Calvin cycle (photosynthetic carbon reduction cycle) The metabolic pathway of the light-independent stage of photosynthesis, which occurs in the stroma of the chloroplasts. The pathway was elucidated by Melvin Calvin and his coworkers and involves the fixation of carbon dioxide and its subsequent reduction to carbohydrate. During the cycle, carbon dioxide combines with ribulose bisphosphate, through the mediation of the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco), to form an unstable six-carbon compound that breaks down to form two molecules of the three-carbon compound glycerate 3-phosphate. This is converted to glyceraldehyde 3-phosphate, which is used to regenerate ribulose bisphosphate and to produce glucose and fructose. The Calvin cycle depends on ATP supplied by the light-dependent reactions, and it generally ceases in the dark.

Calvin cycle A cyclic series of reactions occurring in the stroma of chloroplasts, in which carbon dioxide is fixed and reduced to glucose, using ATP and NADPH formed in the light reaction of photosynthesis. The relatively stable product formed is a 3-carbon sugar.