THE SODIUM-PUMP IN THE EVOLUTION OF ANIMAL-CELLS

Plant cells and bacterial cells are surrounded by a massive cellulose wall, which constrains their high internal osmotic pressure (tens of a tmospheres). Animal cells, in contrast, are in osmotic equilibrium wit h their environment, have no restraining surround, can take on a varie ty of shapes and change these from moment to moment. This osmotic bala nce is achieved by the action of the energy-consuming sodium pump, one of the P-type ATPase transport protein family, members of which are i ndeed also found in bacteria. The pump's action brings about a transme mbranal electrochemical gradient of sodium ions, harnessed in a range of transport systems that couple the dissipation of this gradient to e stablishing a gradient of the coupled substrate. The primary role of t he sodium pump as a regulator of cell volume has evolved to provide th e basis for an enormous variety of physiological functions.