C4 pathway

C₄ carbon fixation or the Hatch-Slack pathway is a photosynthetic process in some plants. It is the first step in extracting carbon from carbon dioxide to be able to use it in sugar and other biomolecules. It is one of three known processes for carbon fixation. The C₄ in one of the names refers to the 4-carbon molecule that is the first product of this type of carbon fixation.

C₄ fixation is an elaboration of the more common C₃ carbon fixation and is believed to have evolved more recently. C₄ overcomes the tendency of the enzyme RuBisCO to wastefully fix oxygen rather than carbon dioxide in the process of photorespiration. This is achieved by ensuring that RuBisCo works in an environment where there is a lot of carbon dioxide and very little oxygen. CO₂ is shuttled via malate or aspartate from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells CO₂ is released by decarboxylation of the malate. C4 plants use PEP carboxylase to capture more CO₂ in the mesophyll cells. PEP Carboxylase (3 carbons) binds to CO₂ to make oxaloacetic acid (OAA). The OAA then makes malate (4 carbons). Malate enters bundle sheath cells and releases the CO₂. These additional steps, however, require more energy in the form of ATP. Using this extra energy, C₄ plants are able to more efficiently fix carbon in drought, high temperatures, and limitations of nitrogen or CO₂. Since the more common C₃ pathway does not require this extra energy, it is more efficient in the other conditions.

The naming Hatch-Slack pathway is in honor of Marshall Davidson Hatch and C. R. Slack, who elucidated it in Australia in 1966.

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