(1) A plant that utilizes the C4 carbon fixation pathway in which the CO2 is first bound to a phosphoenolpyruvate in mesophyll cell resulting in the formation of four-carbon compound (oxaloacetate) that is shuttled to the bundle sheath cell where it will be decarboxylated to liberate the CO2 to be utilized in the C3 pathway.
A C4 plant is better adapted than a C3 plant in an environment with high daytime temperatures, intense sunlight, drought, or nitrogen or CO2 limitation. Most C4 plants have a special leaf anatomy (called Kranz anatomy) in which the vascular bundles are surrounded by bundle sheath cells. Upon fixation of CO2into a 4-carbon compound in the mesophyll cells, this compound is transported to the bundle sheath cells in which it is decarboxylated and the CO2is re-fixed via the C3 pathway. The enzyme involved in this process is PEP carboxylase. In this mechanism, the tendency of rubisco (the first enzyme in the Calvin cycle) to photorespire, or waste energy by using oxygen to break down carbon compounds to CO2, is minimized.