Explain how the structure of a leaf is adapted to facilitate gas exchange between the plant and the atmosphere.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Stomata are pores in the epidermis that allow CO₂ to diffuse into the leaf and O₂ / water vapour to diffuse out.
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.
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.
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.
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.
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.
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.
Examiner tips
- Use the word ‘allow’ to show the function of each structure.
- Mention the short diffusion distance in mesophyll cells.
- Show the leaf shape increases surface area per volume.
- Keep answer concise to fit 4 marks.
Common mistakes
- Confusing stomata with other pores; they are specific to gas exchange.
- Omitting the role of spongy mesophyll air spaces.
- Using vague terms like ‘big’ instead of ‘large air spaces’.
Mark scheme (4 marks)
- Stomata are pores in the epidermis that allow CO₂ to diffuse into the leaf and O₂ / water vapour to diffuse out
- 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
- 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
- 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
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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