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This paper covers the whole of Topic 12. Like a real Cambridge paper it ranges across every sub-topic — 12.1 respiration and the uses of energy, 12.2 aerobic respiration and 12.3 anaerobic respiration — and it mixes them inside single questions. All three Topic 12 papers do; they differ in the angle they come at it from, not in what they cover.
Question 1 — Three Words That Are Not Interchangeable
Total: 12 marks
(a)[2]
Define respiration.
Model Answer — 1(a)
the chemical reactions in cells that break down nutrient molecules [1]
and release energy for metabolism [1]
⚠ If you missed marks here: The verb decides this mark: energy is released, never produced, made or created. It was stored in the glucose long before the cell got hold of it. Note also that the definition begins “in cells” — not in the lungs and not in the blood.
(b)[3]
Distinguish between respiration, ventilation and gas exchange. State where each takes place.
Model Answer — 1(b)
respiration — chemical reactions releasing energy, taking place in every living cell [1]
ventilation — the movement of air into and out of the lungs, brought about by the ribs, intercostal muscles and diaphragm [1]
gas exchange — diffusion of oxygen and carbon dioxide across a surface, taking place at the alveoli [1]
⚠ If you missed marks here: Each mark needs the process and its location. These three words are policed carefully because using the wrong one suggests you think they are the same thing — and a candidate who writes “respiration happens in the lungs” has said exactly that.
(c)[4]
Compare aerobic respiration with anaerobic respiration in human muscle. Give four differences.
Model Answer — 1(c)
aerobic respiration uses oxygen whereas anaerobic respiration does not [1]
aerobic respiration releases much more energy per glucose molecule than anaerobic respiration [1]
aerobic respiration produces carbon dioxide and water whereas anaerobic respiration in muscle produces lactic acid [1]
in aerobic respiration the glucose is completely broken down, whereas in anaerobic respiration it is not, so energy remains in the lactic acid [1]
accept also: aerobic respiration takes place in the mitochondria
⚠ If you missed marks here: The command word is compare, so every sentence must mention both sides — build each one around “whereas” or “than”. A list of four true statements about aerobic respiration alone is a common answer and scores nothing at all.
(d)[3]
State three uses of the energy released by respiration in a plant.
Model Answer — 1(d)
any three of: active transport (for example uptake of ions by root hair cells), protein synthesis, cell division, growth [1 each]
note: muscle contraction, nerve impulses and maintaining a constant body temperature are not creditable for a plant
⚠ If you missed marks here: This is the seven-item list applied to an unfamiliar case, and the challenge is to notice that three of the seven do not apply. A plant has no muscles, no neurones and no constant body temperature, so offering any of those shows the list was memorised without being understood.
Question 2 — How Much Glucose Is Enough?
Total: 12 marks
A student investigated the effect of glucose concentration on the rate of respiration in yeast. Equal volumes of the same yeast suspension were used with glucose solutions of six different concentrations, all at 35 °C. The volume of carbon dioxide produced was measured with a gas syringe and converted to a rate. The results are shown in Fig. 2.1. Two regions of the curve are labelled P and Q.
(a)[3]
State the independent variable, the dependent variable and two variables that must be controlled in this investigation.
Model Answer — 2(a)
independent variable: the concentration of the glucose solution [1]
dependent variable: the rate of carbon dioxide production [1]
any two control variables: temperature; volume and concentration of the yeast suspension; volume of glucose solution; time allowed in the water bath before readings begin; the same apparatus and the same observer [1]
⚠ If you missed marks here: Notice that temperature, which was the independent variable in the classic version of this experiment, has become a control variable here. The labels belong to the investigation, not to the quantity, so read the stem before you decide.
(b)[2]
Use Fig. 2.1 to find the rate at a glucose concentration of 2 % and at 4 %, and calculate the percentage increase between them.
Model Answer — 2(b)
rates read from the graph: about 4.0 and about 6.6 cm³ per minute [1]
⚠ If you missed marks here: A percentage change is always taken of the starting value, so dividing by 6.6 gives 39 % and answers a different question. Read both values off the graph carefully first and write them down before you calculate anything.
(c)[3]
Explain the shape of the curve in region P and in region Q.
