AQA A-level Biology: Respiration (7402) – Practice Questions

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 – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.5.2 Respiration of the AQA A-level Biology (7402) specification, for first teaching 2015, which is A-level only content, and required practical 9. Learn the content first from the study guide and the revision notes. See the AQA A-level Biology hub and the printable checklist. If a question supplies ATP values for the coenzymes, use those.

Questions

1. Glycolysis occurs in the cytoplasm of a cell. Describe the stages of glycolysis. [4]

2. A culture of yeast cells oxidises 45 molecules of glucose by glycolysis. Work out how many molecules are involved of each of these: ATP used, ATP made, net ATP gain, reduced NAD formed and pyruvate formed. [5]

3. A sprinter’s leg muscle cells run short of oxygen. Explain how the formation of lactate allows the muscle cells to go on making some ATP. [4]

4. Compare the formation of ethanol in yeast with the formation of lactate in muscle. [3]

5. (a) State how pyruvate enters the mitochondrial matrix. [1] (b) Describe the link reaction. [3]

6. A cell passes 18 molecules of acetyl CoA through the Krebs cycle. (a) Calculate the number of CO₂, reduced NAD, reduced FAD and ATP molecules produced. [4] (b) Name the process that makes the ATP in the Krebs cycle. [1]

7. Explain how the electron transfer chain and ATP synthase work together to make ATP in a mitochondrion. [6]

8. A chemical makes the inner mitochondrial membrane leaky to protons. Predict and explain the effect on: (a) the rate of ATP synthesis [2] (b) oxygen uptake [2] (c) the energy released by the electron transfer chain. [1]

9. An 18-carbon fatty acid is broken down completely into two-carbon units. (a) Calculate the number of acetyl CoA molecules formed and the number of reduced NAD produced when they enter the Krebs cycle. [2] (b) State the molecule formed from the fatty acid units that enters the Krebs cycle. [1] (c) State where an amino acid enters respiration after its amino group is removed. [1]

10. A student investigated the effect of temperature on the respiration of yeast in a glucose solution. CO₂ was collected in a gas syringe for 4 minutes. Each temperature was repeated three times.

Temperature (°C)	CO₂ volume (cm³)
15	3.0, 3.4, 3.2
25	7.1, 7.5, 7.3
35	14.8, 15.2, 14.6
45	11.0, 19.9, 10.6
55	1.2, 1.0, 1.4

(a) Identify the anomalous result and calculate the mean at 45 °C without it. [2] (b) Calculate the rate of CO₂ production at 35 °C in cm³ per minute. [2] (c) Describe and explain the trend from 15 °C to 55 °C. [3] (d) Suggest one improvement to the method. [1]

11. Assume each reduced NAD gives 2.5 ATP at the electron transfer chain and each reduced FAD gives 1.5 ATP. (a) Calculate the total ATP made per glucose in aerobic respiration, given 10 reduced NAD, 2 reduced FAD and 4 ATP by substrate-level phosphorylation. [3] (b) A cell needs 640 ATP molecules. Calculate the glucose needed aerobically and anaerobically. [2] (c) Explain why anaerobic respiration gives much less ATP per glucose. [1]

12. A mutation stops ATP synthase working in the muscle cells of a patient, while the electron transfer chain stays intact. Explain the likely effects on: (a) the proton gradient across the inner membrane [2] (b) the link reaction and Krebs cycle [3] (c) the muscle cells’ ATP supply and the pyruvate produced. [3]

Answers

1. Phosphorylation of glucose to glucose phosphate using ATP [1]. Production of two triose phosphate molecules [1]. Oxidation of triose phosphate to pyruvate [1], with a net gain of ATP and reduced NAD [1]. [4] Examiner insight: A “describe” answer scores best when the stages are in order and each names its key molecule. Writing only “glucose splits to pyruvate” leaves out the points the question is looking for.

2. ATP used: 45 × 2 = 90 [1]. ATP made: 45 × 4 = 180 [1]. Net ATP: 180 − 90 = 90 [1]. Reduced NAD: 45 × 2 = 90 [1]. Pyruvate: 45 × 2 = 90 [1]. [5] Examiner insight: Show each multiplication. Writing the “used” and “made” values separately shows your method clearly if a single number slips.

3. Without oxygen, reduced NAD cannot be reoxidised at the electron transfer chain [1]. Pyruvate accepts hydrogen from reduced NAD [1] and is converted to lactate [1]. NAD is regenerated and can be used in glycolysis, which continues to make ATP [1]. [4] Examiner insight: Link NAD supply to glycolysis continuing. Stating only that “lactate is made” or that it “makes ATP” misses the central idea.

4. Ethanol formation releases CO₂ (via ethanal) but lactate formation does not [1]. Both use reduced NAD to convert pyruvate [1]. Both regenerate NAD so glycolysis can continue [1]. [3] Examiner insight: A “compare” answer needs at least one similarity and one difference stated clearly. Two separate descriptions without a link back to each other are weaker.

