Calvinsyklens

Calvinsyklens, commonly known as the Calvin cycle, is a set of light-independent reactions in photosynthesis that build carbohydrates from carbon dioxide. Occurring in the stroma of chloroplasts in plants, algae, and some bacteria, it uses ATP and NADPH produced by the light reactions to convert CO2 into glyceraldehyde-3-phosphate (G3P).

The cycle comprises three main phases: carbon fixation, reduction, and regeneration of the CO2 acceptor ribulose-1,5-bisphosphate (RuBP). In carbon fixation, RuBisCO fixes CO2 to RuBP to form two molecules of 3-phosphoglycerate (3-PGA). In the reduction phase, 3-PGA is phosphorylated by ATP and reduced by NADPH to form G3P. Most of the G3P is recycled to regenerate RuBP, a process that consumes additional ATP. For every three CO2 molecules assimilated, the cycle produces one G3P that can exit the cycle to form glucose and other carbohydrates, while the remaining G3P is used for RuBP regeneration. The net energy cost to fix three CO2 is nine ATP and six NADPH.

RuBisCO is the key enzyme that catalyzes the initial carboxylation step, and the cycle operates continuously as long as light reactions supply ATP and NADPH. The Calvin cycle is the primary carbon fixation pathway in many plants. In C4 and CAM plants, spatial or temporal separation reduces photorespiration but still uses the same cycle for producing sugars.

Historically, the cycle was elucidated by Melvin Calvin and colleagues in the 1940s–1950s, earning the 1961 Nobel Prize in Chemistry.