C4 carbon fixation pathway

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A metabolic pathway where CO2 is first added to phosphoenolpyruvate by the enzyme, PEP carboxylase, producing the four-carbon compound within mesophyll cells that is later transported to the bundle sheath cells where the CO2 is to be released for use in the Calvin cycle.


Instead of the direct carbon fixation in the Calvin cycle like in C3 carbon fixation, the C4 pathway involves steps that first converts pyruvate to phosphoenolpyruvate (PEP) to bind with the CO2 forming a four-carbon compound (hence the name C4). As a result, the photorespiration pathway is bypassed, and the wasteful loss of CO2 common in C3 carbon fixation pathway is minimized.

Plants that first go through the C4 pathway are better adapted than plants that solely go through the C3 pathway under conditions of drought, high temperatures and low nitrogen or CO2 concentrations.

Word origin: from the intermediate organic compound, which contains four carbon atoms, hence the name C4.

Synonym: hatch slack kortshak pathway, Hatch-Slack pathway.
Compare: C3 carbon fixation pathway, Crassulacean acid metabolism.
See also: C4 plant.

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