The majority of membrane phospholipids have a glycerol carbons in length and may be saturated or unsaturated (i.e., “backbone” to which are attached the fatty acyl chains, contain one or more double bonds). and an alcohol is linked to glycerol via a phosphate group. The phospholipid composition of the membrane The common alcohols are choline, ethanolamine, serine, varies among different cell types and even between the inositol, and glycerol. Another important phospholipid, bilayer leaflets. For example, in the erythrocyte plasma sphingomyelin, has the amino alcohol sphingosine as its membrane, phosphatidylcholine and sphingomyelin are “backbone” instead of glycerol.
lists these common found predominantly in the outer leaflet of the membrane, phospholipids. The fatty acyl chains are usually 14 to 20 whereas phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol are found in the inner leaflet.
, phosphatidylinositol plays an important role in signal transduction, and its location in the inner leaflet of the membrane facilitates this signaling role.
The sterol molecule cholesterol is also a critical component of the bilayer (see
Fig. 1.3 ). It is found in both leaflets and serves to stabilize the membrane at normal body temperature (37°C). As much as 50% of the lipids found in the membrane can be cholesterol. A minor lipid component of the plasma membrane is glycolipids. These lipids, as their name indicates, consist of two fatty acyl chains linked to polar head groups that consist of carbohydrates (see
Fig.
1.3 ). As discussed in the section on , one glycolipid, glycosylphosphatylinositol (GPI), plays an important role in anchoring proteins to the outer leaflet of the membrane. Both cholesterol and glycolipids, like the phospholipids, are amphipathic, and they are oriented with their polar groups on the outer surface of the leaflet in which they are located. Their hydrophobic portion is thus located within the interior of the bilayer.
The lipid bilayer is not a static structure. The lipids and associated proteins can diffuse within the plane of the membrane. The fluidity of the membrane is determined by temperature and by its lipid composition. As temperature increases, the fluidity of the membrane increases. The presence of unsaturated fatty acyl chains in the phospholipids and glycolipids also increases membrane fluidity. If a fatty acyl chain is unsaturated, the presence of a double bond introduces a “kink” in the molecule (see
Fig. 1.3).
This kink prevents the molecule from associating closely with surrounding lipids, and, as a result, membrane fluidity is increased. Although the lipid bilayer is “fluid,” movement of proteins in the membrane can be constrained or limited. For example, membrane proteins can be anchored to components of the intracellular cytoskeleton, which limits their movement. Membrane domains can also be isolated from one another. An important example of this can be found in epithelial tissues. Junctional complexes (e.g., tight junctions) separate the plasma membrane of epithelial cells into two domains: apical and basolateral (see ). The targeted localization of membrane proteins into one or other of these domains allows epithelial cells to carry