C4 plants chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule phosphoenolpyruvate, a reaction catalyzed by an enzyme called PEP carboxylase and which creates the four-carbon organic acid, oxaloacetic acid. Oxaloacetic acid or malate synthesized by this process is then translocated to specialized bundle sheath cells where the enzyme, rubisco and other Calvin cycle enzymes are located and where CO2 released by decarboxylation of the fourcarbon acids is fixed by rubisco activity to the three-carbon sugar 3 – phosphoglyceric acids.