Biology subjects

Manning, G. S.

Publications and source records attributed to Manning, G. S..

3 recordsLinked to original sources

bioRxiv2024

The Physical Basis of Osmosis in a Donnan Ionic System

Impermeant molecules inside a cell would lead to an inward osmotic flow of water, causing swelling, were it not for the pumping of permeant sodium ions out of the cell as soon as they leak in. The energy barrier model for a semipermeable membrane, first introduced by Debye to provide a molecular-level explanation of the vant Hoff equation for osmotic pressure, can be used to advantage in this situation, since the pump can be conceptualized as increasing the energy barrier for the sodium ion. The Debye model has previously been extended to include osmosis induced by electrostatically neutral solutes. Discussion of the effect of ion pumping on water transport requires an understanding of osmosis in systems containing permeant ions, that is, Donnan systems. We have obtained an equation for Donnan osmosis across a Debye energy barrier that separates an aqueous solution of permeant sodium, potassium, and chloride ions from a solution containing these permeant ions and additionally an impermeant anion, the latter representing intra-cellular impermeant charged species. Donnan osmosis occurs even if osmolarities on the two sides of the membrane are equal. Numerical representation shows that the Donnan-Debye model provides a quantitative theoretical framework for the action of the sodium/potassium/ATPase ion pump as effectively rendering the extracellular sodium ions impermeant, thus balancing the impermeant molecules inside the cell. Another application of Donnan osmosis shows that ion charge effects, missing from lists of Starling forces, are nonetheless expected to be a major contributor to transport across capillary walls. SummaryOsmosis as driven by Starling forces is applicable only if the solute is electrostatically neutral. For ions, Donnan charge effects dominate. An equation for Donnan osmosis is presented and applied to ion pumps and to transport across capillary walls.

biophysics↗

bioRxiv2023

A Hard Sphere Model for Single File Water Transport Across Biological Membranes

We use Gurseys statistical mechanics of a one-dimensional fluid to find a formula for the Pf /Pd ratio in the transport of hard spheres across a membrane through a narrow channel that can accommodate only single file movement. Pf is the membrane permeability for osmotic flow, and Pd the permeability for exchange across the membrane in the absence of osmotic flow. The deviation of the ratio from unity indicates the degree of cooperative transport relative to ordinary diffusion of independent isolated molecules. In contrast to an early idea that Pf /Pd must be equal to the number of molecules in the channel, regardless of the physical nature of the interactions among the molecules, we find a functional dependence on the fractional occupancy of the length of the channel by the hard spheres. We also attempt a random walk calculation for Pd individually, which gives a result for Pf as well when combined with the ratio.

biophysics↗

bioRxiv2023

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.

biophysics↗