# Explain how the gas exchange surface of a leaf is adapted to allow efficient exchange of carbon dioxide and oxygen.

> IB DP Biology Standard Level (2023 syllabus) — B3.1 Gas exchange · Explain · 4 marks

> Plants must exchange gases with the atmosphere for both photosynthesis and cellular respiration. The leaf is the primary organ of gas exchange in most vascular plants.

## Mark scheme (4 marks)

1. Stomata allow gases to diffuse into and out of the leaf, providing a direct pathway between the atmosphere and the internal air spaces.
2. The spongy mesophyll layer contains large interconnected air spaces that increase the surface area available for gas exchange and allow gases to circulate freely to reach the cells.
3. The cell walls of mesophyll cells are moist, allowing gases to dissolve and diffuse across the thin, moist surface into the cells.
4. The mesophyll cells are thin-walled and have a large surface area to volume ratio, reducing the diffusion distance and increasing the rate of gas exchange.

## Related

- [Revision notes for IB DP Biology Standard 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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