Explain why a leaf is adapted to exchange gases efficiently with the surrounding air.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Leaves are the main site of photosynthesis in plants. They must exchange carbon dioxide and oxygen rapidly with the atmosphere.
Model answer (5 marks)
Leaves are thin and flat, giving a large surface area relative to their volume.
The large surface area to volume ratio allows a greater area for gas exchange per unit of leaf mass.
Stomata on the leaf epidermis provide controlled openings through which CO₂ enters and O₂ exits.
Inside the leaf, the spongy mesophyll creates a network of air spaces, increasing the internal surface area for diffusion.
The short diffusion distance and maintained concentration gradient mean gases move rapidly, ensuring efficient exchange with the surrounding air.
The large surface area to volume ratio allows a greater area for gas exchange per unit of leaf mass.
Stomata on the leaf epidermis provide controlled openings through which CO₂ enters and O₂ exits.
Inside the leaf, the spongy mesophyll creates a network of air spaces, increasing the internal surface area for diffusion.
The short diffusion distance and maintained concentration gradient mean gases move rapidly, ensuring efficient exchange with the surrounding air.
Examiner tips
- Use the word "large surface area to volume ratio" to link shape and efficiency. Mention both stomata and spongy mesophyll as key structures. Show understanding of diffusion distance and concentration gradients. Keep the answer concise and use exam‑style terminology.
Common mistakes
- Forgetting to mention stomata or spongy mesophyll. Using vague terms like "big leaf" instead of "large surface area to volume ratio". Failing to explain why a short diffusion distance matters.
Mark scheme (5 marks)
- Leaves are thin / flat / have a large surface area
- Large surface area to volume ratio (allows rapid diffusion / increases rate of exchange)
- Stomata allow gases to enter and leave the leaf
- Air spaces / spongy mesophyll layer increase the internal surface area for gas exchange
- Maintaining a concentration gradient / short diffusion distance means gases diffuse quickly
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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