5. (a) Active transport [1]. [1] (b) Pyruvate is oxidised to acetate [1], producing reduced NAD [1]. Acetate combines with coenzyme A to form acetyl CoA [1]. [3] Examiner insight: Say “oxidised” and “reduced NAD”. “Broken down” is too vague to show you know what the link reaction does.

6. (a) CO₂: 18 × 2 = 36 [1]. Reduced NAD: 18 × 3 = 54 [1]. Reduced FAD: 18 [1]. ATP: 18 [1]. (b) Substrate-level phosphorylation. [1] [5] Examiner insight: Work out the per-turn figure first and then multiply. If a calculation is wrong, written per-turn values show where the error began.

7. Reduced NAD and reduced FAD give up electrons to the electron transfer chain [1]. Electrons pass along carriers in oxidation-reduction reactions, releasing energy [1]. The energy drives proton pumping out of the matrix and across the inner membrane [1]. A proton concentration gradient forms across the inner membrane [1]. Protons move back into the matrix via ATP synthase [1], and the flow drives synthesis of ATP from ADP and inorganic phosphate [1]. [6] Examiner insight: Name which side the protons are pumped to and what moves through ATP synthase. Saying “the electrons make ATP” is a common way to lose these marks.

8. (a) ATP synthesis falls [1] because the proton gradient is not maintained, so fewer protons pass through ATP synthase [1]. [2] (b) Oxygen uptake rises [1] because the chain keeps running and oxidising reduced coenzymes, with electrons still being passed to oxygen [1]. [2] (c) The energy is released as heat rather than used to make ATP. [1] Examiner insight: A “predict and explain” question needs both a direction and a reason. Prediction alone, such as “ATP falls”, is not enough.

9. (a) Acetyl CoA: 18 ÷ 2 = 9 [1]. Reduced NAD in the Krebs cycle: 9 × 3 = 27 [1]. [2] (b) Acetyl CoA. [1] (c) It enters as a Krebs cycle intermediate or as acetyl CoA. [1] Examiner insight: Read what the calculation asks for. The Krebs cycle count excludes reduced coenzymes made earlier while breaking down the fatty acid.

10. (a) 19.9 cm³ is anomalous because it is far from the other two results [1]. Mean = (11.0 + 10.6) ÷ 2 = 10.8 cm³ [1]. [2] (b) Mean at 35 °C = (14.8 + 15.2 + 14.6) ÷ 3 = 14.87 [1]. Rate = 14.87 ÷ 4 = 3.7 cm³ per min [1]. [2] (c) Rate rises from 15 °C to 35 °C [1] because enzymes and substrate have more kinetic energy, so enzyme-substrate complexes form more often [1]. At 45 °C and 55 °C it falls as enzymes denature, with the rate near zero at 55 °C [1]. [3] (d) Use a water bath to keep the temperature constant, or take more repeats. [1] Examiner insight: Give the anomaly’s value and the reason for excluding it. For the trend, quote figures from the table, since a general statement about “enzymes” scores less.

11. (a) Reduced NAD: 10 × 2.5 = 25 [1]. Reduced FAD: 2 × 1.5 = 3 [1]. Total: 25 + 3 + 4 = 32 ATP [1]. [3] (b) Aerobic: 640 ÷ 32 = 20 glucose [1]. Anaerobic: 640 ÷ 2 = 320 glucose [1]. [2] (c) Only glycolysis makes ATP, as the ethanol or lactate step makes none and there is no oxidative phosphorylation. [1] Examiner insight: Show the ATP contribution of each type of coenzyme before adding. Marks are easier to earn from clear working than from a bare total.

12. (a) Protons cannot flow back through ATP synthase [1], so the gradient builds up until pumping is opposed and slows [1]. [2] (b) The chain slows, so reduced NAD and reduced FAD are not oxidised [1]. NAD and FAD run short [1], so the link reaction and Krebs cycle slow down [1]. [3] (c) ATP supply falls to the small amount from glycolysis and substrate-level phosphorylation [1]. Pyruvate cannot be fully oxidised and accumulates [1], so is converted to lactate to regenerate NAD [1]. [3] Examiner insight: In a multi-step “explain” question, chain the effects in order: ATP synthase, then gradient, then reduced coenzymes, then the earlier stages. Jumping to the end loses the middle steps.

Where marks are usually lost

• Giving glycolysis a yield of 2 ATP without the 4 made and 2 used.
• Using two carbons for pyruvate or three for acetate.
• Forgetting that every count doubles for each glucose, because two pyruvate form.
• Saying lactate or ethanol formation makes ATP, rather than regenerating NAD.
• Stating that oxygen is used in the Krebs cycle.
• Saying protons are pumped by ATP synthase or that electrons flow through it.
• Describing a trend without figures from the data.
• Calculating a mean that includes an anomalous result.
• Writing rates without units such as cm³ per min.

Next steps

• Review the revision notes and the study guide.
• Visit the AQA A-level Biology hub and the printable checklist.
• Take one of the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

AQA AS and A-level Biology (7401/7402) specification, for first teaching 2015, published by AQA. These questions follow section 3.5.2 and required practical 9. Check the current document on the AQA website for the latest version.