Explain the role of the Calvin cycle in the fixation of carbon dioxide and the production of organic molecules.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The Calvin cycle fixes CO₂ by combining it with the 5‑carbon acceptor ribulose‑bisphosphate (RuBP) in a reaction catalysed by rubisco, forming an unstable 6‑carbon intermediate that immediately splits into two 3‑carbon molecules of glycerate‑3‑phosphate (G3P). G3P is then reduced to glyceraldehyde‑3‑phosphate using ATP and NADPH produced in the light‑dependent reactions. Some of the G3P is used to synthesise organic molecules such as glucose, carbohydrates, lipids and amino acids, while the rest is used to regenerate RuBP so the cycle can continue.
Examiner tips
- Use the exact terms ‘rubisco’, ‘RuBP’, ‘G3P’, ‘ATP’, ‘NADPH’.
- Show the sequence: CO₂ + RuBP → 6‑C intermediate → 2 G3P → reduction → synthesis/regeneration.
- Mention both the production of organic molecules and the regeneration of RuBP.
- Keep answer to 4–5 short sentences.
Common mistakes
- Confusing G3P with GP or glycerate‑3‑phosphate; using the wrong abbreviation.
- Omitting the role of rubisco or the regeneration of RuBP.
- Failing to mention ATP and NADPH as energy sources.
Mark scheme (4 marks)
- Carbon dioxide combines with / is fixed by ribulose bisphosphate (RuBP), a 5-carbon acceptor molecule, catalysed by the enzyme rubisco.
- This produces an unstable 6-carbon intermediate that immediately splits into two molecules of glycerate-3-phosphate (GP), a 3-carbon compound.
- GP is reduced to glyceraldehyde-3-phosphate (G3P / triose phosphate) using ATP and NADPH from the light-dependent reactions.
- Some G3P is used to synthesise organic molecules such as glucose / carbohydrates / lipids / amino acids, while the remainder is used to regenerate RuBP so the cycle can continue.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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