Model Answer — 2(c)
in region P, glucose is the limiting factor: adding more glucose means more substrate is available, so the enzyme-controlled reactions of respiration go faster [1]
in region Q the rate has levelled off, so glucose is no longer limiting [1]
something else now limits the rate — for example the number of yeast cells, or the number of enzyme molecules, all of which are already working as fast as they can [1]
⚠ If you missed marks here: A plateau always means “something else has become the limiting factor”, and the mark is for naming a plausible candidate rather than just saying the graph flattens. Do not say the yeast has “had enough” — that describes the graph rather than explaining it.
(d)[2]
Predict the shape of the curve if the whole investigation were repeated at 60 °C, and explain your prediction.
Model Answer — 2(d)
the rate would be close to zero at every glucose concentration, so the curve would lie flat along the bottom of the graph [1]
because the enzymes controlling respiration have been denatured at 60 °C, so no amount of substrate can restore the rate [1]
⚠ If you missed marks here: The key idea is that once the enzymes are denatured, glucose concentration stops mattering altogether — the curve does not simply shift downwards, it collapses. Answers predicting “a lower curve of the same shape” miss exactly that point.
(e)[2]
A repeat run gave a rate of 7.4 cm³ per minute at a glucose concentration of 1 %. Explain why this reading should be treated as anomalous, and state what the student should do about it.
Model Answer — 2(e)
at 1 % the other data give about 2.1 cm³ per minute, so 7.4 lies far above the established pattern and is close to the plateau value — it is anomalous [1]
the student should repeat that reading and, if the repeat agrees with the original 2.1, exclude the anomalous value from the mean rather than averaging it in [1]
⚠ If you missed marks here: Identify, repeat, exclude — in that order, and use the word “anomalous”. Quietly averaging a wild value into the mean drags the whole point out of position and hides the very pattern the experiment was designed to reveal.
Question 3 — A Flask of Peas and Two Days of Waiting
Total: 12 marks
Two vacuum flasks were set up. Flask A contained 50 g of peas that had been soaked in water for a day. Flask B contained 50 g of peas that had been soaked and then boiled and cooled. Both flasks were plugged with cotton wool, a thermometer was inserted into each, and both were left in the same room for two days. The temperature in flask A rose from 20 °C to 31 °C. The temperature in flask B stayed at 20 °C. The dry mass of a separate sample of the soaked peas was found to fall by 4 % over the same period.
(a)[3]
Explain why the temperature in flask A rose.
Model Answer — 3(a)
the peas in flask A are alive and are germinating, so they are respiring [1]
respiration releases energy that was stored in the food reserves of the seed [1]
some of that energy is transferred to the surroundings as thermal energy, and the vacuum flask prevents it escaping, so the temperature rises [1]
⚠ If you missed marks here: Two things sink this answer. The first is writing that respiration “produces heat” — the energy was already in the seed. The second is forgetting to mention the flask: an ordinary beaker would lose the energy to the room and show almost no rise, which is why the apparatus is a vacuum flask.
(b)[2]
State the purpose of flask B, and explain why its temperature did not rise.
Model Answer — 3(b)
flask B is the control: it shows that the temperature rise in flask A is caused by the living peas and not by the flask, the cotton wool or a change in the room [1]
the boiled peas are dead and their enzymes have been denatured, so they cannot respire and release no energy [1]
⚠ If you missed marks here: Saying flask B is there “to compare” is too vague to earn the mark. A control exists to rule out an alternative explanation, and here the alternative is that the apparatus itself, rather than the peas, produced the reading.
(c)[3]
Explain why the dry mass of the soaked peas fell by 4 % over the two days.
Model Answer — 3(c)
the peas are respiring, breaking down stored food such as starch or fat [1]
carbon leaves the seed as carbon dioxide, which is a gas and escapes, so the mass of solid material falls [1]
the seedlings cannot yet photosynthesise, so nothing is being added to replace what is used [1]
⚠ If you missed marks here: Dry mass excludes water by definition, so an answer about water being taken up or lost cannot explain a change in it — that is the whole reason dry mass is measured. The mark comes from following the carbon out of the seed as a gas.
(d)[2]
Describe how you would show that the peas in flask A are releasing carbon dioxide, and state the result you would expect.
