WATER TRANSPORT AT CELL MEMBRANES

Abstract

Using cerebral cortex slices in vitro as an example, it is shown that there is no need to postulate active transport of water in the control of cellular volume of most tissues.In the elaboration of secretions of different osmolarity from that of plasma, active transport of water is also no longer invoked. The current theory of formation of hypertonic urine postulates that all the water movements involved remain passive and secondary to active solute transport. Water uptake from the gastrointestinal tract against a concentration gradient is explained in terms of hydrostatic pressure gradients generated within the tissue itself by active solute transport.Water movements involved in changes of mitochondrial volume are explained as a passive consequence of changes in the mitochondrial membrane.It is concluded that in all cases changes in the distribution of water are brought about by passive processes, often in consequence of active transport of ions and metabolites, and that such movement of water does not constitute active transport of water.