A student takes part in a sprint race. During the race, the student's muscles respire anaerobically. Explain why anaerobic respiration occurs during a sprint and describe what happens in the student's body after the race to return conditions to normal.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During intense exercise such as sprinting, the muscles demand energy at a rate that cannot be sustained by aerobic respiration alone. After the race, the student continues to breathe rapidly and deeply for several minutes.
Model answer (5 marks)
During a sprint the muscles need ATP more quickly than oxygen can be supplied, so aerobic respiration cannot keep up.
Because of this, the muscle cells use anaerobic glycolysis, producing lactic acid as a by‑product.
After the race the student breathes rapidly and deeply, taking in more oxygen than at rest. The excess oxygen (the oxygen debt) is used to oxidise the lactic acid that has accumulated, mainly in the liver where it is converted back to pyruvate and then to glucose.
The extra oxygen also helps to replenish the glycogen stores in muscle and liver that were depleted during the sprint, restoring normal energy reserves.
Thus anaerobic respiration occurs because of the immediate high ATP demand, and post‑exercise breathing restores oxygen balance, removes lactic acid and restores glycogen.
Because of this, the muscle cells use anaerobic glycolysis, producing lactic acid as a by‑product.
After the race the student breathes rapidly and deeply, taking in more oxygen than at rest. The excess oxygen (the oxygen debt) is used to oxidise the lactic acid that has accumulated, mainly in the liver where it is converted back to pyruvate and then to glucose.
The extra oxygen also helps to replenish the glycogen stores in muscle and liver that were depleted during the sprint, restoring normal energy reserves.
Thus anaerobic respiration occurs because of the immediate high ATP demand, and post‑exercise breathing restores oxygen balance, removes lactic acid and restores glycogen.
Examiner tips
- Use the exact phrase "oxygen debt" and mention lactic acid formation; explain the role of rapid breathing and oxygen use; include glycogen replenishment; keep answer within 5 points
Common mistakes
- Confusing aerobic with anaerobic respiration; forgetting to mention lactic acid or oxygen debt; writing too long or omitting glycogen restoration
Mark scheme (5 marks)
- During a sprint, the muscles require energy faster than oxygen can be delivered / aerobic respiration cannot supply ATP fast enough
- Anaerobic respiration produces lactic acid in the muscles
- After the race, the student continues to breathe rapidly / takes in extra oxygen (excess post-exercise oxygen consumption / repays the oxygen debt)
- The extra oxygen is used to oxidise / break down lactic acid (in the liver)
- Glycogen stores in the muscles / liver that were broken down during exercise are replenished (as part of recovery)
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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