1015 a with one tail removed the diameter of the head

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the strength of the interaction. 10–15 A. With one tail removed, the diameter of the head group would be much larger than that of the remaining hydrocarbon chain; thus, the shape of the molecule would resemble a cone more than a cylinder and would tend to form micelles rather than bilayers. B. Bilayers formed by lipids with short hydrocarbon tails would be much more fluid. The bilayers would also be less stable, since the shorter tails would be less hydrophobic and, thus, the forces that drive formation of the bilayer would be reduced. C. Bilayers formed by lipids with saturated hydrocarbon tails would be much less fluid. Whereas a normal lipid bilayer has the viscosity of olive oil, a bilayer made of lipids with saturated hydrocarbon tails would have the con- sistency of bacon fat. D. Bilayers formed by lipids with unsaturated hydrocarbon tails would be much more fluid. Also, because the lipids would pack together less well, there would be more gaps and the bilayer would be more permeable to small water-soluble molecules. E. If lipids with two saturated hydrocarbon tails were completely intermixed with lipids carrying two unsaturated hydrocarbon tails, the fluidity of the membrane would be about normal. In such bilayers, the saturated lipid molecules would tend to aggregate with one another because they can pack so much more tightly and would therefore form patches with much reduced fluidity. The bilayer would not, therefore, have uniform properties over its surface. Most phospholipids in normal membranes have one saturated and one unsaturated hydrocarbon chain so they do not tend to segregate; how- ever, sphingolipids often do have long, saturated hydrocarbon chains and do tend to form microdomains, termed lipid rafts. F. If the hydrocarbon tails of the lipids in the two monolayers were covalently linked, the bilayer would have virtually unchanged properties. Each lipid molecule would now span the entire membrane, with one of its two head groups exposed at each surface. Such lipid molecules are actually found in the membranes of thermophilic bacteria, which can live at temperatures approaching that of boiling water. Their bilayers do not come apart at ele- vated temperatures, as usual bilayers do, because the two monolayers are covalently linked into a single membrane. 10–16 In a two-dimensional fluid the molecules are constrained to move in a plane; the molecules in a normal fluid can move in three dimensions. 10–17 Vegetable oil is converted to margarine by reduction of double bonds (by hydrogenation), which converts unsaturated fatty acids to saturated ones.
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THE LIPID BILAYER A223 This change allows the fatty acid chains in the lipid molecules to pack more tightly against one another, increasing the viscosity, turning oil into margarine.
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