The Physical Basis of Osmosis

Osmosis is an important force in all living organisms, yet the molecular basis of osmosis is widely misunderstood as arising from differences in water concentration in solutions of differing osmolarities. In 1923 Debye proposed a physical model for a semipermeable membrane that was hardly noticed at the time and slipped out of view. We show that Debyes analysis of vant Hoffs law for osmotic equilibrium provides a consistent and plausible explanation for osmotic flow. A difference in osmolyte concentrations in solutions separated by a semipermeable membrane generates different pressures at the two water-membrane interfaces. Water is therefore driven through the membrane for exactly the same reason that pure water flows in response to an imposed hydrostatic pressure difference. In this paper we present the Debye model in both equilibrium and flow conditions. We point out its applicability regardless of the nature of the membrane with examples ranging from predominantly convective flow of water through synthetic membranes to purely diffusive flow of independent water molecules through a lipid bilayer and the flow of strongly interacting water molecules in single file across narrow protein channels.