Each living cell must, while performing its life functions, constantly exchange water with its surroundings. This occurs through the cell membrane. In the present paper, we have made an attempt to explain the biophysical basis of this water exchange, realized under stationary conditions, i. e. at constant cell volume. For the investigation, the mechanistic equations for membrane transport have been applied. It has been demonstrated that each living cell which subsists under stationary conditions is capable of water absorption and simultaneous water removal to its surroundings. Water absorption is osmosis-driven, while water removal is driven by the mechanical pressure difference (the turgor pressure). These are new, and very significant, research results. This stationary water exchange cannot be explained on the basis of thermodynamic transport equations.