve the problem completely: every head faces water on one side or the other, and every tail is shielded in the interior. The structure assembles because it is the lowest energy arrangement available, with no instruction, template or energy input required, which is why artificial membranes form as soon as phospholipids are added to water. It is the most stable arrangement, shielding all tails while exposing all heads

7. Explain how membrane structure produces selective permeability.
   Show the full solution

   The core of the membrane is a layer of non-polar fatty acid tails, so substances that dissolve in fat can pass through it directly while charged or strongly polar substances cannot enter that region at all. Permeability therefore depends on a molecule's size and polarity rather than being uniform. Proteins spanning the bilayer provide specific routes for the substances the lipid excludes, so the cell ends up with a barrier that is open to some things, closed to others, and controllable for a third group. A non-polar core admits fat-soluble molecules and blocks charged ones, with proteins supplying exceptions

8. Why is it an advantage that some substances cannot cross without proteins?
   Show the full solution

   Because it gives the cell control. A substance that diffuses freely enters and leaves according to concentration alone, and the cell can do nothing about it, which is why cells cannot regulate their oxygen uptake. A substance that requires a transport protein can be controlled by how many transporters the cell makes, whether they are open, and how much ATP is spent pumping. Glucose uptake, ion balance and nerve signaling all depend on exactly this, and none would be possible through a freely permeable membrane. Protein-dependent entry allows the cell to regulate what comes in and out

9. Predict whether a small fat-soluble drug or a small charged one crosses more easily, and explain.
   Show the full solution

   The fat-soluble one, by a wide margin. It dissolves into the non-polar tail region and diffuses straight through the bilayer without needing any protein, so it can enter essentially any cell in the body. The charged drug cannot enter that region and must find a suitable transport protein, which most cells may not have, so its distribution is limited to tissues carrying the right transporter. This is a central consideration in drug design, and it explains why fat-soluble substances also cross into the brain more readily. The fat-soluble drug, since it dissolves in the non-polar interior

10. Why does membrane fluidity matter for the vesicle transport of lesson 5.3?
    Show the full solution

    Vesicle transport requires membranes to bend, pinch off, travel and then fuse seamlessly with another membrane, and every one of those operations needs the phospholipids to move relative to one another. In a rigid sheet a vesicle could not bud from the endoplasmic reticulum, could not fuse with the Golgi, and could not merge with the cell membrane during exocytosis. Fluidity also allows a membrane to reseal after minor damage rather than leaking, which is what makes the whole compartmental system viable. Budding, fusion and resealing all require the lipids to move

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define diffusion.
   Show the full solution

   Net movement of particles from higher to lower concentration by random motion

2. Name three factors that affect the rate of diffusion.
   Show the full solution

   Steepness of the gradient, temperature, surface area, and distance to travel

3. What makes facilitated diffusion passive?
   Show the full solution

   It moves substances down their gradient and uses no ATP

4. Define osmosis.
   Show the full solution

   Diffusion of water across a partially permeable membrane from higher to lower water potential

5. In the worked example, which way does water move and why?
   Show the full solution

   From A to B, because A has the higher water concentration

6. Explain why "water moves from low to high concentration" is a dangerous way to state osmosis.
   Show the full solution

   Because it omits the substance the word concentration refers to, and the two possible readings give opposite answers. Water moves from high water concentration to low water concentration, which is the same thing as moving toward the higher solute concentration. A student who writes the phrase without specifying gets no credit, and a student who is thinking of solute while writing water will predict the direction backwards. Naming the substance every time removes the ambiguity entirely. It omits which substance, and the two readings give opposite directions

7. Explain why facilitated diffusion has a maximum rate that simple diffusion does not.
   Show the full solution

   Simple diffusion can use any part of the membrane, so raising the gradient keeps raising the rate. Facilitated diffusion depends on a limited number of protein channels or carriers, each of which handles one molecule at a time. As the gradient steepens, more of those proteins are occupied, and once all are working continuously the rate cannot rise further no matter how steep the gradient becomes. The transporters are saturated, which is the same saturation behavior enzymes show in lesson 6.3. A limited number of transport proteins can saturate; the lipid cannot

8. Why does net movement stop at equilibrium even though molecules keep moving?
   Show the full solution

   Diffusion is the statistical result of random individual motion, and individual particles never stop moving or check where they are going. When concentrations are equal, the number crossing in each direction per second is also equal, so the two flows cancel and no net change occurs. Equilibrium is therefore a balance of two continuing processes rather than a halt, which is why it is described as dynamic and why an isolated system can sit at equilibrium indefinitely without anything being still. Equal numbers cross each way, so the flows cancel: it is a dynamic balance

9. A cell is placed in pure water. Predict the direction of water movement and explain.
   Show the full solution

   Water enters the cell. Pure water has the highest possible water concentration, while the cytoplasm contains dissolved salts, sugars and proteins and therefore has a lower one. Water diffuses down that gradient across the partially permeable membrane into the cell, which swells. What happens next depends on whether the cell has a wall: a plant cell becomes turgid and stops, while an animal cell may burst, which is the subject of the next lesson. Into the cell, since pure water has the higher water concentration

10. Why is a large surface area with a short diffusion distance the standard design for an exchange surface?
    Show the full solution

    Rate of diffusion rises with the area available for crossing and falls sharply as the distance to be crossed increases, since diffusion time grows with the square of distance. An exchange surface therefore maximizes the first and minimizes the second, which is why lungs, gills, root hairs and intestinal villi all combine extensive folded surfaces with walls only one cell thick. Maintaining a steep concentration gradient, usually by a blood supply carrying material away, completes the design. Rate rises with area and falls steeply with distance, so both are optimized

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define hypertonic, hypotonic and isotonic.
   Show the full solution

   Hypertonic has more solute than the cell, hypotonic less, isotonic the same

2. What happens to an animal cell in a hypotonic solution?
   Show the full solution

   It swells and may burst

3. What is plasmolysis?
   Show the full solution

   The membrane and contents of a plant cell pulling away from the wall as water leaves

4. How does active transport differ from facilitated diffusion?
   Show the full solution

   It moves substances against the gradient and requires ATP

5. Why is a turgid plant cell the healthy state?
   Show the full solution

   Turgor pressure against the wall supports the tissue and keeps the plant upright

6. Explain why pure water kills an animal cell but suits a plant cell.
   Show the full solution

   Water enters both cells for the same reason, since pure water is hypotonic to both, but the consequences differ because of the wall. An animal cell has only its membrane to resist the increasing volume, so it stretches and eventually ruptures. A plant cell's contents press against a rigid cellulose wall that pushes back, and once that pressure balances the tendency of water to enter, net entry stops before any damage occurs. The wall converts a fatal influx into a useful support mechanism. The rigid wall builds counter-pressure that stops water entry before rupture

7. Why must a root cell absorbing nitrate from dilute soil use active transport?
   Show the full solution

   Because the nitrate concentration inside the root cell is often already higher than in the surrounding soil water, so the gradient runs the wrong way for diffusion. Passive transport can only move substances down a gradient, and would carry nitrate out of the cell rather than in. Taking up more requires carrier proteins that use energy from ATP to move the ions against the gradient, which is why root cells contain many mitochondria and why absorption stops when oxygen is unavailable. The gradient runs the wrong way, so movement against it needs ATP

8. Explain why waterlogged soil causes mineral deficiency in plants even though water is abundant.
   Show the full solution

   Water fills the air spaces in the soil, so oxygen cannot reach the roots. Root cells depend on aerobic respiration to supply the ATP that active transport requires, and without oxygen that supply collapses. Mineral ions such as nitrate can then no longer be taken up against their gradients, so the plant becomes deficient despite being surrounded by soil water containing them. The denitrification of lesson 4.3 makes the situation worse by removing nitrate from the soil at the same time. No oxygen means no ATP, so active uptake of ions stops

9. Why are intravenous drips isotonic rather than pure water?
   Show the full solution

   Because pure water would be strongly hypotonic to the patient's red blood cells, so water would move rapidly into them and they would swell and burst, which is both dangerous and useless for delivering fluid. An isotonic solution has the same solute concentration as the cells, so there is no net water movement across their membranes, and the fluid can be added to the circulation without damaging the cells suspended in it. The same reasoning applies to solutions used to store organs. Pure water would be hypotonic and burst the red blood cells

10. A plant is watered with salty water and wilts. Explain the mechanism.
    Show the full solution

    The salt makes the soil solution hypertonic to the root cells, so water moves out of the roots into the soil rather than in, and the plant loses water it cannot replace. Its cells become flaccid and then plasmolyzed, so they no longer press against their walls and the turgor supporting the tissue is lost, which is what wilting is. The plant is dying of water shortage while standing in wet soil, which is why salinity damages farmland so severely. Salty soil is hypertonic, so water leaves the roots and turgor is lost

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What do the letters ATP stand for, and what are its three parts?
   Show the full solution

   Adenosine triphosphate: an adenine base, a ribose sugar and three phosphate groups

2. How is energy released from ATP?
   Show the full solution

   By hydrolysis, removing the third phosphate to give ADP and a free phosphate

3. Name three kinds of work ATP powers.
   Show the full solution

   Synthesis of molecules, active transport, and movement such as muscle contraction

4. Roughly how many ATP does aerobic respiration of one glucose yield?
   Show the full solution

   About 30, though older textbooks give figures nearer 36 to 38

5. Why is ATP not a long-term energy store?
   Show the full solution

   Cells hold only a few seconds' supply and regenerate it continuously

6. Explain why a cell uses ATP rather than spending glucose directly.
   Show the full solution

   Glucose holds far more energy than any single cellular task requires, so releasing a whole molecule for a small job would waste most of it as heat. ATP divides that store into small units the cell can spend precisely, matching supply to the size of each task. Using one universal currency also means a single respiratory pathway can power every process in the cell, and because ATP is consumed and rebuilt continuously, the rate of respiration can track demand from moment to moment. It breaks a large store into spendable units usable by every process

7. A cell needs 600 ATP of work. Calculate the glucose required aerobically, and explain what the figure assumes.
   Show the full solution

   At about 30 ATP per glucose, 600 divided by 30 gives 20 glucose molecules. The figure assumes complete aerobic breakdown with oxygen freely available, since without oxygen the yield collapses to about 2 ATP per glucose and the same work would need 300 molecules. It also treats 30 as exact when the real yield varies with conditions and with how efficiently the electron transport chain is coupled, so the answer is an estimate rather than a precise count. 20 glucose molecules, assuming complete aerobic respiration

8. Why does a shortage of oxygen affect a cell within seconds rather than hours?
   Show the full solution

   Because the cell's stock of ATP is tiny, amounting to only a few seconds of work, and it is maintained by continuous regeneration rather than held in reserve. Oxygen is required at the final step of aerobic respiration, so removing it halts ATP production almost immediately while consumption continues. Food reserves such as glycogen and fat are untouched by the problem but cannot be used quickly without oxygen, which is why tissues fail long before they run out of fuel. ATP stocks last seconds, and regeneration stops when oxygen does

9. Explain why the ATP to ADP conversion is described as a cycle.
   Show the full solution

   Because the molecule is not consumed, only converted back and forth. Hydrolysis removes the third phosphate, releasing energy for work and leaving ADP; energy released from glucose during respiration then reattaches a phosphate to the ADP, regenerating ATP. The same adenosine unit is reused many thousands of times a day rather than being made afresh, so what flows through the system is energy while the carrier circulates, which is the pattern of lesson 2.7 at the scale of a single molecule. The same molecule is repeatedly broken down and rebuilt rather than consumed

10. Why might a rising concentration of ADP in a cell increase the rate of respiration?
    Show the full solution

    ADP accumulates when ATP is being used quickly, so its concentration is a direct signal of how much work the cell is doing. Respiration requires ADP as a raw material for rebuilding ATP, so more ADP available means the process can run faster, and ADP also acts as a regulator stimulating the pathway. The result is automatic matching of supply to demand without any external instruction, which is a negative feedback loop of the kind described in lesson 4.5. ADP is both the raw material and the signal that ATP is being spent

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What is an enzyme, and what class of molecule is it usually?
   Show the full solution

   A biological catalyst, almost always a protein

2. What is the active site?
   Show the full solution

   The region of the enzyme whose shape and chemistry fit a specific substrate

3. What does an enzyme do to activation energy?
   Show the full solution

   It lowers it

4. What is the induced fit model?
   Show the full solution

   The active site adjusts its shape slightly as the substrate binds, tightening the fit

5. What determines an enzyme's shape?
   Show the full solution

   Its amino acid sequence, which is specified by a gene

6. Explain why saying an enzyme "makes a reaction happen" is wrong.
   Show the full solution

   The reaction is already possible and already occurs without the enzyme, just very slowly, and the products and the overall energy change are identical either way. What the enzyme alters is the height of the energy barrier that must be crossed before the reaction can proceed, so a much larger proportion of molecules have enough energy to react at ordinary temperatures. Saying the enzyme makes it happen implies it supplies something the reaction lacked, which would violate the conservation of energy. The reaction already occurs; the enzyme only lowers the barrier and raises the rate

7. Why does induced fit explain specificity better than a rigid lock and key?
   Show the full solution

   A rigid keyhole explains why the wrong substrate does not fit, but not why binding helps the reaction proceed. Induced fit adds that the active site changes shape as the correct substrate enters, and in closing around it places strain on particular bonds, which is a direct mechanism for lowering activation energy. It also explains why some enzymes accept a small range of closely related substrates, and why binding at one site can change the shape and activity of another part of the molecule. It explains how binding lowers activation energy, not just why the wrong substrate is rejected

8. Why do cells need only small amounts of each enzyme?
   Show the full solution

   Because an enzyme is not consumed by the reaction it catalyzes. Substrate binds, the reaction occurs, products leave, and the active site is left unchanged and immediately available, so a single molecule can process thousands of substrate molecules each second. The quantity needed therefore depends on the rate of turnover required rather than on the amount of substrate present, which is why an enzyme concentration far lower than its substrate concentration is usually sufficient. Enzymes are unchanged by the reaction and turn over repeatedly

