ABSTRACT
Water is the most abundant and also one of the smallest molecules in living organisms. It plays a fundamental role in many biological and physiological processes. A living cell, whether it exists as a separate organism or as a part of a multicellular organ or organism, in the course of all its physiological functions continuously exchanges with the extracellular medium water, nutrients, and metabolic waste products. This exchange takes place across cellular membranes and is controlled by the transport properties of the membranes.
