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

Photosynthesis as an Energy Transfer Process

Objective

Outline the three main stages of the Calvin cycle:

1. Carbon fixation – Rubisco catalyses the fixation of CO₂ by combination with ribulose‑1,5‑bisphosphate (RuBP), a 5‑C compound, to yield two molecules of glycerate‑3‑phosphate (GP), a 3‑C compound.
2. Reduction – GP is reduced to triose phosphate (TP) in reactions involving reduced NADPH and ATP.
3. Regeneration – RuBP is regenerated from TP in reactions that use ATP.

Stage 1 – Carbon Fixation

The enzyme ribulose‑1,5‑bisphosphate carboxylase/oxygenase (Rubisco) catalyses the reaction:

\$\text{RuBP (5C)} + \text{CO}_2 \;\xrightarrow{\text{Rubisco}}\; 2\,\text{GP (3C)}\$

Key points:

• RuBP is a five‑carbon sugar phosphate.
• Each CO₂ molecule yields two three‑carbon molecules of glycerate‑3‑phosphate (GP), also called 3‑phosphoglycerate (3‑PGA).
• This step consumes no ATP or NADPH.

Stage 2 – Reduction

Each GP molecule is phosphorylated by ATP and then reduced by NADPH to produce triose phosphate (TP, commonly glyceraldehyde‑3‑phosphate, G3P).

Overall reduction reaction (per CO₂ fixed):

\$\text{GP} + \text{ATP} + \text{NADPH} \;\longrightarrow\; \text{TP} + \text{ADP} + \text{P}_i + \text{NADP}^+\$

Stage 3 – Regeneration of RuBP

Five of the six TP molecules produced are used to regenerate three molecules of RuBP, allowing the cycle to continue. This regeneration requires ATP.

The net stoichiometry for three CO₂ molecules (the minimal turn of the cycle) is:

\$3\text{CO}2 + 9\text{ATP} + 6\text{NADPH} \;\longrightarrow\; G3P + 8\text{ATP} + 6\text{NADP}^+ + 6\text{P}i\$

Overall Summary of the Calvin Cycle