⚡ Challenge Paper Preparation

Challenge Prep: Respiration

IGCSE Biology 0610 — Topic 12 — Extended

Topic 12 has less content in it than any other topic on the syllabus, and that is exactly what makes a challenge paper on it dangerous. There is not enough recall to fill seven questions, so the marks go somewhere else: into rates that have to be calculated rather than read, into a curve whose two sides have two different explanations, into a lactate trace that peaks after the exercise stops, and into single words that decide whole marks. Respiration releases energy. Yeast makes alcohol; muscle makes lactic acid. The lactic acid goes to the liver. Enzymes are denatured, never killed. Twelve traps, six walkthroughs, six lookalike pairs, a concept map and ten full practice questions below — every one of them aimed at a place where a sensible-sounding sentence earns nothing.

⚠️ Common Traps & Misconceptions

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Twelve traps that cost marks on Topic 12 challenge papers. Every one is an answer that sounds right and that mark schemes refuse.

⚠️ TRAP

Trap 1: Writing that respiration “produces” energy

The Trap“Respiration produces energy in the mitochondria.” It is the first sentence most people write about this topic, it appears in a good many revision websites, and it is refused by every 0610 mark scheme.

The TruthRespiration releases energy. The energy was already there, stored in the chemical bonds of the glucose, put there by photosynthesis in some leaf months or years ago. Respiration is the controlled series of enzyme-catalysed reactions that lets the cell get at it. “Makes”, “creates” and “generates” are refused for the same reason.

Why It MattersIn a topic this small the examiner has to discriminate somehow, and this is the sentence used to do it. It costs a mark in the definition, another in any question about the uses of energy, and another in any explanation of why anaerobic respiration releases less. Read back over every answer you write on Topic 12 and check the verb. It takes three seconds.

Example Question“Define respiration. [2]”

⚠️ TRAP

Trap 2: Treating respiration, ventilation and gas exchange as the same thing

The Trap“Respiration takes place in the lungs.” Or, worse, “the rate of respiration increases during exercise, so she breathes faster” used as though breathing were respiration.

The TruthThree words, three processes, three places. Respiration is a series of chemical reactions in cells. Ventilation is the movement of air into and out of the lungs. Gas exchange is diffusion across the wall of an alveolus. A plant respires; a plant does not breathe.

Why It MattersThis is the boundary between Topics 11 and 12, and examiners police it carefully because using the wrong word suggests you believe they are the same. Any question asking where respiration takes place wants “in the cells” — naming an organ is already the wrong kind of answer.

Example Question“Distinguish between respiration, ventilation and gas exchange, and state where each takes place. [3]”

⚠️ TRAP

Trap 3: Believing that plants respire only at night

The Trap“Plants photosynthesise during the day and respire at night.” It is tidy, it is symmetrical, and it is wrong.

The TruthEvery living plant cell respires continuously, day and night. In daylight the plant photosynthesises as well, and much faster, so the net movement of carbon dioxide is into the leaf. In darkness photosynthesis stops and respiration is the only process left, so carbon dioxide accumulates.

Why It MattersEvery hydrogencarbonate-indicator question is built on this, and so is every graph with a line crossing zero at dawn and dusk. An indicator reports a net change — it never tells you that a process has stopped. Root cells settle the argument by themselves: they have no chloroplasts, they are in permanent darkness, and they respire all day.

Example Question“A tube of hydrogencarbonate indicator containing pondweed turns purple in the light. Explain what this shows. [3]”

⚠️ TRAP

Trap 4: Saying anaerobic respiration releases no energy

The Trap“When the muscles run out of oxygen they cannot release energy, so anaerobic respiration produces lactic acid instead.” As though the lactic acid were the point of the exercise.

The TruthAnaerobic respiration releases energy — that is what makes it respiration. It releases much less energy per glucose molecule, because the glucose is not completely broken down and most of its chemical energy stays locked in the alcohol or the lactic acid. Roughly 150 kJ per mole against about 2880 for aerobic respiration.

Why It MattersIf anaerobic respiration released nothing, a sprinter would collapse the instant her oxygen supply fell short. The examinable phrase is “much less energy per glucose molecule”, and the explanation is always the incomplete breakdown — never “because it is less efficient”, which restates the observation instead of accounting for it.

Example Question“Explain why anaerobic respiration releases less energy per glucose molecule than aerobic respiration. [2]”

⚠️ TRAP

Trap 5: Mixing up the yeast equation and the muscle equation

The Trap“glucose → lactic acid + carbon dioxide”. The carbon dioxide has wandered across from the yeast equation, which is sitting right next to it in your memory.

The TruthTwo organisms, two equations, no overlap. In yeast: glucose → alcohol + carbon dioxide, balanced as C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. In muscle: glucose → lactic acid, and nothing else at all — no carbon dioxide, no water, no alcohol, and no balanced version required.

Why It MattersThis is the single most common error in Topic 12 and it is worth a mark on almost every paper. It also has a practical consequence that questions exploit: because muscle releases no gas, you could not run the classic yeast experiment with muscle tissue — there would be nothing to collect.

Example Question“Compare the products of anaerobic respiration in yeast with those in human muscle. [2]”

⚠️ TRAP

Trap 6: Saying the muscles “switch to” anaerobic respiration

The Trap“During a sprint the muscles run out of oxygen, so they switch to anaerobic respiration.” It sounds like a gearbox, and that picture is wrong.

The TruthAerobic respiration carries on at whatever rate the delivered oxygen allows — and during hard exercise that rate is at its absolute maximum. Anaerobic respiration happens as well, on top, to make up the shortfall in energy. The word to write is “in addition”, never “instead”.

Why It MattersThe oxygen-uptake graph proves it: during exercise the trace sits at a high plateau, not at zero, so the athlete is clearly using a great deal of oxygen. What she cannot do is get enough. The gap between the plateau and the demand line is the debt, and the anaerobic respiration is what fills it.

Example Question“Explain what is happening in the leg muscles during the shaded region of the graph. [3]”

⚠️ TRAP

Trap 7: Dealing with the lactic acid in the muscle

The Trap“When you breathe hard afterwards, the extra oxygen breaks down the lactic acid in the muscles.” Everything about that sentence is right except the last two words, and they are where the mark is.

