Photosynthesis provides all the chemical energy and the carbon compounds needed for all other growth processes. It is thus the central process governing the primary productivity of all green plants. The ultimate substrate for this process is Carbon dioxide (CO2). The ability of leaf photosynthesis to respond to increased CO2 concentration depends on various external and internal limits to CO2 uptake, their dependence on CO2 concentration, and their interactions with other environmental factors. It is convenient to consider CO2 uptake into the leaf in terms of a series of conductances associated with the leaf boundary layer, the stomata, the intercellular air space, the cell wall and intracellular liquid phase. Photosynthetic enhancement by increased CO2 concentration depends strongly on other environmental factors. For C3 plants, the enhancement is least at low temperatures and increases continuously with increasing temperature. Stomatal opening is the result of an accumulation of potassium salts in the guard cells.