# Explain why NAD is essential for cell respiration to continue.

> IB DP Biology Standard Level (2023 syllabus) — C1.2 Cell respiration · Explain · 4 marks

> NAD is a coenzyme present in cells in limited amounts. During cell respiration, NAD accepts hydrogen (electrons and protons) from oxidation reactions, becoming reduced to NADH.

## Mark scheme (4 marks)

1. NAD is needed as a hydrogen/electron acceptor in oxidation reactions during glycolysis and the Krebs cycle.
2. NAD is present in limited/small quantities in the cell, so it must be continuously regenerated/recycled.
3. In aerobic respiration, NADH is re-oxidised to NAD at the inner mitochondrial membrane/electron transport chain, allowing NAD to be reused.
4. In anaerobic respiration, NAD is regenerated by the reduction of pyruvate to lactate (or ethanol and carbon dioxide), allowing glycolysis to continue.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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