A long-distance runner finishes a race and continues to breathe heavily for several minutes afterwards. During the race, the runner's muscles produced lactic acid. Explain why lactic acid is produced during intense exercise and describe what happens to the lactic acid after the race has finished.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During intense exercise, the demand for energy in muscle cells can exceed the rate at which oxygen can be delivered to those cells.
Model answer (5 marks)
During intense exercise the demand for ATP exceeds the rate at which oxygen can be delivered to the muscle cells, so the cells must respire anaerobically.
In anaerobic respiration glucose is only partially oxidised; pyruvate is reduced to lactate, producing lactic acid.
After the race the lactic acid is carried by the bloodstream to the liver.
In the liver it is converted back into glucose via gluconeogenesis (the Cori cycle), using oxygen to complete the oxidation of the lactate.
Thus the lactic acid is removed from the muscle and recycled into glucose, helping to repay the oxygen debt.
In anaerobic respiration glucose is only partially oxidised; pyruvate is reduced to lactate, producing lactic acid.
After the race the lactic acid is carried by the bloodstream to the liver.
In the liver it is converted back into glucose via gluconeogenesis (the Cori cycle), using oxygen to complete the oxidation of the lactate.
Thus the lactic acid is removed from the muscle and recycled into glucose, helping to repay the oxygen debt.
Examiner tips
- Use the phrase ‘oxygen debt’ to link anaerobic respiration to the need for later oxidation.
- Show the Cori cycle: muscle → blood → liver → glucose.
- Mention that lactate is a by‑product of pyruvate reduction.
- Keep the answer concise – 5 points, no extra detail.
Common mistakes
- Confusing lactic acid with lactate; examiners expect ‘lactic acid’ produced in muscle.
- Forgetting that the liver converts lactate back to glucose, not just oxidises it.
- Writing that lactate stays in the muscle instead of being transported to the liver.
Mark scheme (5 marks)
- During intense exercise, oxygen cannot be delivered to muscles fast enough / muscles respire faster than oxygen can be supplied
- Muscles respire anaerobically / anaerobic respiration occurs
- Anaerobic respiration produces lactic acid (from the incomplete breakdown of glucose)
- After exercise, lactic acid is transported (in the blood) to the liver
- In the liver, lactic acid is converted back to glucose / oxidised / broken down using the oxygen debt
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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