The spongy mesophyll layer is an important part of a leaf. Explain how the features of the spongy mesophyll layer and the stomata work together to allow efficient gas exchange in a leaf.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants require carbon dioxide for photosynthesis and must be able to exchange gases efficiently across their internal surfaces.
Model answer (5 marks)
The spongy mesophyll contains loosely packed cells with large intercellular air spaces, providing a vast surface area for gas exchange. Gases dissolve in the thin film of moist cytoplasm on the cell walls before diffusing across the plasma membrane. Stomata, situated in the lower epidermis, are pores that open to the atmosphere, allowing CO₂ to enter and O₂ and water vapour to exit. Because CO₂ is continually consumed in photosynthesis, its concentration inside the leaf remains low, maintaining a concentration gradient that drives diffusion through the stomata and into the spongy mesophyll.
Examiner tips
- Mention the large surface area of air spaces, the moist cell walls, the role of stomata, and the concentration gradient.
- Use the exact terms: spongy mesophyll, air spaces, stomata, concentration gradient, diffusion.
- Show the sequence: stomata → CO₂ enters → dissolves in moist surface → diffuses across cell membrane.
- Keep answer concise and directly linked to each point in the mark scheme.
Common mistakes
- Confusing the role of the epidermis with the spongy mesophyll; forgetting that stomata are in the lower epidermis.
- Failing to mention the moist surface or the concentration gradient that drives diffusion.
- Using vague terms like ‘air spaces’ without explaining their function.
Mark scheme (5 marks)
- The spongy mesophyll has air spaces between its loosely packed cells
- The air spaces create a large surface area for gas exchange / diffusion
- Gases dissolve in the moist surface of the spongy mesophyll cells before diffusing across the cell membrane
- Stomata are pores in the lower epidermis that allow carbon dioxide to diffuse into the leaf and oxygen / water vapour to diffuse out
- A concentration gradient is maintained because carbon dioxide is used up in photosynthesis, keeping its concentration inside the leaf low
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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