BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 10. AN INTRODUCTION TO BIOLOGICAL MEMBRANES

Summary

Introduction/36.html">Biological Membranes are sheetlike structures, about 75 Å thick, composed of protein and lipid molecules held together by noncovalent interactions. Membranes serve as highly selective permeability barriers. They define closed compartments in the form of entire Cells or subcellular Organelles. Pumps and channels embedded in membranes regulate the molecular and ionic composition of these compartments. Membranes also control the flow of information between cells; in particular, some membranes contain Hormone Receptors, such as Insulin receptors. In addition, membranes are directly involved in energy Transduction processes, such as Photosynthesis and Oxidative Phosphorylation.

The Major Classes of Membrane Lipids are Phospholipids, Glycolipids, and Cholesterol. Phospholipids include phosphoglycerides, which are built of a glycerol backbone, two fatty acid chains, and a phosphorylated alcohol. Fatty acid chains typically contain 14 to 24 carbon atoms and can be either saturated or unsaturated. The main phosphoglycerides are phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine. Sphingomyelin is another type of phospholipid, whose backbone is sphingosine rather than glycerol. Glycolipids are carbohydrate-containing lipids derived from sphingosine. All membrane lipids are amphipathic molecules. In aqueous media, they spontaneously form extensive bimolecular sheets consisting of hydrophilic and hydrophobic phases. Lipid bilayers are highly impermeable to ions and most polar molecules; at the same time, the bilayer is fluid, allowing it to serve as a solvent for Membrane Proteins.

Specific Membrane Functions, such as transport, communication, and energy transduction, are carried out by specific proteins. Some membrane proteins are embedded deep within the hydrocarbon core of Cell/29.html">The Lipid Bilayer. Transmembrane proteins, such as the band 3 protein of The erythrocyte membrane, can serve as Ion Channels. Membranes are structurally and functionally asymmetric; this is reflected both in the directionality of ion-transport systems and in the localization of carbohydrate residues exclusively on the outer surface of mammalian Plasma Membranes. Membranes are dynamic structures, and in the absence of specific constraints, their constituent proteins and lipids diffuse rapidly in the plane of the membrane (lateral diffusion). However, the transition of proteins and lipids from one side of the membrane to the other (transverse diffusion, or 'flip-flop') is extremely slow. Membrane fluidity depends in part on the chain length and degree of unsaturation of their constituent Fatty acids.