O O Carbon-fixation reactions are catalyzed by ribulose bisphosphate carboxylase (Rubisco). The carbon-fixation cycle could be divided into three stages: carbon fixation, sugar formation, and regeneration of ribulose 1,5-bisphosphate. 3-Phosphoglycerate is the acceptor for CO during the carbon fixation stage of the carbon-fixation cycle. For every three molecules of CO that enter the cycle, only one molecule of glyceraldehyde 3-phosphate is ultimately produced. Some of the O released to the atmosphere by photosynthesis in chloroplasts is used for oxidative phosphorylation in mitochondria