Protein involved in the cycle of biochemical reactions responsible for photosynthetic CO(2) fixation in many photosynthetic bacteria and in the stroma of plant chloroplasts. The energy and reducing power for this reaction are provided by the ATP and NADPH produced during the light reactions of photosynthesis. The Calvin cycle is the only photosynthetic pathway in C3 plants. In C4 and CAM plants CO(2) is initially fixed into other organic acids that are subsequently decarboxylated to release CO(2) to the Calvin cycle. Non-photosynthetic organism (e.g. Rhizobium) also use the cycle to fix CO(2).