# Guard cells control gas exchange in plant leaves. Explain how the structure of guard cells is suited to their function.

> Pearson Edexcel A-Level Biology A (9BI0) — 2.4 Plant structure and function · Explain · 5 marks

> Guard cells are found in pairs surrounding stomata on the surface of plant leaves. They control the opening and closing of stomata, regulating gas exchange and water loss.

## Mark scheme (5 marks)

1. Guard cells contain chloroplasts, which carry out photosynthesis to produce glucose/ATP
2. The glucose/ATP produced is used for active transport of ions (e.g. potassium ions) into the guard cells
3. This lowers the water potential inside the guard cells, so water enters by osmosis
4. Guard cells have an unevenly thickened cell wall — the inner wall is thicker — so as the cells become turgid they curve/bend outwards, opening the stoma
5. When the stoma is open, carbon dioxide can enter for photosynthesis and oxygen / water vapour can leave, enabling gas exchange

## Key terms

- [guard cells](https://www.gradenine.co.uk/glossary/guard-cells)

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/guard-cells-control-gas-exchange-in-ed9d92e1) · Published by Druglandscape Ltd.