For other meanings of C4, see C4 (disambiguation)

C₄ carbon fixation is a common metabolic pathway found in land [[plant](C4 plants). They are competitively superior to plants possessing the more common C3 pathway under the conditions of drought, high temperatures and nitrogen limitation. It is called "C₄" because the product, oxaloacetate, contains four carbon atoms. It occurs in the mesophyll of the leaf. The chemical equation is:

PEP carboxylase + PEP + CO₂ → oxaloacetate

The product is usually converted to malate, a simple organic compound that gives up its CO₂ to the Calvin cycle after being shipped off to bundle sheath cells surrounding a nearby vein. After losing the CO₂, it becomes pyruvate, and can be phosphorylated into PEP at the cost of a phosphorus group and one ATP. It can then be reused in the above equation.