# The spongy mesophyll and the alveoli in the lungs are both gas exchange surfaces. Explain how the features of the spongy mesophyll layer make it an efficient gas exchange surface.

> WJEC A-Level Biology (Wales) — 2.2 Adaptations for gas exchange · Explain · 5 marks

> Plants exchange gases through internal leaf structures. Carbon dioxide must reach the palisade cells for photosynthesis, and oxygen must be able to leave the leaf. The spongy mesophyll layer plays a key role in this process.

## Mark scheme (5 marks)

1. The spongy mesophyll has air spaces between the loosely packed cells
2. Air spaces increase the surface area available for gas exchange
3. A large concentration gradient is maintained / gases are used up or produced in photosynthesis or respiration
4. The cell walls / surfaces of the spongy mesophyll cells are moist, allowing gases to dissolve before diffusing across the membrane
5. The cells of the spongy mesophyll are thin / have a short diffusion distance, so gases diffuse quickly across the membrane

## Key terms

- [spongy mesophyll](https://www.gradenine.co.uk/glossary/spongy-mesophyll)

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](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-spongy-mesophyll-and-the-alveoli-754a5c6b) · Published by Druglandscape Ltd.