Model Answer — 3(d)
draw air from the flask through limewater, using a pump or syringe, with a control tube of limewater connected to ordinary air [1]
the limewater connected to the flask turns cloudy or milky, whereas the control stays clear [1]
accept: use hydrogencarbonate indicator, which turns from red to yellow
⚠ If you missed marks here: Say what turns cloudy and what does not. “White” is usually refused for limewater, and mixing up the reagents is expensive: limewater goes cloudy, hydrogencarbonate indicator goes yellow, iodine goes blue-black with starch and Benedict’s goes brick red on heating.
(e)[2]
Suggest two ways in which this investigation could be improved.
Model Answer — 3(e)
any two of: repeat with several flasks of each kind and take a mean; use a data logger to record the temperature continuously rather than at the start and end; disinfect the surfaces of the peas so that microorganisms respiring on them do not contribute to the temperature rise; use the same mass and variety of peas in both flasks and check both start at the same temperature [1 each, to a maximum of 2]
⚠ If you missed marks here: Suggestions must be specific enough to be carried out; “be more careful” and “use better equipment” are not credited. The disinfectant point is the strong one here, because until you make it you cannot say whose respiration warmed the flask.
Question 4 — Three Runners and a Table of Numbers
Total: 12 marks
Three students of the same age ran on a treadmill at the same speed for 10 minutes. Their breathing rate, their peak blood lactic acid concentration and the time taken for their blood lactate to return to its resting value were recorded.
Student
Breathing rate at rest / breaths per minute
Breathing rate at the end of the run / breaths per minute
Peak blood lactic acid / arbitrary units
Time for lactate to return to resting / minutes
R
14
36
5.2
19
S
16
44
8.1
38
T
18
49
9.6
52
(a)[3]
Describe the pattern shown by the data, using figures.
Model Answer — 4(a)
breathing rate rose in all three students, for example from 14 to 36 in R and from 18 to 49 in T [1]
the student with the lowest resting breathing rate, R, also had the lowest peak lactate (5.2 against 9.6) [1]
and the shortest recovery time (19 minutes against 52), so the three measures all point the same way [1]
⚠ If you missed marks here: A describe question written without figures rarely scores more than one. Quote a number from at least two of the columns, and say explicitly that the three measures agree with one another — that is the pattern, not just one column read aloud.
(b)[3]
Explain why the breathing rate of all three students increased during the run.
Model Answer — 4(b)
the leg muscles are contracting more, so they are respiring faster and need more oxygen delivered [1]
they also release more carbon dioxide, which raises its concentration in the blood [1]
the increased carbon dioxide concentration is detected by the brain, which increases the rate and depth of breathing so that oxygen is supplied and carbon dioxide removed faster [1]
⚠ If you missed marks here: The trigger is a rise in carbon dioxide, not a fall in oxygen, and that is the mark most often lost. Saying only “because she needs more oxygen” describes the requirement without naming what the body actually detects.
(c)[3]
Explain what is meant by an oxygen debt, and use the data to explain which student built up the largest one.
Model Answer — 4(c)
the oxygen debt is the extra oxygen that must be taken in after exercise in order to deal with the lactic acid that accumulated during it [1]
student T built up the largest debt [1]
because T had the highest peak lactic acid, 9.6 units, and took longest to return to resting, 52 minutes — both of which show that more anaerobic respiration took place [1]
⚠ If you missed marks here: Two features of the definition earn the first mark: extra oxygen and after the exercise. In the second half, name the student and quote the numbers — an answer that says “T, because T is least fit” has used the conclusion as the evidence.
(d)[3]
A teacher concludes from these data that student R is the fittest of the three. Evaluate this conclusion.
Model Answer — 4(d)
the data support it: R has the lowest peak lactate and the shortest recovery time, so R relied least on anaerobic respiration and recovered fastest [1]
but the sample is only three students and each was tested only once, with no repeats and no means [1]
other variables were not controlled — body mass, previous training, how recently they had eaten, whether they were used to a treadmill — so this is a correlation and fitness has not been measured directly [1]
⚠ If you missed marks here: An evaluation must say what the data do support before attacking them, and answers that only disagree score at most one. Naming the uncontrolled variables specifically is what separates a two-mark answer from a three-mark one.