The TruthThe lactic acid is transported in the blood to the liver and respired aerobically there. The syllabus limits the removal of an oxygen debt to exactly three things: a continuing fast heart rate to carry the lactic acid to the liver, continued deeper and faster breathing to supply the oxygen, and aerobic respiration of lactic acid in the liver.

Why It MattersIf the word liver is not in your answer, you cannot reach full marks on any oxygen-debt question, however fluent the rest is. Notice also that each of the three points needs a purpose attached — a bare list of “heart rate high, breathing deep” is half of what is being asked for.

Example Question“Outline how the oxygen debt is removed after vigorous exercise. [3]”

⚠️ TRAP

Trap 8: Blaming lactic acid for aching two days later

The Trap“My legs ache two days after the match because of all the lactic acid.” It is what almost everyone believes, including a good many sports coaches.

The TruthBlood lactate is back at its resting value within about an hour of stopping. It causes muscle fatigue at the time — the muscle cannot go on contracting efficiently — and then it is gone. A substance that has been removed cannot be causing pain 48 hours later.

Why It MattersQuestions give you a lactate graph and then a claim, and the mark is for using the graph against the claim. Point at the value at 60 minutes. That is an evaluation done properly: a figure, a piece of reasoning, and a judgement.

Example Question“A student says the graph explains why her legs ache two days after a run. Evaluate her claim. [3]”

⚠️ TRAP

Trap 9: Saying enzymes are “killed” at high temperature

The Trap“Above 50 °C the enzymes are killed, so respiration stops.” The biology is nearly right and the word is entirely wrong.

The TruthAn enzyme is a molecule and molecules were never alive. At high temperature the enzyme molecule changes shape; the active site no longer fits the substrate, so enzyme–substrate complexes can no longer form. That is denaturation, and it is permanent. You may add that the yeast cells are killed — that is true — but write the enzyme sentence first.

Why It MattersThe whole shape of the yeast temperature curve depends on this. Low temperature slows enzymes reversibly; high temperature destroys them permanently. That asymmetry is why the graph is a gentle climb and a cliff rather than a symmetrical hill — and there is a mark in every full-explanation question for saying so.

Example Question“Explain why the rate falls sharply above 40 °C. [2]”

⚠️ TRAP

Trap 10: Quoting a total volume as though it were a rate

The Trap“35 cm³ of gas was collected, so the rate is 35.” Or, on a graph, comparing two totals that were collected over different lengths of time.

The TruthA rate is a change divided by the time that change took. Both numbers must be differences: (final − initial) ÷ (final time − initial time). A total tells you nothing on its own, because left for long enough even a very slow reaction reaches a large total.

Why It MattersAlmost every calculation in Topic 12 is a rate, and the wrong-answer options in a multiple-choice paper are built from exactly these slips: dividing by the final time instead of the interval, or using the final reading instead of the change. Write the unit you are aiming for before you start — cm³ per minute, or mm³ per gram per hour — and let it tell you which way round to divide.

Example Question“A syringe reads 5.0 cm³ at 2 minutes and 29.0 cm³ at 8 minutes. Calculate the mean rate. [2]”

⚠️ TRAP

Trap 11: Confusing a control with a repeat

The Trap“The tube of boiled seeds is there so we can compare the rate of respiration of live and dead seeds.” Or, “so we can take a mean.”

The TruthA control exists to rule out an alternative explanation. The boiled seeds cannot respire, so if the liquid in that respirometer also moves, the movement in the first tube was never respiration — it was the room warming up, or a leak. Repeats are a different idea altogether: they are about reliability and means.

Why It MattersEvery practical question in this topic has a control in it somewhere: boiled yeast, boiled seeds, dead woodlice, a tube of indicator with nothing in it. State what the control rules out, not just what it contains, and you have the mark.

Example Question“State the purpose of the second respirometer, which contained boiled seeds. [2]”

⚠️ TRAP

Trap 12: Offering a passive process as a use of energy

The Trap“The energy released by respiration is used for diffusion, osmosis and transpiration.” All three are real processes, and none of them costs the organism anything.

The TruthThe syllabus names seven uses: muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses, and the maintenance of a constant body temperature. Diffusion and osmosis run on the random kinetic energy the particles already have — which is exactly why they still happen in a dead cell. Transpiration is evaporation, driven by the sun.

Why It MattersThe two candidates always forget are active transport and maintaining a constant body temperature, so learn those two first. And watch for the trick of asking about a plant: it has no muscles, no neurones and no constant body temperature, so only four of the seven apply.

Example Question“State three uses of the energy released by respiration in a plant. [3]”

🔍 Step-by-Step Walkthroughs

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Six challenge-level questions worked through in the order you should actually think about them. Try each part before revealing the next step.

Walkthrough 1 — A Table That Wants Turning Into RatesYeast and glucose were kept at six temperatures and the volume of carbon dioxide collected in five minutes was recorded. 10 °C: 4 cm³; 20 °C: 10 cm³; 30 °C: 22 cm³; 40 °C: 35 cm³; 50 °C: 12 cm³; 60 °C: 0 cm³. (a) Calculate the rate at 30 and at 40 °C. [2] (b) State the optimum temperature and say how precisely it can be stated. [2] (c) Explain the reading at 60 °C. [2]

Divide by five, every time

All six volumes were collected over the same five minutes, so the rates are 0.8, 2.0, 4.4, 7.0, 2.4 and 0 cm³ per minute. The two the question asks for are 22 ÷ 5 = 4.4 and 35 ÷ 5 = 7.0. Do this conversion first as a matter of habit: it costs nothing here because the time was constant, and it saves you completely on the papers where it was not.

The optimum is 40 °C — but only to the nearest ten degrees

The highest reading is at 40 °C, so the optimum is at or near 40 °C. That is the first mark. The second mark is for the honesty: the readings are 10 °C apart, so all you can safely say is that the true optimum lies somewhere between 30 and 50 °C. The peak of the real curve may fall between two of your readings and never be measured at all.

Denatured, and permanently

At 60 °C the enzymes controlling respiration have been denatured: the shape of the active site has changed, so the substrate no longer fits and no enzyme–substrate complexes can form. The rate is therefore zero. You may add that the yeast cells have been killed, but write the enzyme sentence first, because that is the one the mark scheme is looking for.

Narrower intervals, named range

Any question that follows on will ask how to find the optimum more precisely, and the answer must name both the interval and the range: repeat at 2 °C intervals between 30 and 50 °C, with repeats and a mean at each temperature. “Use more temperatures” is too vague to earn the mark, and testing more temperatures at the ends of the range would not help at all.

