Leaves are adapted to allow efficient gaseous exchange for photosynthesis. Explain how the internal structure of a leaf is adapted to allow carbon dioxide to reach the palisade mesophyll cells as efficiently as possible.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Leaves must obtain carbon dioxide from the air and deliver it to the cells that carry out photosynthesis. Several structural features of a leaf work together to make this process as efficient as possible.
Model answer (5 marks)
Stomata in the lower epidermis are pores that allow CO₂ to enter the leaf by diffusion.
The spongy mesophyll contains large intercellular air spaces that give CO₂ a rapid pathway through the leaf.
A steep concentration gradient is maintained because CO₂ is continuously consumed in the palisade cells during photosynthesis.
The palisade mesophyll lies just above the spongy layer, close to the light‑rich upper surface, and its cells are packed with chloroplasts.
Because the distance from the stomata to the palisade cells is short, CO₂ reaches the photosynthetic cells quickly.
The spongy mesophyll contains large intercellular air spaces that give CO₂ a rapid pathway through the leaf.
A steep concentration gradient is maintained because CO₂ is continuously consumed in the palisade cells during photosynthesis.
The palisade mesophyll lies just above the spongy layer, close to the light‑rich upper surface, and its cells are packed with chloroplasts.
Because the distance from the stomata to the palisade cells is short, CO₂ reaches the photosynthetic cells quickly.
Examiner tips
- Mention stomata, spongy mesophyll air spaces, concentration gradient, palisade location, short diffusion distance
- Use correct terminology: stomata, mesophyll, palisade, spongy, diffusion
- Show logical sequence of adaptations
Common mistakes
- Confusing upper epidermis for stomata location
- Ignoring the role of the spongy mesophyll air spaces
- Failing to note the short diffusion distance
Mark scheme (5 marks)
- Stomata are pores in the lower epidermis that allow carbon dioxide to diffuse into the leaf
- The spongy mesophyll has air spaces which allow carbon dioxide to diffuse quickly through the leaf
- A large concentration gradient is maintained because carbon dioxide is continuously used up in photosynthesis in the palisade cells
- The palisade mesophyll cells are near the top of the leaf, close to where light enters, and contain chloroplasts for photosynthesis
- The short diffusion distance from the stomata to the palisade cells means carbon dioxide reaches them quickly
Key terms in this question
Related
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