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14. Inorganic Pyrophosphatase The enzyme inorganic pyrophosphatase contributes to making many biosynthetic reactions that generate inorganic pyrophosphate essentially irreversible in cells. By keeping the concentration of PPj very low, the enzyme "pulls" these reactions in the direction of PPj formation. The synthesis of ADP-glucose in chloroplasts is one reaction that is pulled in the forward direction by this mechanism. However, the synthesis of UDP-glucose in the plant cytosol, which produces PPj, is readily reversible in vivo. How do you reconcile these two facts?

15. Regulation of Starch and Sucrose Synthesis Sucrose synthesis occurs in the cytosol and starch synthesis in the chloroplast stroma, yet the two processes are intricately balanced. What factors shift the reactions in favor of (a) starch synthesis and (b) sucrose synthesis?

16. Regulation of Sucrose Synthesis In the regulation of sucrose synthesis from the triose phosphates produced during photosynthesis, 3-phosphoglycerate and Pi play critical roles (see Fig. 20-26). Explain why the concentrations of these two regulators reflect the rate of photosynthesis.

17. Sucrose and Dental Caries The most prevalent infection in humans worldwide is dental caries, which stems from the colonization and destruction of tooth enamel by a variety of acidifying microorganisms. These organisms synthesize and live within a water-insoluble network of dextrans, called dental plaque, composed of (a1n6)-linked polymers of glucose with many (a1n3) branch points. Polymerization of dextran requires dietary sucrose, and the reaction is catalyzed by a bacterial enzyme, dextran-sucrose glucosyltransferase.

(a) Write the overall reaction for dextran polymerization.

(b) In addition to providing a substrate for the formation of dental plaque, how does dietary sucrose also provide oral bacteria with an abundant source of metabolic energy?

18. Differences between C3 and C4 Plants The plant genus Atriplex includes some C3 and some C4 species. From the data in the plots below (species 1, black curve; species 2, red curve), identify which is a C3 plant and which is a C4 plant. Justify your answer in molecular terms that account for the data in all three plots.

19. C4 Pathway in a Single Cell In typical C4 plants, the initial capture of CO2 occurs in one cell type, and the Calvin cycle reactions occur in another (see Fig. 20-23). Voznesen-skaya and colleagues11 have described a plant, Bienertia cy-cloptera—which grows in salty depressions of semidesert in Central Asia—that shows the biochemical properties of a C4 plant but unlike typical C4 plants does not segregate the reactions of CO2 fixation into two cell types. PEP carboxylase and rubisco are present in the same cell. However, the cells have two types of chloroplasts, which are localized differently, as shown in the micrograph. One type, relatively poor in grana (thylakoids), is confined to the periphery; the more typical chloroplasts are clustered in the center of the cell, separated from the peripheral chloroplasts by large vacuoles. Thin cy-tosolic bridges pass through the vacuoles, connecting the peripheral and central cytosol.

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In this plant, where would you expect to find (a) PEP carboxylase, (b) rubisco, and (c) starch granules? Explain your answers with a model for CO2 fixation in these C4 cells.

ttVoznesenskaya, E.V., Fraceschi, V.R., Kiirats, O., Artyusheva, E.G., Freitag, H., & Edwards, G.E. (2002) Proof of C4 photosynthesis without Kranz anatomy in Bienertia cycloptera (Chenopodiaceae). Plant J. 31, 649-662.

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