Full Mark-Scheme Answer(a) 22 ÷ 5 = 4.4 cm³ per minute [1]; 35 ÷ 5 = 7.0 cm³ per minute [1]. (b) The optimum is at or near 40 °C [1], but as the readings are 10 °C apart it can only be placed between 30 and 50 °C [1]. (c) The enzymes controlling respiration have been denatured [1]; the active site has changed shape so the substrate no longer fits, and the rate falls to zero [1].

Walkthrough 2 — A Respirometer and Three ConversionsA respirometer contained 2.5 g of germinating seeds above soda lime. In 15 minutes the coloured drop moved 42 mm along a capillary tube of cross-sectional area 1.4 mm². (a) Explain why the soda lime is essential. [2] (b) Calculate the rate of oxygen uptake in mm³ per gram per hour. [3] (c) A second identical tube containing boiled seeds showed no movement. State what this tells you. [2]

Remove one gas so the other becomes visible

Respiring seeds take oxygen out of the sealed air and put carbon dioxide back in. If both gases were free to move, one molecule would simply replace the other, the total volume would barely change and the drop would not move. The soda lime absorbs the carbon dioxide, so the only change left is the oxygen being used up, and the volume falls. That is the whole design in two sentences.

Volume = area × distance

42 × 1.4 = 58.8 mm³ of oxygen used in 15 minutes. Forgetting to multiply by the cross-sectional area is the commonest single error in respirometer questions — the drop moved 42 mm, but 42 is a length, not a volume, and no volume can be quoted in mm.

Per gram, then per hour

Per gram: 58.8 ÷ 2.5 = 23.5 mm³ per gram in 15 minutes. Per hour: fifteen minutes is a quarter of an hour, so multiply by four: 23.5 × 4 = 94 mm³ per gram per hour. Write both lines down. Combined into one calculation this is where the arithmetic goes wrong, and shown working keeps the method marks when it does.

Sanity check: an hour is longer than fifteen minutes, so the per-hour number must be bigger. If you had divided by four and got 5.9 you would have said the seeds respire more slowly the longer you watch them.

It rules out an alternative explanation

The boiled seeds are dead and their enzymes denatured, so they cannot respire. Because the drop in that tube did not move, the movement in the first tube cannot have been caused by the room warming, by a leak or by the apparatus itself — it must have been caused by the seeds respiring. Say what has been ruled out, not just that the tube is “a control”.

Full Mark-Scheme Answer(a) Soda lime absorbs the carbon dioxide released [1], so the change in gas volume is caused only by the oxygen being used up [1]. (b) 42 × 1.4 = 58.8 mm³ [1]; 58.8 ÷ 2.5 = 23.5 mm³ per gram in 15 minutes [1]; × 4 = 94 mm³ per gram per hour [1]. (c) The boiled seeds are dead, with denatured enzymes, so they do not respire [1]; therefore the movement in tube A was caused by respiration and not by leakage or a temperature change [1].

Walkthrough 3 — Reading an Oxygen-Uptake TraceAn athlete has a resting oxygen uptake of 0.4 dm³ per minute. She exercises hard for 8 minutes, during which her muscles require 4.0 dm³ per minute but she can only take in 3.2 dm³ per minute. During recovery she takes in 1.0 dm³ per minute. (a) Calculate the oxygen debt. [2] (b) Estimate the recovery time. [2] (c) Explain what her muscles were doing during the exercise, and name the product. [3]

Subtract first, then multiply

Shortfall = 4.0 − 3.2 = 0.8 dm³ per minute. Over 8 minutes: 0.8 × 8 = 6.4 dm³. The classic error is 4.0 × 8 = 32 dm³, which is the oxygen she needed altogether — a real quantity answering a different question. A debt is only ever the part you did not get.

Only the EXTRA oxygen pays the debt

During recovery she takes in 1.0 but still needs 0.4 simply to stay alive at rest, so the oxygen available for the debt is 1.0 − 0.4 = 0.6 dm³ per minute. Time = 6.4 ÷ 0.6 = 10.7 minutes, so about 11 minutes.

Dividing 6.4 by 1.0 gives 6.4 minutes and quietly assumes every molecule she breathes goes into the debt, leaving nothing to run the rest of her body. Subtracting the resting requirement is the whole point of the step.

Both kinds of respiration at once

She is taking in 3.2 dm³ of oxygen a minute, which is eight times her resting value, so her muscles are clearly respiring aerobically as hard as the delivery system allows. What they cannot get is enough. So they also respire anaerobically, releasing energy without using oxygen — much less per glucose molecule, but fast, and available immediately.

Lactic acid

glucose → lactic acid. Nothing else. No carbon dioxide, no water, no alcohol. If you have written carbon dioxide it has drifted across from the yeast equation, and the mark goes with it.

Full Mark-Scheme Answer(a) Shortfall 4.0 − 3.2 = 0.8 dm³ per minute [1]; × 8 minutes = 6.4 dm³ [1]. (b) Extra oxygen available = 1.0 − 0.4 = 0.6 dm³ per minute [1]; 6.4 ÷ 0.6 = about 11 minutes [1]. (c) The muscles continue to respire aerobically at the maximum rate the oxygen supply allows [1] and respire anaerobically as well to make up the energy shortfall [1]; the product is lactic acid only [1].

Walkthrough 4 — A Lactate Trace With a Late PeakBlood lactic acid was measured during and after a 15-minute run: 0 min 1.0; 5 min 2.6; 10 min 5.9; 15 min 8.6; 18 min 9.4; 25 min 7.1; 40 min 3.2; 60 min 1.2 arbitrary units. The run finished at 15 minutes. (a) Calculate the percentage increase from rest to peak. [2] (b) Explain why the peak is at 18 minutes. [2] (c) Explain the fall between 18 and 60 minutes. [3]

840 per cent

Increase = 9.4 − 1.0 = 8.4 units. As a percentage of the starting value: 8.4 ÷ 1.0 × 100 = 840 %. Dividing by 9.4 gives 89 % and answers a completely different question — what fraction of the final value the rise represented. Read the words “increase from” and let them tell you which number goes on the bottom.

A transport delay, not a production delay

Lactic acid is produced in the muscle cells. The graph measures the blood. It takes a few minutes for the acid already in the muscles to diffuse out and be carried away, so the blood concentration goes on rising after the muscles have stopped making any more.

