Lehninger Principles of Biochemistry Test Bank Ch. 20.pdf

Chapter 20 Carbohydrate Biosynthesis in Plants and Bacteria Multiple Choice Questions 1. Photosynthetic carbohydrate synthesis Page: 753 Difficulty: 1 Ans: C The compound that condenses with CO2 in the first reaction of carbon dioxide assimilation is: A) B) C) D) E)

3-phosphoglycerate. ribose 1,5-bisphosphate. ribulose 1,5-bisphosphate. ribulose 5-phosphate. rubisco.

2. Photosynthetic carbohydrate synthesis Pages: 758-759 Difficulty: 2 Ans: C Which of these enzymes is not part of the Calvin cycle? A) B) C) D) E)

Aldolase Glyceraldehyde 3-phosphate dehydrogenase Phosphofructokinase-1 Ribulose-5-phosphate kinase Transketolase

3. Photosynthetic carbohydrate synthesis Pages: 759-761 Difficulty: 2 Ans: E Transketolase requires the coenzyme:

A) B) C) D) E)

cobalamin (vitamin B12). lipoic acid pyridoxal phosphate. tetrahydrofolic acid. thiamine pyrophosphate.

4. Photosynthetic carbohydrate synthesis Page: 759 Difficulty: 2 Ans: D When transketolase acts on fructose 6-phosphate and glyceraldehyde 3-phosphate, the products are: A) B) C) D) E)

3-phosphoglycerate and glyceraldehyde 3-phosphate. 3-phosphoglycerate and two molecules of glyceraldehyde 3-phosphate. dihydroxyacetone phosphate and glucose 6-phosphate. xylulose 5-phosphate and erythrose 4-phosphate. xylulose 5-phosphate and ribose 5-phosphate.

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5. Photosynthetic carbohydrate synthesis Page: 762 Difficulty: 1 Ans: C Which of these compounds is not directly involved in the Calvin cycle? A) B) C) D) E)

Erythrose 4-phosphate Glyceraldehyde 3-phosphate Mannose 6-phosphate Ribulose 5-phosphate Sedoheptulose 7-phosphate

6. Photosynthetic carbohydrate synthesis Page: 762 Difficulty: 2 Ans: B In the carbon assimilation (“dark”) reactions of photosynthesis, the biosynthesis of 1 mol of hexose from 6 mol of carbon dioxide requires: A) B) C) D) E)

12 mol of NADPH and 12 mol of ATP. 12 mol of NADPH and 18 mol of ATP. 18 mol of NADPH and 12 mol of ATP. 18 mol of NADPH and 18 mol of ATP. no NADPH and 12 mol of ATP.

7. Photosynthetic carbohydrate synthesis Pages: 764, 769 Difficulty: 3 Ans: C The known mechanisms of activation of rubisco or of other enzymes of the Calvin cycle during illumination include all of the following except: A) B) C) D) E)

increased stromal pH. light-driven entry of Mg2+ into the stroma. phosphorylation by cAMP-dependent protein kinase. phosphorylation of phosphoenolpyruvate carboxylase. reduction of a disulfide bridge by thioredoxin.

8. Photosynthetic carbohydrate synthesis Page: 765 Difficulty: 3 Ans: E Which of these chloroplast enzymes is not regulated by light? A) B) C) D) E)

Fructose 1,6-bisphosphatase Glyceraldehyde-phosphate dehydrogenase Ribulose 5-phosphate kinase Sedoheptulose 1,7-bisphosphatase All of the above are regulated by light.

9. Photosynthetic carbohydrate synthesis Page: 765 Difficulty: 2 Ans: A The carbon assimilation (“dark”) reactions of photosynthetic plants: A) B) C) D) E)

are driven ultimately by the energy of sunlight. are important to plants, but ultimately of little significance for bacteria and animals. cannot occur in the light. yield (reduced) NADH. yield ATP, which is required for the light reactions.

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10. Photosynthetic carbohydrate synthesis Page: 765 Difficulty: 2 Ans: B The assimilation of CO2 into organic compounds (triose phosphates) in green plants: A) B) C) D) E)

involves condensation of the two-carbon compound acetate with CO2 to form 3-phosphoglycerate. requires NADPH. results in the production of ATP. takes place at equal rates in light and darkness. takes place in the cytosol.

11. Photorespiration and the C4 and CAM pathways Page: 766 Difficulty: 1 Ans: C All are true of photorespiration except: A) B) C) D) E)

It is driven by light. It oxidizes substrates to CO2. It produces O2. It results from a lack of specificity of the enzyme rubisco. It results in no fixation of carbon.