9. A mutation changes one amino acid in an enzyme's active site. Predict the effect and explain.
   Show the full solution

   The active site's shape or chemistry would change, so the substrate may no longer bind properly or at all, and the reaction rate would fall sharply or stop. The effect depends on which amino acid and where: a change at a position critical to binding can abolish activity entirely, while one at the periphery may barely matter. This is the mechanism connecting a single base change in DNA to a metabolic disorder, and lesson 7.6 develops it. Altered active site shape, so the substrate binds poorly and the rate falls

10. Explain how genes control metabolism, using this lesson.
    Show the full solution

    Each metabolic reaction in a cell is catalyzed by a specific enzyme, and each enzyme is a protein whose amino acid sequence is specified by a gene. The sequence determines how the protein folds, the folding determines the shape of the active site, and the active site determines which reaction is catalyzed and how fast. Controlling which genes are expressed therefore controls which enzymes exist, which controls which reactions can proceed, so the genome sets the cell's entire chemistry indirectly. Genes specify enzymes, enzymes determine which reactions run and how fast

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Why does rate rise as temperature increases below the optimum?
   Show the full solution

   Molecules move faster, so collisions are more frequent and more energetic

2. What happens above the optimum temperature?
   Show the full solution

   The enzyme denatures: bonds holding its shape break and the active site is lost

3. Define denaturation.
   Show the full solution

   Loss of an enzyme's three-dimensional shape, destroying the active site

4. What is saturation?
   Show the full solution

   The point at which all active sites are occupied, so more substrate does not raise the rate

5. How does a competitive inhibitor differ from a non-competitive one?
   Show the full solution

   A competitive inhibitor resembles the substrate and blocks the active site; a non-competitive one binds elsewhere and changes its shape

6. Explain why "heat kills enzymes" is marked wrong.
   Show the full solution

   Enzymes are protein molecules and have never been alive, so they cannot be killed. What heat does is break the relatively weak bonds maintaining the protein's folded shape, so the active site loses the geometry the substrate depends on and the enzyme can no longer catalyze. The correct word is denatured, and using it signals that you know the mechanism is a change of shape rather than a vague destruction, which is what the question is testing. Enzymes were never alive; heat denatures them by destroying their shape

7. Explain why the effect of cold is reversible and the effect of heat usually is not.
   Show the full solution

   Cold reduces the kinetic energy of the molecules, so collisions between enzyme and substrate become less frequent and less energetic and the rate falls. The enzyme's structure is untouched, so warming restores the collision rate and activity returns. Heat above the optimum breaks the bonds holding the folded shape, and once the protein has unfolded it does not generally refold into the same precise arrangement, so the active site is permanently lost and cooling changes nothing. Cold only slows collisions; heat destroys the structure permanently

8. Explain the shape of a graph of rate against substrate concentration.
   Show the full solution

   The rate rises steeply at first because most active sites are free, so adding substrate increases the frequency with which sites are occupied. As concentration rises the sites spend more of their time occupied, so each additional increment of substrate produces a smaller gain and the curve bends. Eventually every active site is working continuously and the enzyme is saturated, at which point the rate plateaus and further substrate has no effect. Raising the rate then requires more enzyme, not more substrate. A steep rise flattening to a plateau as active sites become saturated

9. Why can adding more substrate overcome a competitive inhibitor but not a non-competitive one?
   Show the full solution

   A competitive inhibitor occupies the same active site the substrate uses, so the two are in direct contest for it, and raising the substrate concentration increases the proportion of encounters won by the substrate until the inhibition is effectively swamped. A non-competitive inhibitor binds at a different location and alters the active site's shape, so the site no longer fits the substrate at all. No amount of substrate helps, because the problem is that the site has changed rather than that it is occupied. Competitive inhibitors contest the site; non-competitive ones change its shape

10. Human enzymes have an optimum near 37 degrees. Explain why a fever of 40 degrees is dangerous but a fever of 38 is not.
    Show the full solution

    At 38 degrees most enzymes are still close to their optimum and are working slightly faster, which is part of why a modest fever helps fight infection. By 40 degrees the temperature is approaching the point at which the weak bonds holding protein structure begin to fail, and denaturation is not gradual once it starts, since each denatured molecule is lost entirely. A small further rise therefore causes a disproportionate loss of enzyme activity across many systems at once, which is why high fever is treated urgently. Denaturation begins near 40 and, once started, removes enzymes permanently

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Write the overall equation for photosynthesis in words.
   Show the full solution

   Carbon dioxide plus water, using light energy, gives glucose plus oxygen

2. Where in the cell does photosynthesis take place?
   Show the full solution

   In the chloroplast, in the thylakoid membranes and the stroma

3. Which molecule is the source of the oxygen released?
   Show the full solution

   Water

4. Why do leaves appear green?
   Show the full solution

   Chlorophyll absorbs red and blue light and reflects green

5. Name the three main limiting factors.
   Show the full solution

   Light intensity, carbon dioxide concentration and temperature

6. Explain where the mass of a large tree comes from.
   Show the full solution

   Mostly from the air. The carbon that makes up the bulk of a tree's dry mass enters as carbon dioxide through the stomata and is fixed into carbohydrate by photosynthesis, while the hydrogen comes from water absorbed by the roots. Soil contributes mineral ions such as nitrate and magnesium, which are essential but form only a small fraction of the total. This is why the soil in a pot loses almost no mass while the plant growing in it gains a great deal. Carbon from atmospheric carbon dioxide and hydrogen from water, with minerals a small fraction

7. Explain how isotope experiments showed the oxygen comes from water.
   Show the full solution

   Water was supplied containing a heavier isotope of oxygen while the carbon dioxide contained the ordinary isotope, and the oxygen gas released carried the heavy label. Repeating the experiment with the labels reversed, so that the carbon dioxide was heavy, produced ordinary oxygen gas. Since the label tracked the water in both cases, the released oxygen must originate there. This is a clean example of an experiment with a single variable and a clear falsifying outcome, as lesson 1.2 describes. Labeled heavy oxygen in the water appeared in the gas released, and the reverse did not

8. A greenhouse grower raises light intensity and sees no increase in growth. Suggest why and what to do.
   Show the full solution

   Light is evidently no longer the limiting factor, so raising it further cannot help: something else has become the constraint, most likely carbon dioxide concentration or temperature. The grower should raise carbon dioxide, which is why commercial greenhouses often enrich the air, or check whether temperature is below the optimum for the plant's enzymes. The general principle is that the rate is set by whichever factor is in shortest supply, so increasing any other one has no effect until that one is relieved. Another factor is now limiting; raise carbon dioxide or temperature

9. Why would a plant given green light only grow poorly?
   Show the full solution

   Because chlorophyll absorbs light most strongly in the red and blue regions and reflects or transmits green, which is precisely why leaves look green. Under green light very little of the energy arriving is absorbed by the pigment, so little is available to drive the light-dependent reactions, and photosynthesis runs slowly regardless of how bright the light appears to a human eye. Brightness as we perceive it is not the same as usable energy for the plant. Chlorophyll reflects green, so little of that light is absorbed

10. Explain how this lesson connects to the carbon cycle of lesson 4.2.
    Show the full solution

    Photosynthesis is the flux that removes carbon dioxide from the atmosphere and fixes it into organic molecules, so it is the entry point of the fast carbon cycle. The mass of a tree is atmospheric carbon that has been converted into wood, which is why growing forests act as a carbon sink. That carbon returns when respiration or decomposition breaks the molecules down, closing the loop, and only the small fraction that escapes decomposition and is buried leaves for the slow cycle. It is the flux that fixes atmospheric carbon into the fast cycle

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Where do the light-dependent reactions occur?
   Show the full solution

   In the thylakoid membranes of the chloroplast

2. Name the three products of the light-dependent reactions.
   Show the full solution

   Oxygen, ATP and NADPH

3. Where does the Calvin cycle occur and what does it take in?
   Show the full solution

   In the stroma; it takes in carbon dioxide, ATP and NADPH

4. What does carbon fixation mean?
   Show the full solution

   Attaching carbon dioxide to an existing molecule, converting inorganic carbon to organic

5. Why is "light independent" better than "dark reaction"?
   Show the full solution

   It does not need light directly but needs the products of the light reactions, so it does not run in prolonged darkness

6. Explain why the Calvin cycle stops shortly after nightfall.
   Show the full solution

   The cycle does not use light itself, but it consumes ATP and NADPH, and both are produced only by the light-dependent reactions. When light stops, that supply ends immediately while the cycle continues spending the small quantity already present in the stroma. Within a short time both are exhausted and the cycle halts for lack of energy and reducing power, not for lack of carbon dioxide. This is exactly why the name dark reaction misleads students into predicting the opposite. It runs on ATP and NADPH, which only the light reactions supply

7. Predict what happens to oxygen output if carbon dioxide is removed in bright light, and explain.
   Show the full solution

   Oxygen output falls, even though the reaction producing oxygen does not use carbon dioxide. Without carbon dioxide the Calvin cycle stops, so it no longer consumes ATP and NADPH and no longer returns ADP and the empty carrier to the thylakoid. The light-dependent reactions need those returning molecules as raw materials, so they slow once the supply dries up, and water splitting slows with them. The coupling between the stages makes the second stage able to limit the first. The Calvin cycle stops returning ADP and carrier, so the light reactions slow

8. Why is the oxygen released described as a by-product?
   Show the full solution

   Because the plant splits water for the hydrogen and the electrons it needs to make NADPH and drive ATP synthesis, and the oxygen atoms are simply what is left over. Nothing in photosynthesis requires oxygen, and the plant makes no further use of it, so it diffuses out through the stomata. It is essential to nearly all other life and irrelevant to the process producing it, which is why the history of Earth's atmosphere in lesson 11.7 turns on an accident of plant chemistry. The plant needs the hydrogen and electrons from water; oxygen is left over

9. Why must the two stages occur in different parts of the chloroplast?
   Show the full solution

   The light-dependent reactions need an organized membrane to hold the pigments and electron carriers in the correct order and to maintain a difference in hydrogen ion concentration across it, which is only possible in a sealed membrane system such as the thylakoid. The Calvin cycle is a series of enzyme-controlled reactions in solution and needs a fluid compartment where enzymes and substrates can mix freely, which the stroma provides. This is the compartmentalization argument of lesson 5.2 applied inside one organelle. One needs an organized membrane, the other a fluid compartment for enzymes

10. A plant is kept in constant light. Predict whether it grows faster indefinitely, and explain.
    Show the full solution

    No. Once light is no longer the limiting factor, further illumination cannot raise the rate, and carbon dioxide concentration or temperature will constrain it instead. Very high light intensities can also damage the photosynthetic machinery. Beyond that, plants have internal rhythms and many processes, including some aspects of growth and flowering, depend on a period of darkness, so constant light can disrupt development even where photosynthesis itself is unaffected. No: another factor becomes limiting, and many plants need a dark period

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Write the overall equation for aerobic respiration in words.
   Show the full solution

   Glucose plus oxygen gives carbon dioxide plus water, releasing energy for ATP

2. Where does glycolysis occur and what is the net ATP gain?
   Show the full solution

   In the cytoplasm, with a net gain of 2 ATP

3. Where does the Krebs cycle occur?
   Show the full solution

   In the mitochondrial matrix

4. What is the role of oxygen?
   Show the full solution

   It is the final electron acceptor at the end of the electron transport chain

5. Which stage produces most of the ATP?
   Show the full solution

   The electron transport chain

6. Explain why "oxygen is needed to break down glucose" is inaccurate.
   Show the full solution

   Glucose is broken apart in glycolysis and the Krebs cycle, and neither uses oxygen: all six carbons have been released as carbon dioxide before oxygen appears anywhere in the process. Oxygen acts only at the end of the electron transport chain, accepting the electrons that have been passed along it. Its absence matters enormously, because the chain then backs up and the earlier stages stop, but describing it as the agent of breakdown misplaces it by two stages and obscures why anaerobic respiration is possible at all. Glucose is broken down without oxygen; oxygen only accepts electrons at the end

7. Explain what happens to the whole process if the electron transport chain is blocked.
   Show the full solution

   ATP production collapses almost entirely, since the chain supplies the great majority of it. The electron carriers loaded during glycolysis and the Krebs cycle cannot unload, so they remain full and are unavailable to accept more electrons, which halts the Krebs cycle for lack of empty carriers. Glycolysis can continue only if the cell has another way of regenerating those carriers, which fermentation provides, and that yields just 2 ATP per glucose. This is why cyanide, which blocks the chain, is so rapidly fatal. Carriers stay loaded, the Krebs cycle halts, and yield falls to glycolysis alone

8. Why is respiration described as controlled release rather than burning?
   Show the full solution

   Burning releases all the energy in glucose at once as heat, which a cell could neither capture nor survive. Respiration dismantles the molecule through many small enzyme-controlled steps, each releasing a modest amount of energy, much of which is captured as ATP rather than lost. The stepwise design is what makes capture possible: a single large release would exceed anything the cell could store, and the heat would denature its proteins. The products are identical, which shows the difference is entirely in the pathway. Many small steps allow energy capture, while one burst would be lost as heat

9. All six carbons leave as carbon dioxide. Trace where each pair is released.
   Show the full solution

   Glucose has six carbons, and glycolysis splits it into two pyruvate molecules of three carbons each, releasing none. When each pyruvate enters the mitochondrion it is converted to a two-carbon compound, releasing one carbon dioxide each, so two of the six leave at this step. The remaining two carbons in each of the two compounds are then released during the Krebs cycle, accounting for the final four. Every carbon that entered as glucose leaves as carbon dioxide. Two on entry to the mitochondrion, four in the Krebs cycle, none in glycolysis

10. Why do cells with high energy demands have mitochondria with extensively folded inner membranes?
    Show the full solution

