The Calvin Cycle uses products of the light reaction for energy (ATP and NADPH) and a carbon source (CO2 from the atmosphere), to manufacture high-energy sugar molecules.

The end product is G3P (glyceraldehyde-3-phosphate), a 3-carbon sugar, which is the monomer used to build more complex sugars and starches.

The cyclic process regenerates RuBP (ribulose biphosphate), which is the “pickup molecule” for the next turn of the cycle.

The enzyme rubisco catalyzes the bonding of carbon dioxide to RuBP. This process is known as carbon fixation, the binding of CO2 gas to a solid molecule. The new molecule quickly splits into two 3-carbon molecules of 3-PGA (3-phosphoglyceric acid).

Note that 3 molecules of CO2 enter the cycle, creating 6 molecules of 3-PGA.

ATP energy fuels the next REDOX reactions. NADPH is oxidized (loses electrons) while 3-PGA is reduced (gains electrons) to become the energy-rich sugar molecule, G3P.

Note that 6 ATPs are used, 6 NADPHs are oxidized, and 6 molecules of G3P are formed.

Notice that only one of the six G3P molecules is released from the cycle. This is the only product from one turn of the Calvin Cycle.

The G3P molecule that is released is used by plants to build glucose and other compounds.

The five remaining G3P molecules are used to regenerate three molecules of RuBP. These 3 RuBP molecules pick up three CO2 molecules, and start the Calvin Cycle again.