Structure of a typical eukaryotic cell showing intracellular organelles.
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28 UNIT II Cellular Function
lipids with bent, unsaturated hydrocarbon tails tend to increase fluidity. About 50% of the lipid in eukaryotic cell membranes is cholesterol, which serves to decrease membrane permeability and prevent leakage of small water-soluble molecules. In addition to affecting fluidity by the degree of saturation of tail groups, the phospholipids that inhabit the membrane differ in the size, shape, and charge of the polar head groups. Fig. 3.5 shows the structures of the four most prevalent membrane phospholipids: phosphatidylethanolamine, phosphatidylserine, phos- phatidylcholine, and sphingomyelin. Some membrane-bound proteins require specific phospholipid head groups to function properly. Some lipids—sphingolipids and cholesterol in particular—may bind together transiently to form rafts in the sea of moving lipids. These rafts may surround and help organize membrane proteins into functional units. For example, a membrane receptor and its intracellular target proteins may associate together in a raft to facilitate transfer of information across the membrane.
Glycolipids contain one or more sugar (i.e., carbohydrate) molecules at the polar head region. Glycolipids and glycoproteins are found only in the outer half of the lipid bilayer, with the sugar groups exposed at the cell surface (Fig. 3.6). Membrane glycolipids are involved in cell recognition and cell-to-cell interactions.
A typical phospholipid molecule is shown in Fig. 3.3. The hydrophobic nonpolar tails tend to associate with other hydrophobic nonpolar tail groups to avoid association with polar water molecules. The hydrophilic polar head groups preferentially interact with the surrounding aqueous environment. A bilayer, with tails sandwiched in the middle, allows both portions of the lipid molecules to be chemically “satisfied.” In addition, the lipid bilayers tend to close on themselves, forming sealed, spherical compartments (Fig. 3.4). If the membrane is punctured or torn, it will spontaneously reseal itself to eliminate contact of the hydrophobic tails with water.
For the most part, individual lipid and protein molecules can diffuse freely and rapidly within the plane of the bilayer. The degree of membrane fluidity depends on the lipid composition. Saturated lipids have straight tails that can pack together and tend to stiffen the membrane, whereas