A scientist investigates two samples of muscle tissue. Sample A is taken from a sprinter immediately after a 100 m race. Sample B is taken from a long-distance swimmer after a gentle 30-minute swim. Explain the differences in the products found in the two muscle tissue samples.

WJEC GCSE Biology (Wales) — 1.2.1 Aerobic and anaerobic 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)

Sample A contains lactic acid (lactate) because the sprinter’s muscles worked at very high intensity and the oxygen supply was insufficient, so the muscle cells relied on anaerobic respiration.
Sample B contains carbon dioxide and water because the swimmer’s muscles worked at a lower intensity, the oxygen supply was adequate and the cells used aerobic respiration.

Examiner tips

  • Use the term ‘lactic acid (lactate)’ for anaerobic products. Mention ‘high intensity’ for the sprinter and ‘low intensity’ for the swimmer. State the oxygen supply as the key difference. Show the products of aerobic respiration (CO₂ and H₂O).

Common mistakes

  • Confusing lactate with lactic acid. Forgetting to link intensity to oxygen supply. Listing ATP as a product instead of CO₂ and H₂O for aerobic respiration.

Mark scheme (4 marks)

  1. Sample A contains lactic acid (lactate) because the sprinter used anaerobic respiration
  2. The sprinter's muscles worked at high intensity so oxygen supply was insufficient / muscles respired anaerobically
  3. Sample B contains carbon dioxide and water because the swimmer used aerobic respiration
  4. The swimmer's muscles had sufficient oxygen supply because the exercise was low intensity / aerobic respiration could meet the energy demand

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