Membrane fluidity and cellular functions.

Abstract

An area of intense current interest in molecular and cell biology is the structure of biological membranes. A great change in our picture of membranes has occurred in the last few years, and there is now fairly widespread acceptance of a model for the organization of the lipids and proteins of membranes called the “fluid mosaic model” (Singer and Nicolson, 1972). In this model (Fig. 1) the proteins that are integral to the membrane (Singer, 1971) are proposed to be globular molecules which are partly embedded in the membrane lipid, and partly protrude from it. This partial embedding is determined thermodynamically by the amphipathic character of the integral protein molecule; its hydrophobic end is embedded in the hydrophobic membrane interior and its hydrophilic end protrudes into the aqueous phase. The lipid, arranged largely as a bilayer, forms the matrix of the membrane, and since at physiological temperatures the lipid of most functional membranes is largely fluid, the integral proteins are in principle free to move about laterally and rapidly in the plane of the membrane. There is now a very substantial body of evidence that is consistent with, and strongly supports, the fluid mosaic model. The discussion of this evidence could easily occupy my entire presentation, but I thought that in the context of this symposium it might be more useful to adopt the model as a working hypothesis of membrane structure and consider what it might imply about the mechanisms of a variety of important cellular functions and activities.