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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 — A Water Bath, a Syringe and a Curve You Have Met Before
Total: 12 marks
Fig. 1.1 shows apparatus set up to investigate the effect of temperature on respiration in yeast. A suspension of yeast in glucose solution was used, and the volume of gas collected in five minutes was recorded at each of six temperatures. The results are shown in the table below Fig. 1.1.
Temperature / °C
10
20
30
40
50
60
Volume of gas collected in 5 minutes / cm³
4
10
22
35
12
0
(a)[4]
Name the parts labelled A, B, C and D on Fig. 1.1.
Model Answer — 1(a)
A — a thin layer of oil on the surface of the suspension [1]
B — a thermometer [1]
C — a gas syringe [1]
D — the water bath (accept beaker of water at a set temperature) [1]
⚠ If you missed marks here: The oil is the one that is usually left blank or called a “seal”. Name it as oil and remember what it is for — excluding oxygen, so that the yeast respires anaerobically. Calling C a “measuring cylinder” or a “syringe of water” also loses the mark; a gas syringe has its own name and its own scale.
(b)[3]
Calculate the rate of gas production, in cm³ per minute, at 30 °C and at 40 °C. Use your two values to state how many times faster the reaction is at 40 °C than at 30 °C.
Model Answer — 1(b)
at 30 °C, 22 ÷ 5 = 4.4 cm³ per minute [1]
at 40 °C, 35 ÷ 5 = 7.0 cm³ per minute [1]
7.0 ÷ 4.4 = 1.6 times (accept 1.59, and accept 35 ÷ 22 giving the same value) [1]
⚠ If you missed marks here: Two things cost marks here. First, quoting 22 and 35 as rates — they are totals collected over five minutes, and a total only becomes comparable once it is divided by time. Second, forgetting the unit: cm³ per minute is part of the answer. The ratio in the last part has no unit at all, because it is a number of times, not a quantity.
(c)[2]
Explain why the rate of gas production increases between 10 °C and 40 °C.
Model Answer — 1(c)
the molecules gain kinetic energy and move faster, so enzyme and substrate collide more often [1]
the collisions also have more energy, so more of them are successful and more enzyme–substrate complexes form, increasing the rate of the enzyme-controlled reactions of respiration [1]
⚠ If you missed marks here: Saying only that “the enzymes work faster” describes the result rather than explaining it. The mark scheme wants movement, collisions and successful reactions. Note also that warming does not create extra enzymes — it makes the ones already there meet their substrate more often.
(d)[2]
Explain why the rate falls sharply between 40 °C and 60 °C.
Model Answer — 1(d)
the enzymes controlling respiration are denatured [1]
the shape of the active site changes, so the substrate no longer fits and enzyme–substrate complexes can no longer form [1]
accept also: the yeast cells themselves are killed at the highest temperatures
⚠ If you missed marks here: Never write that the enzymes are “killed” — a molecule was never alive. Notice too that this fall and the rise in part (c) have two different causes: the rise is about collisions and is reversible on cooling, the fall is about denaturation and is not.
(e)[1]
State the purpose of the layer of oil labelled A, and state which type of respiration the yeast is therefore carrying out.
Model Answer — 1(e)
the oil prevents oxygen from the air dissolving into the suspension, so the yeast respires anaerobically [1]
⚠ If you missed marks here: The favourite wrong answer is that the oil prevents evaporation — that is its job in a transpiration experiment, not this one. Under oil the yeast is anaerobic, so its products are alcohol and carbon dioxide; lactic acid belongs to muscle and never appears in yeast.
Question 2 — Two Equations, Seven Uses and One Chain of Reasoning
Total: 12 marks
(a)[2]
Define aerobic respiration.
Model Answer — 2(a)
the chemical reactions in cells that break down nutrient molecules [1]
that use oxygen to release energy [1]
⚠ If you missed marks here: Two marks means two ideas, and the phrase “use oxygen” is what makes this the definition of aerobic respiration rather than of respiration in general. The verb must be release: “produce energy” is refused wherever it appears in this topic.
(b)[3]
Write the word equation and the balanced chemical equation for aerobic respiration.
equation correctly balanced, with 6 in front of the oxygen, the carbon dioxide and the water [1]
⚠ If you missed marks here: Adding “+ energy” to the right-hand side is not credited, because energy is not a chemical substance. Write the equation clean and then add the sentence “energy is released” underneath. In the balanced version, the commonest slip is 12 water molecules — glucose supplies only twelve hydrogen atoms, so only six waters can form.