    The electron transport chain, which makes most of the ATP, is embedded in the inner mitochondrial membrane, so the quantity of ATP a mitochondrion can produce depends on how much of that membrane it has. Folding it into cristae packs a much larger area into the same volume, exactly the surface area strategy of lesson 5.4, allowing many more chains to operate at once. Cells such as muscle and liver cells that demand ATP continuously show the most extensive folding. Folding packs more electron transport chain membrane into the same volume

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What is the ATP yield of anaerobic respiration per glucose?
   Show the full solution

   About 2

2. Name the products of anaerobic respiration in animals and in yeast.
   Show the full solution

   Lactate in animals; ethanol and carbon dioxide in yeast

3. Roughly how many times more efficient is aerobic respiration?
   Show the full solution

   About fifteen times

4. A cell needs 900 ATP. How much glucose aerobically, and how much anaerobically?
   Show the full solution

   30 molecules aerobically and 450 anaerobically

5. Do plants respire at night?
   Show the full solution

   Yes, constantly, in every living cell, day and night

6. Explain where the missing energy goes in anaerobic respiration.
   Show the full solution

   It stays in the products. Anaerobic respiration only completes glycolysis, which splits glucose into two three-carbon molecules, so the great majority of the chemical energy originally in the glucose remains locked in the bonds of lactate or ethanol. Nothing has been destroyed and little has been lost as heat; the pathway simply stops early. This is why ethanol is a usable fuel and why the liver can later process lactate to recover energy from it when oxygen becomes available. It remains in the bonds of lactate or ethanol, which are still energy-rich

7. Why does a sprinter use anaerobic respiration even though it wastes glucose?
   Show the full solution

   Because the limiting factor during a sprint is not fuel but the rate at which ATP can be supplied. Aerobic respiration depends on oxygen reaching the muscle through the blood, and that delivery cannot rise fast enough to meet the demand of maximal effort. Anaerobic respiration produces ATP quickly and without oxygen, so it can bridge the gap. The cost is inefficient use of glucose and the accumulation of lactate, which is acceptable for a short burst. It supplies ATP quickly without oxygen, which delivery cannot match in a sprint

8. Explain why calling photosynthesis and respiration opposites is only partly right.
   Show the full solution

   The overall equations are near reverses, so the reactants of one are the products of the other, and in that limited sense they are opposites. But they are not one reaction run in two directions: they occur in different organelles, use different enzymes, proceed through entirely different intermediates, and differ in what happens to energy, since photosynthesis converts light into stored chemical energy while respiration releases stored chemical energy into a usable form. Treating them as a single reversible process leads to wrong predictions about both. The equations reverse, but the locations, enzymes, intermediates and energy direction all differ

9. Why does bread rise, and why does the alcohol not remain?
   Show the full solution

   Yeast respires anaerobically in the dough, producing carbon dioxide and ethanol. The carbon dioxide forms bubbles trapped by the gluten network, which expands the dough and gives bread its texture. The ethanol is also present but boils at a much lower temperature than water, so it evaporates during baking and almost none remains in the finished loaf. The same process in a sealed vessel, where the gas cannot escape and the liquid is not heated, produces beer instead. Carbon dioxide from yeast fermentation raises it; the ethanol evaporates in the oven

10. A sealed jar containing a green plant is kept in darkness. Predict what happens to the oxygen and carbon dioxide, and explain.
    Show the full solution

    Oxygen falls and carbon dioxide rises. In darkness photosynthesis stops entirely, but respiration continues in every living cell of the plant, consuming oxygen and releasing carbon dioxide, so the exchange runs one way only. In the light the same jar would show the reverse, because photosynthesis normally exceeds respiration during the day, but the plant would still be respiring throughout. This is the experiment that disproves the idea that plants only photosynthesize. Oxygen falls and carbon dioxide rises, since respiration continues without photosynthesis

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Why did many scientists expect protein to be the genetic material?
   Show the full solution

   Proteins are built from twenty amino acids and DNA from only four bases, so protein looked capable of carrying more information

2. Describe the two bacterial strains Griffith used.
   Show the full solution

   S with a smooth capsule and lethal; R rough, without a capsule, and harmless

3. What happened when heat-killed S was injected with living R?
   Show the full solution

   The mice died, and living S bacteria were recovered from them

4. What did Avery's group do to identify the transforming principle?
   Show the full solution

   Destroyed each candidate molecule in turn with enzymes; transformation failed only when DNA was destroyed

5. What did Hershey and Chase show?
   Show the full solution

   That only viral DNA, not viral protein, entered the infected bacterial cell

6. Explain why the recovery of living S bacteria mattered more than the death of the mice.
   Show the full solution

   The deaths alone could have had several explanations, including a toxin surviving the heat treatment or some chemical from the dead cells making the R bacteria temporarily dangerous. Recovering living S bacteria showed that the R cells themselves had been converted into a different strain, and because those bacteria bred true in later generations the change was inherited. Only a transfer of genetic material explains a permanent heritable conversion, so that observation is what makes the experiment about heredity rather than about toxicity. It showed a permanent heritable conversion rather than a temporary toxic effect

7. Identify the variables and controls in Griffith's design.
   Show the full solution

   The independent variable is what is injected, with four levels: living S, living R, heat-killed S, and heat-killed S combined with living R. The dependent variable is whether the mouse survives, with recovery of bacteria from the body as a second measurement. The first three groups act as controls establishing what each component does alone, which is what makes the fourth interpretable: since neither heat-killed S nor living R kills by itself, the deaths in the combined group must come from their interaction. Injection contents is independent, survival is dependent, and the three single-component groups are controls

8. Why was Avery's enzyme approach more decisive than Griffith's?
   Show the full solution

   Griffith showed that some material transferred information but had no way to say which molecule. Avery's group took the purified extract and destroyed one candidate at a time, using enzymes that break down protein, RNA or DNA specifically, then tested whether transformation still occurred. Transformation survived the destruction of protein and RNA and failed only when DNA was destroyed, which isolates a single variable and points at one molecule. It moves from detecting an effect to identifying its cause. Destroying one candidate at a time isolated DNA as the necessary molecule

9. Why does the agreement of three independent experiments matter more than any one of them?
   Show the full solution

   Each experiment has its own possible weaknesses: Griffith's could be explained by an unknown transferable substance, Avery's by traces of contaminating protein in a supposedly pure DNA preparation, and Hershey and Chase's by imperfect labeling or incomplete separation. Those weaknesses are unrelated to one another, so a single explanation that accounts for all three results while leaving DNA out becomes very hard to construct. Independent lines converging on the same answer is the strongest form of evidence in biology. Their weaknesses are unrelated, so an alternative explanation would have to defeat all three

10. The four-base objection turned out to be wrong. Suggest why four symbols are enough.
    Show the full solution

    Because information capacity depends on sequence length as well as on the number of symbols available, and long sequences of few symbols carry enormous amounts of information. Four bases read in groups of three give sixty four combinations, comfortably more than the twenty amino acids that need specifying, and a chromosome contains millions of bases in a defined order. The same principle lets two symbols encode anything in a computer. The objection confused the size of the alphabet with the length of the message. Sequence length carries the information; four symbols in long strings are ample

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Name the three parts of a nucleotide.
   Show the full solution

   A phosphate group, a deoxyribose sugar, and a nitrogenous base

2. Which bases pair with which?
   Show the full solution

   A with T, and C with G

3. What holds the two strands together?
   Show the full solution

   Hydrogen bonds between the paired bases

4. A sample contains 22 percent guanine. Find the other three percentages.
   Show the full solution

   C is 22 percent; A and T are 28 percent each

5. What does antiparallel mean?
   Show the full solution

   The two strands run in opposite directions

6. Explain why it is an advantage that the base pair bonds are weak.
   Show the full solution

   The strands must be separated every time the molecule is copied or a gene is transcribed, and that has to be possible at body temperature using enzymes rather than extreme conditions. Individually weak hydrogen bonds can be broken locally with little energy, so a region can be opened without disturbing the rest. The molecule stays stable overall because there are so many of these bonds along its length, so strength comes from number rather than from each bond being strong. Strands must separate easily for copying; stability comes from the number of bonds

7. Explain why the sugar-phosphate backbone carries no information.
   Show the full solution

   The backbone is identical everywhere along the molecule, an unvarying alternation of the same sugar and the same phosphate group, so reading it at one position tells you nothing that reading it at any other would not. Information requires something that can differ from place to place, and in DNA only the bases do. This is the reason a four-letter sequence of bases, rather than the structure holding them, is what a gene consists of. It is identical along its length, and information requires variation

8. Explain how the structure itself suggests a copying mechanism.
   Show the full solution

   Because pairing is specific, each strand's sequence dictates its partner's exactly, so the molecule effectively carries the same message twice in complementary form. Separating the two strands therefore yields two complete templates, each able to direct the assembly of a new partner by the same pairing rules. No additional information is needed and no separate copying machinery has to store the sequence, which is why the structure was regarded as solving the problem of heredity as soon as it was proposed. Specific pairing means each strand is a complete template for its partner

9. Chargaff's ratios were known before the structure was proposed. Explain their significance.
   Show the full solution

   They were an unexplained regularity: across very different organisms the amount of adenine always matched thymine and cytosine always matched guanine, even though the overall proportions varied between species. Any correct structure had to account for that, and specific pairing explains it exactly, since pairing A with T necessarily makes their quantities equal. The ratios therefore acted as a strong constraint that ruled out proposed structures and supported the one that predicted them. They were a regularity any correct structure had to explain, and specific pairing does

10. Two species have DNA with very different proportions of A and C. Does this conflict with Chargaff? Explain.
    Show the full solution

    No. Chargaff's rules state that within any one sample the amount of A equals the amount of T and C equals G, which follows from pairing. They say nothing about the ratio of A to C, which varies considerably between species and is a genuine difference in DNA composition. Confusing the two is a standard error: the rules constrain each pair to be balanced, not the two pairs to be equal to each other. The rules equate A with T and C with G, not A with C

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What does semi-conservative mean?
   Show the full solution

   Each new double helix has one original strand and one new strand

2. What does helicase do?
   Show the full solution

   Unwinds the helix and separates the strands by breaking hydrogen bonds

3. What does DNA polymerase do?
   Show the full solution

   Builds new strands by adding nucleotides that pair with the template bases

4. Which bonds break during replication?
   Show the full solution

   The hydrogen bonds between paired bases, not the backbone

5. Why is replication accurate?
   Show the full solution

   The original strand acts as a template and polymerase proofreads

6. Explain why the first round of the labeling experiment could not distinguish semi-conservative from dispersive.
   Show the full solution

   Both models predict that after one round every molecule contains equal amounts of old heavy material and new light material, so every molecule has the same intermediate density and all of them form one band in the same position. The models disagree about how that material is arranged, with semi-conservative putting it in two intact strands and dispersive scattering it in fragments, but density measures only the proportions. A second round is needed because only then do the two arrangements produce different distributions. Both give molecules of identical intermediate density after one round

7. Explain how the second round separates the two.
   Show the full solution

   Under semi-conservative replication each intermediate molecule separates into one heavy strand and one light strand, and each acquires a new light partner, producing half intermediate molecules and half entirely light ones, which form two distinct bands. Under dispersive replication the old material is broken up and distributed among all the products, so every molecule is again uniform in composition, merely lighter than before, and a single band appears. Two bands against one is a clean qualitative difference. Semi-conservative gives two bands; dispersive gives one uniformly lighter band

8. Why does it matter that the original strands survive intact?
   Show the full solution

   It means the information is never reconstructed from a copy of a copy within a single round, since each new molecule retains an original strand as its reference. Errors therefore cannot compound in the way they would if each generation were built entirely from the previous generation's copy. It also means a cell's DNA contains physically old material inherited across many divisions, and it is what makes the labeling experiment possible at all, since the label stays in identifiable strands instead of being scattered. An original template is retained each round, so copying errors cannot compound

9. DNA polymerase proofreads but the error rate is not zero. Explain why that matters for unit 10.
   Show the full solution

   Uncorrected errors are mutations, and mutation is the only original source of new genetic variation. Crossing over and independent assortment in meiosis rearrange existing alleles but cannot create one that never existed, so without replication errors there would be nothing for natural selection to act on and no evolution. A perfect copying system would therefore be an evolutionary dead end. The error rate is low enough to protect the individual and high enough to supply variation to the population. Uncorrected errors are mutations, the only original source of new variation

10. A student says replication is how organisms reproduce. Correct them.
    Show the full solution

    Replication is the copying of a DNA molecule, which happens inside a cell before it divides. Reproduction is the production of new organisms, and it requires cell division, and in sexually reproducing species also meiosis, fertilization and development. Replication is a necessary step within those processes rather than the process itself, and confusing the two loses the distinction between copying information and building an organism, which is the subject of the next four lessons. Replication copies DNA within a cell; reproduction produces new organisms

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Name three differences between RNA and DNA.
   Show the full solution

   Ribose instead of deoxyribose, uracil instead of thymine, and single stranded

2. Which enzyme carries out transcription?
   Show the full solution

   RNA polymerase

3. What is a promoter?
   Show the full solution

   A region marking where a gene begins and which strand to read

4. Transcribe the template strand TAC CGA GTA ATT.
   Show the full solution

   AUG GCU CAU UAA

5. Where does transcription occur in a eukaryotic cell?
   Show the full solution

   In the nucleus

6. Explain why the cell makes a disposable RNA copy rather than sending the DNA.
   Show the full solution

   The DNA is the cell's only master copy of its entire genetic information, and the cytoplasm contains enzymes and chemical conditions that could damage it, so sending it out would risk the permanent loss of information that cannot be recovered. A short mRNA copy carries only the one gene needed, can be made in large numbers when a protein is required in quantity, and is broken down afterward, which also gives the cell a way of switching production off. The original stays protected in the nucleus throughout. The master copy stays protected, and disposable copies allow control of output

