Asymmetric Diffusion

TL;DR

This paper demonstrates how asymmetric diffusion through membranes occurs due to particle shape and pore structure, leading to potential biological implications, using simulations of particle systems and geometric considerations.

Contribution

It introduces a deterministic simulation approach to show asymmetric diffusion caused by particle shape and membrane structure, highlighting a geometric mechanism for directional flow.

Findings

Asymmetric diffusion can rapidly seal off flow in one direction.

Particle shape and pore geometry significantly influence diffusion asymmetry.

The effect may have biological relevance in membrane ion channels.

Abstract

Diffusion rates through a membrane can be asymmetric, if the diffusing particles are spatially extended and the pores in the membrane have asymmetric structure. This phenomenon is demonstrated here via a deterministic simulation of a two-species hard-disk gas, and via simulations of two species in Brownian motion, diffusing through a membrane that is permeable to one species but not the other. In its extreme form, this effect will rapidly seal off flow in one direction through a membrane, while allowing free flow in the other direction. The system thus relaxes to disequilibrium, with very different densities of the permeable species on each side of the membrane. A single species of appropriately shaped particles will exhibit the same effect when diffusing through appropriately shaped pores. We hypothesize that purely geometric effects discussed here may play a role in common biological contexts such as membrane ion channels.