ABSTRACT
In higher plants and fungi many essential transport processes are in some way linked to a transmembrane proton gradient that is built up and maintained by a single transporter, the H+-ATPase (proton pump). Typically, the circulation of protons or, more generally, that of electrical charge represents the electrochemical driving belt by which a variety of matter is translocated against its concentration gradient. Because this primary active H+ transport is the dominating force, it is not surprising that a good deal of the cell’s energy is fed into such action [1, 2]. Not all plants live in this proton world. With a few exceptions (Acetabularia has a Cl− import pump), marine algae posses Na+ export pumps, which enables them to deal directly with the high external Na+ concentration. Higher plants or fungi that secondarily experience such salt stress remain with the proton pump and solve that problem through reducing their relative permeabilities (e.g., PNa/PK and possibly through exchanging Na+ for H+ [3]. During the conquest of land, the salt stress ceased to exist, and plants switched to H+ as the master ion. The reason for this farreaching step we can only speculate on, but it is very likely based on the necessity to deal with large water potential gradients between the cell interior and the atmosphere. This forced the plants to create a space in which an aqueous layer adjacent to the plasma membrane could provide the conditions required for transport and a variety of extracellular enzymatic reactions to take place in an osmotically and structurally protected area. The exchange of Na+ for H+ was energetically favorable for two major reasons:
1. Whereas in seawater high external [Na+] provides a sufficient inwardly directed electrochemical Na+ gradient to drive (co-)transport, such a gradient cannot be built up in plants because [Na+] is usually higher in cells than on the outside, and even a substantial membrane potential, built up by an Na+ pump, would yield an electrochemical Na+ gradient that is only a fraction of the electrochemical H+ gradient built up by a proton pump.
