Photosynthesis of C4 plants.

CO2 is bound to phosphoenolpyruvate (PEP) in mesophyll cells. The product is oxaloacetate. The next step generates malate. In the cells of the vascular bundle sheath, the 'Kranz' cells, is carbon dioxide split off the malate and fed into the CALVIN cycle. The pyruvate is transported back into the mesophyll cells (active transport) and is with the help of additional ATP phosphorylated to PEP


The Crassulacean Acid Metabolism (CAM)

CAM is the abbreviation of Crassulacean acid metabolism. The name points at the fact that this pathway occurs mainly in Crassulacean species (and other succulent plants). The chemical reaction of the carbon dioxide accumulation is similar to that of C4 plants but here are carbon dioxide fixation and its assimilation not separated spatially but in time. CAM plants occur mainly in arid regions. The opening of the stomata to take up carbon dioxide is always connected with large losses of water. To inhibit this loss during intense sun (the transpiration via the cuticle remains intact) has a mechanism developed that allows the uptake of carbon dioxide during the night. The prefixed carbon dioxide is stored in the vacuoles as malate (and isocitrate) and is used during the daytime for photosynthesis.

Influence of different parameters on the efficiency of the carbon dioxide uptake (ordinate) of a C3 plant (Atriplex patula, yellow line) and a C4 plant (Atriplex rosea, green line). Measured parameters (from left to right): light intensity, leaf temperature and concentration of carbon dioxide within the intercellular space.