1976 true in addition to these differences

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19–76 True. In addition to these differences, proteoglycans can contain as much as 95% carbohydrate by weight, whereas other glycoproteins generally contain a lower fraction of carbohydrate (1–60%). 19–77 True. If the collagen in adults did not turn over, there would be no need for new collagen synthesis, which is where vitamin C is required. 19–78 False. The elasticity of elastin fibers derives from their lack of secondary structure: elastin forms random coils that are easily stretched. The set of hydrogen bonds that stabilizes an a helix is too strong, in aggregate, to be disrupted by the kinds of forces that deform elastin. 19–79 True. There is a single fibronectin gene in the human genome. This gene, which contains about 50 exons, can be spliced in many different ways to produce many different fibronectin isoforms. THOUGHT PROBLEMS 19–80 Individual beads of carboxymethyl Sephadex swell because of osmotic effects. The negative charges on the cross-linked polymer trap an equal number of cations (which are present in the dry gel) to maintain electrical neutrality. These charges—both the fixed negative charges on the polymer and the mobile cations—are confined by electrostatic forces to the volume occupied by the cross-linked polymer. In a solution of pure water, the con- centration of particles in the volume of the gel is higher than in the water; A450 Chapter 19: Cell Junctions, Cell Adhesion, and the Extracellular Matrix
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hence, the water flows into the gel to try to equalize the concentrations inside and outside the gel bead. The beads swell as the water enters until they reach a maximum size dictated by their cross-linked structure. At that point, further entry of water molecules is opposed by the pressure exerted by the cross-linked structure of the polymer. It is more common to think of osmotic effects across a semi-permeable membrane, but the effects are exactly analogous in a cross-linked gel. When a salt solution is added to the swollen gel, the osmotic effects are reversed. The concentration of particles in 50 mM NaCl is much higher than in the swollen gel, so water flows out of the gel (and Na + and Cl ions flow in) to equalize the concentration of ions. In the absence of the osmotic water pressure, the gel shrinks. 19–81 Because the racemization of L -aspartate to D -aspartate occurs slowly, pro- teins that turn over rapidly will have very low levels of D -aspartate, if it can be detected at all. Proteins that are degraded and replaced more slowly will be expected to have a higher percentage of D -aspartate, with the absolute level depending on the rate of turnover. What makes the observations on elastin remarkable is the age dependence of the D -aspartate levels. These observations have been interpreted to mean that our lifetime supply of elastin is made early on and never degraded. In addition, studies in humans that made use of the inadvertent metabolic 14 C-labeling due to atmospheric testing of nuclear weapons, led to the conclusion that elastin synthesis
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