1994 false although the major components of plant

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19–94 False. Although the major components of plant cell walls are polysaccha- rides, structural proteins make a small contribution (on the order of 5% of the dry mass of the cell wall). These proteins are thought to strengthen the wall. They are also produced in higher amounts as a local response to attack by pathogens. 19–95 False. In the absence of cortical microtubules, new cellulose microfibrils tend to be deposited in the same orientation as the preexisting microfibrils. The cortical array of microtubules seems to be critically important for the initial deposition of cellulose microfibrils, as well as for any developmentally programmed change in orientation. A454 Chapter 19: Cell Junctions, Cell Adhesion, and the Extracellular Matrix
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THOUGHT PROBLEMS 19–96 If you soak the lettuce in tap water, it will take up water due to osmosis and become crisper. Soaking the lettuce in salt water or sugar water will have the opposite effect, sucking even more water out of the lettuce, making it even limper. Your day-old lettuce is long past the point where photosynthesis can do it any good, and the bright light will dry it out even more. 19–97 The orientation of cellulose microfibrils follows the orientation of the corti- cal array of microtubules. Thus, in gibberellic-acid-treated cells the cellulose microfibrils will be oriented perpendicular to the long axis of the cell, and in ethylene-treated cells the microfibrils will be oriented parallel to the long axis. Because cell elongation occurs perpendicular to the orientation of the cellulose microfibrils, gibberellic acid-treated cells will elongate in the direc- tion of the long axis of the cell, producing long, thin shoots. By contrast, ethylene-treated cells will elongate perpendicular to the long axis, produc- ing short, fat shoots. CALCULATIONS 19–98 At 0.1 MPa the hydraulic conductivity of a single water channel is 4.4 ¥ 10 –23 m 3 /s [(4.4 ¥ 10 –22 m 3 /s MPa) ¥ 0.1 MPa = 4.4 ¥ 10 –23 m 3 /s]. Thus, the ques- tion becomes how many water molecules are in 4.4 ¥ 10 –23 m 3 . There are 3.33 ¥ 10 28 water molecules/m 3 [(55.5 moles/L)(10 3 L/m 3 )(6 ¥ 10 23 water mole- cules/mole)]. Therefore, 1.5 ¥ 10 6 water molecules flow through a water channel each second at 1 atmosphere of pressure [(4.4 ¥ 10 –23 m 3 /s)(3.33 ¥ 10 23 water molecules/m 3 )]. Reference: Tyerman SD, Bohnert HJ, Maurel C, Steudle S & Smith JAC (1999) Plant aquaporins: their molecular biology, biophysics and significance for plant water relations. J. Exp. Bot. 50, 1055–1071. DATA HANDLING 19–99 The 83-kd band in Figure 19–22B is the cellulose synthase. Several experi- mental results point to this conclusion. First, it is the only band that is labeled in the purified fraction. A faint band at the same position is visible in the soluble fraction in the presence of cyclic di-GMP, suggesting that the 83- kd protein is also present at lower concentration (or activity) in the soluble fraction. Second, addition of cyclic di-GMP increases its activity by about 50%. Presumably, the effect of added cyclic di-GMP is not greater because there is already some present in the enzyme preparation. Third, the 83-kd
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  • Spring '04
  • EricLander
  • cell biology, Collagen, basal lamina, cell adhesion, J. Cell Biol.

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