The tempting wrong answer is that the muscles kept respiring anaerobically for three more minutes. They did not — the exercise had stopped, and so had the demand that caused the anaerobic respiration in the first place.

Production has stopped AND removal continues

Two things are true at once, and a three-mark question wants both. No more lactic acid is being made, because the exercise is over. And the acid already present is being carried in the blood to the liver, where it is respired aerobically using the extra oxygen supplied by the continued deep breathing. Describe the fall with figures — 9.4 down to 1.2 — and then give the mechanism.

Use the 60-minute value against any claim about soreness

By 60 minutes the concentration is 1.2, effectively the resting value of 1.0. Any claim that lactic acid causes aching two days later dies on that number. That is what an evaluation looks like: quote the figure, state what it rules out, then reach a judgement — and be fair, because the data do explain the fatigue she felt at the time.

Full Mark-Scheme Answer(a) 9.4 − 1.0 = 8.4 [1]; 8.4 ÷ 1.0 × 100 = 840 % [1]. (b) Lactic acid is made in the muscles and not in the blood [1]; it takes time to pass into the blood, so the blood concentration peaks after the exercise has ended [1]. (c) It falls from 9.4 to 1.2, almost back to resting [1]; no more lactic acid is being produced, and that already present is carried in the blood to the liver [1]; where it is respired aerobically using the extra oxygen taken in [1].

Walkthrough 5 — Three Tubes of IndicatorHydrogencarbonate indicator is red in ordinary air, yellow when carbon dioxide rises and purple when it falls. Tube A contains a woodlouse in the light; tube B contains a leaf in the light; tube C contains a leaf wrapped in foil; tube D contains indicator only. (a) Predict the colour in A, B and C. [3] (b) Explain B fully. [2] (c) State the purpose of D. [1]

Animals only respire; plants do both

A woodlouse has one option, so tube A can only go yellow. A leaf in the light has two processes running at once. A leaf in foil has light excluded, so photosynthesis stops and only respiration is left — tube C goes yellow as well. Tube B is the interesting one, and it goes purple.

Purple does not mean respiration has stopped

In tube B the leaf is photosynthesising and respiring at the same time. Photosynthesis is much faster, so it uses carbon dioxide faster than respiration releases it, and the concentration falls overall. Both processes are running throughout, and saying so explicitly is where the second mark lives.

Tube C, not tube B

If you want to demonstrate that a leaf respires, the darkened tube is the only one that can do it, because in the light photosynthesis masks the result completely. That is a lovely point to make in an experimental-design question: the tube you are tempted to ignore is the one carrying the argument.

Tube D is the control

It shows that any colour change is caused by the organism and not by handling the tubes, by breathing near them, by the temperature of the room or by the indicator itself changing over time. One sentence, one mark, and it applies to every version of this experiment you will ever meet.

Full Mark-Scheme Answer(a) A yellow [1]; B purple [1]; C yellow [1]. (b) The leaf photosynthesises and respires at the same time [1]; photosynthesis uses carbon dioxide faster than respiration releases it, so the concentration falls overall [1]. (c) Tube D is a control, showing that any colour change is caused by the organism and not by the apparatus or the surroundings [1].

Walkthrough 6 — Comparing Two Energy YieldsAerobic respiration releases about 2880 kJ per mole of glucose; anaerobic respiration in yeast releases about 118 kJ per mole and in human muscle about 150 kJ per mole. (a) How many times more energy does aerobic respiration release than anaerobic respiration in muscle? [1] (b) What percentage of the aerobic value is released anaerobically in yeast? [2] (c) Explain the difference. [2] (d) Explain why a sprinter uses anaerobic respiration at all. [2]

“How many times more” puts the big number on top

2880 ÷ 150 = 19.2, so about 19 times. And a ratio has no unit — writing 19 kJ is a genuine error, not a harmless extra, because the units cancelled when you divided.

Four per cent

118 ÷ 2880 × 100 = 4.1 %. Notice how close these two questions look and how different the sums are. A challenge paper will offer you 24, 4.1 and 96 as options, and all three are correct calculations — only one answers the question that was asked.

The glucose is not completely broken down

In aerobic respiration the glucose ends up as carbon dioxide and water, which have almost no chemical energy left in them. In anaerobic respiration it ends up as alcohol or lactic acid — molecules still carrying most of the original energy. That is why the liver bothers to respire lactic acid rather than excreting it: throwing it away would waste 95 % of the glucose.

Two different meanings of “more”

Anaerobic respiration releases far less energy per glucose molecule, but it releases it quickly and it does not wait for oxygen to be delivered. In a ten-second race, the rate at which energy becomes available matters far more than how much is squeezed out of each molecule. Keeping those two senses of “more” apart is exactly what a challenge paper is testing.

Full Mark-Scheme Answer(a) 2880 ÷ 150 = 19 times, no unit [1]. (b) 118 ÷ 2880 × 100 [1] = 4.1 % [1]. (c) In anaerobic respiration the glucose is not completely broken down [1], so much of the chemical energy remains in the alcohol or lactic acid [1]. (d) Anaerobic respiration releases energy rapidly [1] and does not depend on how fast oxygen can be delivered, so it supplies the extra energy a sprint demands [1].

🔍 Spot the Difference

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Six pairs that look almost identical and have different answers. The distinction is where the marks live.

Question A

Write the word equation for anaerobic respiration in yeast.

glucose → alcohol + carbon dioxide. Ethanol is accepted for alcohol. Carbon dioxide is released, which is why the yeast experiment can be measured with a gas syringe.

Question B

Write the word equation for anaerobic respiration in muscle.

glucose → lactic acid. That is the entire equation. No carbon dioxide, no water, no alcohol, and no balanced version is required by the syllabus.

Key DifferenceTwo organisms, two equations, and they never share a product. Because muscle releases no gas, you could not run the classic yeast experiment using muscle tissue — there would be nothing to collect. That consequence is a favourite second mark.

Question A

Explain why the rate of respiration in yeast is low at 10 °C.

The molecules have less kinetic energy, so enzyme and substrate collide less often and with less energy. Warm it up and the rate comes straight back — the effect is reversible.

Question B

Explain why the rate of respiration in yeast is low at 60 °C.

The enzymes have been denatured: the shape of the active site has changed so the substrate no longer fits. Cool it down and nothing returns — the change is permanent.

