The Calvin cycle acts like a sugar factory within a chloroplast; it is an essential part of photosynthesis. It synthesizes carbon from CO2, ATP energy, and NADPH energy to construct the three-carbon sugar G3P. This sugar can then be used to make glucose. First, rubisco attaches CO2 to RuBP. 3-PGA is then reduced to G3P. The energy released helps to regenerate RuBP (to make a cycle). However, the majority of the energy is stored for future use by the plant. All autotrophs (such as the one displayed above) that generate energy from photosynthesis possess cells that undergo the Calvin cycle.