(c)[3]
State three uses, in a human, of the energy released by respiration. For each one, give a specific example.
Model Answer — 2(c)
any three of: muscle contraction (for example the heart beating), protein synthesis (joining amino acids to build an enzyme), cell division (replacing red blood cells), active transport (absorbing glucose from the small intestine), growth, passage of nerve impulses, maintenance of a constant body temperature [1 each, to a maximum of 3]
⚠ If you missed marks here: Vague words such as “for living”, “for energy” or “for moving about” are not credited — an examiner cannot award a mark for what you nearly wrote. Each of the seven syllabus uses is at most three words long, so there is no reason not to use them exactly.
(d)[4]
A root hair cell takes up nitrate ions from soil water in which the concentration of nitrate ions is lower than the concentration inside the cell. When the soil becomes waterlogged for several days, the uptake of nitrate ions falls almost to zero. Explain fully.
Model Answer — 2(d)
uptake is against the concentration gradient, so it is by active transport [1]
active transport requires energy released by respiration [1]
waterlogged soil has no air spaces, so there is little or no oxygen available to the root [1]
so little or no aerobic respiration can take place, much less energy is released, and the protein carriers cannot move the ions across the membrane [1]
⚠ If you missed marks here: This is a four-link chain, and answers that jump straight from “waterlogged” to “the plant cannot absorb nitrate” score one. Write the links out one at a time: against the gradient, therefore active transport, therefore energy from respiration, therefore no oxygen means no energy. Do not say the root “drowns” — that is a description, not a mechanism.
Question 3 — A Sealed Tube, Some Soda Lime and a Drop of Coloured Liquid
Total: 12 marks
A respirometer was set up. Tube A contained 4.0 g of germinating pea seeds resting on a wire platform above a layer of soda lime, which absorbs carbon dioxide. Tube A was connected to a horizontal capillary tube containing a drop of coloured liquid. Tube B was identical in every way except that the seeds had been boiled and cooled before use. Both tubes were placed in a water bath at 20 °C. After 20 minutes the liquid in tube A had moved 36 mm towards the seeds. The liquid in tube B had not moved. The capillary tube has a cross-sectional area of 1.5 mm².
(a)[2]
Explain why the soda lime is necessary in tube A.
Model Answer — 3(a)
it absorbs the carbon dioxide released by the respiring seeds [1]
so the only gas whose volume changes is the oxygen being used up, and the fall in volume — and therefore the movement of the liquid — measures oxygen uptake [1]
⚠ If you missed marks here: A common answer says the soda lime is there to “keep the air pure” or “stop the seeds being poisoned”. The point is arithmetical: without it, every molecule of oxygen removed is replaced by a molecule of carbon dioxide, the total volume hardly changes and the liquid does not move at all.
(b)[2]
State the purpose of tube B and explain why the liquid in it did not move.
Model Answer — 3(b)
tube B is a control: it shows that any movement in tube A is caused by the seeds respiring and not by a physical change such as the room warming or the apparatus leaking [1]
the boiled seeds are dead, their enzymes have been denatured, so no respiration takes place and no oxygen is used [1]
⚠ If you missed marks here: A control is not a repeat and it is not a comparison. Writing “to compare the rate of respiration of live and dead seeds” scores nothing, because dead seeds have no rate to compare. The purpose is always to rule out an alternative explanation for the result.
(c)[3]
Calculate the rate of oxygen uptake by the seeds in tube A, in mm³ per gram per hour. Show your working.
Model Answer — 3(c)
volume of oxygen used = 36 × 1.5 = 54 mm³ [1]
per gram = 54 ÷ 4.0 = 13.5 mm³ per gram in 20 minutes [1]
per hour = 13.5 × 3 = 40.5 mm³ per gram per hour [1]
⚠ If you missed marks here: Do the three conversions separately and write each one down — combined into one line, this is where the arithmetic goes wrong, and shown working earns the method marks even if the final number slips. Check the direction at the end: an hour is longer than twenty minutes, so the per-hour figure must be the larger one.
(d)[3]
Predict how the distance moved by the liquid in tube A would change if the whole apparatus were kept at 30 °C instead of 20 °C, and how it would change at 60 °C. Explain both predictions.