Key DifferenceThe two ends of the curve look similar on the graph and have completely different explanations. The test that separates them is to move both tubes to the optimum temperature: the cold one recovers, the hot one does not. That asymmetry is also why the fall is steeper than the rise.

Question A

What is the purpose of the control tube containing boiled seeds?

To rule out an alternative explanation — to show that the movement of the liquid in the first tube was caused by respiration and not by a leak or a change in room temperature.

Question B

What is the purpose of repeating the measurement three times?

To improve reliability — to allow a mean to be calculated and any anomalous reading to be spotted and excluded.

Key DifferenceA control and a repeat answer two different worries. The control asks “did the thing I think caused this really cause it?”; the repeat asks “would I get the same number again?”. Offering repeats when a control is wanted scores nothing, and it is one of the easiest marks in the topic to give away.

Question A

A tube of indicator with pondweed in the light turns purple. What does this show?

Photosynthesis is using carbon dioxide faster than respiration releases it. Both processes are happening; only the balance has tipped.

Question B

A tube of indicator with pondweed in the dark turns yellow. What does this show?

Only respiration is taking place, because there is no light for photosynthesis, so carbon dioxide accumulates. This is the tube that provides evidence of respiration.

Key DifferenceAn indicator reports a net change, never a single process. The lit tube tells you which of two processes is faster; the darkened tube is the only one that isolates respiration. Any answer saying the plant “stops respiring in the light” has misread what the colour means.

Question A

Calculate the oxygen debt when the demand is 4.0 and the intake is 3.2 dm³ per minute for 8 minutes.

Shortfall 4.0 − 3.2 = 0.8; × 8 = 6.4 dm³. Only the part she failed to get counts.

Question B

Calculate the total oxygen used over the same 8 minutes.

3.2 × 8 = 25.6 dm³ — what she actually took in. Or 4.0 × 8 = 32 dm³ if you are asked what she required.

Key DifferenceThree different, perfectly correct calculations sit in these two boxes, and multiple-choice papers will offer you all three. Read whether the question wants the debt, the intake or the requirement before you touch the calculator — the arithmetic is trivial and the reading is where the mark is.

Question A

State three uses of energy in a human.

Any three of the seven: muscle contraction, protein synthesis, cell division, active transport, growth, passage of nerve impulses, maintenance of a constant body temperature.

Question B

State three uses of energy in a plant.

Only four of the seven apply: active transport, protein synthesis, cell division and growth. A plant has no muscles, no neurones and no constant body temperature.

Key DifferenceThe plant version is the challenge-paper version, and it catches candidates who memorised a list without thinking about what is in it. It is also a reminder that transpiration is not on the list at all: it is evaporation driven by the sun and costs the plant nothing.

🔗 Respiration Concept Map

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Click each node to see how the three sub-topics connect into one story: energy released, energy spent, and what happens when the oxygen cannot arrive fast enough.

⭐ CORE FRAMEWORK 1

Energy is released, not made → and it is spent on seven things

Where the Energy Was Before the Cell Got Hold of It ▶

The energy was stored in the chemical bonds of glucose, put there by photosynthesis. Respiration is the series of enzyme-controlled reactions that releases it in a form the cell can use. Nothing is created at any point.

The definition Cambridge wants has three parts: chemical reactions in cells → break down nutrient molecules → release energy for metabolism. Each part is doing work, and “in cells” is the one that keeps respiration out of the lungs.

Because respiration is a chain of enzyme-controlled reactions, everything you learned in Topic 5 applies to it: an optimum temperature, a gentle rise, a steep fall, and denaturation on the high side.

The Seven Uses, and the Two Everyone Forgets ▶

Muscle contraction, protein synthesis, cell division, active transport, growth, passage of nerve impulses, maintenance of a constant body temperature. Seven, and a three-mark question wants three of them named exactly.

The two left out are active transport and keeping warm. Learn those two first, because everyone remembers muscles. Keeping warm is also the answer to why a bird eats ten times as much per gram as a lizard of the same mass.

Diffusion, osmosis and transpiration are not on the list and never will be. They are passive: they run on kinetic energy the particles already have, which is why they still happen in a dead cell.

The Active Transport Chain, Which Links Four Topics ▶

Uptake against a concentration gradient → must be active transport (3.3) → requires energy from respiration (12.1) → which requires oxygen for the aerobic route (12.2) → so anything that removes the oxygen stops the uptake.

The standard contexts are a root hair cell in waterlogged soil, a gill cell in poorly oxygenated water, and a gut lining cell absorbing glucose. All three are the same four links in a different coat.

The visible sign is many mitochondria. If a question shows you a cell packed with them and asks you to suggest its job, the answer is always “something requiring a great deal of energy” — then name the process from the context you were given.

⭐ CORE FRAMEWORK 2

Two equations, one organelle, and a curve with two different sides

Aerobic Respiration, Word for Word and Atom for Atom ▶

glucose + oxygen → carbon dioxide + water. Energy is released and does not appear in the equation, because it is not a chemical substance. Write “energy is released” underneath as a separate sentence.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Six of everything except the glucose. Check by counting atoms: 6 C, 12 H and 18 O on each side, where the 18 includes the six oxygen atoms inside the glucose itself.

The site is the mitochondrion, and that is all 0610 asks about it. Every living cell respires — an oak leaf at midday, a bacterium, a root, your own brain. The one human cell that cannot is the mature red blood cell, which has no mitochondria.

The Same Curve as Topic 5, Wearing a Different Label ▶

Rising side: more kinetic energy → enzyme and substrate collide more often and with more energy → more enzyme–substrate complexes → faster rate. Fully reversible on cooling.

Falling side: denaturation → the active site changes shape → the substrate no longer fits. Permanent. That difference is why the curve is a gentle climb and a cliff rather than a symmetrical hill.

Everything else in the yeast practical follows from measuring a gas: independent variable temperature, dependent variable rate of carbon dioxide production, controls the yeast, the glucose and the timing, five minutes in the bath before you start, and oil on top to exclude oxygen.

Four Ways to Detect Something You Cannot See ▶

Carbon dioxide released — limewater turns cloudy or milky; hydrogencarbonate indicator turns from red to yellow. Both are net measures, so in a lit plant they report the balance of two processes, not one.

Oxygen used — a respirometer, in which soda lime absorbs the carbon dioxide so the only volume change left is the oxygen. Volume = cross-sectional area × distance moved.

