# Explain why a muscle cell that is actively contracting continues to produce lactate even when oxygen is available.

> OCR A-Level Biology A (H420) — 5.7 Respiration · Explain · 4 marks

> During intense exercise, skeletal muscle cells can produce lactate despite being supplied with oxygen via the bloodstream. This observation puzzled early physiologists who expected aerobic respiration to dominate whenever oxygen was present.

## Mark scheme (4 marks)

1. The rate of ATP demand during intense contraction exceeds the maximum rate at which aerobic respiration can supply ATP
2. Anaerobic glycolysis (lactate fermentation) runs simultaneously to produce additional ATP quickly
3. Pyruvate is produced faster than the mitochondria can accept and oxidise it, so excess pyruvate is reduced to lactate
4. Lactate production regenerates NAD+, allowing glycolysis to continue producing ATP

## Key terms

- [lactate](https://www.gradenine.co.uk/glossary/lactate)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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/explain-why-a-muscle-cell-that-cebf4ca9) · Published by Druglandscape Ltd.