A student goes for a long run. Towards the end of the run, the student's leg muscles begin to feel tired and painful. Explain why the student's leg muscles feel tired and painful, referring to the process of aerobic respiration in your answer.

Cambridge International IGCSE Biology (0610) — 12.2 Aerobic respiration · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

During a long run the demand for ATP in the leg muscles rises sharply. The blood flow cannot supply enough oxygen to support aerobic respiration at the required rate, so the muscle cells switch to anaerobic glycolysis. In this pathway glucose is broken down to pyruvate and then to lactic acid, producing only 2 ATP per glucose instead of the 36–38 ATP from aerobic respiration. The lactic acid accumulates in the muscle, lowering the pH and causing the characteristic fatigue and pain (cramps). Thus the lack of oxygen forces a less efficient energy‑producing pathway, leading to muscle tiredness and discomfort.

Examiner tips

  • Mention the oxygen supply problem first, then the switch to anaerobic glycolysis, and finally the build‑up of lactic acid causing fatigue.
  • Use the terms ‘aerobic respiration’, ‘anaerobic respiration’, ‘lactic acid’, ‘ATP’ and ‘muscle fatigue’ as the mark scheme expects.
  • Keep the answer concise – 4 points can be covered in 3–4 short sentences.

Common mistakes

  • Confusing lactic acid with lactate; examiners expect the term ‘lactic acid’. Forgetting to mention that aerobic respiration releases far more ATP per glucose. Using vague phrases like ‘muscle tiredness’ without linking it to oxygen supply and lactic acid buildup.

Mark scheme (4 marks)

  1. During vigorous exercise, not enough oxygen is delivered to the muscle cells for aerobic respiration to continue at the required rate.
  2. The muscle cells switch to anaerobic respiration, breaking down glucose to produce lactic acid.
  3. Lactic acid builds up in the muscles, causing fatigue and pain / cramp.
  4. Aerobic respiration releases energy from glucose using oxygen; without sufficient oxygen, less energy is released (anaerobic respiration is less efficient), contributing to muscle fatigue.

Key terms in this question

aerobic respiration

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