# Explain how substances move across cell membranes by facilitated diffusion and describe how this differs from simple diffusion.

> AQA A-Level Biology (7402) — 3.2.3 Transport Across Membranes · Explain · 5 marks

## Mark scheme (5 marks)

1. Facilitated diffusion moves substances from a higher concentration to a lower concentration (down a concentration gradient)
2. Facilitated diffusion uses protein channels (carrier proteins / channel proteins) in the membrane
3. Facilitated diffusion does not require energy (ATP) / is a passive process
4. Simple diffusion does not use protein channels — substances pass directly through the phospholipid bilayer
5. Facilitated diffusion allows larger or charged/polar molecules (e.g. glucose, ions) to cross the membrane that cannot pass directly through the bilayer

## Key terms

- [facilitated diffusion](https://www.gradenine.co.uk/glossary/facilitated-diffusion)
- [simple diffusion](https://www.gradenine.co.uk/glossary/simple-diffusion)

## Related

- [Revision notes for AQA A-Level Biology (7402)](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-how-substances-move-across-cell-3c2d1a96) · Published by Druglandscape Ltd.