Lipid molecules constitute about 50% of the mass of most animal cell membranes, nearly all of the remainder being protein. There are approximately 5 × 106 lipid molecules in a 1 μm × 1 μm area of lipid bilayer, or about 109 lipid molecules in the plasma membrane of a small animal cell. All of the lipid molecules in cell membranes are amphiphilic—that is, they have a hydrophilic (“water-loving”) or polar end and a hydrophobic (“water-fearing”) or nonpolar end.
The most abundant membrane lipids are the phospholipids. These have a polar head group containing a phosphate group and two hydrophobic hydrocarbon tails. In animal, plant, and bacterial cells, the tails are usually fatty acids, and they can differ in length (they normally contain between 14 and 24 carbon atoms). One tail typically has one or more cis-double bonds (that is, it is unsaturated), while the other tail does not (that is, it is saturated). As shown in Figure 10–2, each cis-double bond creates a kink in the tail. Differences in the length and saturation of the fatty acid tails influence how phospholipid molecules pack against one another, thereby affecting the fluidity of the membrane, as we discuss later.
The main phospholipids in most animal cell membranes are the phosphoglycerides, which have a three-carbon glycerol backbone (see Figure 10–2). Two long-chain fatty acids are linked through ester bonds to adjacent carbon atoms of the glycerol, and the third carbon atom of the glycerol is attached to a phosphate group, which in turn is linked to one of several types of head group. By combining several different fatty acids and head groups, cells make many different phosphoglycerides. Phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine are the most abundant ones in mammalian cell membranes (Figure 10–3A–C).
Another important class of phospholipids are the sphingolipids, which are built from sphingosine rather than glycerol (Figure10–3D–E). Sphingosine is a long acyl chain with an amino group (NH2) and two hydroxyl groups (OH) at one end. In sphingomyelin, the most common sphingolipid, a fatty acid tail is attached to the amino group, and a phosphocholine group is attached to the terminal hydroxyl group. Together, the phospholipids phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin constitute more than half the mass of lipid in most mammalian cell membranes (see Table 10–1, p. 571).
Figure 10–2 The parts of a typical phospholipid molecule. this example is a phosphatidylcholine, represented (A) schematically, (B) by a formula, (c) as a space-filling model (Movie 10.1), and hydrophilicheadhydrophobictails(D) (D) as a symbol.
Figure 10–3 Four major phospholipids in mammalian plasma membranes. Different head groups are represented by different colors in the symbols. the lipid molecules shown in (A–c) are phosphoglycerides, which are derived from glycerol. the molecule in (D) is sphingomyelin, which is derived from sphingosine (e) and is therefore a sphingolipid. note that only phosphatidylserine carries a net negative charge, the importance of which we discuss later; the other three are electrically neutral at physiological ph, carrying one positive and one negative charge.
Figure 10–4 The structure of cholesterol.
polar head group cholesterol is represented (A) by a formula, (B) by a schematic drawing, and (c) as a space-filling model.