Model Answer — 3(d)
at 30 °C the liquid would move further in the same time [1]
because the enzymes and substrate have more kinetic energy, so successful collisions are more frequent and the rate of respiration is higher [1]
at 60 °C the liquid would move much less or not at all, because the enzymes have been denatured and the seeds are killed, so respiration stops [1]
⚠ If you missed marks here: Predicting the direction of change is only half the mark; the explanation carries the rest. Take care not to give the same explanation twice — the rise is about collisions and the fall is about denaturation, and a paper that asks for both is testing precisely whether you know they are different.
(e)[2]
Suggest two ways in which the reliability of this investigation could be improved.
Model Answer — 3(e)
any two of: repeat the measurement several times at each temperature and calculate a mean; use a larger number of seeds of the same species and age so that one unusual seed matters less; leave the apparatus in the water bath for several minutes before starting so that it reaches the set temperature; check the water bath temperature at the start and end; take readings at fixed time intervals rather than only at the end [1 each, to a maximum of 2]
⚠ If you missed marks here: Suggestions must be specific enough to carry out. “Be more accurate” and “do the experiment properly” earn nothing at all. Notice that repeating and taking a mean is about reliability, whereas controlling a variable is about validity — questions ask for one or the other, so read which.
Question 4 — Eight Minutes of Hard Exercise, and What Happens Afterwards
Total: 12 marks
Fig. 4.1 shows the volume of oxygen taken in per minute by an athlete before, during and after a period of hard exercise. Two regions of the graph have been shaded and labelled X and Y.
(a)[3]
Describe what Fig. 4.1 shows about the oxygen taken in by the athlete. Use figures from the graph in your answer.
Model Answer — 4(a)
at rest the uptake is steady at about 0.4 dm³ per minute [1]
during the exercise it rises steeply and then levels off at about 3.2 dm³ per minute, which is below the 4.0 dm³ per minute the muscles required [1]
after the exercise it falls, but gradually, and does not return to the resting value until about 8 minutes after the exercise finished [1]
⚠ If you missed marks here: The command word is describe, so figures are the marks — an answer written entirely in words such as “it goes up and then comes down” typically scores one out of three. The detail most often missed is that the uptake during exercise plateaus below the demand line, which is the whole reason there is a debt at all.
(b)[3]
State what regions X and Y represent, and calculate the volume of oxygen represented by region X.
Model Answer — 4(b)
X is the oxygen debt being built up — oxygen the muscles needed but did not receive [1]
Y is the extra oxygen taken in after the exercise, which repays that debt [1]
shortfall = 4.0 − 3.2 = 0.8 dm³ per minute; over the 8 minutes of exercise this gives 6.4 dm³ (allow a small allowance for the rise at the start, so accept 5.5–6.4) [1]
⚠ If you missed marks here: The debt is the shortfall multiplied by the time, never the whole requirement. Using 4.0 × 8 = 32 dm³ is the standard error and it describes the oxygen she needed altogether, not the part she failed to get.
(c)[3]
Explain what is happening in the athlete’s leg muscles during region X, and name the product formed.
Model Answer — 4(c)
oxygen cannot be delivered fast enough to meet the demand, so as well as respiring aerobically the muscles also respire anaerobically [1]
anaerobic respiration releases energy without using oxygen, though much less per glucose molecule than aerobic respiration [1]
the product is lactic acid — and nothing else [1]
⚠ If you missed marks here: Two words decide this answer. The first is “as well”: the muscles do not stop respiring aerobically and switch over, they add anaerobic respiration on top. The second is “only”: lactic acid is the sole product, and adding carbon dioxide imports it from the yeast equation.
(d)[3]
Outline how the oxygen debt is removed after the exercise has finished.
Model Answer — 4(d)
the heart rate stays fast, so the lactic acid is transported in the blood from the muscles to the liver [1]
breathing stays deeper and faster, so extra oxygen continues to be taken in and delivered [1]
the lactic acid is respired aerobically in the liver [1]
⚠ If you missed marks here: The word that must appear is liver. Answers that keep the lactic acid in the muscle score at most one, however fluently they describe the breathing. Also attach a purpose to each continuing change: a bare list of “heart rate high, breathing deep” is only half of what is being asked for.
Question 5 — Reading a Lactate Trace
Total: 10 marks
A student ran hard for 15 minutes. Samples of her blood were taken at intervals and the concentration of lactic acid measured. The results are shown in the table. The exercise began at 0 minutes and finished at 15 minutes.
