Explain why muscle cells have large numbers of mitochondria and describe what happens in muscle cells when the supply of oxygen becomes insufficient during vigorous exercise.

Cambridge International IGCSE Biology (0610) — 12.1 Respiration · Explain and Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Muscle cells need a large amount of energy for contraction, so they contain many mitochondria where aerobic respiration takes place.
When oxygen supply is insufficient during vigorous exercise, the cells switch to anaerobic respiration. Glucose is broken down to produce lactic acid (and a small amount of energy). The build‑up of lactic acid leads to muscle fatigue and cramps.

Examiner tips

  • Use the word ‘aerobic’ and ‘anaerobic’ to show understanding of respiration types.
  • Explain the role of mitochondria in energy production.
  • Mention lactic acid and its effect on muscle fatigue.
  • Keep the answer concise and use correct terminology.

Common mistakes

  • Confusing glycolysis with the entire anaerobic pathway.
  • Saying ‘oxygen is not needed’ instead of ‘oxygen supply is insufficient’.
  • Omitting the link between lactic acid build‑up and muscle fatigue.

Mark scheme (4 marks)

  1. Muscle cells require a large amount of energy (for contraction), so they need many mitochondria where aerobic respiration occurs.
  2. When oxygen supply is insufficient, muscle cells switch to anaerobic respiration.
  3. Glucose is broken down to produce lactic acid (and energy) during anaerobic respiration in muscle cells.
  4. Build-up of lactic acid causes muscle fatigue / cramp.

Key terms in this question

mitochondria

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