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Light Independent Steps (Carbon Fixation)
The Calvin Cycle
So far (from the light reactions), energy from light has been stored in the chemical bonds of ATP and NADPH. In the Calvin cycle, this stored energy is used to produce sugar molecules. The Calvin cycle is a complex series of chemical reactions carried out in the stroma. The Calvin cycle begins with carbon fixation. Three molecules of carbon dioxide are added to three molecules of a 5-carbon sugar abbreviated RuBP. These molecules then split up to form six 3-carbon molecules. In the next two reactions, products from the light reactions are used to boost the energy of these three-carbon molecules. First, high-energy phosphate groups are added. One of the G3P molecules represents the three carbon dioxide molecules fixed so far. The other five G3Ps are reshuffled to regenerate the original RuBP molecules. Thus The Calvin cycle has used the energy of the light reactions to reduce three molecules of carbon dioxide and produce one molecule of G3P. Three more carbon dioxide molecules are fixed to form G3P in this same complicated way. To make each G3P, the Calvin Cycle consumes 9 ATP molecules and 6 NADPH molecules. These are regenerated in the light reactions. G3P is the actual final product of the Calvin cycle. The cell can combine two G3Ps to make glucose, which stores the energy that chlorophyll originally captured from the sun.
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