The Possible Role of Various Membrane Transport Mechanisms in Adaptation to Salinity

Abstract

Peter Mitchell’s chemiosmotic hypothesis (Mitchell, 1986) has now dominated our thinking about membrane transport processes for nearly two decades. It envisages a relatively small number of primary energy transducers in the cell membranes, the “primary pumps”, which generate transmembrane ion gradients by transferring specific ions energetically uphill. These ions are the “working” ions — their return flux, downhill, can serve as the direct source of energy for the transmembrane flux of numerous other metabolites and ions if there are “porter” molecules in the membrane which couple the two fluxes, those of the “driving” and “driven” solute respectively (Fig. 1). These two fluxes may be in the same (symport) or opposing (antiport) direction, and in addition there may be “uniport” of ions, that is electrophoretic flux through specific channels, driven by the membrane potential (Δψ) generated by the electrogenic primary pump. The principal primary pump in the animal cell is the Na⁺K⁺ATPase which ejects Na⁺ from the cell and builds up an inwardly directed Na⁺ gradient. Its functional counterpart in the plant cell is the proton pump which is responsible for the electrogenic extrusion of protons, thus generating the “protonmotive force”, or pmf (ΔpH +Δψ).