C₄ Carbon Fixation

C₄ Carbon Fixation

C₄ carbon fixation is one of three biochemical mechanisms, along with C₃ and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon molecule present in the first product of carbon fixation in the small subset of plants known as C₄ plants, in contrast to the 3-carbon molecule products in C₃ plants.

C₄ fixation is an elaboration of the more common C₃ carbon fixation and is believed to have evolved more recently. C₄ and CAM overcome the tendency of the enzyme RuBisCO to wastefully fix oxygen rather than carbon dioxide in what is called photorespiration. This is achieved by using a more efficient enzyme to fix CO₂ in mesophyll cells and shuttling this fixed carbon via malate or asparate to bundle-sheath cells. In these bundle-sheath cells, RuBisCO is isolated from atmospheric oxygen and saturated with the CO₂ released by decarboxylation of the malate or oxaloacetate. These additional steps, however, require more energy in the form of ATP. Because of this extra energy requirement, C₄ plants are able to more efficiently fix carbon in only certain conditions, with the more common C₃ pathway being more efficient in other conditions.