Explain how the concentration of carbon dioxide affects the rate of photosynthesis in a C3 plant.

IB DP Biology Standard 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).

C3 plants fix carbon dioxide via the Calvin cycle using the enzyme RuBisCO. At current atmospheric CO₂ concentrations, RuBisCO operates below its maximum reaction rate.

Model answer (4 marks)

CO₂ is fixed by RuBisCO, which combines it with ribulose‑1,5‑bisphosphate (RuBP) to produce two molecules of 3‑phosphoglycerate (3‑PGA) that are then converted to glycerate‑3‑phosphate (G3P) in the Calvin cycle.

When CO₂ concentration is increased, more substrate is available for RuBisCO, so the rate of carbon fixation rises and the overall rate of the Calvin cycle increases.

At low or limiting CO₂ levels, the availability of CO₂ is the limiting factor; therefore, increasing CO₂ concentration directly increases the rate of photosynthesis.

Once CO₂ reaches a saturating concentration, another factor (e.g. light intensity, RuBisCO enzyme concentration or ATP/NADPH supply) limits the cycle, so the rate plateaus and further increases in CO₂ give no additional rise in photosynthetic rate.

Examiner tips

  • Use the term RuBisCO and RuBP; link CO₂ fixation to G3P production.
  • Show the two regimes: CO₂‑limited and CO₂‑saturated.
  • Mention that the rate plateaus when another factor limits the cycle.
  • Keep the answer concise – 4 marks, one point each.

Common mistakes

  • Confusing RuBisCO with other enzymes; forgetting RuBP as the acceptor.
  • Saying CO₂ always increases the rate without recognising saturation.
  • Using vague terms like ‘more CO₂’ without linking to the Calvin cycle or limiting factors.

Mark scheme (4 marks)

  1. CO₂ is fixed by RuBisCO / combined with RuBP to form glycerate-3-phosphate (G3P) in the Calvin cycle
  2. Increasing CO₂ concentration increases the rate of carbon fixation / increases the rate of the Calvin cycle
  3. At low/limiting CO₂ concentrations, CO₂ availability is the limiting factor, so increasing CO₂ increases the rate of photosynthesis
  4. Above a saturating CO₂ concentration, another factor (e.g. light intensity or RuBisCO enzyme concentration) becomes limiting, so the rate plateaus / no further increase occurs

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