During a sprint race, a runner's muscles switch from aerobic to anaerobic respiration. Describe what happens to the lactic acid that accumulates in the muscles during and after the race.

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

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

Model answer (4 marks)

During anaerobic respiration, pyruvate is converted to lactic acid, which accumulates in the muscle cells.

The excess lactic acid lowers the pH of the muscle, causing fatigue and cramps.

After the race, lactic acid is transported via the bloodstream to the liver.

In the liver, lactic acid is oxidised (the Cori cycle) and converted back to glucose using oxygen, thereby repaying the oxygen debt.

Examiner tips

  • Use the phrase "lactic acid builds up" and "causes fatigue" to hit the first two points. Show the transport to the liver and the Cori cycle to cover the last two. Keep each point brief but include the key terms: "oxidised", "converted to glucose", "oxygen debt".

Common mistakes

  • Confusing lactic acid with lactate or omitting the liver step. Forgetting to mention the oxygen debt or the Cori cycle. Using vague terms like "used up" instead of "converted to glucose".

Mark scheme (4 marks)

  1. Lactic acid builds up / accumulates in the muscles during anaerobic respiration
  2. Lactic acid causes muscle fatigue / cramp
  3. Lactic acid is transported to the liver
  4. In the liver, lactic acid is oxidised / broken down / converted to glucose using oxygen (repaying the oxygen debt)

Key terms in this question

lactic acid · anaerobic respiration

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