Question 5 — A Leaf Over Twenty-Four Hours
Total: 10 marks
Fig. 5.1 shows the net exchange of carbon dioxide between a leaf and the surrounding air over 24 hours. Points P and Q are the two moments at which the net exchange is zero.
(a)[2]
Explain what is happening in the leaf at points P and Q.
Model Answer — 5(a)
the rate of photosynthesis exactly equals the rate of respiration [1]
so all the carbon dioxide released by respiration is used by photosynthesis and none is exchanged with the surrounding air — both processes are still taking place [1]
⚠ If you missed marks here: The trap is reading “no net exchange” as “nothing is happening”. Both processes are running flat out at P and Q; they simply cancel. Say so explicitly, because that clause is the second mark.
(b)[3]
Describe and explain the net exchange of carbon dioxide during the hours of darkness and during the middle of the day.
Model Answer — 5(b)
in darkness the leaf gives out carbon dioxide overall, at about 2.0 units [1]
because there is no light, so no photosynthesis; only respiration takes place and its carbon dioxide is released to the air [1]
in the middle of the day the leaf takes carbon dioxide in, peaking at about 5.5 units, because photosynthesis is much faster than respiration and uses carbon dioxide faster than respiration releases it [1]
⚠ If you missed marks here: Answers that say the plant “respires at night and photosynthesises in the day” lose marks because respiration never stops. What changes is which process is faster, and the graph shows the balance, not either process on its own.
(c)[2]
The roots of the same plant are in the soil, in complete darkness, throughout the 24 hours. State what the root cells are doing during the middle of the day, and explain why they need to do it.
Model Answer — 5(c)
they are respiring, continuously, exactly as they are at night [1]
they need the energy released for processes such as active transport of ions from the soil, cell division and growth — and they have no chloroplasts, so they cannot photosynthesise at all [1]
⚠ If you missed marks here: This part checks whether you believe respiration is something plants do only when they cannot photosynthesise. A root never photosynthesises and never stops respiring, which is why waterlogged soil damages a plant so quickly.
(d)[3]
Describe an investigation, using hydrogencarbonate indicator, that would demonstrate that a leaf respires during the middle of the day as well as photosynthesising.
Model Answer — 5(d)
set up two sealed tubes of indicator, each containing a leaf: one in the light and one wrapped in foil to exclude light, plus a third tube of indicator alone as a control [1]
leave for a fixed time at the same temperature, then compare the colours [1]
the lit tube turns purple and the darkened tube turns yellow, showing that a leaf deprived only of light still releases carbon dioxide — so respiration is taking place in leaves at all times [1]
⚠ If you missed marks here: The darkened tube is the whole experiment: it is the only way to observe respiration in a leaf without photosynthesis masking it. An answer that describes only the lit tube cannot demonstrate anything about respiration, because that tube shows the net result of two processes.
Question 6 — Without Oxygen
Total: 12 marks
A biologist compared the energy released per mole of glucose by three processes: aerobic respiration, 2880 kJ; anaerobic respiration in yeast, 118 kJ; anaerobic respiration in human muscle, 150 kJ.
(a)[2]
Write the word equation for anaerobic respiration in yeast and the word equation for anaerobic respiration in human muscle.
⚠ If you missed marks here: Adding carbon dioxide to the muscle equation is the single most common error in Topic 12, because the yeast equation is sitting in your memory next to it. In muscle, lactic acid is the only product — no carbon dioxide, no water, no alcohol.
(b)[2]
Write the balanced chemical equation for anaerobic respiration in yeast and show that it balances.
Model Answer — 6(b)
C6H12O6 → 2C2H5OH + 2CO2 [1]
checked: 6 C on each side, 12 H on each side, 6 O on each side [1]
⚠ If you missed marks here: Counting atoms rather than molecules is the only reliable way to check an equation, and it is what the second mark is for. The unbalanced version, with one ethanol and one carbon dioxide, is the popular answer because it looks neater — but it has only three carbon atoms on the right.
(c)[3]
Calculate how many times more energy aerobic respiration releases per mole of glucose than anaerobic respiration in yeast, and than anaerobic respiration in muscle. Explain why anaerobic respiration releases so much less.
