Aerobic respiration - AQA A-Level Biology (Paper 2) Exam Questions

Practise 20 exam-style questions on Aerobic respiration for AQA A-Level Biology (Paper 2). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with the Study Helper on hand if you get stuck.

20 questions
1–8 marks each
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20 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

In the electron transfer chain, electrons are passed along a series of carrier proteins, losing a little energy at each step, rather than being passed directly from reduced NAD to oxygen in a single step.

Suggest the advantage of this.

Model answer

Passing electrons along a chain releases the energy in small amounts. Much of this energy can be used, at several points along the chain, to move protons across the inner membrane and so make ATP, and less is lost as heat than if all the energy were released in one step.

Explanation

A single large transfer of energy would release most of it as heat, which cannot be used to make ATP. In the electron transfer chain, each carrier is at a slightly lower energy level than the one before, and the energy released at several steps is used to move protons into the intermembrane space. The resulting proton gradient drives ATP synthesis.

Mark scheme

1 mark
Energy is released in small amounts / gradually
Max 2.
1 mark
(So) more of the energy can be used to move protons / make ATP
Accept 'energy is used at several points to pump protons'.
1 mark
Less energy is lost / released as heat
Accept 'avoids releasing a large amount of heat that could damage the cell / denature enzymes'.

Question 2

3 marks

ATP is produced in respiration by substrate-level phosphorylation and by oxidative phosphorylation.

Give three differences between substrate-level phosphorylation and oxidative phosphorylation.

Model answer

1. Substrate-level phosphorylation takes place in the cytoplasm (in glycolysis) and in the mitochondrial matrix (in the Krebs cycle), but oxidative phosphorylation takes place on the inner mitochondrial membrane.
2. Oxidative phosphorylation uses energy from electrons passing along the electron transfer chain, but substrate-level phosphorylation does not.
3. Oxidative phosphorylation needs oxygen as the final electron acceptor, but substrate-level phosphorylation in glycolysis can take place without oxygen, in anaerobic respiration.

Explanation

Both processes add phosphate to ADP, but in substrate-level phosphorylation a phosphate group is transferred directly to ADP during a reaction of glycolysis or the Krebs cycle, whereas oxidative phosphorylation uses a proton gradient set up by the electron transfer chain and ATP synthase. Each difference should say something about both processes.

Mark scheme

1 mark
Substrate-level takes place in the cytoplasm (glycolysis) and matrix (Krebs cycle) AND oxidative takes place on the inner mitochondrial membrane / cristae
Max 3. List rule: if more than three differences are given, each wrong or contradictory extra difference cancels one correct difference. Each difference must refer to both processes, or be a clear comparison.
1 mark
Oxidative involves the electron transfer chain AND substrate-level does not
1 mark
Oxidative uses a proton gradient / ATP synthase / chemiosmosis AND substrate-level does not (phosphate is transferred directly from a substrate molecule to ADP)
1 mark
Oxidative directly requires oxygen (as the final electron acceptor) AND substrate-level does not / substrate-level (in glycolysis) can take place in anaerobic respiration
1 mark
Oxidative produces most of the ATP / more ATP AND substrate-level produces little / less ATP

Question 3

4 marks

2,4-dinitrophenol (DNP) is a chemical that makes the inner mitochondrial membrane permeable to protons. DNP was once sold as a slimming drug, but it was banned because some people who took it died from overheating.

Explain why people who took DNP lost body mass and had a raised body temperature.

Model answer

DNP lets protons diffuse back into the matrix across the inner membrane without passing through ATP synthase, so the proton gradient is reduced and much less ATP is made from each molecule of respiratory substrate. To make enough ATP, the body respires more of its stores, such as fat, so body mass falls. The energy released by electron transfer is no longer used to make ATP but is released as heat, so body temperature rises.

Explanation

Oxidative phosphorylation depends on the inner membrane being impermeable to protons except through ATP synthase. A substance that lets protons leak back 'uncouples' electron transfer from ATP synthesis: respiration continues, and even speeds up, but the energy is released as heat. A common error is to say that DNP stops respiration; in fact oxygen use and substrate use increase.

Mark scheme

1 mark
Protons diffuse back into the matrix through the membrane, not through ATP synthase / the proton gradient is reduced
Max 4. Accept H⁺ for protons.
1 mark
Less ATP is produced (per molecule of respiratory substrate)
Accept 'less oxidative phosphorylation'.
1 mark
So more respiratory substrate / fat / glucose / glycogen is respired (to produce enough ATP), reducing body mass
Accept 'fat stores are used up' or 'the rate of respiration increases'.
1 mark
Energy (from the electron transfer chain) is released as heat instead of being used to make ATP
Accept 'heat is produced'. Reject the idea of heat energy being produced.

Question 4

6 marks

Describe how ATP is produced by oxidative phosphorylation in a mitochondrion.

Model answer

Reduced NAD and reduced FAD from the link reaction and Krebs cycle transfer electrons to the electron transfer chain on the inner mitochondrial membrane (the cristae). The electrons pass along a series of carrier proteins in oxidation-reduction reactions, releasing energy. This energy is used to move protons from the matrix into the intermembrane space, so a proton gradient forms across the inner membrane. The protons diffuse back into the matrix through ATP synthase, which catalyses the formation of ATP from ADP and inorganic phosphate. At the end of the chain, oxygen is the final electron acceptor: it combines with electrons and protons to form water.

Explanation

This is the chemiosmotic theory. Energy from electrons moving along the chain is stored as a proton gradient across the inner membrane, and the flow of protons back through ATP synthase provides the energy for ATP synthesis. Be precise about the direction of proton movement and the location (inner membrane, intermembrane space, matrix), and say that oxygen accepts electrons - it does not 'make' ATP.

Mark scheme

1 mark
Reduced NAD / reduced FAD transfer electrons / hydrogen to the electron transfer chain
Max 6. Reject NADP. Accept 'reduced coenzymes are oxidised by the electron transfer chain'.
1 mark
(Electron transfer chain) on the inner mitochondrial membrane / cristae
1 mark
Electrons pass along a series of carriers (in oxidation-reduction reactions)
Accept 'electrons lose energy as they pass along the chain'.
1 mark
Energy released (from electrons) is used to move / pump protons from the matrix into the intermembrane space
Accept H⁺ for protons. Ignore 'actively transported'. Reject 'ATP is used to pump protons'. Reject 'hydrogen atoms' for protons once only. Reject the idea of energy being produced.
1 mark
(Creating) a proton gradient / higher concentration of protons in the intermembrane space
Accept 'electrochemical gradient'.
1 mark
Protons (diffuse) back into the matrix through ATP synthase
Accept 'chemiosmosis' with a correct description.
1 mark
ATP synthase catalyses the synthesis of ATP from ADP and Pi
Accept 'phosphorylation of ADP'.
1 mark
Oxygen is the final electron acceptor
1 mark
Oxygen combines with electrons and protons to form water

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