A student goes for a brisk walk up a steep hill. During the walk, the student's muscles are supplied with enough oxygen to meet the demand for energy. Explain how the student's muscle cells release energy during the walk, and describe what would happen to the process of respiration in the muscles if the student began to run up the hill and oxygen supply to the muscles became insufficient.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.1 Cellular respiration · Explain · 5 marks · View as Markdown

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

During moderate exercise, muscles rely on aerobic respiration. During intense exercise, the oxygen supply to muscles may not meet the demand.

Model answer (5 marks)

During the walk the muscle cells use aerobic respiration: glucose is oxidised in the mitochondria with oxygen to give ATP, CO₂ and H₂O. The ATP produced is used by myosin‑actin cross‑bridge cycling to produce muscle contraction.

If the student runs, the oxygen supply can no longer keep up. The cells then switch to anaerobic respiration – glycolysis only – converting glucose to lactic acid (lactate). Lactic acid accumulates in the muscle, lowering pH and leading to fatigue, muscle ache and cramps.

Examiner tips

  • Use the term ‘aerobic respiration’ for the walk and ‘anaerobic respiration’ for the run; mention ATP, CO₂, H₂O and lactic acid; link lactic acid to fatigue.
  • Show the sequence: glucose → ATP (aerobic) → lactate (anaerobic).
  • Use UK spelling and correct terminology.

Mark scheme (5 marks)

  1. During the walk, glucose is broken down by aerobic respiration
  2. Aerobic respiration produces ATP / energy, carbon dioxide and water
  3. When running, oxygen supply is insufficient so anaerobic respiration occurs
  4. During anaerobic respiration, glucose is converted to lactic acid
  5. Lactic acid builds up in the muscles, causing fatigue / muscle ache / cramp

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