7. Why is only one DNA strand transcribed?
   Show the full solution

   The two strands are complementary rather than identical, so they carry different base sequences and would specify different and mostly meaningless amino acid sequences. Only one of them corresponds to the protein the gene encodes, so transcribing the other would produce a useless or harmful product. The promoter region determines which strand RNA polymerase reads for a given gene, and different genes along the same chromosome can use either strand as their template. The strands are complementary, so only one carries the correct sequence

8. Explain why the mRNA matches the coding strand rather than the template.
   Show the full solution

   The mRNA is built by pairing with the template strand, and the coding strand is also the complement of that same template. Two sequences complementary to the same third sequence must match each other, so the mRNA reproduces the coding strand exactly, with uracil in every position where the DNA has thymine. This is a useful check when working a question: if the mRNA does not read like the coding strand with T replaced by U, a pairing error has been made somewhere. Both are complementary to the template, so they match each other

9. How does transcription allow cells with identical DNA to be different?
   Show the full solution

   Every cell carries the same genes, but transcription is selective: only the genes a particular cell needs are copied into mRNA, so only those proteins are made. A muscle cell transcribes the genes for contractile proteins and leaves the insulin gene untouched, while a pancreatic cell does the reverse. The difference between cell types is therefore a difference in which genes are being read rather than in which genes are present, which is the subject of lesson 7.7. Only the genes a cell needs are transcribed, so different proteins are made from the same genome

10. A student writes an mRNA sequence containing thymine. Explain the error and its consequence.
    Show the full solution

    RNA does not contain thymine at all; it uses uracil in that role, so a T in an mRNA sequence means adenine on the template was paired incorrectly. The consequence in an exam is that the sequence is wrong from that point and any amino acids read from it afterward may also be wrong, so a single slip can cost the whole answer. Checking a finished mRNA sequence for the letter T takes a moment and catches the error reliably. RNA uses uracil, so a T means adenine was paired wrongly

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. How many bases make a codon, and how many codons are there?
   Show the full solution

   Three bases, giving 64 codons

2. What is the start codon and which amino acid does it specify?
   Show the full solution

   AUG, which specifies methionine

3. What does a stop codon do?
   Show the full solution

   Ends translation; it codes for no amino acid

4. What is an anticodon, and where is it found?
   Show the full solution

   A sequence of three bases on a tRNA, complementary to a codon on the mRNA

5. Using the table, translate AUG GCU CAU UAA.
   Show the full solution

   Methionine, alanine, histidine, then stop

6. Explain why codons must be three bases rather than two.
   Show the full solution

   Four different bases read in groups of two give four times four, which is sixteen possible combinations, and twenty amino acids must be specified, so a two-base code is simply too small. Groups of three give four times four times four, which is sixty four, comfortably enough. Three is therefore the smallest group size that can work, and the surplus explains the redundancy in the code, since sixty four codons must cover twenty amino acids plus a stop signal. Two bases give only 16 combinations for 20 amino acids; three give 64

7. Explain what redundancy in the genetic code means and why it is useful.
   Show the full solution

   Redundancy means most amino acids are specified by more than one codon, since sixty four codons cover twenty amino acids. It is useful because it buffers against mutation: a base change that converts one codon into another for the same amino acid leaves the protein completely unaltered. Such silent mutations are common, and the redundancy is concentrated in the third base of many codons, so changes there are especially likely to be harmless. The code has a degree of built-in error tolerance. Several codons per amino acid, so many base changes leave the protein unchanged

8. Why is the near-universality of the code strong evidence for common ancestry?
   Show the full solution

   The assignment of particular codons to particular amino acids is arbitrary in the sense that a different assignment would work just as well, so there is no chemical necessity making AUG mean methionine. If life had arisen independently several times, those separate origins would have had no reason to settle on the same arbitrary scheme. Finding essentially the same code in bacteria, plants, fungi and animals is therefore best explained by inheritance from a single ancestral population that already used it. The assignments are arbitrary, so shared ones point to a single origin

9. A tRNA has the anticodon UUA. Which codon does it read and which amino acid does it carry?
   Show the full solution

   The anticodon pairs with its complementary codon, so UUA pairs with AAU, using the pairing rules with uracil in place of thymine: U pairs with A, U with A, and A with U. From the table AAU specifies asparagine, so this tRNA carries asparagine and delivers it whenever the ribosome reaches an AAU codon. Working from anticodon to codon requires taking the complement, which is why keeping the two terms distinct matters. It reads the codon AAU and carries asparagine

10. Explain why the reading frame matters, using the worked example.
    Show the full solution

    The ribosome reads in consecutive groups of three from the start codon, so where it starts determines how every subsequent base is grouped. Reading AUG CCU AAU gives methionine, proline, asparagine, but starting one base later would group the same sequence as UGC CUA AUU and give entirely different amino acids. The bases have not changed; only the grouping has. This is why an insertion or deletion, which shifts the frame, is so much more damaging than a substitution, as the next lesson shows. The same bases grouped differently specify entirely different amino acids

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define a mutation.
   Show the full solution

   A change in the base sequence of DNA

2. Name the three kinds of substitution.
   Show the full solution

   Silent, missense and nonsense

3. What is a frameshift?
   Show the full solution

   A shift in the reading frame caused by an insertion or deletion not a multiple of three

4. Why can a substitution be silent?
   Show the full solution

   Because the code is redundant, so the new codon may specify the same amino acid

5. Which mutations can be inherited?
   Show the full solution

   Those occurring in gametes or the cells that produce them

6. Explain why a frameshift is usually more damaging than a substitution.
   Show the full solution

   A substitution alters a single base and therefore at most one codon, so at worst one amino acid in the protein changes. An insertion or deletion shifts every base after it into a different grouping, so every codon from that point to the end of the gene is read differently and specifies a different amino acid. The original stop codon is usually destroyed as well, so the protein may also be the wrong length. The result is a protein that is entirely wrong downstream rather than slightly altered. It changes every codon after it, while a substitution changes at most one

7. Explain why a deletion of three bases is often much milder than a deletion of one.
   Show the full solution

   Codons are groups of three, so removing three bases removes exactly one whole codon and leaves the grouping of everything after it unchanged. The protein is produced with one amino acid missing but is otherwise correct, and depending on which amino acid it was, it may still function reasonably well. Removing one base leaves the remaining bases regrouped from that point onward, so the entire downstream sequence is wrong. The multiple of three is what preserves the reading frame. Three bases remove a whole codon without shifting the frame

8. Two missense mutations in the same gene have very different effects. Suggest why.
   Show the full solution

   Because a protein's function depends far more on some positions than others. An amino acid change within the active site of an enzyme alters the shape or chemistry that binds the substrate, and can abolish activity entirely, as lesson 6.2 describes. A change on an outer surface loop far from the active site may leave the folding and the binding unaffected, so the protein works normally. The severity depends on the position and on how chemically different the substituted amino acid is. The effect depends on whether the position is critical to folding or the active site

9. Explain why "mutations are harmful" is a poor summary.
   Show the full solution

   Most mutations have no detectable effect at all, either because they fall in non-coding DNA or because they are silent substitutions that leave the protein unchanged. Of those that do change a protein, many are tolerated. Harmful ones exist and are important, but a rare few improve function or fitness in a particular environment, and those are the only original source of the variation natural selection acts on. Describing mutation as harmful also invites the idea that organisms mutate in response to need, which is false. Most are neutral, and the rare beneficial ones are the raw material of evolution

10. A mutation occurs in a skin cell and another in a sperm cell. Compare the consequences.
    Show the full solution

    The skin cell mutation affects only that cell and the cells it divides to produce, so its consequences are confined to the individual and disappear when they die. It can matter greatly to that person if it disrupts the control of cell division, which is how cancers begin. The sperm cell mutation is not present in the parent's body tissues at all, but if that sperm fertilizes an egg it will be present in every cell of the offspring and can be passed to further generations. The skin mutation affects one individual; the gamete mutation can be inherited by every cell of offspring

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Do specialized cells contain the whole genome?
   Show the full solution

   Yes, almost all of them do

2. Define differentiation.
   Show the full solution

   Specialization of a cell by switching on some genes and silencing others

3. At which stage is gene expression mainly regulated?
   Show the full solution

   At transcription

4. How does a regulatory protein control a gene?
   Show the full solution

   By binding to DNA near the gene and blocking or promoting RNA polymerase

5. What is a stem cell?
   Show the full solution

   An undifferentiated cell that retains the ability to become other cell types

6. Explain why a bacterium benefits from switching the lactose enzymes off.
   Show the full solution

   Making a protein costs amino acids and ATP, and enzymes that are never used return nothing for that investment. A cell that produced them constantly would divert resources from growth and reproduction and would be outcompeted by cells that did not, so selection favors regulation. Switching the genes on only when lactose is present means the cost is paid only when there is a benefit, which is why regulated genes are the rule rather than the exception even in organisms with small genomes. Making unused proteins wastes amino acids and ATP that competitors spend on growth

7. Explain why "specialized cells lose the genes they do not need" is wrong, and give the evidence.
   Show the full solution

   The genes are retained and silenced rather than removed, so a liver cell still contains the genes for making nerve or muscle proteins but does not transcribe them. The decisive evidence is that a nucleus taken from a fully specialized adult cell and transferred into an egg cell can direct the development of a complete organism with every tissue type. That is only possible if the nucleus still carries the complete genome, so differentiation must be a change in expression rather than in content. Genes are silenced, not deleted, as nuclear transfer producing a whole organism shows

8. How does the lactose switch illustrate negative feedback?
   Show the full solution

   The presence of lactose removes the block on the genes, so the enzymes that digest lactose are produced. Those enzymes then break the lactose down, and as its concentration falls the regulatory protein is no longer prevented from binding, so it blocks the genes again and production ceases. The output of the process reduces the signal that triggered it, which is the definition of negative feedback, and the result is that enzyme levels track the need for them automatically. The enzymes destroy the substance that switched them on, so the signal is self-limiting

9. Why are stem cells medically interesting?
   Show the full solution

   Because they have not yet committed to a single fate and can be directed to become particular cell types, which raises the possibility of replacing tissue that the body cannot regenerate, such as damaged nerve or heart muscle. They are also valuable for studying how differentiation is controlled and for testing drugs on human cells of a specific type. The difficulties are practical and ethical rather than conceptual, including controlling what the cells become and the source from which they are obtained. They can be directed to become tissue types the body cannot regenerate

10. Connect this lesson to lesson 6.2 on enzymes.
    Show the full solution

    Lesson 6.2 established that each metabolic reaction is catalyzed by a specific enzyme whose shape is determined by its gene, so genes control chemistry through the enzymes they specify. This lesson adds the layer above: which genes are transcribed determines which enzymes exist in a given cell at a given moment. A cell's chemistry is therefore set by regulation as much as by the genome, which is how two cells with identical genes carry out entirely different reactions. Genes specify enzymes, and regulation decides which enzymes a cell actually has

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Name the three parts of interphase and what happens in each.
   Show the full solution

   G1 growth, S DNA replication, G2 final preparation

2. What are the two components of M phase?
   Show the full solution

   Mitosis, which separates chromosomes, and cytokinesis, which divides the cytoplasm

3. What happens to the amount of DNA during S phase?
   Show the full solution

   It doubles

4. Name two things a checkpoint verifies.
   Show the full solution

   That DNA replication is complete and undamaged, and that chromosomes are correctly attached

5. In the worked example, how long is M phase if the cycle is 20 hours?
   Show the full solution

   8 percent of 20 hours, which is 1.6 hours or 96 minutes

6. Explain why calling interphase a resting phase is wrong.
   Show the full solution

   Interphase is when the cell does almost everything: it grows, manufactures proteins and organelles, carries out whatever specialized function it has, and replicates its entire genome during S phase. Replication alone is an enormous undertaking involving millions of base pairs copied accurately. The impression of rest comes from microscope images, in which interphase nuclei look uniform while mitotic chromosomes are dramatic, so the visually quiet stage is mistaken for an inactive one. It is the most active stage, including replication of the entire genome

7. Explain why the proportion of cells in a stage indicates its duration.
   Show the full solution

   In a large population of cells dividing independently, each cell is at a random point in the cycle at any moment. The chance of finding a given cell in a particular stage is therefore proportional to the fraction of the cycle that stage occupies, so counting many cells estimates those fractions directly. It is the same logic as photographing a busy road and inferring how long traffic lights stay red from the proportion of images showing red. It requires the population to be unsynchronized. Cells are at random points, so the fraction seen in a stage estimates its share of the cycle

8. Why must the checkpoint before division confirm that DNA replication is complete?
   Show the full solution

   Dividing with incompletely copied DNA would give at least one daughter cell a partial genome, missing genes it cannot recover since there is no other copy to restore from. That cell would lack proteins essential to its function and would usually die, and if it survived and divided the defect would be inherited by every descendant. Checking before the irreversible step of separation is far cheaper than dealing with the consequences, which is the general logic of every checkpoint. A daughter cell would inherit a partial genome with no way to recover the missing genes

9. A cell is found to have twice the normal DNA content. Which stages could it be in?
   Show the full solution

   It has completed S phase, so it could be in G2 or in M phase up to the point where the chromosomes are separated. It cannot be in G1, which precedes replication, and it cannot be a cell that has finished dividing, since separation halves the DNA content again. If it were partway through S phase the content would be between the two values rather than exactly double, so the measurement places it fairly precisely. G2, or M phase before the chromosomes separate

10. Why might a cell that has permanently stopped dividing distort the worked-example method?
    Show the full solution

    Cells that exit the cycle permanently, such as mature nerve cells, remain in a G1-like state indefinitely and never progress. Counting them alongside actively cycling cells adds a large number to the G1 tally that has nothing to do with how long G1 takes, so G1 appears far longer than it is and every other stage appears correspondingly shorter. The method assumes every cell counted is moving through the cycle, and a tissue containing many permanently arrested cells breaks that assumption. They accumulate in G1 without cycling, inflating its apparent duration

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. List the four stages of mitosis in order.
   Show the full solution

