# Explain the role of the Calvin cycle in the fixation of carbon dioxide and the production of organic molecules.

> IB DP Biology Standard Level (2023 syllabus) — C1.3 Photosynthesis · Explain · 4 marks

## Mark scheme (4 marks)

1. Carbon dioxide combines with / is fixed by ribulose bisphosphate (RuBP), a 5-carbon acceptor molecule, catalysed by the enzyme rubisco.
2. This produces an unstable 6-carbon intermediate that immediately splits into two molecules of glycerate-3-phosphate (GP), a 3-carbon compound.
3. GP is reduced to glyceraldehyde-3-phosphate (G3P / triose phosphate) using ATP and NADPH from the light-dependent reactions.
4. 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

- [Calvin cycle](https://www.gradenine.co.uk/glossary/calvin-cycle)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-role-of-the-calvin-602e2920) · Published by Druglandscape Ltd.