Carbon dioxide is a substrate of the Calvin cycle. The enzyme RuBisCO combines carbon dioxide with ribulose bisphosphate (RuBP), a 5-carbon compound, to form two molecules of glycerate 3-phosphate (GP). This is carbon fixation.The concentration of carbon dioxide in the atmosphere is only about 0.04%, so on bright, warm days it is often the limiting factor. As carbon dioxide concentration increases, the rate of photosynthesis increases, because RuBisCO fixes carbon dioxide faster. The rate levels off at around 0.1% as another factor, such as light intensity or temperature, becomes limiting. Very high concentrations cause stomata to close, which reduces the rate.
Effect on the Calvin cycle. If carbon dioxide concentration falls, less RuBP is converted into GP:• RuBP accumulates, because it is still regenerated from TP but is used up more slowly.
• GP falls, because less is made, while it is still being reduced to TP.
• TP falls, because less GP is available to be reduced.Note that low light has the opposite effect on GP and RuBP: low light makes GP rise and RuBP fall. Remembering which step each factor affects lets you work out the changes: carbon dioxide is used in the step RuBP → GP, whereas ATP and reduced NADP from the light-dependent stage are used in GP → TP and in regenerating RuBP.
Effect on the Calvin cycle. If carbon dioxide concentration falls, less RuBP is converted into GP:• RuBP accumulates, because it is still regenerated from TP but is used up more slowly.• GP falls, because less is made, while it is still being reduced to TP.
• TP falls, because less GP is available to be reduced.Note that low light has the opposite effect on GP and RuBP: low light makes GP rise and RuBP fall. Remembering which step each factor affects lets you work out the changes: carbon dioxide is used in the step RuBP → GP, whereas ATP and reduced NADP from the light-dependent stage are used in GP → TP and in regenerating RuBP.