All major lipids in membranes contain both hydrophobic and hydrophilic regions and are therefore termed amphipathic. If the hydrophobic region were separated from the rest of the molecule, it would be insoluble in water but soluble in organic solvents. Conversely, if the hydrophilic region were separated from the rest of the molecule, it would be insoluble in organic solvents but soluble in water. The amphipathic nature of a phospholipid is represented in Figure1. Thus, the polar head groups of the phospholipids and the hydroxyl group of cholesterol interface with the aqueous environment; a similar situation applies to the sugar moieties of the GSLs.

Fig1. Diagrammatic representation of a phospholipid or other membrane lipid. The polar head group is hydrophilic, and the hydrocarbon tails are hydrophobic or lipophilic. The fatty acids in the tails are saturated (S) or unsaturated (U); the former is usually attached to carbon 1 of glycerol and the latter to carbon 2 (see Figure 40–2). Note the kink in the tail of the unsaturated fatty acid (U), which is important in conferring increased membrane fluidity.

The S-U phospholipid on the left contains the C16 saturated lipid palmitic acid, and the monounsaturated C18 lipid cis-oleic acid; both are esterified to glycerol. The S-S phospholipid schematized on the right contains the C16 saturated lipid palmitic acid and the saturated C 18 lipid, stearic acid.

Saturated fatty acids form relatively straight tails, whereas unsaturated fatty acids, which generally exist in the cis form in membranes, form “kinked” tails. As the number of double bonds within the lipid side chains increase, the number of kinks in the tails increases. As a consequence, the membrane lipids become less tightly packed and the membrane more fluid.

Detergents are amphipathic molecules that are important in biochemistry as well as in the household. The molecular structure of a detergent is not unlike that of a phospholipid. Certain detergents are widely used to solubilize and purify membrane proteins. The hydrophobic end of the detergent binds to hydrophobic regions of the proteins, displacing most of their bound lipids. The polar end of the detergent is free, bringing the proteins into solution as detergent-protein complexes, usually also containing some residual lipids.

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