Explain why the rate of photosynthesis in a C3 plant decreases during a period of water stress, even when light intensity and carbon dioxide concentration remain constant.

OCR A-Level Biology A (H420) — 5.6 Photosynthesis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

During hot, dry conditions, land plants close their stomata to reduce water loss by transpiration. Researchers observed that despite unchanged light intensity and atmospheric CO₂ levels, the net rate of photosynthesis fell significantly within 30 minutes of stomatal closure.

Model answer (4 marks)

Stomatal closure limits the diffusion of CO₂ into the leaf, so the internal CO₂ concentration falls.

The Calvin cycle depends on the availability of CO₂; a lower internal CO₂ reduces the rate of carbon fixation.

Because the reduction in CO₂ is rapid, the regeneration of RuBP (the electron‑transport‑dependent step) cannot keep pace, so RuBP is not regenerated quickly enough.

With less RuBP available, fewer triose phosphates are produced, leading to a decline in overall carbohydrate synthesis and thus a lower net rate of photosynthesis.

Examiner tips

  • State the sequence: stomatal closure → ↓ internal CO₂ → ↓ Calvin cycle → ↓ RuBP regeneration → ↓ triose phosphate → ↓ photosynthesis.
  • Use the exact terms: stomatal closure, internal CO₂, Calvin cycle, RuBP, triose phosphate.
  • Keep the answer concise and directly linked to the marks scheme points.

Common mistakes

  • Confusing stomatal closure with light intensity changes.
  • Mentioning photorespiration without linking it to CO₂ limitation.
  • Using vague terms like "less energy" instead of specific biochemical steps.

Mark scheme (4 marks)

  1. Stomatal closure reduces CO₂ entry into the leaf
  2. Lower internal CO₂ reduces the rate of the Calvin cycle / carbon fixation
  3. GP cannot be reduced at the same rate, so RuBP is not regenerated quickly enough
  4. Reduced carbon fixation means fewer / less triose phosphate produced, so overall carbohydrate synthesis decreases

Key terms in this question

C3 plant · water stress

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