Temperature rise in a vacuum flask of germinating seeds, and dry mass loss in stored seeds or tubers. Dry mass excludes water by definition, which is exactly why it is measured — a loss of dry mass is carbon leaving as carbon dioxide.

⭐ CORE FRAMEWORK 3

When the oxygen cannot arrive fast enough

Two Organisms, Two Equations, No Overlap ▶

Yeast: glucose → alcohol + carbon dioxide; C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Yeast also respires aerobically when oxygen is available, which is why the bung on a flask decides which equation applies.

Muscle: glucose → lactic acid. One product, no gas, no balanced version required.

Both release much less energy per glucose molecule than aerobic respiration, and for the same reason: the glucose is not completely broken down, so most of its energy stays in the alcohol or the lactic acid.

Why a Sprinter Uses a Wasteful Process ▶

During hard exercise the demand for oxygen outruns what the heart and lungs can deliver. Aerobic respiration continues at its maximum rate; anaerobic respiration happens as well, to fill the gap. Never “instead”.

Anaerobic respiration releases far less energy per molecule but releases it quickly and without waiting for delivery. In a ten-second race that is the only thing that matters.

The cost is lactic acid, which builds up in the muscles and the blood and causes muscle fatigue — the muscle can no longer contract efficiently. Temporary, and gone within about an hour.

The Oxygen Debt, in Exactly Three Sentences ▶

1. The heart rate stays fast, so the lactic acid is transported in the blood from the muscles to the liver.

2. Breathing stays deeper and faster, so extra oxygen is supplied for the aerobic respiration of the lactic acid.

3. The lactic acid is respired aerobically in the liver. It is respired rather than excreted because it still contains most of the energy from the original glucose — throwing it away would waste about 95 % of it.

Reading the Two Graphs That Always Appear ▶

Oxygen uptake against time: resting value, a plateau during exercise that sits below the demand line, and a slow return to rest afterwards. The gap during exercise is the debt built up; the bulge afterwards is the same debt repaid. Debt = shortfall × time.

Blood lactate against time: a rise during exercise, a peak a few minutes after it stops, and a slow fall back to resting within about an hour. The late peak is a transport delay — the acid is made in muscle and measured in blood.

Recovery time is the single most useful measure of fitness in either graph, exactly as it was for heart rate in Topic 9. A lower peak and a shorter recovery both point the same way.

❌ “Why Is This Wrong?” Exercises

▼

Six real student answers. Find the fault before you reveal it.

Exercise 1: “Define respiration. [2]”

Student’s Answer“Respiration is when your body takes in oxygen and produces energy so that you can move around.”

The FlawThree faults in one sentence. “Produces energy” is refused — energy is released, not made. “Takes in oxygen” describes ventilation and gas exchange, not respiration, and it also excludes anaerobic respiration, which is still respiration. And “so that you can move around” is only one of seven uses, expressed vaguely.

Correct Answer“Respiration is the chemical reactions in cells that break down nutrient molecules [1] and release energy for metabolism [1].”

Key RuleLearn the definition as three phrases and check them off. Notice that the correct version never mentions oxygen, because the general definition has to cover the anaerobic route as well. Oxygen belongs in the definition of aerobic respiration only.

Exercise 2: “Explain why the rate of respiration in yeast falls above 45 °C. [3]”

Student’s Answer“The yeast gets too hot so the enzymes are killed and stop working, and the yeast also runs out of glucose because it has been respiring so fast.”

The Flaw“Killed” is the wrong word for a molecule, and the glucose claim can be disproved from the student’s own data: every tube was given the same glucose, and the cooler tubes did not slow down. The answer also has no mechanism — nothing about shape, nothing about the active site.

Correct Answer“The enzymes controlling respiration are denatured [1]. The shape of the active site changes, so the substrate no longer fits and enzyme–substrate complexes cannot form [1]. The change is permanent, so the rate falls sharply and does not recover on cooling [1].”

Key RuleTwo habits are worth building here. Never write “killed” of an enzyme, and always check a proposed explanation against the rest of the data before you offer it — if glucose were running out, every tube would show it.

Exercise 3: “Explain what happens in the leg muscles during a sprint. [3]”

Student’s Answer“The muscles use up all the oxygen, so they stop respiring aerobically and switch to anaerobic respiration, which produces lactic acid and carbon dioxide.”

The FlawTwo errors. The muscles do not switch — aerobic respiration carries on at its maximum rate and anaerobic respiration is added on top. And anaerobic respiration in muscle produces lactic acid only; the carbon dioxide has drifted in from the yeast equation.

Correct Answer“Oxygen cannot be delivered fast enough to meet the demand, so the muscles continue to respire aerobically as fast as the oxygen supply allows and respire anaerobically as well [1]. Anaerobic respiration releases energy without using oxygen, though much less per glucose molecule [1]. The product is lactic acid, and nothing else [1].”

Key RuleThe oxygen-uptake graph settles the first point for you: during the sprint the trace is at a high plateau, not at zero, so a great deal of oxygen is clearly being used. The problem is never that there is none — it is that there is not enough.

Exercise 4: “Describe how the oxygen debt is removed after exercise. [3]”

Student’s Answer“You keep breathing hard, and the extra oxygen you breathe in gets rid of the lactic acid in the muscles until it has all gone.”

The FlawThe answer gets one of the three limited points and puts the process in the wrong organ. The lactic acid is not dealt with in the muscles. There is also no mention of the heart rate, which is the mechanism that gets the acid to where it can be dealt with, and “gets rid of” is too vague to count as a fate.

Correct Answer“The heart rate stays fast, transporting the lactic acid in the blood from the muscles to the liver [1]. Breathing stays deeper and faster, supplying the extra oxygen needed [1]. The lactic acid is respired aerobically in the liver [1].”

Key RuleThree points, three marks, and each one needs a purpose attached. If the word liver is missing you cannot reach full marks on any oxygen-debt question, however well the rest reads.

Exercise 5: “A tube of hydrogencarbonate indicator containing pondweed turns purple in the light. Explain what this shows. [3]”

Student’s Answer“Purple means the carbon dioxide has gone down, so the pondweed is photosynthesising and is not respiring because it does not need to during the day.”

