# Explain how the compartmentalisation provided by the inner mitochondrial membrane contributes to ATP synthesis during aerobic respiration.

> IB DP Biology Higher Level (2023 syllabus) — B2.2 Organelles and compartmentalisation · Explain · 4 marks

## Mark scheme (4 marks)

1. The inner mitochondrial membrane is impermeable to protons (H⁺ / hydrogen ions), so protons cannot diffuse freely across it.
2. Electron carriers in the inner membrane pump protons from the matrix into the intermembrane space, establishing a proton gradient (or electrochemical / proton-motive force).
3. Protons can only re-enter the matrix through ATP synthase (chemiosmosis), so the energy of the gradient drives ATP synthesis from ADP and inorganic phosphate (Pᵢ).
4. Compartmentalisation is essential because it allows a localised, concentrated proton gradient to be maintained between two distinct spaces (intermembrane space and matrix), which would be impossible in an unpartitioned system.

## Key terms

- [compartmentalisation](https://www.gradenine.co.uk/glossary/compartmentalisation)
- [inner mitochondrial membrane](https://www.gradenine.co.uk/glossary/inner-mitochondrial-membrane)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-compartmentalisation-provided-by-a4eb6102) · Published by Druglandscape Ltd.