The Physical Basis of Osmosis in a Donnan Ionic System

The osmotic flow of water is driven by the repulsive force between semipermeable membrane and impermeable solute that creates a pressure drop at the interface between membrane and aqueous solution. An analysis of the physical principles involved is relatively simple for a neutral solute that is absolutely impermeable. Extension to partially permeable neutral solutes is more complicated but has been accomplished. In this paper the analysis is further extended to include Donnan ionic systems. The electric forces due to the charges on the ions give rise to non-uniform ionic concentrations and trans-membrane electric voltages as well as pressure gradients, and the relationships among all of these Donnan effects have an impact on osmosis. The supplementary effect of an ion pump, acting to prevent the osmotic danger of cell swelling and rupture, is explored.