The FlawThe colour is read correctly and then the conclusion goes wrong. An indicator reports a net change, so it can never show that a process has stopped. Respiration continues in every living plant cell, day and night, and a plant certainly does “need to” — it has to pay for active transport, protein synthesis, cell division and growth.

Correct Answer“Purple shows the carbon dioxide concentration has fallen [1]. The pondweed is photosynthesising and respiring at the same time [1], and photosynthesis is using carbon dioxide faster than respiration releases it, so the concentration falls overall [1].”

Key RuleWhenever a measurement reports a balance rather than a process, say so in the answer. The same reasoning covers the compensation points at dawn and dusk, where the two rates are equal and there is no net exchange at all — while both processes are running flat out.

Exercise 6: “A gas syringe reads 6 cm³ at 3 minutes and 30 cm³ at 9 minutes. Calculate the mean rate of gas production over that interval. [2]”

Student’s Answer“30 ÷ 9 = 3.3, so the rate is 3.3.”

The FlawTwo faults. The student used the final reading instead of the change, and the final time instead of the interval — so both numbers are wrong, and the answer happens to look plausible, which is worse. There is also no unit, which is very often the second mark.

Correct Answer“Change in volume = 30 − 6 = 24 cm³ [1]. Time = 9 − 3 = 6 minutes. Rate = 24 ÷ 6 = 4.0 cm³ per minute [1].”

Key RuleIn a rate question, both numbers must be differences or neither is. Write the unit you are aiming for before you start — cm³ per minute — and it will tell you exactly which quantities to divide.

✍️ Ultra-Detailed Practice Questions

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Ten Cambridge-style challenge questions. Write your answer first, then reveal the model answer and the examiner’s notes.

Question 1

[6 marks]

(a) Define respiration. [2] (b) State three uses, in a human, of the energy released by respiration. [3] (c) State one process that is often wrongly given as a use of this energy, and explain why it is not. [1]

Model Answer(a) The chemical reactions in cells that break down nutrient molecules [1] and release energy for metabolism [1].
(b) Any three of: muscle contraction; protein synthesis; cell division; active transport; growth; passage of nerve impulses; maintenance of a constant body temperature [1 each].
(c) Diffusion (or osmosis, or transpiration) [1] — it is passive, driven by the random kinetic energy the particles already have, which is why it still happens in a dead cell.

Examiner’s NotesIn (a) the verb decides one of the two marks. In (b), each of the seven answers is at most three words, so there is no excuse for writing something vague like “for living”, which earns nothing. Part (c) is the challenge-level twist and it tests whether the list was understood or only memorised.

Question 2

[7 marks]

(a) Define aerobic respiration. [2] (b) Write the word equation and the balanced chemical equation. [3] (c) State the total number of oxygen atoms on each side of the balanced equation and show how you counted them. [2]

Model Answer(a) The chemical reactions in cells that break down nutrient molecules [1] using oxygen to release energy [1].
(b) glucose + oxygen → carbon dioxide + water [1]; C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, formulae correct [1] and correctly balanced [1].
(c) 18 on each side [1]; left, 6 in the glucose plus 12 in the six oxygen molecules; right, 12 in the six carbon dioxides plus 6 in the six waters [1].

Examiner’s NotesThe words “use oxygen” in (a) are what make it a definition of aerobic respiration. In (b), adding “+ energy” is not credited. In (c) the commonest answer is 12, from counting only the 6O₂ and forgetting that glucose contains six oxygen atoms of its own.

Question 3

[8 marks]

A student investigates the effect of temperature on respiration in yeast, using a gas syringe. (a) State the independent, dependent and two control variables. [3] (b) Explain why the tube is left in the water bath for five minutes before the first reading. [2] (c) Explain why a layer of oil is floated on the suspension, and name the products of respiration under those conditions. [3]

Model Answer(a) Independent: the temperature of the water bath [1]. Dependent: the rate of carbon dioxide production [1]. Any two controls: volume and concentration of yeast suspension; volume and concentration of glucose; same batch of yeast; same equilibration time; same apparatus and observer [1].
(b) So that the contents of the tube reach the temperature of the water bath [1]; otherwise the mixture is not at the temperature recorded in the results table and every reading is wrong in the same direction [1].
(c) The oil prevents oxygen dissolving in from the air [1], so the yeast respires anaerobically [1]; the products are alcohol and carbon dioxide [1].

Examiner’s NotesThe dependent variable is the rate, not the volume — say rate and you have also shown you know the volume alone means nothing. In (c) the popular wrong answer is that the oil stops evaporation, which is its job in a transpiration experiment, not this one.

Question 4

[7 marks]

The rate of respiration in yeast rises from 10 °C to a maximum at 40 °C and then falls to zero by 60 °C. (a) Explain the rise. [2] (b) Explain the fall. [2] (c) Explain why the fall is much steeper than the rise. [2] (d) A tube from 60 °C is cooled to 40 °C. Predict what happens. [1]

Model Answer(a) The molecules gain kinetic energy and move faster, so enzyme and substrate collide more often [1]; the collisions carry more energy, so more are successful and more enzyme–substrate complexes form [1].
(b) The enzymes are denatured [1]; the shape of the active site changes so the substrate no longer fits [1].
(c) The slowing caused by cooling is only a reduction in kinetic energy and is fully reversible [1], whereas denaturation is a permanent change of shape, so once the optimum is passed the rate collapses rather than tailing off [1].
(d) Nothing — the rate stays at or near zero, because denaturation cannot be reversed by cooling [1].

Examiner’s NotesParts (a) and (b) must not share an explanation; giving “the enzymes work faster or slower” for both scores about one out of four. Part (c) is the mark almost nobody takes, and part (d) is the experiment that proves it — worth remembering as a demonstration rather than a fact.

Question 5

[8 marks]

A respirometer contains 5.0 g of germinating peas above soda lime, connected to a capillary tube of cross-sectional area 1.2 mm². The drop moves 40 mm in 20 minutes. (a) Explain why the soda lime is necessary. [2] (b) Calculate the rate of oxygen uptake in mm³ per gram per hour. [3] (c) Describe the control needed and explain why the apparatus must be in a water bath. [3]

Model Answer(a) It absorbs the carbon dioxide released [1], so the only change in gas volume is caused by the oxygen being used up [1].
(b) 40 × 1.2 = 48 mm³ [1]; 48 ÷ 5.0 = 9.6 mm³ per gram in 20 minutes [1]; × 3 = 28.8 mm³ per gram per hour [1].
(c) An identical respirometer containing the same mass of boiled peas or glass beads [1], to show that any movement is caused by respiration and not by a leak [1]; the water bath keeps the temperature constant because gases expand and contract with temperature, and a change in the room would move the drop and be mistaken for oxygen uptake [1].