   Prophase, metaphase, anaphase, telophase

2. What happens in metaphase?
   Show the full solution

   Chromosomes line up along the equator of the cell

3. What separates during anaphase?
   Show the full solution

   Sister chromatids

4. Name three purposes of mitosis.
   Show the full solution

   Growth, repair and asexual reproduction

5. How does cytokinesis differ between plant and animal cells?
   Show the full solution

   Animal cells pinch inward; plant cells build a new cell plate because of the wall

6. Explain how to tell metaphase from anaphase in a photograph.
   Show the full solution

   Count the groups of chromosomes. Metaphase shows a single line across the middle of the cell with every chromosome aligned on it, and nothing has yet separated. Anaphase shows two distinct groups moving apart toward opposite poles, often with the chromosomes forming V shapes because the centromere leads and the arms trail behind. The key difference is whether separation has begun, not exactly where the chromosomes sit, since early anaphase chromosomes are still near the middle. One aligned row is metaphase; two separating groups is anaphase

7. Explain why chromosomes condense before being moved.
   Show the full solution

   Uncondensed DNA is extremely long and fine, and a human cell contains around two meters of it. Attempting to drag threads of that length to opposite poles would tangle and break them, losing or damaging genes. Condensing each chromosome into a short, compact structure makes it a manageable object that can be attached to a spindle fiber and pulled cleanly. The visibility that makes prophase recognizable under a microscope is a side effect of that packaging. Long fine DNA would tangle and break; condensing makes it movable

8. A cell with 12 chromosomes undergoes mitosis. State the number in each daughter cell and explain.
   Show the full solution

   Each daughter cell has 12 chromosomes. Before division the cell replicated its DNA, so each of the 12 chromosomes consisted of two identical sister chromatids, giving 24 chromatids in total. Anaphase separates the chromatids rather than the chromosomes, sending one of each pair to each pole, so each daughter receives 12 single-chromatid chromosomes. The chromosome number per cell is therefore unchanged, which is what makes mitosis suitable for growth. 12, because chromatids separate rather than chromosomes

9. Why is it important that mitosis produces identical cells?
   Show the full solution

   Because its purposes require sameness. Growth adds more cells of the existing types, repair replaces damaged cells with functional equivalents, and asexual reproduction produces an offspring suited to the same environment as the parent. A skin cell dividing to give something other than skin cells would be a failure, and in a multicellular body genetic differences between cells are usually the beginning of a tumor rather than a source of useful variety. Growth and repair need replacements that do the same job

10. Explain why a drug that prevents spindle formation stops cell division.
    Show the full solution

    The spindle is the apparatus that attaches to chromosomes at the centromere and pulls the sister chromatids to opposite poles. Without it the chromosomes can condense and the nuclear envelope can break down, but they cannot be aligned at metaphase or separated at anaphase, so the cell arrests at the checkpoint that verifies correct attachment. Since rapidly dividing cells are the most affected, such drugs are used against cancers, which also explains why they damage hair follicles and gut lining. Without a spindle, chromosomes cannot align or separate, so the cell arrests

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. How many cells does meiosis produce, and are they haploid or diploid?
   Show the full solution

   Four, and they are haploid

2. What are homologous chromosomes?
   Show the full solution

   Matching pairs, one from each parent, carrying the same genes in the same order

3. What separates in meiosis I, and what in meiosis II?
   Show the full solution

   Homologous chromosomes in meiosis I; sister chromatids in meiosis II

4. How many chromosomes are in a human body cell and in a human gamete?
   Show the full solution

   46 in a body cell, 23 in a gamete

5. What restores the diploid number?
   Show the full solution

   Fertilization

6. Explain why chromosome number would double each generation without meiosis.
   Show the full solution

   Fertilization combines the nuclei of two cells, so the offspring receives the sum of the chromosomes in each gamete. If gametes were produced by mitosis they would carry the full diploid number, and fusing two of them would give an offspring with twice the parental number, which would double again in the next generation and continue indefinitely. Meiosis halves the number before fertilization doubles it, so the two processes cancel and the species number stays constant. Fertilization adds two sets, so gametes must carry half

7. Explain why meiosis II resembles mitosis but is not the same.
   Show the full solution

   Both separate sister chromatids and both produce two cells from one, so the mechanics look alike. The difference is the starting material: meiosis II begins with a haploid cell that has already lost one member of each homologous pair, so the products are haploid rather than diploid. The chromatids being separated are also no longer identical, because crossing over in meiosis I exchanged segments between homologues, so the resulting cells differ genetically. It starts from a haploid cell whose chromatids are no longer identical

8. A cell with 8 chromosomes undergoes meiosis. Describe the products.
   Show the full solution

   Four cells, each containing 4 chromosomes, and all four genetically different from each other and from the parent cell. The 8 chromosomes form 4 homologous pairs, meiosis I separates the members of each pair to give two cells of 4 chromosomes each, and meiosis II separates the sister chromatids to give four cells of 4 single-chromatid chromosomes. The number has halved from 8 to 4, which is the definition of a reduction division. Four genetically different haploid cells with 4 chromosomes each

9. Why is the pairing of homologues in meiosis I essential?
   Show the full solution

   Pairing is what allows the two members of each homologous pair to be directed to opposite cells, which is how the chromosome number is halved while ensuring each daughter gets one complete set rather than a random selection. If chromosomes lined up individually, as in mitosis, separation would be arbitrary and a cell might receive both copies of one chromosome and neither of another. Pairing also brings homologues into contact so that crossing over can occur. It ensures each cell gets one complete set, and enables crossing over

10. Explain why meiosis is described as a reduction division but mitosis is not.
    Show the full solution

    Meiosis reduces the chromosome number per cell from diploid to haploid, because meiosis I separates whole homologous chromosomes so each daughter receives only one member of each pair. Mitosis separates sister chromatids instead, and since each chromatid becomes a full chromosome, each daughter ends with the same number the parent had. The distinction lies entirely in what is pulled apart, which is why confusing meiosis I with meiosis II makes the whole account incoherent. Meiosis I separates homologues and halves the number; mitosis separates chromatids and maintains it

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. When and where does crossing over occur?
   Show the full solution

   In meiosis I, between non-sister chromatids of paired homologous chromosomes

2. What is a chiasma?
   Show the full solution

   A point at which paired homologues are joined and exchange segments

3. State the formula for combinations from independent assortment.
   Show the full solution

   2 raised to the power of the number of homologous pairs

4. How many combinations does an organism with 5 pairs produce?
   Show the full solution

   2 to the power 5, which is 32

5. Which process creates genuinely new alleles?
   Show the full solution

   Mutation only

6. Explain why crossing over between sister chromatids would achieve nothing.
   Show the full solution

   Sister chromatids are the two identical copies produced when a chromosome was replicated in S phase, so they carry exactly the same alleles at every position. Exchanging a segment between them swaps material that is identical to the material it replaces, leaving both chromatids unchanged. Variation requires exchange between chromosomes that differ, which is why crossing over occurs between non-sister chromatids of homologous pairs, where one came from each parent and the alleles may differ. Sister chromatids are identical, so exchanging segments changes nothing

7. Explain why independent assortment gives 2 to the power n rather than 2 times n.
   Show the full solution

   Each homologous pair contributes an independent two-way choice, and independent choices combine by multiplication rather than addition. With two pairs, each of the two options for the first pair can be combined with either option for the second, giving two times two rather than two plus two. Adding a third pair doubles the total again. Repeated doubling is a power, which is why the count rises so steeply and why 23 pairs give over eight million rather than forty six. Independent choices multiply, and repeated doubling is a power

8. Why do siblings resemble each other more than unrelated people, yet differ?
   Show the full solution

   They draw their alleles from the same two parents, so the pool available to each is the same and many alleles are shared, which produces the resemblance. But each sibling received a different combination, because independent assortment sent a different selection of homologues into each gamete, crossing over rearranged the alleles within chromosomes differently each time, and fertilization paired a different sperm with a different egg. The pool is shared and the sample from it is not. Same parental allele pool, different combination in each gamete

9. Explain why shuffling alleles is useful even though it creates nothing new.
   Show the full solution

   Because a population's capacity to respond to change depends on the range of combinations present, not only on the range of individual alleles. Two alleles that are individually advantageous may never have occurred together in one organism, and recombination can bring them into the same individual, or separate a beneficial allele from a harmful one it was linked to. Mutation supplies new variants slowly and rarely, so shuffling is what turns a small number of new alleles into a large number of tested combinations. It creates new combinations of existing alleles for selection to act on

10. Two genes are very close together on the same chromosome. Predict how often they are separated by crossing over, and explain.
    Show the full solution

    Rarely. Crossing over occurs at positions scattered along the chromosome, so the chance that a crossover point falls between two particular genes depends on the distance separating them. Genes close together are nearly always inherited as a unit because few crossovers occur in the short stretch between them, while genes far apart are separated often. This relationship between distance and recombination frequency is what allowed the first genetic maps to be built from breeding data alone. Rarely, since few crossover points fall in the short distance between them

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. List the four sources of genetic variation.
   Show the full solution

   Mutation, crossing over, independent assortment and random fertilization

2. Which of them creates new alleles?
   Show the full solution

   Mutation only

3. Which three only rearrange existing alleles?
   Show the full solution

   Crossing over, independent assortment and random fertilization

4. Is an acquired characteristic inherited?
   Show the full solution

   No, because it does not change the alleles in the gametes

5. In which type of cell must a mutation occur to be inherited?
   Show the full solution

   A gamete, or a cell that produces gametes

6. Explain why a population with no suitable allele cannot adapt, however strong the selection.
   Show the full solution

   Natural selection acts by favoring individuals that already differ, so it can only increase the frequency of variants that exist. Recombination rearranges existing alleles and cannot invent one, so if the allele is absent from the entire population there is nothing for selection to favor no matter how severe the pressure. The population is limited by its variation rather than by the strength of the selection, and extinction is a realistic outcome, which is why small isolated populations are so vulnerable. Selection can only favor variants that already exist

7. Explain why the arrival of a predator does not make a useful mutation more likely.
   Show the full solution

   Mutations arise from copying errors and chemical damage to DNA, processes that have no connection to what an organism is experiencing or needs. A predator changes which existing variants survive to reproduce, but it cannot reach into a gamete and alter a base sequence in a helpful direction. This is the central correction of the course: variation arises first and independently, and the environment then filters it. Reversing that order is the teleology error of lesson 10.2. Mutation is random with respect to need; the environment only filters

8. A weightlifter develops large muscles. Explain why their children do not inherit them.
   Show the full solution

   Training changes the muscle cells of the body, but inheritance passes through the alleles carried in gametes, and nothing about lifting weights alters the DNA sequence in those cells. The children inherit whatever alleles the parent already carried, including any tendency toward a particular build, but not the developed muscle itself. They would need to train to achieve the same result, which is the distinction between an acquired characteristic and an inherited one. Body cells changed, but the alleles in the gametes did not

9. Explain what recombination contributes if a rare beneficial allele already exists.
   Show the full solution

   It separates that allele from the particular genetic background it arose in and spreads it into many different combinations. The individual who first carried it also carried thousands of other alleles, some of which may be harmful, and without recombination the beneficial allele would be inherited together with them as a package. Recombination allows it to be tested on its own merits and to be combined with other favorable alleles arising elsewhere in the population, which speeds adaptation considerably. It frees the allele from its original background and combines it with others

10. Why is sexual reproduction common despite costing more than asexual reproduction?
    Show the full solution

    Sexual reproduction requires finding a mate, produces offspring carrying only half of each parent's alleles, and takes longer, all of which are real costs relative to simply copying oneself. The compensating benefit is variation: it generates offspring with combinations never tested before, so a population can respond to a changing environment, to new diseases and to new competitors. Asexual populations are efficient while conditions hold steady and vulnerable when they change, since every individual is susceptible to the same threats. The variation it generates lets populations respond to changing conditions

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define nondisjunction.
   Show the full solution

   Failure of chromosomes to separate properly during meiosis

2. What is trisomy?
   Show the full solution

   Having three copies of a chromosome instead of two

3. What is a karyotype?
   Show the full solution

   A display of an individual's chromosomes arranged in pairs by size

4. What is the underlying problem in cancer?
   Show the full solution

   Loss of control over cell division, through mutations in the genes that regulate it

5. Why does cancer risk rise with age?
   Show the full solution

   Several mutations are usually needed, and they accumulate over time

6. Explain why an extra chromosome is usually more serious than a single gene mutation.
   Show the full solution

   A gene mutation alters one protein, and the effect may range from nothing to severe depending on that protein's role. An extra chromosome changes the number of copies of every gene it carries, which can be hundreds at once, so the cell produces the wrong quantity of a great many proteins simultaneously. Development depends on those quantities being balanced against each other, so a whole-chromosome imbalance disrupts many interacting systems at the same time and is rarely survivable. It changes the dosage of hundreds of genes at once rather than one

7. Explain why trisomy 21 is survivable when most trisomies are not.
   Show the full solution

   Chromosome 21 is among the smallest human chromosomes and carries comparatively few genes, so an extra copy alters the dosage of a smaller number of proteins than an extra copy of a large chromosome would. The imbalance is therefore milder and development can proceed, although it is still substantially affected. Trisomies of large gene-rich chromosomes disrupt far more systems at once and almost always end in early loss of the pregnancy. Chromosome 21 is small and carries few genes, so the imbalance is smaller

8. Explain why "cancer cells grow too fast" is an inadequate description.
   Show the full solution

   Some cancer cells divide no faster than healthy cells of the same tissue, and some healthy tissues such as bone marrow and gut lining divide extremely rapidly without being cancerous. The defining problem is the loss of control: the cells divide when signals say they should not, fail to stop when crowded by neighbors, ignore the checkpoints that would arrest a damaged cell, and escape the programmed death that should remove them. Speed is a symptom in some cancers rather than the defect. The defect is loss of control over division, not the rate of it

