Proton Co‐transport of Organic Solutes by Plant Cells

I. SUMMARY The possible role of proton co‐transport in the uptake of sugars and amino acids by plant cells is discussed. Evidence is presented for sodium or proton co‐transport of organic solutes in animal, bacterial and algal systems and this is discussed in relation to evidence for a similar mechanism operating in the cells of higher plants. It appears that in plant cells an electrogenic proton efflux pump generates a gradient of electrochemical potential for protons which provides energy for the movement of an organic solute‐proton‐carrier complex across the membrane. The large sugar transport associated with the loading of phloem sieve elements is postulated to be driven by a proton co‐transport system and the relation of this loading process to transport and subsequent unloading of the phloem is considered.