Calvin cycle

The Calvin cycle — also called the Calvin-Benson-Bassham cycle — is the metabolic pathway that fixes atmospheric CO₂ into organic carbon compounds during photosynthesis. It operates in the stroma of chloroplasts and is powered by ATP and NADPH generated by the light reactions. The cycle's net output is glyceraldehyde-3-phosphate, a three-carbon sugar that feeds into starch synthesis, cellulose production, and the broader metabolic economy of the plant.

The cycle is regulated at multiple points. The enzyme RuBisCO catalyzes the first committed step — the carboxylation of ribulose-1,5-bisphosphate — but its activity is modulated by light intensity, stromal pH, and magnesium concentration. This makes the Calvin cycle a metabolic control system rather than a passive conveyor belt: it adjusts its throughput in real time to match the supply of reducing power from the thylakoids. When the light reactions outpace carbon fixation, the cycle slows to prevent the accumulation of reactive intermediates that could damage the chloroplast.