The Calvin cycle

The light reaction gives ATP, NADPH and O₂. The oxygen leaves the chloroplast. The ATP and NADPH are used to make sugar from CO₂: this is the biosynthetic phase.

Melvin Calvin traced what happens to CO₂ by feeding algae carbon dioxide made with radioactive ¹⁴C. The first stable product was a 3-carbon acid, 3-phosphoglyceric acid (PGA). So this pathway is called the C3 pathway. In some plants, the first product is instead a 4-carbon acid, oxaloacetic acid (OAA): the C4 pathway.

What does CO₂ join with? Scientists first guessed a 2-carbon compound, since the product had 3 carbons. It turned out to be a 5-carbon sugar, ribulose bisphosphate (RuBP).

The Calvin cycle has three stages:

• Carboxylation: the enzyme RuBisCO fixes CO₂ onto RuBP, giving two molecules of 3-PGA.
• Reduction: using 2 ATP and 2 NADPH for each CO₂ fixed, PGA is turned into sugar.
• Regeneration: RuBP is remade so the cycle can continue, using 1 more ATP per CO₂.

So each CO₂ needs 3 ATP and 2 NADPH. Making one glucose (6 carbons) needs six turns of the cycle: 6 CO₂, 18 ATP and 12 NADPH. The Calvin cycle runs in all photosynthetic plants, C3 and C4 alike.