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The physical basis of osmosis
Journal article   Open access   Peer reviewed

The physical basis of osmosis

Gerald S. Manning and Alan R. Kay
The Journal of general physiology, Vol.155(10), e202313332
10/02/2023
DOI: 10.1085/jgp.202313332
PMCID: PMC10457415
PMID: 37624228
url
https://doi.org/https://doi.org/10.1085/jgp.202313332View
Published (Version of record) Open Access

Abstract

It is surprising that osmosis, a phenomenon so central to biology, has been cloaked in misunderstanding for so long. The authors show that the most plausible account for what drives water fluxes is one put forward by Peter Debye in 1923, where the repulsion of solute molecules from the semipermeable membrane generates a pressure drop, which draws water from a chamber with low solute concentration to one that is high. Osmosis is an important force in all living organisms, yet the molecular basis of osmosis is widely misunderstood as arising from diffusion of water across a membrane separating solutions of differing osmolarities, and hence different water concentrations. In 1923, Peter Debye proposed a physical model for a semipermeable membrane emphasizing the repulsive forces between solute molecules and membrane that prevent the solute from entering the membrane. His work was hardly noticed at the time and slipped out of view. We show that Debye’s analysis of van ’t Hoff’s law for osmotic equilibrium also provides a consistent and plausible mechanism for osmotic flow. A difference in osmolyte concentrations in solutions separated by a semipermeable membrane leads to different pressures at the two water–membrane interfaces because the total repulsive force between solute molecules and the membrane is different at the two 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 the predominantly convective flow of water through synthetic membranes and capillary walls to the purely diffusive flow of independent water molecules through a lipid bilayer and the flow of a single-file column of water molecules in narrow protein channels.
 Biophysics Membrane Transport Cellular Physiology Lipids and Membranes

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