Introduction to Biomembranes

Abstract

A biological membrane or biomembrane separates the internal environment of the cell from the outside. It acts as a selectively permeable barrier which defines a living cell or a cellular organelle. It maintains the cellular/organelle integrity and identity. Many cellular functions are associated with cell membranes. Today we know that biomembranes are phospholipid bilayers with associated proteins and carbohydrates. This chapter traces back the history and discoveries which led to the widely accepted fluid mosaic model of membranes. According to this model, phospholipids form the membrane matrix in which the proteins are embedded or associated. Phosphoglycerides form the major class of phospholipids; other class includes phosphosphingolipids. Sterols and glycosphingolipids also form a major component of membranes. The membranes from different sources vary in their lipid and protein composition. Biomembranes also show considerable transbilayer asymmetry in terms of lipid composition; for example, in the plasma membrane, the exoplasmic leaflet is predominantly rich in lipids like phosphatidylcholine (PC), sphingomyelin (SM), and glycosphingolipids, while aminophospholipids and other charged lipids like phosphatidylserine (PS) and phosphatidylethanolamine (PE) are predominantly found in the cytoplasmic leaflet. Carbohydrates in membranes are mainly present as oligosaccharides/glycans covalently linked to lipids and/or proteins forming glycoconjugates.