Explain how lactic acid is produced during vigorous exercise and describe what happens to the lactic acid after the exercise stops.

AQA GCSE Biology (8461) — 4.4.2 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 vigorous exercise the oxygen supply to the working muscles is insufficient, so the muscles rely on anaerobic respiration. In this process glucose is broken down to pyruvate, which is then reduced to lactic acid, producing ATP.
After the exercise stops the accumulated lactic acid is carried in the blood to the liver where it is converted back to glucose via the Cori cycle. The body also consumes the extra oxygen required to break down the lactic acid – the so‑called oxygen debt.

Examiner tips

  • Use the phrase "oxygen supply is insufficient" to show understanding of the cause of anaerobic respiration.
  • Mention the Cori cycle or liver conversion to glucose to hit the key point.
  • Show the link between oxygen debt and lactic acid breakdown.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Confusing lactic acid with lactic acid production; students often say it is produced after exercise rather than during.
  • Forgetting to mention the liver or the Cori cycle as the site of conversion back to glucose.
  • Using vague terms like "fatigue" instead of "oxygen supply is insufficient".

Mark scheme (4 marks)

  1. During vigorous exercise, oxygen supply to muscles is insufficient / muscles cannot get enough oxygen
  2. Anaerobic respiration occurs in the muscles, producing lactic acid
  3. After exercise, extra oxygen is needed to break down / react with the accumulated lactic acid — this is the oxygen debt
  4. Lactic acid is transported (in the blood) to the liver where it is converted back to glucose

Key terms in this question

lactic acid

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