Calvin Cycle

The Calvin cycle, also called the Calvin–Benson–Bassham cycle, is the series of reactions in the stroma of the chloroplast that fixes carbon dioxide into sugar. It uses the ATP and NADPH made by the light reactions of photosynthesis.

In carboxylation, the enzyme rubisco attaches CO₂ to the five-carbon sugar ribulose-1,5-bisphosphate, giving two molecules of 3-phosphoglycerate. In reduction, ATP and NADPH convert them to glyceraldehyde-3-phosphate (G3P). In regeneration, most G3P is rearranged, using more ATP, back into ribulose bisphosphate so the cycle can continue.

Three turns fix three CO₂ molecules and yield one net G3P, spending nine ATP and six NADPH. G3P can be made into glucose, starch or sucrose. The cycle is switched on in the light, because light changes the redox state and pH of the stroma and activates several of its enzymes.

Melvin Calvin, Andrew Benson and James Bassham traced the path of radioactive ¹⁴C through algae in the 1950s, and Calvin received the Nobel Prize in Chemistry in 1961. Rubisco can also react with oxygen, which wastes energy in a process called photorespiration, and the efficiency of the whole cycle matters for plants and for the carbon cycle.

• Rubisco — the enzyme that fixes the carbon
• Photosynthesis — the process it completes
• Photorespiration — the wasteful side reaction