Use of ATP and NADPH: the Calvin cycle (C₃ pathway)

Using radioactive ¹⁴C to trace carbon in algae, Melvin Calvin found that the first product of CO₂ fixation is a 3-carbon organic acid, 3-phosphoglyceric acid (PGA).

Later, a different group of plants was found whose first stable product is a 4-carbon acid, oxaloacetic acid (OAA). Plants of the first kind are called C₃ plants and the second kind C₄ plants.

In the Calvin cycle, CO₂ is first accepted by ribulose bisphosphate (RuBP), a ketose sugar with 5 carbons, and not by a 2-carbon molecule as was once expected.

The Calvin cycle occurs in all photosynthetic plants, whether C₃ or C₄. It runs in three stages: carboxylation, reduction and regeneration.

Carboxylation: CO₂ is fixed onto RuBP by RuBisCO (RuBP carboxylase-oxygenase), producing two molecules of 3-PGA. This is the most crucial step, and RuBisCO is described as the most abundant enzyme in the world.

Reduction: 3-PGA is converted to carbohydrate using 2 ATP (for phosphorylation) and 2 NADPH (for reduction) per CO₂ fixed.

Regeneration: RuBP is remade so the cycle can continue; this step needs one more ATP per CO₂ for phosphorylation.

So each CO₂ fixed costs 3 ATP and 2 NADPH. Making one glucose needs six turns of the cycle: 6 CO₂ in, 18 ATP and 12 NADPH used, 1 glucose out.