# Explain why a muscle cell that is actively contracting requires a continuous supply of reduced NAD (NADH), and describe what happens to the reduced NAD produced during glycolysis when oxygen is unavailable.

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

> During intense exercise, skeletal muscle cells can continue contracting even when oxygen delivery is insufficient to meet the demand of aerobic respiration. Under these conditions, the cell must rely on anaerobic pathways to sustain ATP production.

## Mark scheme (4 marks)

1. Reduced NAD is required as a hydrogen/electron carrier to be oxidised in the electron transport chain, regenerating NAD for use in glycolysis (and the Krebs cycle)
2. Without oxygen, the electron transport chain cannot function, so reduced NAD cannot be oxidised by this route and NAD becomes unavailable
3. Reduced NAD from glycolysis donates its hydrogen to pyruvate, reducing pyruvate to lactate (catalysed by lactate dehydrogenase)
4. This process regenerates oxidised NAD, allowing glycolysis to continue and ATP to continue being produced

## Key terms

- [reduced NAD](https://www.gradenine.co.uk/glossary/reduced-nad)
- [glycolysis](https://www.gradenine.co.uk/glossary/glycolysis)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-muscle-cell-that-ded03d30) · Published by Druglandscape Ltd.