The C4 pathway: an efficient CO2 pump

The C4 pathway is a complex combination of both biochemical and morphological specialisation, which provides an elevation of the CO2 concentration at the site of Rubisco. We review the key parameters necessary to make the C4 pathway function efficiently, focussing on the diffusion of CO2 out of the bundle sheath compartment. Measurements of cell wall thickness show that the thickness of bundle sheath cell walls in C4 species is similar to cell wall thickness of C3 mesophyll cells. Furthermore, NAD-ME type C4 species, which do not have suberin in their bundle sheath cell walls, do not appear to compensate for this with thicker bundle sheath cell walls. Uncertainties in the CO2 diffusion properties of membranes, such as the plasmalemma, choroplast and mitochondrial membranes make it difficult to estimate bundle sheath diffusion resistance from anatomical measurements, but the cytosol itself may account for more than half of the final calculated resistance value for CO2 leakage. We conclude that the location of the site of decarboxylation, its distance from the mesophyll interface and the physical arrangement of chloroplasts and mitochondria in the bundle sheath cell are as important to the efficiency of the process as the properties of the bundle sheath cell wall. Using a mathemathical model of C4 photosynthesis, we also examine the relationship between bundle sheath resistance to CO2 diffusion and the biochemical capacity of the C4 photosynthetic pathway and conclude that bundle sheath resistance to CO2 diffusion must vary with biochemical capacity if the efficiency of the C4 pump is to be maintained. Finally, we construct a mathematical model of single cell C4 photosynthesis in a C3 mesophyll cell and examine the theoretical efficiency of such a C4 photosynthetic CO2 pump.