On diffusion in organized assemblies and in biological membranes

The following paper is a brief presentation of problems related to the concepts of diffusion coefficient D and so-called viscosity eta used to characterize the cohesion of biological membranes. The first approach to this problem is a recall of the definition of D and eta in liquids. It appears that the models developed with exogenous probes to account for the diffusion-viscosity relationship are not verified in membranes. The existence of complex diffusional mechanisms, the influence of the size of the probe are presented. The results of a model calculation suggest that there is no direct correlation other than great simplifications, between the diffusion coefficient and viscosity. The calculations are then extended to actual biological assemblies and the influence of proteins on the motion of the probe considered. The limitations of the methods involving exogenous probes for determining the cohesion of biological membranes are discussed.