Time / minutes
0
5
10
15
18
25
40
60
Blood lactic acid / arbitrary units
1.0
2.6
5.9
8.6
9.4
7.1
3.2
1.2
(a)[2]
State the resting concentration and the highest concentration recorded, and calculate the increase.
Model Answer — 5(a)
resting 1.0 units and highest 9.4 units [1]
increase = 9.4 − 1.0 = 8.4 units [1]
⚠ If you missed marks here: Quoting both figures as well as the difference is worth doing even when the question only asks for the increase — “describe with figures” is a mark that costs five seconds. Watch the arithmetic: subtracting the resting value is what makes it an increase rather than a value.
(b)[2]
The highest concentration was recorded at 18 minutes, three minutes after the exercise had finished. Explain why.
Model Answer — 5(b)
lactic acid is produced in the muscles, not in the blood [1]
it takes time to pass out of the muscle cells and be carried away in the blood, so the concentration in the blood goes on rising for a few minutes after the muscles have stopped producing more [1]
⚠ If you missed marks here: The commonest wrong answer is that the muscles carried on respiring anaerobically after she stopped running. They did not — the delay is a transport delay, not a production delay. Distinguishing where a substance is made from where it is measured is the whole point of the question.
(c)[3]
Describe and explain what happens to the concentration of lactic acid between 18 and 60 minutes.
Model Answer — 5(c)
it falls steadily from 9.4 to 1.2 units, almost back to the resting value [1]
the lactic acid is carried in the blood to the liver [1]
where it is respired aerobically, using the extra oxygen taken in by the continued deep and rapid breathing [1]
⚠ If you missed marks here: Two commands in one question, so answer both: describe wants the figures and the direction, explain wants the liver and the aerobic respiration. Answers that give only the mechanism, or only the numbers, routinely lose a mark they had the knowledge to earn.
(d)[3]
The student says that these results explain why her legs ache badly two days after a hard run. Evaluate her claim, using the data.
Model Answer — 5(d)
the data show the concentration back to 1.2 units at 60 minutes, effectively the resting value [1]
so the lactic acid has been removed within about an hour, long before the aching appears, and cannot be causing pain two days later [1]
a fair judgement: the claim is not supported by these data, although they do explain the muscle fatigue she felt during and just after the run [1]
⚠ If you missed marks here: An evaluation needs a figure, a piece of reasoning and a judgement, and answers that only say “she is wrong” score one at most. The third mark here rewards fairness: her data do explain something real, just not the thing she claimed.
Question 6 — Yeast in a Sealed Flask
Total: 12 marks
Yeast was added to a glucose solution in a flask which was then sealed with a bung carrying a delivery tube. The flask was kept at 30 °C. The volume of carbon dioxide released was measured every 30 minutes. Production was rapid at first, became slower after about two hours, and had stopped completely after four hours. Chemical tests showed that glucose was still present in the solution at the end of the experiment. It is given that aerobic respiration releases about 2880 kJ per mole of glucose and anaerobic respiration in yeast releases about 118 kJ per mole.
(a)[3]
Write the word equation and the balanced chemical equation for anaerobic respiration in yeast.
correctly balanced, with 2 in front of both products [1]
⚠ If you missed marks here: The unbalanced version, with one ethanol and one carbon dioxide, is the one most candidates write because it looks tidier. Count the carbon: six atoms on the left means six on the right, which forces the two in front of each product. Do not put oxygen on the left-hand side — the word anaerobic rules it out.
(b)[3]
Using the figures given, calculate how many times more energy is released per mole of glucose by aerobic respiration than by anaerobic respiration in yeast, and calculate the percentage of the aerobic value that is released anaerobically. Give each answer to two significant figures.
Model Answer — 6(b)
2880 ÷ 118 = 24 times (24.4, so 24 to two significant figures) [1]
118 ÷ 2880 × 100 = 4.1 % [1]
both answers given to two significant figures, and the ratio correctly given without a unit [1]
⚠ If you missed marks here: Read which way round the division goes before touching the calculator: “how many times more” wants the large number on top, “what percentage of” wants the small one. A ratio has no unit at all, and writing kJ after it is a genuine error rather than a harmless extra.
(c)[2]
Explain why anaerobic respiration releases so much less energy per mole of glucose than aerobic respiration.
