13.1.11—Calvin cycle three main stages

The Calvin cycle takes place in the chloroplast stroma and uses carbon dioxide, ATP and reduced NADP to produce triose phosphate while regenerating RuBP. ATP and reduced NADP come from the light-dependent stage, linking light energy capture to carbon fixation.

• Fixation: rubisco catalyses the combination of carbon dioxide with the five-carbon acceptor RuBP. The unstable product separates to form glycerate 3-phosphate (GP), so inorganic carbon becomes part of an organic compound.
• Reduction: ATP supplies energy and reduced NADP supplies hydrogen/reducing power to convert GP into triose phosphate (TP), a three-carbon product of the cycle.
• Use of TP: some TP leaves the cycle to contribute to useful organic molecules, including carbohydrate products such as starch, sucrose or cellulose. The exact allocation is not the focus of this high-level chain.
• Regeneration: the remaining TP is rearranged using ATP to regenerate RuBP, allowing the cycle to accept more carbon dioxide.
• Stage connection: the Calvin cycle is light-independent in its immediate reactions, but it depends on ATP and reduced NADP supplied by the light-dependent stage. It therefore cannot continue indefinitely without those inputs.

Do not treat rubisco as producing carbohydrate in one step, omit RuBP regeneration, or describe “light-independent” as independent of the light-dependent stage. Detailed limiting-factor investigations and exact molecule counts belong outside this high-level card.