A leaf is adapted for efficient gas exchange. Describe the features of a leaf that make it well-adapted for gas exchange, and explain how each feature helps gases to be exchanged efficiently.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Stomata on the lower epidermis provide pores through which CO₂ enters and O₂ exits the leaf.
2. The spongy mesophyll contains large air spaces that increase the surface area for gas exchange and allow rapid diffusion of gases within the leaf.
3. The cells forming the exchange surface are thin and have a short diffusion distance, so gases cross the membrane quickly.
4. The moist cell walls lining the air spaces dissolve gases before they diffuse across the membrane, facilitating transfer.
5. A large concentration gradient is maintained – CO₂ is consumed in photosynthesis and O₂ is produced, while O₂ is consumed in respiration – which drives the rate of diffusion.
2. The spongy mesophyll contains large air spaces that increase the surface area for gas exchange and allow rapid diffusion of gases within the leaf.
3. The cells forming the exchange surface are thin and have a short diffusion distance, so gases cross the membrane quickly.
4. The moist cell walls lining the air spaces dissolve gases before they diffuse across the membrane, facilitating transfer.
5. A large concentration gradient is maintained – CO₂ is consumed in photosynthesis and O₂ is produced, while O₂ is consumed in respiration – which drives the rate of diffusion.
Examiner tips
- Use the exact terminology (stomata, spongy mesophyll, thin cells, moist surface, concentration gradient).
- Link each feature directly to its functional benefit for gas exchange.
- Keep each point brief but complete – 5 marks total.
Common mistakes
- Confusing the upper epidermis for the stomata location.
- Forgetting to mention the short diffusion distance of thin cells.
Mark scheme (5 marks)
- Stomata (pores) in the lower epidermis allow gases to diffuse into and out of the leaf
- The spongy mesophyll has air spaces which increase the surface area for gas exchange / allow fast diffusion of gases inside the leaf
- The cells forming the exchange surface are thin / have a short diffusion distance, so gases diffuse across quickly
- The moist surface of cells lining the air spaces allows gases to dissolve before diffusing across the membrane
- A large concentration gradient is maintained (e.g. carbon dioxide is used up in photosynthesis / oxygen is used in respiration) which increases the rate of diffusion
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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