Model Answer — 6(c)
yeast: 2880 ÷ 118 = 24 times (24.4) [1]
muscle: 2880 ÷ 150 = 19 times (19.2) [1]
because in anaerobic respiration the glucose is not completely broken down, so a great deal of chemical energy remains in the alcohol or the lactic acid [1]
⚠ If you missed marks here: The ratios have no units — writing kJ after them is a real error rather than a harmless extra. In the explanation, avoid the words “less efficient” on their own: they restate the observation instead of accounting for it.
(d)[3]
Outline how an oxygen debt is removed after vigorous exercise.
Model Answer — 6(d)
the heart rate remains fast, transporting lactic acid in the blood from the muscles to the liver [1]
breathing remains deeper and faster, supplying the extra oxygen needed [1]
the lactic acid is respired aerobically in the liver [1]
⚠ If you missed marks here: If the word liver does not appear, the answer cannot reach full marks however well it describes the breathing. Attach a purpose to each continuing change — a bare list of two changes is half of what is being asked for.
(e)[2]
Suggest why the body respires lactic acid rather than excreting it in the urine.
Model Answer — 6(e)
lactic acid still contains most of the chemical energy that was in the original glucose molecule [1]
so respiring it aerobically releases that energy for the body to use, whereas excreting it would waste it [1]
⚠ If you missed marks here: This is a suggest question, so the mark is for the reasoning rather than for a remembered fact. The chain runs from part (c): if anaerobic respiration leaves most of the energy in the lactic acid, then that lactic acid is worth keeping.
Question 7 — Three Situations, One Idea
Total: 10 marks
(a)[3]
Fish in a pond are much more likely to die from lack of oxygen in warm weather than in cold weather, even though the amount of dissolved oxygen in the water falls only slightly. Suggest an explanation.
Model Answer — 7(a)
at a higher temperature the enzymes of the fish work faster, so the rate of respiration rises [1]
so the fish use oxygen faster, and their demand rises steeply while the supply does not [1]
other organisms in the pond, such as bacteria and other animals, also respire faster and take oxygen from the same water, so it is used up more quickly than it can dissolve in [1]
⚠ If you missed marks here: The stem tells you the supply barely changed, so an answer built entirely on “there is less oxygen in warm water” has ignored the information given. The gap opens because demand rises, and noticing that is what the question is testing.
(b)[3]
Potatoes are stored in a cool, dark, well-ventilated store at about 4 °C rather than at room temperature. Explain why cooling them reduces the loss of stored starch, and suggest why they are not simply frozen.
Model Answer — 7(b)
at 4 °C the enzymes controlling respiration work more slowly, so the rate of respiration is much lower and less stored starch is broken down [1]
so less carbon leaves the tuber as carbon dioxide and the dry mass falls more slowly, keeping the potatoes usable for longer [1]
freezing would kill the cells (ice damages the cell membranes), so the tubers would spoil once thawed and could not be planted [1]
⚠ If you missed marks here: Notice that low temperature slows enzymes and does not denature them — that is why the effect is reversible and why refrigeration is not the same as sterilising. The last mark is a suggest, so any sensible consequence of freezing damage is credited.
(c)[4]
A sports drink is advertised with the words: “Clinically shown to remove lactic acid — in a trial, 10 athletes who drank it recovered in 24 minutes compared with 31 minutes for 10 who drank water.” Evaluate this claim.
Model Answer — 7(c)
the data do show a difference: 24 minutes against 31, a reduction of 7 minutes or about 23 % [1]
but the samples are small — 10 in each group — and no spread of results is given, so the difference could be due to chance [1]
other variables may not have been controlled: the fitness of the two groups, how hard each athlete exercised, what they had eaten, and whether they knew which drink they were given [1]
a fair judgement: the trial is consistent with the drink shortening recovery, but the wording “clinically shown to remove lactic acid” claims far more than the evidence supports, since lactic acid is removed by the liver in any case [1]
⚠ If you missed marks here: The fourth mark rewards attacking the wording rather than the numbers: lactic acid is removed anyway, so shortening the process is not the same as causing it. An evaluation that only lists faults, without first crediting the difference the data really show, will not reach full marks.
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