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The Calvin cycle is a metabolic pathway found in the stroma of the chloroplast in which carbon enters in the form of CO2 and leaves in the form of sugar. The Calvin Cycle The cycle spends ATP as an energy source and consumes NADPH2 as reducing power for adding high energy electrons to make the sugar. There are three phases of the cycle. In phase 1 (Carbon Fixation), CO2 is incorporated into a five-carbon sugar named ribulose bisphosphate (RuBP). The enzyme which catalyzes this first step is RuBP carboxylase or rubisco. It is the most abundant protein in chloroplasts and probably the most abundant protein on Earth. The product of the reaction is a six-carbon intermediate which immediately splits in half to form two molecules of 3-phosphoglycerate. In phase 2 ( Reduction), ATP and NADPH2 from the light reactions are used to convert 3-phosphoglycerate to glyceraldehyde 3-phosphate, the three-carbon carbohydrate precursor to glucose and other sugars. In phase 3 (Regeneration), more ATP is used to convert some of the of the pool of glyceraldehyde 3-phosphate back to RuBP, the acceptor for CO2, thereby completing the cycle. For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3-phosphate (G3P). For each G3P synthesized, the cycle spends nine molecules of ATP and six molecules of NADPH2. The light reactions sustain the Calvin cycle by regenerating the ATP and NADPH2.