9. Why does a single mutation rarely cause cancer?
   Show the full solution

   The controls on cell division are redundant and overlapping, so damage to one of them is usually detected by another. A cell with a broken checkpoint is typically arrested or destroyed by programmed cell death before it can form a tumor, and the immune system removes many abnormal cells as well. Escaping requires several of those safeguards to fail in the same cell lineage, which is improbable as a single event but becomes likely over decades of divisions and mutagen exposure. Control mechanisms are redundant, so several must fail in the same lineage

10. Why is a mutagen, a substance that increases mutation rate, also usually a carcinogen?
    Show the full solution

    Cancer arises from mutations in genes controlling cell division, so anything raising the overall mutation rate increases the chance that those particular genes are hit. Since several such mutations are normally required in one cell lineage, and each is individually unlikely, raising the rate has a disproportionate effect on the probability of the full combination occurring. The link is statistical rather than specific: the mutagen does not target the control genes, it simply damages DNA more often. More mutations overall means a higher chance of hitting the control genes

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What is a true-breeding line?
   Show the full solution

   A line producing offspring identical to itself over many generations

2. What did Mendel observe in the F1 generation of a tall by short cross?
   Show the full solution

   All offspring were tall, with no intermediates

3. What ratio appeared in the F2?
   Show the full solution

   About three tall to one short

4. Give two features of peas that suited the work.
   Show the full solution

   They can self-pollinate or be crossed by hand, and their characteristics come in two clear forms

5. What does the word latent tell you about a recessive character?
   Show the full solution

   It is hidden rather than lost, so it can reappear unchanged

6. Explain why the F2 result contradicts blending inheritance.
   Show the full solution

   Under blending, a tall plant crossed with a short one would give an intermediate offspring, and crossing those intermediates would give further intermediates, so the original forms would be diluted away and could never return. Mendel's F1 plants were all tall rather than intermediate, and short plants reappeared in the F2 exactly as short as the original grandparent. A character that disappears for a generation and comes back unchanged must have been carried intact, which blending cannot accommodate. Short plants reappeared unchanged, so the factor persisted rather than blending away

7. Explain why Mendel's choice of characteristics was as important as his method.
   Show the full solution

   Each characteristic he studied is controlled largely by a single gene with two clearly different forms, so every plant could be placed unambiguously in one of two categories and counted. That makes a numerical ratio visible. Most visible traits, including height in many species, are influenced by many genes and by the environment, producing a continuous range in which no ratio can be seen. Investigators studying such traits found only blurred averages, which is precisely why blending seemed correct. Single-gene either-or traits produce countable ratios; continuous traits hide them

8. Why did Mendel need to count thousands of plants?
   Show the full solution

   Because the three to one ratio is a statement about probabilities, and small samples depart from expected proportions by chance. Twelve offspring might easily give seven tall and five short, which suggests nothing, whereas several thousand will approach three to one closely enough that the pattern cannot be accidental. This is the sample size argument of lesson 1.3 applied to inheritance, and it is a large part of why Mendel saw a law where others saw noise. Small samples vary by chance; large ones reveal the underlying ratio

9. Explain what it means that Mendel inferred an unseen mechanism.
   Show the full solution

   He never observed a gene, a chromosome or a gamete carrying a factor. What he observed were counts of plants across generations, and from the regularity of those numbers he reasoned that the results would follow if each plant carried two discrete factors for each characteristic, one from each parent, which separated when gametes formed. The mechanism was a hypothesis proposed to explain a pattern, exactly as lesson 1.1 describes, and it was confirmed by cytology decades later. He proposed discrete paired factors to explain numerical patterns, with no direct observation

10. Suggest why Mendel's work was ignored for over thirty years.
    Show the full solution

    His argument was quantitative at a time when most biology was descriptive, so readers were unprepared for a conclusion resting on ratios rather than on observations of structures. Blending inheritance also fitted everyday experience with traits such as human height and skin color, which are polygenic, so his results looked like a peculiarity of peas rather than a general law. Without chromosomes and meiosis, which were described later, there was no visible mechanism his factors could correspond to. Its statistical argument had no known mechanism and conflicted with everyday impressions

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Distinguish a gene from an allele.
   Show the full solution

   A gene is a length of DNA coding for a characteristic; an allele is one version of that gene

2. Distinguish genotype from phenotype.
   Show the full solution

   Genotype is the alleles carried; phenotype is the observable characteristic

3. What does heterozygous mean?
   Show the full solution

   Carrying two different alleles of a gene

4. How many copies of a recessive allele are needed for it to be expressed?
   Show the full solution

   Two

5. Does dominant mean common?
   Show the full solution

   No; it means one copy is enough for expression

6. Explain why a dominant phenotype does not reveal the genotype but a recessive one does.
   Show the full solution

   A dominant allele is expressed whenever it is present, so both the homozygous dominant and the heterozygous genotype produce the same appearance and cannot be told apart by looking. A recessive phenotype requires two copies of the recessive allele, since a single dominant allele would have masked it, so an organism showing the recessive characteristic must be homozygous recessive. This asymmetry is why test crosses exist and why recessive individuals are so useful in pedigree analysis. Two genotypes give the dominant phenotype but only one gives the recessive

7. A short plant crossed with a tall plant gives some short offspring. Deduce both genotypes and explain.
   Show the full solution

   The short parent must be tt, since short is recessive and requires two copies. Each short offspring must also be tt, and it received one allele from each parent, so it must have received a t from the tall parent as well. Since that parent is tall it must carry at least one T, so its genotype is Tt. The appearance of a recessive phenotype among the offspring is what proves the dominant-looking parent is heterozygous. Short parent tt and tall parent Tt, since the short offspring needed a t from each

8. Explain why polydactyly being dominant and rare is not a contradiction.
   Show the full solution

   Dominance describes how an allele behaves when present with a different allele, specifically that one copy suffices for expression. It says nothing about how many people carry it. Frequency depends on how often the allele arises by mutation and on whether it has been favored or disfavored by selection over many generations. A dominant allele that is disadvantageous or simply rare in origin will stay rare, while a common recessive allele can be carried by a large fraction of a population without being expressed. Dominance concerns expression with one copy; frequency depends on mutation and selection

9. Why must the same letter be used for both alleles of a gene?
   Show the full solution

   Because the two alleles are versions of the same gene occupying the same position on homologous chromosomes, and the notation is meant to show that relationship. Writing T for tall and s for short would suggest two separate genes with no connection, and the Punnett square built from it would be meaningless. Using T and t makes it immediately clear which alleles are alternatives at one locus, which is the information a cross depends on. They are alternatives at the same locus, and the notation must show that

10. An organism is described as homozygous. What can and cannot be concluded about its phenotype?
    Show the full solution

    Only that its two alleles for that gene are identical, which does not by itself say which phenotype it shows. Homozygous dominant and homozygous recessive are both homozygous and give opposite appearances. What can be concluded is that it is true-breeding for that characteristic, since every gamete it produces carries the same allele, so crossing it with an identical organism gives offspring all showing the same phenotype. It is true-breeding for that gene, but the phenotype depends on which allele is doubled

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What gametes does a Tt parent produce, and in what proportion?
   Show the full solution

   T and t, in equal numbers

2. Give the genotypic and phenotypic ratios for Tt by Tt.
   Show the full solution

   Genotypic 1 TT to 2 Tt to 1 tt; phenotypic 3 tall to 1 short

3. What are the offspring of TT by tt?
   Show the full solution

   All Tt, and all showing the dominant phenotype

4. What is the probability that a Tt by tt cross gives a short offspring?
   Show the full solution

   One half

5. From 200 offspring of Tt by Tt, how many are expected to be short?
   Show the full solution

   About 50, since one quarter of 200 is 50

6. Explain why the Punnett square works, in terms of meiosis.
   Show the full solution

   Each parent carries two alleles on homologous chromosomes, and meiosis I separates those homologues so that every gamete receives exactly one. The two rows and two columns of the square represent the equally likely gamete types each parent produces. Fertilization brings one gamete from each parent together at random, which is what each box represents. The grid is therefore a direct picture of segregation followed by random fertilization, not an arbitrary convention. Meiosis separates the alleles into gametes and fertilization recombines them at random

7. A cross of two tall plants produces some short offspring. Deduce the parental genotypes and justify.
   Show the full solution

   Both parents must be Tt. Each short offspring is tt, since short is recessive and requires two copies, and it received one allele from each parent, so each parent must have supplied a t. Both parents are tall, so each must also carry a T, making both heterozygous. Neither could be TT, because a TT parent can supply only T and no short offspring would be possible at all. Both Tt, since each short offspring needed a t from each tall parent

8. Explain why four offspring of a Tt by Tt cross are often not three tall and one short.
   Show the full solution

   The three to one ratio is a probability per offspring rather than a quota to be filled. Each fertilization independently has a three in four chance of giving a tall plant, so with only four offspring the outcome is subject to considerable chance, in the same way that four coin tosses frequently do not give exactly two heads. All four could be tall. The ratio becomes reliable only over large numbers, which is why Mendel counted thousands. It is a probability per offspring, and small samples vary by chance

9. A couple have three children with a recessive condition. Is the fourth less likely to be affected? Explain.
   Show the full solution

   No. If both parents are carriers, each child independently has a one in four chance of inheriting two recessive alleles, and the gametes involved in the fourth conception are unaffected by what happened in the first three. The intuition that the odds must even out treats past independent events as though they influenced future ones. The only thing the three affected children change is the confidence that both parents really are carriers. Each conception is independent, so the chance remains one in four

10. Why does a cross between a homozygous dominant and a homozygous recessive parent hide the recessive allele?
    Show the full solution

    The TT parent can only supply T and the tt parent can only supply t, so every offspring is Tt. Each of them carries the recessive allele but also carries a dominant one, which is expressed, so all the offspring show the dominant phenotype and none reveals the recessive. The allele has not been lost and reappears in the next generation when two heterozygotes are crossed, which is exactly the F1 and F2 pattern Mendel observed. All offspring are heterozygous, so the dominant allele masks the recessive one

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What gametes does a TtRr parent produce?
   Show the full solution

   TR, Tr, tR and tr, in equal numbers

2. State the phenotypic ratio from TtRr by TtRr.
   Show the full solution

   9 to 3 to 3 to 1

3. How many boxes does a dihybrid Punnett square have?
   Show the full solution

   Sixteen

4. What condition must hold for the 9:3:3:1 ratio?
   Show the full solution

   The two genes must assort independently, usually meaning they are on different chromosomes

5. From 160 offspring, how many are expected to show both recessive characteristics?
   Show the full solution

   One sixteenth of 160, which is 10

6. Explain where the 9:3:3:1 ratio comes from.
   Show the full solution

   It is the product of two independent three to one ratios. Considered alone, each gene in a heterozygous cross gives three quarters showing the dominant characteristic and one quarter showing the recessive. Because the genes assort independently, the probability of any combination is the product of the separate probabilities: three quarters times three quarters gives nine sixteenths, three quarters times one quarter gives three sixteenths twice over, and one quarter times one quarter gives one sixteenth. Those four fractions are the ratio. Two independent 3:1 ratios multiplied together

7. Explain why the probability method is better than drawing the square.
   Show the full solution

   It is faster, less error prone and it scales. A sixteen-box square requires listing four gamete types twice and filling sixteen cells correctly, and a single slip in the gamete list corrupts everything after it. The probability method needs four multiplications. More importantly, following three genes would need a sixty four box square, which is impractical, while the probability method simply multiplies a third fraction into each result and works for any number of independently assorting genes. It is quicker, less error prone, and extends to three or more genes

8. Why can a gamete never be TT or rr?
   Show the full solution

   A gamete is haploid and carries one allele of each gene, because meiosis I separates the homologous chromosomes that bear the two alleles. Writing TT would mean the gamete carried both copies of the height gene, which happens only in nondisjunction, and writing rr would mean the same for seed shape. Each gamete must contain exactly one allele from each gene, which is why the four types from TtRr each pair one height allele with one shape allele. Gametes are haploid and carry one allele of each gene

9. A dihybrid cross gives a ratio close to 3:1 rather than 9:3:3:1. Suggest why.
   Show the full solution

   The two genes are probably linked, meaning they sit close together on the same chromosome and are inherited as a unit rather than assorting independently. Under linkage the parental combinations appear far more often than expected and the four-category ratio collapses toward the pattern of a single gene, with only a small number of recombinant offspring produced by crossing over between the two loci. The departure from the expected ratio is itself the evidence for linkage. The genes are linked on the same chromosome and do not assort independently

10. Calculate the expected number of tall wrinkled offspring from 480 offspring of TtRr by TtRr.
    Show the full solution

    Tall requires at least one T, which has probability three quarters, and wrinkled requires two r alleles, which has probability one quarter. Multiplying gives three sixteenths for the combination. Three sixteenths of 480 is 480 divided by 16, which is 30, multiplied by 3, giving 90. So about 90 offspring are expected to be tall and wrinkled, remembering that this is an expectation rather than a guarantee. 90, since three sixteenths of 480 is 90

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What is a test cross, and what is it used for?
   Show the full solution

   Crossing an unknown dominant phenotype with a homozygous recessive to determine its genotype

2. Why is a homozygous recessive used as the partner?
   Show the full solution

   It can only contribute recessive alleles, so it cannot mask anything

3. What do squares and circles mean in a pedigree?
   Show the full solution

   Squares are male and circles female

4. What does an affected child of two unaffected parents prove?
   Show the full solution

   That the condition is recessive and both parents are carriers

5. What result from a test cross proves the unknown is heterozygous?
   Show the full solution

   Any offspring showing the recessive phenotype

6. Explain why one recessive offspring is conclusive but ten dominant ones are not.
   Show the full solution

   A recessive offspring must be homozygous recessive, so it received a recessive allele from each parent, and the unknown parent must therefore carry one. That is a logical certainty from a single observation. A run of dominant offspring is only probabilistic: a heterozygote crossed with a homozygous recessive has a one in two chance of a dominant offspring each time, so ten in a row has a probability of about one in a thousand. Unlikely is not impossible. One recessive offspring is logically decisive; a run of dominants is only improbable

