# The stomata in a leaf open and close to control gas exchange. Explain how the structure and position of the stomata and guard cells make the leaf well-adapted for gas exchange, and how a large concentration gradient is maintained across the stomata.

> Eduqas A-Level Biology — 2.2 Adaptations for gas exchange · Explain · 5 marks

## Mark scheme (5 marks)

1. Stomata are located on the lower epidermis / underside of the leaf
2. Stomata are small pores / openings that allow gases (carbon dioxide and oxygen) to diffuse in and out of the leaf
3. Guard cells control the opening and closing of the stomata / regulate gas exchange
4. The spongy mesophyll has air spaces which allow gases to diffuse rapidly to and from the stomata / increases surface area for gas exchange inside the leaf
5. A large concentration gradient is maintained across the stomata because carbon dioxide is used up in photosynthesis (keeping internal CO₂ concentration low) OR because oxygen produced by photosynthesis diffuses out, maintaining a low internal oxygen concentration relative to outside

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Biology](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/the-stomata-in-a-leaf-open-c8090b6b) · Published by Druglandscape Ltd.