Osmotic Flow in Membrane Pores of Molecular Size

Water transfer by osmosis through pores occurs either by viscous flow or diffusion depending on whether the driving osmolyte is able to enter the pore. Analysis of osmotic permeabilities (Pos) measured in antibiotic and cellular pore systems supports this distinction, showing that Pos approaches either the viscous value (Pf) or the diffusive value (Pd) depending on the size of the osmolyte in relation to the pore radius. Macroscopic hydrodynamics and diffusion theory, when used with drag and steric coefficients within an appropriate osmotic model, apply with remarkable accuracy to channels of molecular dimensions where water molecules cannot pass each other, without the need to postulate any special flow regimens. It becomes apparent that the true viscous to diffusive flow ratio, Pf/Pd, can be separated from the effects of tracer filing by osmotic measurements alone. It does not monotonically decrease with the pore radius but rises steeply at the smaller radii which would apply to pores in cell membranes. Consequently, the application of the theory to osmotic and diffusive flow data for the red cell predicts a pore radius of 0.2 nm in agreement with other recent measurements on isolated components of the system, showing that the viscous-diffusive distinction applies even in molecular pores.