There are three main phases of the Calvin cycle: carbon fixation, reduction, and regeneration. During carbon fixation, a CO2 molecule combines with a RuBP molecule. This forms 3-PGA, which is catalyzed by rubisco. Reduction is the second stage in which ATP and NADPH are used to convert the 3-PGA into G3P. This is because the NADPH donates electrons to the 3-PGA to reduce it to G3P. Finally, during regeneration, some of the G3P molecules are used to make glucose, while the others are recycled to regenerate the RuBP acceptor. This process requires ATP and involves a complex network of reactions. This allows the cycle to start over again and continue on.