The functional roles of lipids in biological membranes

Publisher Summary Biological membranes are fluid (liquid–crystalline) lipid bilayers, into which, proteins can insert or associate at the surface. The membrane research has focused primarily on the protein components, with the lipid portion viewed as a convenient barrier and environment for enzymes. However, biological membranes contain a wide diversity of lipids, far more than are needed to perform structural functions, and these lipids require elaborate metabolic pathways for their synthesis and transport. The main classes of lipids found in eukaryotic biological membranes include the glycerophospholipids, the sphingolipids, and cholesterol (Chol). Of the former group, phosphatidylcholine (PC) is the major lipid, but phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), and cardiolipin (CL) are also major lipid species in biological membranes. A representative chemical structure of a common phospholipid is 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The sphingolipids include sphingomyelin (SPM), ceramide (CER), and the glycosphingolipids (GSLs). Most biological membranes are fluid at physiological temperatures—a requirement for proper function. The fluid-membrane phase usually refers to the liquid–crystalline bilayer phase, although membranes that contain large quantities of Chol can adopt a different fluid-phase known as liquid-ordered.