7. Explain why a characteristic appearing in every generation does not prove it is dominant.
   Show the full solution

   Appearing in every generation is what a dominant allele typically produces, but a recessive allele that is common in the population can do the same, because affected individuals frequently have partners who are carriers or affected themselves. The pattern is therefore consistent with both possibilities and cannot distinguish them. What does distinguish them is an affected child born to two unaffected parents, which is impossible under dominance and straightforward under recessive inheritance. A common recessive allele produces the same pattern; only unaffected parents with an affected child is decisive

8. A tall plant crossed with a short plant gives 12 tall and 0 short. What can be concluded?
   Show the full solution

   That the tall parent is very probably homozygous dominant, but not certainly. If it were heterozygous, each offspring would have a one in two chance of being short, so twelve consecutive tall offspring would have a probability of about one in four thousand. That is strong evidence for TT without being proof, and the honest answer states the conclusion with its strength rather than asserting certainty, which is the habit lesson 1.5 requires. Very probably TT, since twelve tall offspring from a heterozygote is about one in four thousand

9. In a pedigree, an affected son has an unaffected mother and unaffected father. What are the possibilities?
   Show the full solution

   The condition is recessive, since neither parent shows it. It could be autosomal recessive, in which case both parents are carriers and the son is homozygous recessive. It could equally be X-linked recessive, in which case the mother is a carrier and the son inherited her affected X while receiving a Y from his father, so the father need carry nothing. Distinguishing them requires more of the family: affected daughters would argue for autosomal inheritance. Autosomal recessive with both parents carriers, or X-linked recessive with a carrier mother

10. Two carriers of a recessive condition have a child. State the probabilities and explain.
    Show the full solution

    Both parents are heterozygous, so the cross is equivalent to Tt by Tt. The child has a one in four chance of being homozygous recessive and affected, a two in four chance of being a heterozygous carrier, and a one in four chance of being homozygous unaffected. Expressed as phenotypes, there is a three in four chance the child is unaffected, though two thirds of those unaffected children will be carriers, which matters for the next generation. One in four affected, two in four carriers, one in four homozygous unaffected

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What phenotype does incomplete dominance give in the heterozygote?
   Show the full solution

   An intermediate between the two homozygous phenotypes

2. How does codominance differ from incomplete dominance?
   Show the full solution

   Both alleles are fully expressed rather than producing an intermediate

3. How many ABO alleles exist, and how many does one person carry?
   Show the full solution

   Three exist; each person carries two

4. Give the genotypes for blood group O and group AB.
   Show the full solution

   O is ii; AB is I^AI^B

5. What kind of variation do polygenic characteristics produce?
   Show the full solution

   Continuous variation rather than distinct categories

6. Explain why incomplete dominance is not blending inheritance.
   Show the full solution

   Blending inheritance claims the hereditary material itself mixes, so the parental forms are permanently diluted and cannot return. In incomplete dominance only the phenotype is intermediate: the two alleles remain distinct in the heterozygote and separate unchanged during meiosis. Crossing two pink snapdragons therefore produces red and white offspring again, in a quarter of cases each, which is impossible under blending and is decisive evidence that the alleles stayed intact. The alleles stay distinct and separate unchanged, so red and white reappear

7. Explain why the F2 ratio for incomplete dominance is 1:2:1 rather than 3:1.
   Show the full solution

   The genotypic ratio from a heterozygous cross is always 1 to 2 to 1, whatever the dominance relationship. Under complete dominance the homozygous dominant and the heterozygote look identical, so two of the three categories merge and the phenotypic ratio collapses to 3 to 1. Under incomplete dominance each genotype produces a distinguishable appearance, so nothing merges and the phenotypic ratio simply matches the genotypic one. Each genotype is visibly different, so no categories merge

8. A group A parent and a group B parent have a group O child. Explain how.
   Show the full solution

   Both parents must be heterozygous, carrying the recessive i allele alongside their expressed allele, so their genotypes are I^Ai and I^Bi. Each can pass on i, and a child receiving i from both parents is ii and therefore group O. The parents' own appearance gives no hint of this, because a single I^A or I^B allele is enough to determine their group, which is the standard behavior of a dominant allele. Both parents are heterozygous carriers of i and each passed it on

9. Explain why human height shows a continuous range rather than tall and short categories.
   Show the full solution

   Height is polygenic, influenced by many genes each contributing a small amount, so the possible totals form a near continuous scale rather than two groups. With many genes involved, most people inherit a mixture of increasing and decreasing alleles and fall near the middle, while extremes require an unusual combination and are rarer. Environment adds further variation through nutrition and health, smoothing the distribution further. This is why Mendel's approach would have failed on such a trait. Many genes each contribute a little, and environment adds more variation

10. Why can blood groups exclude a parent but not prove parentage?
    Show the full solution

    Certain combinations are impossible, so finding one excludes a person definitively: a group AB individual cannot be the parent of a group O child, because they carry no i allele to pass on. But the possible combinations are shared by very large numbers of people, since only four groups exist, so a compatible result shows only that the person is not excluded. Exclusion is logically decisive while inclusion is not, which is a general feature of tests with few categories. Impossible combinations exclude definitively, but compatibility is shared by millions

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What are the sex chromosome genotypes of females and males?
   Show the full solution

   Females XX, males XY

2. Why can a male not be a carrier of an X-linked condition?
   Show the full solution

   He has only one X, so any allele on it is expressed

3. Write the genotype of a carrier female for an X-linked recessive condition.
   Show the full solution

   X^NXⁿ

4. From which parent does a son inherit his X chromosome?
   Show the full solution

   His mother

5. In the worked cross, what proportion of sons are affected?
   Show the full solution

   One half

6. Explain why X-linked recessive conditions are commoner in males.
   Show the full solution

   A male has a single X chromosome and a Y that carries no corresponding allele, so whatever version of the gene sits on his X is expressed with nothing to mask it. One copy of the recessive allele is therefore sufficient. A female has two X chromosomes, so a recessive allele on one is masked by a normal allele on the other unless she happens to inherit the recessive version from both parents, which is far less likely. The requirement is one copy against two. Males need one copy to be affected, females need two

7. Explain why an affected father cannot pass the condition to his sons.
   Show the full solution

   A father gives each son a Y chromosome and each daughter his X. Since the gene is carried on the X, the allele he possesses goes only to his daughters, who receive it alongside an X from their mother and are usually carriers rather than affected. His sons receive their single X from their mother, so whether they are affected depends entirely on her. This is why X-linked conditions appear to skip from grandfather to grandson through an unaffected daughter. Sons receive his Y, not his X, so the allele goes only to daughters

8. A color blind woman has sons with a man of normal vision. Predict the outcome.
   Show the full solution

   She is XⁿXⁿ and he is X^NY. Every son receives an Xⁿ from his mother and a Y from his father, so every son is XⁿY and color blind. Every daughter receives an Xⁿ from her mother and an X^N from her father, so every daughter is X^NXⁿ, a carrier with normal vision. All sons affected and all daughters carriers is the characteristic signature of an affected mother. All sons color blind, all daughters unaffected carriers

9. Why does writing the genotypes as Nn and nn lose marks?
   Show the full solution

   Because that notation describes an autosomal gene, where both sexes carry two alleles, and it therefore cannot represent the situation that makes sex linkage work. It gives no way to show that a male has a single allele and a Y with no partner, so it predicts the same outcome for sons and daughters, which is exactly the result the question is testing. Writing X^N, Xⁿ and Y makes the asymmetry explicit and produces the correct ratios automatically. It cannot represent the Y chromosome, so it predicts no difference between the sexes

10. A condition appears in a pedigree only in males, passed through unaffected females. What does this suggest?
    Show the full solution

    X-linked recessive inheritance. Affected males inherit the allele on the single X they receive from their mothers, who are unaffected carriers because their second X carries a normal allele. The condition therefore appears to skip generations, passing invisibly through women and surfacing in their sons. An autosomal recessive condition would affect both sexes about equally, so a strong male bias combined with transmission through unaffected women is the characteristic signature. X-linked recessive, carried invisibly by female carriers and expressed in their sons

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. List the four conditions for natural selection.
   Show the full solution

   Variation, heritability, overproduction, and differential survival and reproduction

2. What evolves: an individual or a population?
   Show the full solution

   A population

3. Where does overproduction lead, according to unit 3?
   Show the full solution

   To competition, because populations exceed what resources can support

4. Did Darwin know the source of variation?
   Show the full solution

   No; he wrote "from whatever cause proceeding"

5. What happens if a variation is not heritable?
   Show the full solution

   Natural selection cannot change the population, since survivors cannot pass it on

6. Explain why all four conditions are necessary.
   Show the full solution

   Without variation every individual is equivalent, so survival cannot depend on any difference. Without heritability the survivors' advantages die with them and the next generation starts over. Without overproduction almost everyone survives and there is no differential to act on. Without differential survival related to the variation, who reproduces is effectively random and allele frequencies drift rather than adapt. Remove any one and the conclusion fails, which is why the list is a set of conditions rather than a description. Removing any one breaks the chain from variation to changed frequencies

7. Explain why Darwin's argument survived the discovery of genetics.
   Show the full solution

   His argument requires only that variation exists and is heritable, not any particular explanation of how. Genetics supplied the mechanism he lacked, showing that variation arises through mutation and is shuffled by recombination, and that inheritance is particulate rather than blending. Far from overturning the argument, this strengthened it: blending inheritance would actually have destroyed variation each generation and undermined selection, so Mendel's results rescued a difficulty Darwin could not resolve. It assumed only that variation is heritable, and genetics supplied the mechanism

8. A farmer breeds only from the highest-yielding wheat plants each year. Identify the four conditions.
   Show the full solution

   Variation: the plants differ in yield. Heritability: some of that difference is genetic and passes to the next generation. Overproduction: far more seed is produced than the farmer replants, so most does not contribute. Differential reproduction: which plants contribute seed is decided by yield rather than at random. All four hold, so the population changes, and the only difference from natural selection is that a farmer rather than the environment is doing the selecting. All four hold, with the farmer supplying the differential reproduction

9. Explain why "the individual evolved to be darker" is wrong.
   Show the full solution

   An individual's genotype is fixed at fertilization and does not change over its life in response to the environment, so a light beetle cannot become dark. What changes is the proportion of dark individuals in the population across generations, as light ones are eaten more often and leave fewer offspring. Evolution is a change in allele frequencies within a population over time, so an individual is the thing selected rather than the thing that evolves, and the two must not be run together. An individual's genotype is fixed; only population frequencies change

10. Why does Darwin's phrase "in its infinitely complex relations" matter?
    Show the full solution

    It makes clear that whether a variation is profitable depends entirely on circumstances rather than being a fixed property of the organism. A dark shell helps on dark bark and harms on pale bark; thick fur helps in cold and harms in heat. Advantage is defined relative to a particular environment, including other organisms, so there is no such thing as a generally superior variant. This is why selection produces organisms suited to conditions rather than progressively better organisms. Advantage is relative to a specific environment, not an absolute property

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Is mutation directed toward what an organism needs?
   Show the full solution

   No; it is random with respect to need

2. Which comes first, the variation or the selection pressure?
   Show the full solution

   The variation

3. Name three phrases that signal the teleology error.
   Show the full solution

   Any three of: in order to, so that, needed to, wanted to, tried to

4. Does the environment instruct or select?
   Show the full solution

   It selects

5. Why might a resistant bacterium be rare before the antibiotic appears?
   Show the full solution

   Resistance often carries a cost, so those cells reproduce more slowly without the drug

6. Explain why the antibiotic cannot cause the useful mutation.
   Show the full solution

   Mutations arise from errors during DNA replication and from chemical or radiation damage, and none of those processes has any means of identifying which change would help the cell survive a particular poison. An antibiotic interferes with a bacterial process in order to kill the cell; it has no mechanism for editing a specific base in a specific gene in a beneficial direction. Even mutagens that raise the overall mutation rate raise it indiscriminately, producing far more harmful changes than useful ones. No process connects a poison to a specific beneficial base change

7. Rewrite correctly: "Giraffes stretched their necks to reach higher leaves, so their necks got longer."
   Show the full solution

   A correct version: neck length varied among ancestral giraffes and that variation was heritable. Where food was concentrated in higher branches, individuals with longer necks obtained more food, survived better and left more offspring, so the alleles associated with longer necks became commoner over many generations. The original sentence contains two errors: it treats a characteristic acquired by stretching as heritable, which lesson 8.5 rules out, and it makes the need the cause of the change. Existing heritable variation in neck length was favored, rather than stretching being inherited

8. Why is finishing a course of antibiotics important, in these terms?
   Show the full solution

   A short course kills the most susceptible bacteria first, leaving those with partial resistance alive in a body where competition has just been removed. Those survivors then multiply freely, and the population that regrows is enriched for resistance, which is selection operating exactly as described. Completing the course aims to eliminate the partially resistant cells as well, before they can reproduce and before further mutations can add to their resistance. Stopping early leaves the most resistant cells to multiply without competition

9. An experiment shows resistant bacteria exist in a culture never exposed to the antibiotic. Explain its significance.
   Show the full solution

   It demonstrates directly that resistance arises before exposure rather than in response to it, which is the whole question at issue. If the drug caused the mutation, a culture that had never encountered it should contain no resistant cells at all. Finding them shows that random mutation supplies the variation independently and that the antibiotic's only role is to select among variants that already exist. It is a designed test with a clear falsifying outcome, in the sense of lesson 1.1. Resistance arising without exposure shows mutation is independent of the pressure

10. Explain why teleological language is treated so seriously rather than as a stylistic problem.
    Show the full solution

    Because it describes a mechanism that does not exist and generates wrong predictions. A student who believes organisms produce the variation they need will expect any population under pressure to adapt, and will be unable to explain why species go extinct, why the same pressure produces different outcomes in different populations, or why a population lacking a suitable allele cannot respond at all. The phrasing is the visible symptom of a model that fails on exactly the cases the subject cares about. It encodes a false mechanism that predicts adaptation is always available

