# Explain the role of the electron transport chain in linking the oxidation of NADH to the net synthesis of ATP during aerobic respiration.

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

## Mark scheme (4 marks)

1. NADH donates electrons (and protons/H⁺) to Complex I (NADH dehydrogenase) at the start of the electron transport chain, becoming oxidised to NAD⁺.
2. Electrons are passed along a series of carriers (e.g. FMN, iron–sulfur clusters, ubiquinone, cytochromes), losing energy at each transfer.
3. The energy released at Complexes I, III, and IV is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, establishing an electrochemical / proton-motive gradient.
4. Protons flow back into the matrix down their electrochemical gradient through ATP synthase (Complex V), driving the phosphorylation of ADP to ATP (chemiosmosis).

## Key terms

- [electron transport chain](https://www.gradenine.co.uk/glossary/electron-transport-chain)
- [NADH](https://www.gradenine.co.uk/glossary/nadh)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](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/explain-the-role-of-the-electron-4b9302d0) · Published by Druglandscape Ltd.