CO2 uptake and transport in leaf mesophyll cells

The acquisition of inorganic carbon for photosynthetic assimilation by leaf mesophyll cells and chloroplasts is discussed with particular reference to membrane permeation of CO2 and HCO−3. Experimental evidence indicates that at the apoplast pH normally experienced by leaf mesophyll cells (pH 6–7) CO2 is the principal species of inorganic carbon taken up. Uptake of HCO−3 may also occur under certain circumstances (i.e. pH 8.5), but its contribution to the net flux of inorganic carbon is small and HCO−3 uptake does not function as a CO2‐concentrating mechanism. Similarly, CO2 rather than HCO−3 appears to be the species of inorganic carbon which permeates the chloroplast envelope. In contrast to many C3 aquatic plants and C4 plants, C3 terrestrial plants lack specialized mechanisms for the acquisition and transport of inorganic carbon from the intercellular environment to the site of photosynthetic carboxylation, but rely upon the diffusive uptake of CO2.