Explain how changes in carbon dioxide concentration affect the rate of the Calvin cycle in a C3 plant.

IB DP Biology Higher Level (2023 syllabus) — C1.3 Photosynthesis · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

CO₂ is fixed by RuBisCO, which combines CO₂ with RuBP to form 3‑phosphoglycerate (G3P). A higher CO₂ concentration increases the rate of RuBisCO carboxylation, so more G3P is produced per unit time and the Calvin cycle runs faster. When CO₂ is very low, RuBP cannot be efficiently converted to G3P; RuBP accumulates and the cycle slows, leaving excess ATP and NADPH from the light reactions unused and potentially limiting the light reactions. Above a saturation point the rate no longer rises because RuBisCO (or the availability of RuBP, ATP or NADPH) becomes limiting.

Examiner tips

  • Use the term RuBisCO and G3P; link CO₂ to carboxylation rate; mention RuBP accumulation at low CO₂; note saturation and limiting factors

Common mistakes

  • Confusing RuBisCO with Rubisco; forgetting RuBP accumulation; not mentioning the saturation point or limiting factors

Mark scheme (4 marks)

  1. CO₂ is fixed by RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) combining CO₂ with RuBP to form glycerate-3-phosphate (G3P/3-PGA)
  2. Higher CO₂ concentration increases the rate of carboxylation / RuBisCO activity, so more G3P is produced per unit time
  3. At very low CO₂, RuBP accumulates / cannot be regenerated into G3P efficiently, so the cycle slows; NADPH and ATP from the light reactions are not consumed and may limit/feedback on light reactions
  4. Above a CO₂ saturation point, the rate no longer increases because RuBisCO / another factor (e.g., available RuBP, ATP, NADPH) becomes limiting

Key terms in this question

Calvin cycle

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