For every three turns of the Calvin cycle, three atoms of carbon are fixed from three molecules of carbon dioxide.
Explanation:
In the carbon fixation reactions that occur in the stroma, NADPH and ATP, produced in the energy capture reactions, are used to reduce a three-carbon compound, glyceraldehyde phosphate. This route in which carbon is fixed by means of glyceraldehyde phosphate is called the three-carbon route or C3. In this case, carbon fixation is carried out through the Calvin cycle, in which the ribulose bisphosphate (RuBP) carboxylase enzyme combines a carbon dioxide molecule with the starting material, a five-carbon sugar called ribulose bisphosphate.
In each complete cycle, enter a molecule of carbon dioxide. The number required to make two glyceraldehyde phosphate molecules, equivalent to a six-carbon sugar, is six turns. Six molecules of RuBP, a compound of five carbons, are combined with six molecules of carbon dioxide, producing six molecules of an unstable intermediate that is soon cleaved into twelve molecules of phosphoglycerate, a compound of three carbons. The latter are reduced to twelve molecules of glyceraldehyde phosphate. Ten of these three-carbon molecules combine and regenerate to form six five-carbon RuBP molecules. The two "extra" molecules of glyceraldehyde phosphate represent the net gain of the Calvin cycle. These molecules are the starting point of numerous reactions that can involve, for example, the synthesis of carbohydrates, amino acids and fatty acids.
The energy that drives the Calvin cycle is the ATP and NADPH produced by the energy capture reactions in the first stage of photosynthesis.
Gliceraldehyde phosphate can also be used as a starting material for other organic compounds necessary for the cell. Other plants that live in dry and warm environments have mechanisms that allow them to initially fix CO2 in one of two ways, and thus minimize water loss. These pathways are known as the four-carbon pathway, or C4 and the CAM plant pathway, and precede the Calvin cycle.
