C4 carbon fixation

For other meanings of C4, see C4 (disambiguation)

C₄ carbon fixation is a common metabolic pathway found in land plants (C4 plants). They are competitively superior to plants possessing the more common C3 pathway under the conditions of drought, high temperatures and nitrogen limitation. The C₄ cycle allows for a spacial separation of carbon fixation from respiration, thus allowing C₄ plants to increase concentration of CO₂ within their leaves. This increases the amount of photosynthesis and decreases the chances of photorespiration, a harmful process in which organic material and energy is lost from the plant due to high concentrations of oxygen. It is called "C₄" because the product, oxaloacetate, contains four carbon atoms. It occurs in the mesophyll of the leaf, specifically in the mesophyll cells and the bundle sheath cells. 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.