Chapter_10_Solutions

Chapter_10_Solutions - Chapter 10 Membrane Structure THE...

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THE LIPID BILAYER DEFINITIONS 10–1 Black membrane 10–2 Liposome 10–3 Hydrophobic 10–4 Lipid raft 10–5 Ganglioside 10–6 Amphiphilic 10–7 Phosphoglyceride 10–8 Cholesterol TRUE/FALSE 10–9 True. The hydrophobic interior of the lipid bilayer acts as a barrier to the passage of the hydrophilic lipid head groups that must occur during flip- flop. The energetic cost of this movement effectively prevents spontaneous flip-flop of lipids, so that it occurs extremely rarely in the absence of specific catalysts known as phospholipid translocators. 10–10 True. The positively charged moieties in all cases are balanced by the nega- tive charge on the phosphate group; thus, none of the common phospho- lipids carries a net positive charge. 10–11 True. Glycolipids are synthesized in the lumen of the Golgi apparatus, which is topologically equivalent to the outside of the cell, and cannot flip-flop across the bilayer. THOUGHT PROBLEMS 10–12 Water is a liquid, and thus hydrogen bonds between water molecules are not static; they are continually formed and broken again by thermal motion. When a water molecule happens to be next to a hydrophobic solute, it is more restricted in motion and has fewer neighbors with which it can interact because it cannot form any hydrogen bonds in the direction of the hydropho- bic solute. It will therefore form hydrogen bonds to the more limited number of water molecules in its proximity. Bonding to fewer partners results in a more ordered water structure, which constitutes the icelike cage in Figure 10–1. The true cage of water molecules is substantially different from that represented in the figure; the real cage exists in three dimensions, forming a In This Chapter THE LIPID BILAYER A221 MEMBRANE PROTEINS A227 A221 Chapter 10 10 Membrane Structure
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A222 Chapter 10: Membrane Structure pentagonal dodecahedron (like a soccer ball) or clusters of them that enclose the hydrophobic solute. The structure is similar to ice, although it is a more transient, less organized, and less extensive network than even a tiny ice crystal. The formation of any ordered structure decreases the entropy of the system, which is energetically unfavorable. 10–13 The same forces that dictate that certain lipids will form a bilayer, as opposed to micelles, operate in the repair of a tear in the bilayer. The tear will heal spontaneously because a bilayer is the most energetically favorable arrangement. The lipids that make up a bilayer are cylindrical in shape and therefore do not readily form a micelle (or a hemi-micelle), which would require cone-shaped lipids. 10–14 Lipid bilayers assemble because the surrounding water molecules exclude the component lipids; thus, analogy (2) is the correct one. If bilayers formed because of attractive forces among the lipids—analogy (1)—the properties of the bilayer would likely be quite different. Molecules ‘attract’ one another by forming specific bonds that hold them together. Such bonding among lipids would make the bilayer less fluid, perhaps even rigid, depending on the strength of the interaction.
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