# A student claims that the electron transport chain is responsible for most of the ATP produced during aerobic respiration. Explain why this claim is correct, referring to the role of reduced NAD in this process.

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

> Aerobic respiration occurs in stages, each producing different amounts of ATP. The electron transport chain, located in the inner mitochondrial membrane, is the final stage and accounts for the majority of ATP yield per glucose molecule.

## Mark scheme (4 marks)

1. Reduced NAD donates electrons (and/or hydrogen) to the electron transport chain
2. Energy released from electrons passing along the chain is used to actively pump protons (H⁺) across the inner mitochondrial membrane, from the matrix to the intermembrane space
3. A proton gradient (chemiosmotic gradient / electrochemical gradient) is established across the inner mitochondrial membrane
4. Protons flow back into the matrix through ATP synthase (chemiosmosis), driving the synthesis of a large quantity of ATP, accounting for the majority of ATP yield

## Key terms

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

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-claims-that-the-electron-9c0d548c) · Published by Druglandscape Ltd.