# Explain how the structure of a leaf is adapted to facilitate gas exchange between the plant and the atmosphere.

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

## Mark scheme (4 marks)

1. Stomata are pores in the epidermis that allow CO₂ to diffuse into the leaf and O₂ / water vapour to diffuse out
2. The spongy mesophyll layer contains large air spaces that increase the surface area available for gas exchange and allow gases to move between cells and stomata
3. Mesophyll cells have a thin, moist cell wall that allows gases to dissolve and diffuse rapidly across the short diffusion distance into / out of the cell
4. The broad, flat shape of the leaf maximises the ratio of surface area to volume, increasing the area through which gases can enter and the number of stomata present

## Related

- [Revision notes for IB DP Biology Higher 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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