<P> There are several variants of this pathway: </P> <Ol> <Li> The 4 - carbon acid transported from mesophyll cells may be malate, as above, or aspartate . </Li> <Li> The 3 - carbon acid transported back from bundle - sheath cells may be pyruvate, as above, or alanine . </Li> <Li> The enzyme that catalyses decarboxylation in bundle - sheath cells differs . In maize and sugarcane, the enzyme is NADP - malic enzyme; in millet, it is NAD - malic enzyme; and, in Panicum maximum, it is PEP carboxykinase . </Li> </Ol> <Li> The 4 - carbon acid transported from mesophyll cells may be malate, as above, or aspartate . </Li> <Li> The 3 - carbon acid transported back from bundle - sheath cells may be pyruvate, as above, or alanine . </Li>

The c-4 photosynthesis pathway is the most common form of carbon fixation