outline the three main stages of the Calvin cycle: rubisco catalyses the fixation of carbon dioxide by combination with a molecule of ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of glycerate 3-phosphate (GP), a 3C compound, GP

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Photosynthesis: Calvin Cycle Stages

Photosynthesis as an Energy‑Transfer Process

The Calvin cycle (also called the C₃ cycle) operates in the stroma of chloroplasts and uses the ATP and NADPH produced in the light‑dependent reactions to convert atmospheric CO₂ into carbohydrate.

Three Main Stages of the Calvin Cycle

  1. Carbon Fixation

    Ribulose‑1,5‑bisphosphate (RuBP), a five‑carbon (\$5C\$) molecule, combines with CO₂ in a reaction catalysed by the enzyme ribulose‑bisphosphate carboxylase/oxygenase (rubisco). The result is an unstable six‑carbon intermediate that immediately splits into two molecules of 3‑phosphoglycerate (glycerate‑3‑phosphate, GP), each containing three carbons (\$3C\$).

  2. Reduction

    Each GP molecule is phosphorylated by ATP to form 1,3‑bisphosphoglycerate and then reduced by NADPH (the reduced form of NADP⁺) to produce triose phosphate (TP, also called glyceraldehyde‑3‑phosphate). This step stores the energy from ATP and the reducing power of NADPH in the carbon skeleton of TP.

  3. Regeneration of RuBP

    For the cycle to continue, five of the six TP molecules generated are rearranged through a series of reactions that consume additional ATP, ultimately reforming three molecules of RuBP. The remaining TP can be exported from the chloroplast for biosynthesis of glucose and other carbohydrates.

Summary Table of Each Stage

StageKey Enzyme(s)Primary Substrate(s)ProductsEnergy Input
Carbon FixationRubiscoCO₂ + RuBP (\$5C\$)2 × GP (\$3C\$ each)None (uses only substrate binding)
ReductionPhosphoglycerate kinase, Glyceraldehyde‑3‑phosphate dehydrogenaseGP + ATP + NADPH2 × TP2 ATP, 2 NADPH per CO₂ fixed
Regeneration of RuBPVarious trans‑ketolase and aldolase reactions5 × TP + ATP3 × RuBP3 ATP per CO₂ fixed

Suggested diagram: Flowchart of the Calvin cycle showing carbon fixation, reduction, and RuBP regeneration, with ATP and NADPH inputs highlighted.

Key Points to Remember

  • Rubisco is the most abundant enzyme on Earth and catalyses the first, irreversible step of carbon fixation.

  • ATP provides the phosphate groups needed for activation of carbon intermediates, while NADPH supplies the electrons for reduction.

  • The cycle must turn three times to fix three molecules of CO₂, producing one net molecule of glyceraldehyde‑3‑phosphate that can be used for glucose synthesis.

  • Regeneration of RuBP is essential; without it the cycle would halt despite the availability of ATP and NADPH.