There are three phases of the Calvin cycle. In phase l(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. 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-sphate back to RuBP, the acceptor for Co2, thereby completing the cycle.