The Calvin cycle (named after the American biochemist Melvin Calvin) is the second stage in photosynthesis and occurs in the stroma of the chloroplast. It is a cyclic series of reactions that assembles sugar molecules using CO2 and the energy-containing products of the light reactions. First, in carbon fixation, the enzyme rubisco attaches CO2 to RuBP. In the next step, a reduction, NADPH reduces 3-PGA to G3P (with the help of ATP). For every three CO2 molecules fixed, one G3P molecule leaves the cycle as product, and the remaining five G3P molecules are rearranged. Energy is used from ATP to regenerate three molecules of RuBP. The plant above uses the Calvin cycle to make its sugars.