Chapter 1 Physical properties and functional roles of lipids in membranes

Abstract

Biological membranes contain an astonishing variety of lipids. As detailed throughout this book, generation of this diversity requires elaborate metabolic pathways. The lipid compounds representing the end products of these pathways must bestow significant evolutionary advantages to the cellular or multicellular systems in which they reside, implying particular functional roles for each component. However, clarification of the functional roles of individual lipid species has proven a difficult problem. Here we present a synopsis of the physical properties of lipid systems and indicate how they may relate to the functional capacities of biological membranes. The major role of membrane lipids has been understood in broad outline since the early experiments of Gorter and Grendell [l], who extracted lipids from the erythrocyte membrane and measured the areas these lipids were able to cover as a monolayer at an air-water interface. This work led to the conclusion that the erythrocytes contained sufficient lipid to provide a bilayer lipid matrix surrounding the red blood cell. This bilayer lipid organization, which provides a permeability barrier between exterior and interior compartments, has remained a dominant theme in our understanding of the organization and function of biological membranes. Subsequent observations that such bilayers are fluid, allowing rapid lateral diffusion of lipid and protein in the plane of the membrane, and that membrane proteins are often inserted into and through the lipid matrix, have further contributed to our present understanding of membranes, resulting in the Singer and Nicholson [2] fluid mosaic model, a refined version of which is shown in Fig.1