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define biological fitness.
   Show the full solution

   The number of surviving offspring an individual produces relative to others in the population

2. What is the fitness of an organism that lives long but never reproduces?
   Show the full solution

   Zero

3. Why is survival relevant at all?
   Show the full solution

   Only because it enables reproduction

4. In the worked example, which male has the highest fitness?
   Show the full solution

   B, with 11 surviving offspring

5. What does sexual selection act on?
   Show the full solution

   Mating success

6. Explain why "survival of the fittest" is an unsatisfactory summary.
   Show the full solution

   It emphasizes survival, which matters only as a means to reproduction, and it invites the everyday reading of fittest as strongest or healthiest. As usually stated it is also close to circular, since the fittest are identified by their survival and their survival is then explained by their fitness. Describing the process as differential reproductive success avoids all three problems, because offspring number is measurable independently of the outcome being explained. It stresses survival over reproduction, invites the wrong sense of fittest, and risks circularity

7. Explain how a trait that shortens life can be favored.
   Show the full solution

   Selection counts offspring, not years. If a characteristic causes an organism to invest very heavily in a single large reproductive effort, it may leave far more surviving young than a cautious individual that lives twice as long and breeds modestly, and the alleles involved will therefore increase in frequency. Salmon that spawn once and die illustrate the extreme case. The cost in lifespan is simply irrelevant provided the reproductive gain exceeds it. If it raises offspring number enough, the loss of lifespan does not matter

8. Explain how an elaborate tail that makes escape harder can still be favored.
   Show the full solution

   Such a feature reduces survival but can increase mating success enough to more than compensate. If females prefer larger displays, males carrying the alleles for them obtain more matings and leave more offspring despite a higher risk of predation, so those alleles spread. Fitness is the net outcome of all effects on reproductive success, and a survival cost is only decisive if it is not outweighed. This is sexual selection, and it explains features that look positively unhelpful. The gain in mating success outweighs the cost to survival

9. Why is fitness described as relative rather than absolute?
   Show the full solution

   Because what matters to the composition of the next generation is how an individual's reproductive output compares with that of others in the same population. Two surviving offspring is high fitness where the average is one and low where the average is ten, so the same number means opposite things in different contexts. Fitness also depends on the environment, since a characteristic favored on dark bark is penalized on pale bark, so a genotype has no fixed fitness value at all. It is measured against the rest of the population and depends on the environment

10. A sterile worker bee raises its mother's offspring. Explain how this can make evolutionary sense.
    Show the full solution

    Its own direct fitness is zero, since it never reproduces, but the siblings it helps raise carry many of the same alleles, having the same mother. Alleles causing an individual to help close relatives reproduce can therefore increase in frequency even when their carrier has no offspring, because copies of those alleles are being passed on through the relatives. Fitness is counted in copies of alleles reaching the next generation, and helping relatives is one route to that. Relatives carry the same alleles, so helping them raise young passes copies on

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Name the three patterns of selection.
   Show the full solution

   Directional, stabilizing and disruptive

2. Which pattern shifts the mean toward one extreme?
   Show the full solution

   Directional

3. What happens to the spread under stabilizing selection?
   Show the full solution

   It narrows, while the mean stays about the same

4. What shape of distribution does disruptive selection produce?
   Show the full solution

   A two-peaked distribution

5. State the test for classifying a case.
   Show the full solution

   Ask which part of the distribution is being removed: one tail, both tails, or the middle

6. Explain why human birth mass is a case of stabilizing selection.
   Show the full solution

   Both extremes suffer higher mortality while the middle does best. Very small babies have poorer survival because they are less able to maintain temperature and are more vulnerable to infection, and very large babies historically made delivery dangerous for both mother and infant. Infants of intermediate mass therefore survived most often, removing both tails of the distribution. The mean stays roughly where it is and the spread narrows, which is exactly the stabilizing signature. Both extremes have higher mortality, so the spread narrows around an unchanged mean

7. Explain why stabilizing selection is the commonest pattern.
   Show the full solution

   A population that has lived in an environment for many generations has already been shaped by selection toward the best available compromise for those conditions, so the current mean is close to optimal. Most individuals departing from it in either direction do slightly worse, and those departures are removed each generation. Directional selection requires something to have changed, such as a new predator or a shifting climate, and environments are stable more often than they change. Populations are usually already near the optimum for a stable environment

8. Explain how disruptive selection can begin the formation of two species.
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   It favors individuals at both extremes while removing intermediates, so the population develops two increasingly distinct groups. If members of each group also begin to mate preferentially with others like themselves, perhaps because they feed in different places or breed at different times, gene flow between the groups falls. With reduced interbreeding the two groups accumulate further genetic differences independently, and if the separation becomes complete they may eventually be unable to interbreed at all. It splits the population into two groups, and reduced gene flow lets them diverge

9. Artificial selection produces change very quickly. Explain why.
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   Because the selection differential is far stronger and more consistent than anything natural conditions usually impose. A breeder may allow only the most extreme few percent of individuals to reproduce, and applies that same criterion every generation without interruption, whereas natural selection typically acts weakly, inconsistently, and on many characteristics at once that trade off against each other. Concentrating intense selection on a single trait produces rapid change, and the variety of dog breeds shows how much. Breeders impose far stronger, more consistent selection on one trait

10. A population of moths shifts from mostly pale to mostly dark within fifty years. Classify and explain.
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    Directional selection, since one extreme of the existing variation is being favored and the whole distribution has moved toward it. Heritable variation in color was already present, and a change in the environment, such as darkening of the surfaces the moths rest on, made pale individuals more conspicuous to predators hunting by sight. Pale moths left fewer offspring, the proportion of dark moths rose each generation, and the population mean shifted without any individual moth changing color. Directional: an environmental change favored one existing extreme and shifted the mean

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Name two conditions required for fossilization.
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   Rapid burial in sediment and absence of oxygen

2. What is relative dating based on?
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   The order of rock layers, with deeper layers older

3. What does absolute dating use?
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   Radioactive decay

4. Which organisms are over-represented in the fossil record?
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   Marine organisms and those with hard parts such as shells and bones

5. What is a transitional form?
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   An organism showing a combination of features found in different later groups

6. Explain why the order of appearance is stronger evidence than any single fossil.
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   A single fossil can always be questioned on dating, identification or context, and an individual specimen shows only that one organism existed. The order is a pattern across millions of specimens from sites all over the world, and it matches a prediction that could have failed at any time: no group appears in rocks older than the group it descends from. That the prediction has held across such an enormous and independently collected sample is far harder to explain away than any one find. It is a testable pattern across millions of specimens, not a single claim

7. Explain why gaps in the record are expected.
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   Fossilization requires a rare combination of circumstances: the organism must die where sediment accumulates rapidly, be buried before scavengers and decomposers destroy it, avoid oxygen, and then survive hundreds of millions of years of geological activity without being eroded or melted. Most organisms die in places where none of this happens. Soft-bodied organisms and those living in uplands are especially unlikely to be preserved, so absence from the record is weak evidence that something never existed. The conditions for preservation are rare and biased toward particular environments

8. Explain why "half reptile and half bird" is a poor description of a transitional fossil.
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   It suggests an incomplete or intermediate creature assembled from two modern categories, when in fact the animal was a functioning organism well suited to its own environment. The categories reptile and bird are labels applied afterward to groups defined by features that had not yet fully separated at the time the animal lived. A better description is that it shows a combination of characteristics later found in different groups, which is exactly what descent with modification predicts should have existed. It was a complete organism showing a mixture of features, not a half-formed hybrid

9. Why does finding a fossil rarely prove direct ancestry?
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   Because the fraction of organisms preserved is tiny and the fraction discovered smaller still, so the probability that any particular specimen belonged to the actual ancestral population rather than to a related side branch is low. A fossil showing the right combination of features at the right time demonstrates that such forms existed and shows the kind of change that occurred, which is what the argument needs. Claiming a specific ancestor requires much stronger evidence and is usually avoided. Most fossils are from related branches, not the exact ancestral line

10. What would a mammal fossil in rock older than the first fish imply?
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    It would be a serious problem for common descent as currently understood, since mammals are held to descend from earlier vertebrate groups that descend in turn from fish, and nothing can precede its own ancestors. A single well-documented case would demand either a major revision of the sequence or an explanation of how the specimen came to be in that layer. The fact that such a find is possible in principle, and has not occurred despite enormous sampling, is what makes the order genuine evidence. It would contradict common descent, which is why the untroubled order is meaningful

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. Define homologous structures.
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   Structures sharing underlying structure and origin, though possibly with different functions

2. Define analogous structures.
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   Structures sharing a function but not underlying structure or origin

3. Which type indicates common ancestry?
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   Homologous

4. Give an example of a vestigial structure.
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   The pelvic bones of whales

5. What does convergent evolution produce?
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   Analogous structures, from similar selective pressures on unrelated lineages

6. Explain why homologous structures with different functions are stronger evidence than similar structures doing the same job.
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   When two organisms do the same job, similarity might simply reflect the best solution to that problem, so it could arise independently and says little about relatedness. When the jobs are entirely different, as with a bat wing and a human arm, there is no functional reason for the same bone arrangement to appear in both, and design for the task would predict different structures. Inheritance from a shared ancestor that already had that arrangement is the remaining explanation. Different functions rule out the possibility that the similarity is required by the job

7. A dolphin and a shark have similar body shapes. Explain what this does and does not show.
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   It shows that fast swimming in open water imposes strong constraints, so a streamlined shape with fins is favored in any lineage occupying that role. It does not show close relatedness, because the underlying structures differ fundamentally: a dolphin is a mammal that breathes air, bears live young and has the bone arrangement of a four-limbed ancestor in its flippers, while a shark is a fish with a cartilaginous skeleton. This is analogy produced by convergent evolution. Shared selective pressure, not shared ancestry: the structures differ

8. Why are vestigial structures difficult to explain without common descent?
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   They are features that cost something to build and maintain while serving little or no current purpose, which is what an organism designed for its present way of life would not have. Pelvic bones in a whale, unconnected to any limb, make sense as a reduced remnant inherited from four-legged ancestors and progressively lost as the lineage returned to water. Their presence is predicted by descent with modification, which can only modify what is already there, and is otherwise puzzling. They are useless leftovers that descent predicts and design does not

9. What does shared early embryonic development suggest?
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   That related groups inherit not only adult structures but the developmental programs that build them. Vertebrate embryos pass through stages showing features such as pharyngeal arches and a tail even in species whose adults have neither, because the early stages of a shared program are harder to modify than the later ones without disrupting everything downstream. The resemblance is therefore a trace of common ancestry preserved in development rather than in the finished animal. Related groups inherit shared developmental programs, whose early stages resist change

10. A student says a bird wing and a bat wing are analogous because both fly. Correct them.
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    The function is indeed the same, but classification depends on underlying structure and origin rather than on use. Both wings contain the same basic vertebrate limb arrangement of one upper bone, two lower bones, wrist bones and digits, inherited from a common four-limbed ancestor, and they differ only in how that framework has been modified, with feathers in the bird and a membrane stretched between elongated fingers in the bat. They are therefore homologous structures that happen also to share a function. Both share the vertebrate limb arrangement, so they are homologous despite the shared function

Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.

1. What does the near-universality of the genetic code suggest?
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   A single common origin for life

2. How does the number of sequence differences relate to relatedness?
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   More closely related species have fewer differences

3. From the table, which organism differs most from humans?
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   Yeast, with 45 differences

4. Why is DNA comparison more sensitive than protein comparison?
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   It detects changes that do not alter the amino acid sequence

5. What makes molecular evidence powerful alongside anatomical evidence?
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   It is independent, so it could have disagreed and did not

6. Explain why independent agreement between two methods is stronger than either alone.
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   Each method has its own assumptions and its own possible errors, and those are unrelated between anatomy and molecular sequencing. A mistaken interpretation of a skeleton has no reason to produce a matching mistake in an amino acid sequence, so an alternative explanation would have to account for both patterns separately while denying the conclusion they share. That is far harder than questioning one method. Convergence of unrelated approaches is the general structure of strong evidence in science. Unrelated methods have unrelated errors, so agreement is hard to explain away

7. Human and chimpanzee cytochrome c are identical. Does this mean the species are identical? Explain.
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   No. It means this particular protein has not accumulated any differences between the two lineages since they diverged, which is expected for a slowly changing protein over a relatively short period. The species differ at a great many other positions in their genomes, and comparisons using faster-changing sequences reveal those differences clearly. The result is a statement about the resolution of one molecule, not about overall similarity. Only that this slowly changing protein has not diverged; other sequences have

8. Explain why cytochrome c changes so slowly over evolutionary time.
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   It has an essential role in the electron transport chain, which supplies most of a cell's ATP, and its function depends on a precise structure. Most mutations altering it therefore impair respiration and are strongly selected against, so they do not persist in populations. Only the rare changes that leave function intact accumulate. Proteins under weaker constraint, or regions of DNA that do not code for anything, change much faster because harmful variants are not removed as efficiently. Strong selection removes most changes to an essential protein

9. Which molecule would you choose to compare two closely related species, and why?
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   A fast-changing sequence, such as a region of DNA that does not code for a protein or a gene under weak selective constraint. Closely related species have had little time since diverging, so a slowly changing protein like cytochrome c may show no differences at all and provide no resolution, as the human and chimpanzee result illustrates. A sequence that accumulates changes quickly will have registered enough differences in that short interval to allow a meaningful comparison. A fast-changing sequence, since a conserved one may show no differences at all

10. Summarize the three lines of evidence and state what each contributes.
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    Fossils show that life has changed over time and establish the order in which groups appeared, which no other evidence can supply directly. Comparative anatomy shows shared underlying structure where the function does not require it, along with vestigial features and shared embryonic development, pointing to inheritance rather than independent design. Molecular comparison shows a pattern of sequence differences matching relatedness, and does so independently of the other two. The argument rests on their convergence rather than on any one. Fossils give the sequence, anatomy gives unnecessary shared structure, molecules confirm it independently