Primate cells evolved a plasma membrane to restrict the loss of important molecules. The osmotic problems that then arose were solved in one of several ways. Of major importance was the evolution of specific ion pumps, to actively extrude those salts whose inward diffusion would have led to swelling and lysis. In addition, these pumps allowed the cell to store energy in the form of ion gradients across the membrane. Thus, even in the earliest stages, the evolution of ion transport systems coincided with the development of mechanisms which catalyzes the energy transformations. It is postulated that an "ATP"-driven proton pump was one of the first ion transport systems. Such a proton pump would extrude hydrogen ions from the cell, establishing both a transmembrane pH gradient (alkaline inside) and a membrane potential (negative inside). This difference in electrochemical potential for protons (the proton-motive force) could then drive a variety of essential membrane functions, such as the active transport of ions and nutrients. A second major advance was the evolution of an ion transport system that converted light energy into a form which could be used by the cell. The modern model for this is the "purple membrane" of Halobacterium halobium, which catalyzes the extrusion of protons after the capture of light. The protonmotive force generated by such a light-driven proton pump could then power net synthesis of ATP by a reversal of the ATP-driven proton pump. A third important evolutionary step associated with ion transport was the development of a system to harness energy released by biological oxidations. Again, the solution of this problem was to conserve energy as a protonmotive force by coupling the activity of a respiratory chain to the extrusion of protons. Finally, with the development of animal cells a more careful regulation of internal and external pH was required. Thus, an ATP-driven Na+-K+ pump replaced the proton-translocating ATPase as the major ion pump found in plasma membranes.