In the carbon fixation cycle, CO2 is added by the enzyme, RuBisCO, to a 5- carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. This is then converted to sugar by the Calvin cycle, and the RuBP is regenerated. During this process, ATP and NADPH synthesised in the light reaction are utilised. RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants: photorespiration.
Some tropical plants show a special type of photosynthesis called C4 pathway. In these plants the first product of CO2 fixation that takes place in the mesophyll, is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carried out for the synthesis of carbohydrates.