Much more is known about the specific lipid composition of cell membranes, because the lipids are more readily extracted from the membranes using a variety of organic solvents.
Once extracted from isolated membranes, the lipids may be separated and identified using chromatographic or other procedures.
Nearly all the membranes studied so far appear to contain the same types of lipid molecules.
Phospholipids such as phosphatidyl ethanolamine, phosphatidyl serine, phosphatidyl inositol, phosphatidyl choline (lecithin), and sphingomyelin are the most common constituents, but cholesterol is also present.
Table 15-2 lists the most common lipids found in a variety of cell membranes and also shows their protein-to-lipid weight ratios; the latter vary considerably. In addition to its widespread occurrence in plasma membranes, cholesterol is also found in many intracellular membranes. The rigid (i.e., planar) nature of cholesterol imparts an ordering effect to those cellular membranes that contain this lipid.
Mobility of Membrane Lipids:
Lipids exhibit a higher degree of mobility in membranes than do proteins, although lateral mobility is very much greater than transverse (“flip-flop”) mobility. A single lipid molecule may move several microns laterally through the membrane in just 1 or 2 seconds! The mobility of lipid and protein molecules in the plasma membrane attests to the membrane’s fluidity. C. F. Fox and H. M. McConnell have shown that the degree of fluidity is dependent, in turn, on the fatty acid contents of side chains of phospholipids in the membrane.
Fatty acid side chains of membrane phospholipids can be either saturated or unsaturated. In saturated side chains, all the carbon-carbon bonds are single, with the remaining carbon bonds carrying hydrogen atoms; in unsaturated side chains, one or more pairs of neighboring carbon atoms are linked by double bonds.
In phospholipid layers consisting exclusively of saturated fatty acids, the side chains are aligned next to one another in an ordered, crystalline array; the result is a relatively rigid structure (Fig. 15-15). In phospholipid layers consisting of a mixture of saturated and unsaturated fatty acid side chains, the packing of neighboring molecules is less orderly (and therefore more fluid).
The double bonds of the unsaturated side chains produce bends in the hydrocarbon chains, and these give rise to structural deformations that prevent formation of the more rigid crystalline structure. The greater the number of double bonds, the more disordered (and fluid) is the lipid bilayer (Fig. 15-15).
The rigidity of lipid layers is also affected by temperature. Almost everyone is familiar with the “melting” of fats and waxes at elevated temperatures. To maintain membrane fluidity, cells living at low temperatures have higher proportions of unsaturated fatty acids in their membranes than do cells at higher temperatures. Evidence also exists suggesting that cells can alter the balance of saturated and unsaturated fatty acids in their membranes as an adjustment to changing temperature or other factors.
In recent years, the degree of membrane fluidity has been linked to the capability of various metabolites and hormones to bind to surface receptors. An increase in membrane fluidity may be accompanied by the withdrawal of exposed receptors (i.e., they are drawn deeper into the lipid bilayer), whereas a decrease in membrane fluidity is accompanied by greater accessibility of the receptor through increased exposure above the bilayer.
The various membrane lipids are not equally distributed in both monolayers, although the asymmetry is not nearly as marked as in the case of protein. The distribution of lipids in the erythrocyte membrane is shown in Table 15-3 and reveals that the choline phosphatides are primarily in the outer monolayer and the amino phosphatides are in the inner monolayer.
Although lipid asymmetry is a general property of membranes, the type of asymmetry varies considerably from one membrane to another. Asymmetry, once established, is most likely maintained because of the high activation energy that would be required to move the polar groups through the hydrophobic center of the bilayer.
Just as proteins are differentially distributed in the plasma membrane areas comprising the various functional faces of a tissue cell, so are the lipids. This is vividly seen in Figure 15-16, which shows the distributions of phosphatidyl choline, sphingomyelin, phosphatidyl ethanolamine, and phosphatidyl serine in the three major plasma membrane regions (faces) of the liver parenchymal cell.