The uptake of inorganic carbon by freshwater plants
Abstract. The uptake of bicarbonate for photosynthetic assimilation by charophyte plants uses a chemiosmotic mechanism involving primary outward active transport of H+ in ‘acid zones’ of the membrane, and passive H+ re-entry in spatially separate ‘alkaline’ zones. In the process large electric currents circulate in the medium, and large local pH changes occur; bicarbonate ions, diffusing inwards across the unstirred layer of medium, encounter a number of competing mechanisms for the transfer of the carbon across the membrane. These are: 
 
i.transport of H2CO3 by diffusion 
ii.transport of CO2 by diffusion; or 
iii.co-transport of HCO3 and H+. 
 
 
The decision amongst these mechanisms is not yet possible. There are parallels between the charophytes and aquatic angiosperms that are indicated, and contrasts with the chlorophytes.