Explain why carbon dioxide produced during respiration in the leaf's palisade mesophyll cells can be used directly for photosynthesis during daylight, without the gas needing to leave the leaf.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During the day, a flowering plant carries out both photosynthesis and respiration simultaneously. The palisade mesophyll and spongy mesophyll are the two main photosynthetic tissues of the leaf.
Model answer (4 marks)
1. Respiration in the palisade mesophyll cells produces CO₂ as a waste product.
2. The CO₂ can diffuse directly from these cells to the chloroplasts within the same leaf, because the diffusion distance is very short.
3. Photosynthesis in the chloroplasts consumes CO₂, keeping its concentration lower than in the respiring cells and maintaining a concentration gradient that drives the diffusion.
4. Thus, less CO₂ has to leave the leaf through the stomata, improving the plant’s efficiency during daylight.
2. The CO₂ can diffuse directly from these cells to the chloroplasts within the same leaf, because the diffusion distance is very short.
3. Photosynthesis in the chloroplasts consumes CO₂, keeping its concentration lower than in the respiring cells and maintaining a concentration gradient that drives the diffusion.
4. Thus, less CO₂ has to leave the leaf through the stomata, improving the plant’s efficiency during daylight.
Examiner tips
- Use the word ‘diffuse’ and mention the short distance; link respiration to photosynthesis; explain the concentration gradient; note the efficiency benefit.
Common mistakes
- Claiming CO₂ must leave the leaf before photosynthesis; confusing stomatal conductance with internal diffusion; omitting the role of the concentration gradient.
Mark scheme (4 marks)
- Respiration produces carbon dioxide (as a waste gas) inside the palisade mesophyll cells / leaf cells
- Carbon dioxide can diffuse directly from the respiring cells to the chloroplasts / site of photosynthesis within the same leaf / short diffusion distance
- Photosynthesis uses / consumes carbon dioxide, maintaining a concentration gradient so carbon dioxide moves from the respiring cells towards the chloroplasts
- This means the rate of carbon dioxide loss through stomata is reduced / less carbon dioxide needs to be taken in from the atmosphere / the plant is more efficient
Key terms in this question
palisade mesophyll · photosynthesis
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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