(Benson- Calvin- Bassham cycle) The sequence of reactions, making up the dark or light-independent reactions of photosynthesis, in which carbon dioxide is reduced to carbohydrate using ATP and NADPH derived from the light-dependent reactions (see photo-phosphorylation). The Calvin cycle takes place in the chloroplast stroma and begins with the carboxylation and cleavage of ribulose bisphosphate (RUBP) to form two molecules of phosphoglyceric acid. The RUBP is then regenerated by a complex series of reactions involving 3-, 4-, 5-, 6-, and 7-carbon sugar phosphates. During this process glucose is formed by reversal of the glycolytic reactions (see diagram). The overall series of reactions can be written as: 6 ribulose 1,5-bisphosphate + 6C02
+ 18 ATP + 12NADPH + 12H+
0 → 6 ribulose 1,5-bisphosphate + glucose + 18Pi
+ 18 ADP + 12NADP+
. The glucose is subsequently converted to starch, cellulose, and other polysaccharides.
Calvin and his associates worked out this cycle of reactions by illuminating green algae in the presence of radioactive carbon dioxide for a couple of seconds and then immersing the cells in boiling water to prevent further reaction. They then found which metabolites first became radioactively labelled using chromatography. See also Hatch-Slack pathway
The Calvin cycle.