12. Photorespiration and the C4 and CAM pathways Page: 767 Difficulty: 1 Ans: D The three subcellular organelles involved in the phosphoglycolate salvage pathway are: A) B) C) D) E)

endoplasmic reticulum, chloroplast, and mitochondrion. nucleus, endoplasmic reticulum, and chloroplast. golgi apparatus, chloroplast, and mitochondrion. mitochondrion, peroxisome, and chloroplast. peroxisome, endoplasmic reticulum, and chloroplast.

13. Photorespiration and the C4 and CAM pathways Page: 767 Difficulty: 1 Ans: C The glycine decarboxylase complex in the leaves of pea or spinach plants is localized mainly in the: A) B) C) D) E)

chloroplast. endoplasmic reticulum. mitochondrion. cell membrane. peroxisome.

14. Photorespiration and the C4 and CAM pathways Page: 769 Difficulty: 1 Ans: C In “C4” plants of tropical origin, the first intermediate into which 14CO2 is fixed is: A) B) C) D) E)

aspartate. phosphoenolpyruvate. oxaloacetate. malate. 3-phosphoglycerate.

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15. Biosynthesis of starch and sucrose Page: 771 Difficulty: 2 Ans: C The synthesis of glycogen, starch, and sucrose all: A) B) C) D) E)

involve addition of a sugar residue at the reducing end of the growing polymer. take place in liver and muscle of mammals. use a sugar nucleotide as substrate. use glucose 1-phosphate as the only substrate. use glucose-6-phosphate as substrate.

16. Biosynthesis of starch and sucrose Page: 771 Difficulty: 2 Ans: B The synthesis of starch and sucrose in plants uses _________ as the substrate, rather than _________, which is used in the synthesis of glycogen in animal cells. A) B) C) D) E)

ADP-fructose; UDP-glucose ADP-glucose; UDP-glucose fructose 1-phosphate; glucose 1-phosphate glucose 1-phosphate; glucose 6-phosphate UDP-glucose; ADP-glucose

17. Synthesis of cell wall polysaccharides: plant cellulose and bacterial peptidoglycan Pages: 777-778 Difficulty: 2 Ans: D A precursor in the synthesis of the peptidoglycan of bacterial cell walls is UDP-: A) B) C) D) E)

galactose. glucose. glucuronic acid. N-acetylglucosamine. penicillin.

18. Synthesis of cell wall polysaccharides: plant cellulose and bacterial peptidoglycan Page: 778 Difficulty: 2 Ans: C Penicillin inhibits the synthesis of peptidoglycan: A) B) C) D) E)

branches. chains. crosslinks. precursors. all of the above.

19. Integration of carbohydrate metabolism in the plant cell Page: 780 Difficulty: 1 Ans: E Which one of the following reactions, cycles, or pathways is not found in plant systems? A) B) C) D) E)

The Calvin cycle The gluconeogenesis pathway The glyoxalate cycle The rubisco reaction The urea cycle

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20. Integration of carbohydrate metabolism in the plant cell Page: 780 Difficulty: 1 Ans: D Which one of the following cellular organelles is not unique to plant cells, in carrying out the indicated pathway or function of carbohydrate metabolism? A) B) C) D) E)

Amyloplasts (starch synthesis) Chloroplasts (Calvin cycle) Glyoxysomes (glyoxlate cycle) Mitochondria (citric acid cycle) Vacuoles (organic acid storage)

21. Integration of carbohydrate metabolism in the plant cell Pages: 781-782 Difficulty: 2 Ans: B Which one of the following sugar phosphates is not part of the pool of readily interconvertible metabolites used by the plant cell? A) B) C) D) E)

Dihydroxyacetone phosphate Fructose 2,6-bisphosphate Glucose 1-phosphate 6-phosphogluconate Xylulose 5-phosphate

22. Integration of carbohydrate metabolism in the plant cell Page: 782 Difficulty: 3 Ans: B When glycerol is converted to glucose via gluconeogenesis in germinating seeds, the first glycolytic intermediate formed is: A) B) C) D) E)

1,3-bisphosphoglycerate. dihydroxyacetone phosphate. glycerol 1,3-bisphosphate. glycerol 3-phosphate. ribulose 1,5-bisphosphate.

Short Answer Questions 23. Photosynthetic carbohydrate synthesis Page: 754 Difficulty: 2 Show the reaction catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco). Ans: Ribulose 1,5-bisphosphate + CO2 → 2 (3-phosphoglycerate) (For the details of the reaction, see Fig. 20-7, p. 756.) 24. Photosynthetic carbohydrate synthesis Page: 754 Difficulty: 2 Draw the structure of 3-phosphoglycerate. Circle the atom(s) that would be labeled first in plants grown in CO2 labeled with radioactive carbon.