Examiner’s NotesShow all three lines of the calculation. Combined into one, a slip loses everything; written out, the method marks survive. The temperature point in (c) is physics rather than biology and it is the one most often left out.

Question 6

[8 marks]

(a) Write the word equation for anaerobic respiration in yeast and in human muscle. [2] (b) Write the balanced equation for anaerobic respiration in yeast. [1] (c) Explain why anaerobic respiration releases much less energy per glucose molecule than aerobic respiration. [2] (d) Aerobic respiration releases 2880 kJ per mole and anaerobic respiration in muscle 150 kJ per mole. Calculate the ratio and the percentage. [3]

Model Answer(a) Yeast: glucose → alcohol + carbon dioxide [1]. Muscle: glucose → lactic acid [1].
(b) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [1].
(c) The glucose is not completely broken down [1], so a great deal of chemical energy remains in the alcohol or the lactic acid [1].
(d) 2880 ÷ 150 = 19.2 times, with no unit [1]; 150 ÷ 2880 × 100 = 5.2 % [1]; both correctly rounded and correctly identified as ratio and percentage [1].

Examiner’s NotesAdding carbon dioxide to the muscle equation is the most common single error in Topic 12. In (d), read which way round the division goes before touching the calculator — 19.2 and 5.2 are both correct answers to different questions, and a multiple-choice paper will offer you both.

Question 7

[8 marks]

An athlete requires 4.5 dm³ of oxygen per minute during a 6-minute run but can only take in 3.3 dm³ per minute. Her resting uptake is 0.4 dm³ per minute and during recovery she takes in 1.2 dm³ per minute. (a) Calculate the oxygen debt. [2] (b) Estimate the recovery time. [2] (c) Explain what her muscles are doing during the run and name the product. [2] (d) Outline how the debt is removed. [2]

Model Answer(a) Shortfall = 4.5 − 3.3 = 1.2 dm³ per minute [1]; × 6 = 7.2 dm³ [1].
(b) Extra oxygen during recovery = 1.2 − 0.4 = 0.8 dm³ per minute [1]; 7.2 ÷ 0.8 = 9 minutes [1].
(c) Her muscles respire aerobically as fast as the oxygen supply allows and respire anaerobically as well [1]; the product is lactic acid only [1].
(d) The heart rate stays fast, carrying the lactic acid to the liver, and breathing stays deep and fast to supply extra oxygen [1]; the lactic acid is respired aerobically in the liver [1].

Examiner’s NotesThe two traps sit in (a) and (b): using 4.5 rather than the shortfall, and dividing by 1.2 rather than subtracting the resting requirement first. Both give plausible-looking numbers, which is exactly why they are set.

Question 8

[7 marks]

Blood lactate was measured during and after a run: 0 min 1.0; 10 min 5.9; 15 min 8.6 (run ends); 18 min 9.4; 40 min 3.2; 60 min 1.2 arbitrary units. (a) Calculate the percentage increase from rest to peak. [2] (b) Explain the late peak. [2] (c) A student says these data explain aching two days later. Evaluate. [3]

Model Answer(a) 9.4 − 1.0 = 8.4 [1]; 8.4 ÷ 1.0 × 100 = 840 % [1].
(b) Lactic acid is produced in the muscles, not in the blood [1]; it takes time to pass into the blood, so the blood concentration continues to rise for a few minutes after the exercise has stopped [1].
(c) The data show the concentration back to 1.2 at 60 minutes, effectively the resting value [1]; so the lactic acid has been removed within about an hour and cannot be causing pain 48 hours later [1]; judgement — the claim is not supported, although these data do explain the fatigue she felt during and just after the run [1].

Examiner’s NotesAn evaluation needs a figure, a piece of reasoning and a judgement. The third mark here rewards fairness: the data explain something real, just not what was claimed. Answers that only say “she is wrong” score one.

Question 9

[7 marks]

(a) A root hair cell takes up nitrate ions from soil in which the nitrate concentration is lower than inside the cell. Waterlogging the soil stops the uptake. Explain fully. [4] (b) Cells lining the small intestine, which absorb glucose by active transport, contain many mitochondria. Explain the connection. [3]

Model Answer(a) Uptake is against the concentration gradient, so it is active transport [1]; active transport requires energy released by respiration [1]; waterlogged soil has no air spaces and therefore little or no oxygen [1]; so little aerobic respiration occurs, much less energy is released and the uptake of ions stops [1].
(b) Mitochondria are the site of aerobic respiration [1]; active transport requires a great deal of energy, released by respiration [1]; so a cell carrying out a high rate of active transport needs a high rate of energy release and therefore contains many mitochondria [1].

Examiner’s NotesPart (a) is a four-link chain and answers that jump from “waterlogged” to “no uptake” score one. Write the links out one at a time. Do not say the root “drowns” — that is a description with no mechanism in it.

Question 10

[8 marks]

Yeast in a sealed flask of glucose solution at 30 °C produces carbon dioxide steadily, then more slowly, then stops after four hours, although glucose remains. (a) Name the process and write the word equation. [2] (b) Suggest why gas production stopped. [2] (c) Predict what would happen if air were bubbled through the flask throughout, and explain. [2] (d) Describe a control for this investigation and state what it would show. [2]

Model Answer(a) Anaerobic respiration in yeast [1]; glucose → alcohol + carbon dioxide [1].
(b) The alcohol produced has accumulated [1] to a concentration that kills or inhibits the yeast, so respiration stops even though glucose remains [1].
(c) Much less alcohol would be produced and the glucose would be used more quickly [1], because with oxygen available the yeast would respire aerobically, giving carbon dioxide and water and releasing far more energy per glucose molecule [1].
(d) An identical sealed flask of glucose solution containing boiled yeast or no yeast, at the same temperature [1]; no alcohol and no gas would be produced, showing that the changes in the first flask were caused by living yeast respiring [1].

Examiner’s NotesThe stem rules out glucose deliberately, so any answer blaming a lack of food has ignored the evidence it was given. In (c), remember that yeast is not restricted to the anaerobic route — it uses whichever the conditions allow, which is why the bung matters so much.