A Level Biology: Energy and respiration — Practice Questions (Cambridge 9700)

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Each question practises a skill tested in the June 2024 Paper 42. After each answer there is an examiner insight, a mark-scheme insight or a tip and, where one matches, the real question to try next.

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Questions

1. In a muscle cell, carbon dioxide is produced inside mitochondria and then leaves them.

(a) Name the two stages of aerobic respiration that release carbon dioxide inside a mitochondrion, and state where they happen.

(b) Describe the route taken by a carbon dioxide molecule from where it is made to the cytoplasm, and state how it crosses the membranes. [3]

2. Describe how the energy in reduced NAD is used to make ATP by oxidative phosphorylation. [6]

3. Cyanide stops the last carrier in the electron transport chain from passing electrons to oxygen. Explain why cyanide stops ATP production by oxidative phosphorylation, and why the Krebs cycle also stops. [4]

4. A chemical called an uncoupler makes the inner mitochondrial membrane leaky to protons (H⁺). Suggest and explain the effects of an uncoupler on the production of ATP and on the temperature of the cell. [3]

5. In a respirometer, some germinating seeds took in 4.8 cm³ of oxygen and gave out 3.4 cm³ of carbon dioxide in one hour. Calculate the respiratory quotient (RQ) and suggest which respiratory substrate the seeds were using. [2]

6. Yeast cells are kept in a sealed flask with no oxygen. Explain how they can still make ATP, and why they make much less ATP per glucose molecule than in aerobic conditions. [4]

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Answers

1. (a) The link reaction and the Krebs cycle, both in the mitochondrial matrix [1].

(b) The carbon dioxide molecule diffuses from the matrix through the inner mitochondrial membrane into the intermembrane space, then through the outer mitochondrial membrane into the cytoplasm [1]. It crosses both membranes by simple diffusion through the phospholipid bilayer (it is small and non-polar), not through membrane proteins [1].

Mark-scheme insight (Cambridge 9700 June 2024 mark scheme, Paper 42, Question 6(a)): In the similar question, which traced oxygen from the cytoplasm into the matrix, the mark needed arrows through the phospholipid layer of both the outer and the inner membrane; arrows through membrane proteins were rejected.

Source for the mark-scheme insights on this page: Cambridge International AS & A Level Biology 9700 June 2024 mark scheme for Paper 42 (9700/42), paraphrased. Cambridge’s 9700 past papers page publishes the Paper 41 mark scheme from this series, not the Paper 42 one.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 42, Question 6(a).

2. Any six: reduced NAD gives up hydrogen, which splits into protons (H⁺) and electrons, at the inner mitochondrial membrane [1]; the electrons pass along the electron transport chain (electron carriers) [1]; energy is released as electrons move from carrier to carrier [1]; this energy is used to actively transport (pump) protons from the matrix into the intermembrane space [1]; this builds up a high concentration of protons (a proton gradient) in the intermembrane space [1]; protons diffuse back into the matrix through ATP synthase [1]; this drives ATP synthase to make ATP from ADP and Pi [1]; the process is called chemiosmosis [1]; oxygen is the final electron acceptor, forming water [1].

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 42, Question 6(b)): Common errors were calling the chain “carrier proteins” or “proton pumps” instead of electron carriers, and not saying whether protons move by active transport or by diffusion. Energy is released as electrons move along the chain; it is not “produced” or “lost”.

Source for the examiner insights on this page: Cambridge International AS & A Level Biology 9700 June 2024 Principal Examiner Report for Teachers, Paper 9700/42 section, paraphrased.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 42, Question 6(b).

3. Any four: electrons cannot leave the chain, so electron flow along the electron transport chain stops [1]; no energy is released to pump protons, so no proton gradient is kept up [1]; no protons diffuse through ATP synthase, so no ATP is made by oxidative phosphorylation [1]; reduced NAD and reduced FAD are not oxidised, so NAD and FAD are not regenerated [1]; the Krebs cycle (and the link reaction) need NAD and FAD to accept hydrogen, so they stop [1].

Tip: When a step in a pathway is blocked, trace the effect in both directions: what cannot be made after the block, and what cannot be recycled before it.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 42, Question 6(b).

4. Any three: protons leak back into the matrix without passing through ATP synthase [1]; so the proton gradient is reduced and less ATP is made [1]; the electron transport chain keeps running (oxygen is still used), but the energy released is lost as heat [1]; so the temperature of the cell rises [1].

Tip: ATP synthase makes ATP only when protons flow through it. Any route that lets protons bypass it wastes the gradient.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 42, Question 6(b).

5. RQ = CO₂ given out ÷ O₂ taken in = 3.4 ÷ 4.8 = 0.71 [1]. An RQ of about 0.7 suggests the substrate is lipid [1].

Tip: Learn the typical RQ values: carbohydrate 1.0, protein about 0.9, lipid about 0.7. Always divide carbon dioxide by oxygen, not the other way round.

6. Any four: without oxygen, the electron transport chain and Krebs cycle stop, so ATP is made only by glycolysis (substrate-level phosphorylation) [1]; pyruvate is converted to ethanal, releasing carbon dioxide [1]; ethanal is reduced to ethanol by reduced NAD [1]; this regenerates NAD, so glycolysis can continue [1]; glycolysis gives a net gain of only 2 ATP per glucose, compared with many more when oxidative phosphorylation also happens [1].

Tip: Say why NAD must be regenerated: without it, glycolysis would stop and no ATP at all could be made.

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Where marks are usually lost

• Showing small molecules such as oxygen or carbon dioxide passing through membrane proteins, or giving only part of their route across the two mitochondrial membranes.
• Calling the electron transport chain “carrier proteins” or “proton pumps” instead of electron carriers.
• Not saying that protons are actively transported into the intermembrane space but diffuse back through ATP synthase.
• Saying energy is “produced” or “lost” rather than released.
• Dividing oxygen by carbon dioxide when calculating RQ.