Describe how the structure of a leaf is adapted to allow carbon dioxide to reach the chloroplasts in the palisade mesophyll cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Stomata in the lower epidermis allow CO₂ to enter the leaf by diffusion.
Air spaces in the spongy mesophyll provide a pathway for CO₂ to diffuse towards the palisade mesophyll.
The large surface area of the leaf and the spongy mesophyll cells increases the rate of CO₂ diffusion.
Chloroplasts, the site of photosynthesis, are abundant in the palisade mesophyll cells, so CO₂ is used there, maintaining a concentration gradient for continued diffusion.
Air spaces in the spongy mesophyll provide a pathway for CO₂ to diffuse towards the palisade mesophyll.
The large surface area of the leaf and the spongy mesophyll cells increases the rate of CO₂ diffusion.
Chloroplasts, the site of photosynthesis, are abundant in the palisade mesophyll cells, so CO₂ is used there, maintaining a concentration gradient for continued diffusion.
Examiner tips
- Use the word "diffusion" when describing CO₂ movement. Mention stomata are in the lower epidermis. Show the link between large surface area and diffusion rate. State chloroplasts are in palisade cells and are the site of photosynthesis.
Common mistakes
- Forgetting to mention stomata are in the lower epidermis. Confusing spongy mesophyll with palisade mesophyll. Not linking chloroplasts to photosynthesis or to CO₂ use.
Mark scheme (4 marks)
- Stomata (in the lower epidermis) allow carbon dioxide to enter the leaf by diffusion
- Air spaces in the spongy mesophyll allow carbon dioxide to diffuse towards the palisade mesophyll
- The large surface area of the leaf / spongy mesophyll cells increases the rate of diffusion of carbon dioxide
- Chloroplasts are the site of photosynthesis and are present in the palisade mesophyll cells, so carbon dioxide is used there, maintaining a concentration gradient for continued diffusion
Key terms in this question
palisade mesophyll · chloroplast
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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