# A athlete is training indoors on a stationary bicycle. At the start of the session, the athlete pedals at a gentle pace and breathes comfortably. After 20 minutes, the trainer increases the resistance significantly, and the athlete finds they cannot take in oxygen fast enough to meet the muscles' energy demands. Explain what happens in the athlete's muscle cells to continue releasing energy, and describe one consequence of this process for the athlete.

> OCR GCSE Biology A: Gateway Science (J247) — B1.3 Respiration · Explain · 4 marks

> During intense exercise, muscle cells may not receive enough oxygen to sustain aerobic respiration alone.

## Mark scheme (4 marks)

1. Anaerobic respiration occurs (in the muscle cells) when oxygen supply is insufficient
2. Glucose is broken down to produce lactic acid (releasing energy)
3. Lactic acid builds up in the muscles causing muscle fatigue / pain / aching
4. Less energy (ATP) is released from glucose compared to aerobic respiration / anaerobic respiration is less efficient

## Related

- [Revision notes for OCR GCSE Biology A: Gateway Science (J247)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-athlete-is-training-indoors-on-f8fbb190) · Published by Druglandscape Ltd.