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Ans:

25. Photosynthetic carbohydrate synthesis Page: 758 Difficulty: 3 Show the reaction in which 3-phosphoglycerate is converted into glyceraldehyde 3-phosphate. Show all required cofactors, and circle the carbon atom(s) in glyceraldehyde 3-phosphate that is (are) derived from CO2 during the photosynthetic fixation of CO2. Ans: (1) 3-phosphoglycerate + ATP → 1,3-bisphosphoglycerate + ADP (2) 1,3-bisphosphoglycerate + NADPH + H+ → glyceraldehyde 3-phosphate + NADP+ + Pi The carbon atom at C-1 of glyceraldehyde 3-phosphate is derived from CO2.

26. Photosynthetic carbohydrate synthesis Page: 760 Difficulty: 2 Diagram the reaction catalyzed by transketolase when fructose 6-phosphate and glyceraldehyde 3phosphate are the substrates. Ans: Fructose 6-phosphate + glyceraldehyde 3-phosphate → xylulose 5-phosphate + erythrose 4-phosphate (See Fig. 20-11, p. 760, for structures.) 27. Photosynthetic carbohydrate synthesis Page: 762 Difficulty: 2 Explain why both ATP and NADPH are required for the operation of the Calvin cycle, and why these two reactants are required in different amounts. Ans: ATP is required in two reactions of the Calvin cycle: the formation of 1,3-bisphosphoglycerate from 3-phosphoglycerate (2 ATP per CO2 fixed) and the conversion of ribulose 5-phosphate into ribulose 1,5-bisphosphate (1 ATP per CO2 fixed). NADPH is required in the reduction of 1,3bisphosphoglycerate to glyceraldehyde 3-phosphate (2 NADPH per CO2 fixed). Thus the operation of the cycle consumes 3 ATP and 2 NADPH per CO2 fixed. (See Fig. 20-14, p. 762.) 28. Photosynthetic carbohydrate synthesis Page: 764 Difficulty: 2 How does glyceraldehyde 3-phosphate formed in the chloroplast stroma by the Calvin cycle reactions enter the cytosol?

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Ans: It is converted to dihydroxyacetone phosphate or to 3-phosphoglycerate and carried into the cytosol by the Pi-triose phosphate antiporter, a specific transporter in the inner chloroplast membrane. (See Fig. 20-16, p. 764.) 29. Photosynthetic carbohydrate synthesis Pages: 764-765 Difficulty: 3 Describe how thioredoxin participates in the regulation of several chloroplast enzymes by light. Ans: Thioredoxin is an electron-carrying protein that is reduced by electrons from ferredoxin during illumination. Electrons from thioredoxin reduce critical disulfide bonds in key enzymes of the Calvin cycle, activating those enzymes. (See Fig. 20-19, p. 765.) 30. Photorespiration and the C4 and CAM pathways Pages: 766-768 Difficulty: 3 Describe the oxygenase activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) and explain why this reaction is undesirable from the point of view of a plant. Ans: The condensation of molecular oxygen with ribulose 1,5-bisphosphate yields 3phosphoglycerate and the two-carbon compound phosphoglycolate. Phosphoglycolate has no known metabolic role; its carbon is salvaged by a series of reactions that consume O2 and produce CO2 − the “photorespiration” process. The salvage pathway requires energy, and therefore the oxygenase reaction of rubisco represents a net energy cost to the plant cell in which it occurs. 31. Photorespiration and the C4 and CAM pathways Page: 767 Difficulty: 1 Describe the reaction sequence by which 2-phosphoglycolate (produced when O2 replaces CO2 as substrate for rubisco) is converted to serine. Name each enzyme and any cofactors required and indicate the subcellular compartment in which the reaction takes place. Ans: 2-phosphoglycolate is converted to glycolate by a phosphatase in the chloroplast. Glycolate is transported to the peroxisome and converted to glyoxylate by glycolate oxidase. The glyoxylate is then converted in the peroxisome to glycine by a transaminase that requires pyridoxal phosphate. Finally, two molecules of glycine are converted to serine + NH3 + CO2 by the enzyme glycine decarboxylase, which is located in the mitochondrion. 32. Photorespiration and the C4 and CAM pathways Page: 770 Difficulty: 1 CAM plants, such as cactus and pineapple, are native to very hot and dry environments. Briefly describe the biochemical events that allow CAM plants to minimize water loss by closing their stroma during daylight hours. Ans: CAM plants fix CO2 into malate in the dark when the stroma are open. The resulting malate is stored in vacuoles. During daylight hours, the CO2 is released from malate by the action of the NADP-linked malic enzyme, and the CO2 serves as substrate for rubisco. 33. Biosynthesis of starch and sucrose Pages: 772-773 Difficulty: 2 Diagram the pathway by which sucrose is synthesized from glucose 6-phosphate; indicate how any required cofactors are involved.

