As indicated above, biological membranes are composed of lipids, proteins, and carbohydrates. The carbohydrates of membranes are attached either to lipid forming glycolipids of various classes, or to proteins formingglycoproteins. The lipid and protein compositions of membranes vary from cell type to cell type as well as within the various intracellular compartments that are defined by intracellular membranes. Protein concentrations can range from around 20% to as much as 70% of the total mass of a particular membrane.
The lipids making up components of membranes are of three major classes that includes glycerophospholipids, sphingolipids, and cholesterol. For information on the structures of these different lipid classes see the Lipids page,Lipid Synthesis page, Sphingolipids page and the Cholesterol page. Sphingolipids and glycerophospholipids constitute the largest percentage of the lipid weight of biological membranes. The hydrocarbon tails of these two classes of lipid result in steric limitations to their packing such that they will form disk-like micelles. The structure of these micelles results from the interactions of the hydrophobic tails of the lipids and the exposure of the polar head groups to the aqueous environment. This orientation results in what is referred to as a lipid bilayer and is diagrammed in the figure below. Lipid bilayers are essentially two-dimensional fluids and the lipid components of the bilayer can diffuse laterally and in fact evidence demonstrates that this lateral diffusion occurs readily. Lipids in the bilayer can also undergo transverse diffusion (also called a flip-flop) where the lipid diffuses from one surface to the other. However, because the flip-flop requires the polar head group to pass through the hydrocarbon core of the bilayer the process is extremely rare. Enzymes have been identified that facilitate the flip-flop process and these enzymes are referred to as flipases.
Structure of the typical lipid bilayer of the plasma membrane. Integral proteins are those that pass through the bilayer. Peripheral proteins are associated with the inner surface of the plasma membrane. Most integral proteins are modified by carbohydrate addition to their extracellular domains. Membranes also contain carbohydrate modified lipids (glycolipids) in addition to the more common phoshopholipids and cholesterol that constitute the bulk of the lipid content of the membrane.
Biological membranes also contain proteins, glycoproteins, and lipoproteins (see the Glycoproteins and Protein Modifications pages). Proteins associated with membranes are of two general types: integral and peripheral. Integral membrane proteins (also called intrinsic proteins) are tightly bound to the membrane through hydrophobic interactions and are inserted into and/or penetrate the lipid bilayer. In contrast, peripheral membrane proteins (also called extrinsic proteins) are only loosely associated with the membrane either through interactions with the polar head groups of the lipids or through interactions with integral membrane proteins. Peripheral membrane proteins are most often, if not exclusively, found on the cytosolic face of the plasma membrane or the lumenal surface of subcellular organelle membranes.
Proteins that are found associated with membranes can also be modified by lipid attachment (lipoproteins). The lipid portion of a lipoprotein anchors the protein to the membrane either through interaction with the lipid bilayer directly or through interactions with integral membrane proteins. Lipoproteins associated with membranes contain one of three types of covalent lipid attachment. The lipids are isoprenoids such as farnesyl and geranylgeranyl residues (see theProtein Modifications page for the mechanism of protein prenylation), fatty acids such as myristic and palmitic acid, and glycosylphosphatidylinositol, GPI (termed glipiated proteins: see the Glycoproteins page for details).