# During a long-distance cycling race, a competitor's muscles must sustain contraction for several hours. At the start of the race, the cyclist's muscles respire aerobically. As the race intensifies, the muscles begin to respire anaerobically. Explain how muscle cells obtain energy for contraction during both aerobic and anaerobic respiration, and describe what happens in the body after the race to deal with the products of anaerobic respiration.

> Pearson Edexcel A-Level Biology A (9BI0) — 4.2 Muscle contraction · Explain · 5 marks

> A competitive cyclist completes a gruelling four-hour race. During the final sprint finish, the cyclist pushes their leg muscles to maximum effort. After crossing the finish line, the cyclist continues to breathe rapidly and deeply for several minutes.

## Mark scheme (5 marks)

1. Aerobic respiration breaks down glucose using oxygen to release energy / ATP for muscle contraction
2. During anaerobic respiration, glucose is broken down without oxygen
3. Anaerobic respiration in animal / muscle cells produces lactic acid
4. After exercise, lactic acid must be broken down / oxidised, which requires extra oxygen — this is the oxygen debt
5. The cyclist continues to breathe rapidly / deeply after the race to take in the extra oxygen needed to repay the oxygen debt

## Key terms

- [aerobic respiration](https://www.gradenine.co.uk/glossary/aerobic-respiration)
- [anaerobic respiration](https://www.gradenine.co.uk/glossary/anaerobic-respiration)

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/during-a-long-distance-cycling-race-a-930cf7e7) · Published by Druglandscape Ltd.