The starting point of the Calvin cycle is carbon fixation. Rubisco catalyzes the reaction between RuBP, a 5-carbon sugar, and CO₂. The resulting 6-carbon compound is unstable and splits into 3-carbon sugars. This is why the reactions of the Calvin cycle are termed C₃ photosynthesis. ATP and NADPH provide the energy to form PGAL, the carbohydrate product of the Calvin cycle, and to rearrange the remaining molecules to form RuBP that recycles back into carbon fixation.