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Ans: (See Fig. 20-25, p. 773.) Glucose 6-phosphate is converted into glucose 1-phosphate by phosphoglucomutase, and into fructose 6-phosphate by phosphohexose isomerase. Then, Glucose 1-phosphate + UTP → UDP-glucose + PPi UDP-glucose + fructose 6-phosphate → sucrose 6-phosphate + UDP Sucrose 6-phosphate → sucrose + Pi 34. Biosynthesis of starch and sucrose Pages: 771-772 Difficulty: 2 Explain the utility to plants in using sucrose as the transport form of carbon. Ans: Sucrose is a nonreducing disaccharide, with its glycosidic linkage between the anomeric carbons of glucose and fructose. The unavailability of these normally reactive carbons prevents sucrose from reacting nonenzymatically with amino acids or proteins, and this unusual bond is not hydrolyzed by amylases or other common carbohydrate-cleaving enzymes. 35. Biosynthesis of starch and sucrose Page: 772 Difficulty: 2 Explain how starch synthase, in contrast to glycogen synthase in animals, can lengthen starch molecules from the reducing end of the polysaccharide chain. Ans: There are two equivalent active sites in starch synthase that alternate in being covalently attached to the reducing end of the growing chain, with nucleophilic displacement of the enzyme by a glucosyl moiety bound at the other active site. (See Fig. 20-24.) 36. Biosynthesis of starch and sucrose Page: 773 Difficulty: 2 Describe briefly how the allosteric effector fructose 2,6-bisphosphate (F2,6BP) regulates starch and sucrose synthesis. Ans: In vascular plants, the concentration of F2,6BP varies inversely with the rate of photosynthesis. At high levels of photosynthesis, the dihydroxyacetone phosphate produced inhibits phosphofructokinase-2, which makes F2,6BP, thus favoring greater flux of triose phosphate into fructose 6phosphate formation and sucrose synthesis. 37. Biosynthesis of starch and sucrose Page: 773 Difficulty: 3 Describe the role of fructose 2,6-bisphosphate in the regulation of sucrose biosynthesis in plant cells. Ans: In the light, 3-phosphoglycerate and dihydroxyacetone phosphate produced by the Calvin cycle inhibit PFK-2, slowing the conversion of fructose 6-phosphate to fructose 2,6-bisphosphate. The fructose 6-phosphate is then used to synthesize sucrose. In the dark, the fructose 2,6-bisphosphate level rises. Elevated levels of fructose 2,6-bisphosphate inhibit FBPase-1 and activate PFK-1, which results in a decrease in the level of fructose 6-phosphate and hence in the synthesis of sucrose. (See Fig. 20-26, p. 773.) 38. Integration of carbohydrate metabolism in the plant cell Page: 781 Difficulty: 3 Describe how plants and some microorganisms can, unlike animals, convert acetyl-CoA derived from fatty acids into glucose or sucrose.

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Ans: The integration of reaction sequences in three subcellular compartments is required for the production of fructose 6-phosphate or sucrose from stored lipids. Some of the essential enzymes are sequestered in glyoxysomes, where glyoxysome-specific isozymes of β-oxidation break down fatty acids to acetyl-CoA. The physical sepration of the glyoxylate cycle and β -oxidation enzymes from the mitochondrial citric acid cycle enzymes prevents further oxidation of acetyl-CoA to CO2. Instead, the acetyl-CoA is converted to succinate in the glyoxylate cycle, which passes into the mitochondrial matrix, where it is converted by citric acid cycle enzymes to oxaloacetate, which moves into the cytosol. Cyotsolic oxaloacetate is converted by gluconeogenesis to fructose 6-phosphate, a precursor for glucose or sucrose. 39. Integration of carbohydrate metabolism in the plant cell Page: 782 Difficulty: 2 Describe the interconversions between the triose-, pentose- and hexose-phosphate pools in the plant cell. Ans: Compounds in each pool are readily interconverted by enzymatic reactions with small standard free-energy changes. Thus a temporary imbalance is rapidly corrected to achieve a new equilibrium state. Specific interconversions are depicted in Fig. 20-37, p. 782.