During a long-distance cycling race, a cyclist's muscles switch from aerobic to anaerobic respiration as they sprint uphill. Explain why this switch occurs and describe one consequence for the cyclist's muscles.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During intense exercise, muscles may not receive oxygen fast enough to meet their energy demands.
Model answer (4 marks)
During intense exercise the muscles cannot receive enough oxygen to match the energy demand, so the cells switch from aerobic to anaerobic respiration.
Anaerobic respiration does not require oxygen; glucose is broken down in the cytoplasm to produce a small amount of ATP.
In muscle cells this pathway produces lactic acid as a by‑product.
The accumulation of lactic acid lowers the pH of the muscle, causing fatigue, pain or cramps, which reduces the muscles’ ability to contract efficiently.
Anaerobic respiration does not require oxygen; glucose is broken down in the cytoplasm to produce a small amount of ATP.
In muscle cells this pathway produces lactic acid as a by‑product.
The accumulation of lactic acid lowers the pH of the muscle, causing fatigue, pain or cramps, which reduces the muscles’ ability to contract efficiently.
Examiner tips
- Use the phrase ‘cannot receive enough oxygen’ to show the cause of the switch.
- Mention that anaerobic respiration is oxygen‑independent and produces lactic acid.
- Explain the effect of lactic acid on muscle contraction.
- Keep the answer concise – 4 short sentences will earn full marks.
Mark scheme (4 marks)
- During intense exercise, oxygen cannot be delivered to the muscles fast enough (to keep up with demand)
- Anaerobic respiration occurs without oxygen / glucose is broken down without oxygen
- Anaerobic respiration in muscle cells produces lactic acid (as well as energy)
- Lactic acid build-up causes muscle fatigue / pain / cramp, meaning muscles cannot contract as effectively
Key terms in this question
aerobic respiration · anaerobic respiration
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Aerobic and anaerobic respiration questions
- Yeast cells are used in bread making. Explain how yeast cells produce carbon dio…
- Explain why aerobic respiration releases more energy than anaerobic respiration …
- A sprinter completes a 100 m race in under 12 seconds. Describe what happens in …
- After a period of intense exercise, a person continues to breathe deeply and rap…