▲ FIGURE 2-19 Phosphatidylcholine, a typical phospho-glyceride. All phosphoglycerides are amphipathic, having a hydrophobic tail (yellow) and a hydrophilic head (blue) in which glycerol is linked via a phosphate group to an alcohol. Either of or both the fatty acyl side chains in a phosphoglyceride may be saturated or unsaturated. In phosphatidic acid (red), the simplest phospholipid, the phosphate is not linked to an alcohol.

phosphate and its associated esterified group, the "head" group of a phospholipid, is hydrophilic, whereas the fatty acyl chains, the "tails," are hydrophobic.

The amphipathic nature of phospholipids, which governs their interactions, is critical to the structure of biomembranes. When a suspension of phospholipids is mechanically dispersed in aqueous solution, the phospholipids aggregate into one of three forms: spherical micelles and liposomes and sheetlike, two-molecule-thick phospholipid bilayers (Figure 2-20). The type of structure formed by a pure phospholipid or a mixture of phospholipids depends on several factors, including the length of the fatty acyl chains, their degree of saturation, and temperature. In all three structures, the hy-drophobic effect causes the fatty acyl chains to aggregate and exclude water molecules from the "core." Micelles are rarely formed from natural phosphoglycerides, whose fatty acyl chains generally are too bulky to fit into the interior of a micelle. If one of the two fatty acyl chains is removed by hydrolysis, forming a lysophospholipid, the predominant type of aggregate that forms is the micelle. Common detergents and soaps form micelles in aqueous solution that behave as tiny ball bearings, thus giving soap solutions their slippery feel and lubricating properties.

Under suitable conditions, phospholipids of the composition present in cells spontaneously form symmetric phos-pholipid bilayers. Each phospholipid layer in this lamellar

▲ FIGURE 2-20 Cross-sectional views of the three structures formed by phospholipids in aqueous solutions. The white spheres depict the hydrophilic heads of the phospholipids, and the squiggly black lines (in the yellow regions) represent the hydrophobic tails. Shown are a spherical micelle with a hydrophobic interior composed entirely of fatty acyl chains; a spherical liposome, which has two phospholipid layers and an aqueous center; and a two-molecule-thick sheet of phospholipids, or bilayer, the basic structural unit of biomembranes.

structure is called a leaflet. The fatty acyl chains in each leaflet minimize contact with water by aligning themselves tightly together in the center of the bilayer, forming a hydrophobic core that is about 3 nm thick (see Figure 2-20). The close packing of these nonpolar tails is stabilized by the hydrophobic effect and van der Waals interactions between them. Ionic and hydrogen bonds stabilize the interaction of the phospholipid polar head groups with one another and with water.

A phospholipid bilayer can be of almost unlimited size— from micrometers (^m) to millimeters (mm) in length or width—and can contain tens of millions of phospholipid molecules. Because of their hydrophobic core, bilayers are virtually impermeable to salts, sugars, and most other small hydrophilic molecules. The phospholipid bilayer is the basic structural unit of nearly all biological membranes; thus, although they contain other molecules (e.g., cholesterol, gly-colipids, proteins), biomembranes have a hydrophobic core that separates two aqueous solutions and acts as a permeability barrier. The structural organization of biomembranes and the general properties of membrane proteins are described in Chapter 5.

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