Explain how the structure of a leaf is adapted to maximise the rate of photosynthesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. The upper epidermis is transparent, allowing light to pass through to the palisade mesophyll.
2. The palisade mesophyll layer lies just below the upper epidermis and contains the greatest concentration of chloroplasts, so it absorbs the most light for photosynthesis.
3. The spongy mesophyll layer is loosely packed with air pockets, increasing the surface area for gas exchange.
4. The increased surface area allows carbon dioxide to reach the photosynthetic cells more efficiently.
5. Stomata, controlled by guard cells, open to let carbon dioxide enter the leaf for photosynthesis.
2. The palisade mesophyll layer lies just below the upper epidermis and contains the greatest concentration of chloroplasts, so it absorbs the most light for photosynthesis.
3. The spongy mesophyll layer is loosely packed with air pockets, increasing the surface area for gas exchange.
4. The increased surface area allows carbon dioxide to reach the photosynthetic cells more efficiently.
5. Stomata, controlled by guard cells, open to let carbon dioxide enter the leaf for photosynthesis.
Examiner tips
- Use the exact terms: palisade mesophyll, spongy mesophyll, stomata, guard cells. Explain the function of each structure. Show the logical sequence from light entry to CO₂ uptake.
Common mistakes
- Confusing the location of the palisade layer (placing it below the lower epidermis). Forgetting to mention the role of stomata or guard cells. Using vague phrases like "more light" without linking to chloroplast concentration.
Mark scheme (5 marks)
- The palisade mesophyll layer is located near the top of the leaf / below the upper epidermis
- The palisade mesophyll layer contains the greatest concentration of chloroplasts, so it absorbs the most light for photosynthesis
- The upper epidermis is transparent, allowing light to pass through to the palisade mesophyll tissue
- The spongy mesophyll layer has air pockets / is loosely packed, which increases the surface area for gas exchange, allowing carbon dioxide to reach photosynthetic cells
- Stomata (controlled by guard cells) open to allow carbon dioxide to enter the leaf for photosynthesis
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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