Muscles require energy for contraction. Explain how the body meets the energy demands of skeletal muscle during vigorous exercise, including what happens when oxygen supply becomes insufficient.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During a 400-metre sprint, a runner's muscles contract repeatedly and with great force. The cardiovascular system attempts to deliver more oxygen to the working muscles, but demand can outstrip supply.
Model answer (5 marks)
Skeletal muscle contraction requires ATP.
1. Under normal conditions ATP is produced mainly by aerobic respiration in mitochondria, where glucose is oxidised with oxygen to give ATP.
2. During a 400‑metre sprint the demand for ATP rises sharply and the cardiovascular system cannot supply oxygen fast enough.
3. Consequently the muscle fibres switch to anaerobic glycolysis, producing ATP without oxygen.
4. Anaerobic glycolysis yields lactic acid (lactate + H⁺) as a by‑product.
5. Accumulation of lactic acid lowers pH, leading to muscle fatigue, cramp and pain, and creates an oxygen debt that must be repaid after exercise to oxidise the lactate.
1. Under normal conditions ATP is produced mainly by aerobic respiration in mitochondria, where glucose is oxidised with oxygen to give ATP.
2. During a 400‑metre sprint the demand for ATP rises sharply and the cardiovascular system cannot supply oxygen fast enough.
3. Consequently the muscle fibres switch to anaerobic glycolysis, producing ATP without oxygen.
4. Anaerobic glycolysis yields lactic acid (lactate + H⁺) as a by‑product.
5. Accumulation of lactic acid lowers pH, leading to muscle fatigue, cramp and pain, and creates an oxygen debt that must be repaid after exercise to oxidise the lactate.
Examiner tips
- Use the word "ATP" and "aerobic respiration" first. Show the switch to anaerobic glycolysis and the production of lactic acid. Explain the effect of lactic acid and the concept of oxygen debt. Keep the answer concise and use the exact terminology.
- common_mistakes
- :
- Forgetting to mention ATP as the energy currency. Confusing anaerobic respiration with aerobic respiration. Not linking lactic acid to muscle fatigue and oxygen debt.
Mark scheme (5 marks)
- Muscles need ATP / energy for contraction
- Aerobic respiration occurs / glucose and oxygen are used to release energy (ATP) in mitochondria
- During vigorous exercise, oxygen cannot be delivered fast enough to meet demand / muscles respire anaerobically
- Anaerobic respiration produces lactic acid (in animals/humans)
- Lactic acid causes muscle fatigue / cramp / pain, and an oxygen debt is created which must be repaid after exercise to oxidise the lactic acid
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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