The Calvin Cycle

The Calvin cycle regenerates its starting material after molecules enter and leave the cycle. The Calvin cycle is anabolic, using energy to build sugar from smaller molecules. Carbon enters the cycle as CO2 and leaves as sugar. The cycle spends the energy of ATP and the reducing power of electrons carried by NADPH to make sugar. The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).To make one glucose molecule requires six cycles and the fixation of six CO2 molecules.

The Calvin cycle has three phases. In the carbon fixation phase, each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP). This is catalyzed by RuBP carboxylase or rubisco. The six-carbon intermediate is unstable and splits in half to form two molecules of 3-phosphoglycerate for each CO2.During reduction, each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3-bisphosphoglycerate.A pair of electrons from NADPH reduces each 1,3-bisphosphoglycerate to G3P. After fixation and reduction, we would have six molecules of G3P (18C).The other five G3P (15C) remain in the cycle to regenerate three RuBP. In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle to regenerate three molecules of RuBP.

-Biology 8th edition textbook, Solomon Berg Martin