Model Answer — 6(c)
in anaerobic respiration the glucose molecule is not completely broken down [1]
so a great deal of chemical energy remains locked in the products — in the alcohol in yeast, or in the lactic acid in muscle [1]
⚠ If you missed marks here: Answers that say anaerobic respiration is “slower” or “less efficient” describe the result rather than explaining it, and the first of those is actually false — anaerobic respiration is the faster process, which is why a sprinter uses it. The explanation lives in the products.
(d)[2]
Suggest why carbon dioxide production stopped completely after four hours even though glucose was still present.
Model Answer — 6(d)
the alcohol produced has accumulated in the flask [1]
to a concentration that is toxic to the yeast and kills the cells, so respiration stops even though substrate remains [1]
accept: the accumulating product inhibits the enzymes of respiration
⚠ If you missed marks here: The stem rules out glucose deliberately, and any answer that says the yeast ran out of food ignores the sentence it was given. Do not say the yeast ran out of oxygen either — it was never using oxygen, so running out of it would change nothing.
(e)[2]
Compare the products of anaerobic respiration in yeast with the products of anaerobic respiration in human muscle.
Model Answer — 6(e)
yeast produces alcohol and carbon dioxide, whereas human muscle produces lactic acid [1]
muscle produces no carbon dioxide and no alcohol, and yeast produces no lactic acid [1]
⚠ If you missed marks here: The command word is compare, so a sentence describing only one organism cannot earn the mark however correct it is. Build the answer around “whereas”, and use the second mark to state explicitly what each organism does not produce — that is where the misconception lives.
Question 7 — Putting It Together
Total: 10 marks
(a)[4]
A student wrote: “Respiration produces energy in the mitochondria of cells. During hard exercise the muscles run out of oxygen, so they switch to anaerobic respiration, which produces lactic acid and carbon dioxide. The lactic acid is then broken down in the muscles once breathing returns to normal.” Identify four errors in this passage and give the correct version of each.
Model Answer — 7(a)
“produces energy” is wrong: respiration releases energy, which was already stored in the glucose [1]
“switch to” is wrong: the muscles continue to respire aerobically at whatever rate the oxygen supply allows, and respire anaerobically as well [1]
“lactic acid and carbon dioxide” is wrong: anaerobic respiration in muscle produces lactic acid only [1]
“broken down in the muscles” is wrong: the lactic acid is carried in the blood to the liver and respired aerobically there [1]
⚠ If you missed marks here: Every one of these four errors is a single word or phrase, which is exactly how a short topic is examined. Get into the habit of checking four things in any answer you write on respiration: the verb, whether you have written “as well” or “instead”, the list of products, and the organ.
(b)[3]
Hydrogencarbonate indicator is red in ordinary air, turns yellow when the carbon dioxide concentration rises and purple when it falls. Tube 1 contains pondweed and is left in bright light. Tube 2 contains identical pondweed wrapped in foil. Tube 3 contains indicator only. Predict the colour in tubes 1 and 2 after two hours, explain each prediction, and state the purpose of tube 3.
Model Answer — 7(b)
tube 1 turns purple, because the plant is photosynthesising as well as respiring and photosynthesis uses carbon dioxide faster than respiration releases it [1]
tube 2 turns yellow, because in the dark there is no photosynthesis, so only respiration occurs and carbon dioxide accumulates [1]
tube 3 is the control, showing that any colour change is caused by the pondweed and not by handling, temperature or the indicator itself [1]
⚠ If you missed marks here: An indicator reports a net change and never a single process, so an answer saying the plant in the light “has stopped respiring” loses the mark even though the colour prediction is right. Respiration continues in both tubes throughout.
(c)[3]
A magazine reports: “Runners who train at altitude clear lactic acid faster, which proves altitude training removes the oxygen debt.” The evidence given is that eight altitude-trained runners had a mean recovery time of 26 minutes after a test run, compared with 41 minutes for eight sea-level runners. Evaluate this claim.
Model Answer — 7(c)
the data do show a difference: 26 minutes against 41, a reduction of 15 minutes or about 37 % [1]
but a shorter recovery means a smaller debt repaid more quickly, not no debt — both groups clearly built one up [1]
and the sample is small (eight in each group), no spread is given, and other variables such as prior fitness, age and how hard each runner ran were not controlled, so this is a correlation rather than proof of cause [1]
⚠ If you missed marks here: The word “proves” is always the weak point of a claim like this, and attacking it is where the second mark sits. Notice that a good evaluation still credits the data with what they do show: quoting the two means and the difference is the first mark and it is free.
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