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Chapter 20 Carbohydrate Biosynthesis in Plants and Bacteria
Multiple Choice Questions
1. Which of the following is not true about plastids?
A) Most of the proteins in a plastid are encoded in nuclear genes.
B) Plastids are encoded by a double membrane.
C) Plastids are self-replicating.
D) Plastids contain a small circular genome.
E) The inner membrane of plastids is permeable to polar and charged molecules.
2. The compound that condenses with CO2 in the first reaction of carbon dioxide assimilation is:
A) 3-phosphoglycerate.
B) ribose 1,5-bisphosphate.
C) ribulose 1,5-bisphosphate.
D) ribulose 5-phosphate.
E) rubisco.
3. The final product that is formed by the enzyme rubisco is:
A) 3-phosphoglycerate.
B) ATP.
C) ribulose 1,5-bisphosphate.
D) glyceraldehyde-3-phosphate.
E) CO2.
4. Which of the following is not required to activate rubisco?
A) ATP hydrolysis
B) Carbamoylation of the active site lysine by rubisco activase
C) Release of ribulose 1,5-bisphosphate from the active site
D) Release of 2-carboxyarabinitol from the active site
E) Binding of Mg2+
5. Which of these enzymes is not part of the Calvin cycle?
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A) Aldolase
B) Glyceraldehyde 3-phosphate dehydrogenase
C) Phosphofructokinase-1
D) Ribulose-5-phosphate kinase
E) Transketolase
6. Transketolase requires the coenzyme:
A) cobalamin (vitamin B12).
B) lipoic acid
C) pyridoxal phosphate.
D) tetrahydrofolic acid.
E) thiamine pyrophosphate.
Pages: 805–807 Difficulty: 2 Ans: E
7. When transketolase acts on fructose 6-phosphate and glyceraldehyde 3-phosphate, the products are:
A) 3-phosphoglycerate and glyceraldehyde 3-phosphate.
B) 3-phosphoglycerate and two molecules of glyceraldehyde 3-phosphate.
C) dihydroxyacetone phosphate and glucose 6-phosphate.
D) xylulose 5-phosphate and erythrose 4-phosphate.
E) xylulose 5-phosphate and ribose 5-phosphate.
Pages: 805–807 Difficulty: 2 Ans: D
8. Which of these compounds is not directly involved in the Calvin cycle?
A) Erythrose 4-phosphate
B) Glyceraldehyde 3-phosphate
C) Mannose 6-phosphate
D) Ribulose 5-phosphate
E) Sedoheptulose 7-phosphate
9. In the carbon assimilation (“dark”) reactions of photosynthesis, the biosynthesis of 1 mol of hexose
from 6 mol of carbon dioxide requires:
A) 12 mol of NADPH and 12 mol of ATP.
B) 12 mol of NADPH and 18 mol of ATP.
C) 18 mol of NADPH and 12 mol of ATP.
D) 18 mol of NADPH and 18 mol of ATP.
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E) no NADPH and 12 mol of ATP.
10. The known mechanisms of activation of rubisco or of other enzymes of the Calvin cycle during
illumination include all of the following except:
A) increased stromal pH.
B) light-driven entry of Mg2+ into the stroma.
C) phosphorylation by cAMP-dependent protein kinase.
D) phosphorylation of phosphoenolpyruvate carboxylase.
E) reduction of a disulfide bridge by thioredoxin.
11. Which of these chloroplast enzymes is not regulated by light?
A) Fructose 1,6-bisphosphatase
B) Glyceraldehyde-phosphate dehydrogenase
C) Ribulose 5-phosphate kinase
D) Sedoheptulose 1,7-bisphosphatase
E) All of the above
12. The carbon assimilation (“dark”) reactions of photosynthetic plants:
A) are driven ultimately by the energy of sunlight.
B) are important to plants, but ultimately of little significance for bacteria and animals.
C) cannot occur in the light.
D) yield (reduced) NADH.
E) yield ATP, which is required for the light reactions.
13. The assimilation of CO2 into organic compounds (triose phosphates) in green plants:
A) involves condensation of the two-carbon compound acetate with CO2 to form 3-phosphoglycerate.
B) requires NADPH.
C) results in the production of ATP.
D) takes place at equal rates in light and darkness.
E) takes place in the cytosol.
14. Which of the following is not true of the Pi-triose phosphate antiporter?
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241
A) The antiporter can move phosphate in or out of the chloroplast.
B) When Pi is moved into the chloroplast, it supports synthesis of ATP.
C) When triose phosphates are moved out of the chloroplast, they become incorporated into sucrose.
D) ATP and reducing equivalents are moved into the chloroplast by the antiporter.
E) The antiporter is found in the inner membrane of chloroplasts.
15. All are true of photorespiration except:
A) It is driven by light.
B) It oxidizes substrates to CO2.
C) It produces O2.
D) It results from a lack of specificity of the enzyme rubisco.
E) It results in no fixation of carbon.
16. The three subcellular organelles involved in the phosphoglycolate salvage pathway are:
A) endoplasmic reticulum, chloroplast, and mitochondrion.
B) nucleus, endoplasmic reticulum, and chloroplast.
C) golgi apparatus, chloroplast, and mitochondrion.
D) mitochondrion, peroxisome, and chloroplast.
E) peroxisome, endoplasmic reticulum, and chloroplast.
17. The glycine decarboxylase complex in the leaves of pea or spinach plants is localized mainly in the:
A) chloroplast.
B) endoplasmic reticulum.
C) mitochondrion.
D) cell membrane.
E) peroxisome.
18. In “C4” plants of tropical origin, the first intermediate into which 14CO2 is fixed is:
A) aspartate.
B) phosphoenolpyruvate.
C) oxaloacetate.
D) malate.
E) 3-phosphoglycerate.
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19. The synthesis of glycogen, starch, and sucrose all:
A) involve addition of a sugar residue at the reducing end of the growing polymer.
B) take place in liver and muscle of mammals.
C) use a sugar nucleotide as substrate.
D) use glucose 1-phosphate as the only substrate.
E) use glucose-6-phosphate as substrate.
20. 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) ADP-fructose; UDP-glucose
B) ADP-glucose; UDP-glucose
C) fructose 1-phosphate; glucose 1-phosphate
D) glucose 1-phosphate; glucose 6-phosphate
E) UDP-glucose; ADP-glucose
21. The precursors for sucrose biosynthesis are:
A) glucose and fructose.
B) UDP-glucose and fructose 6-phosphate.
C) UDP-fructose and glucose 6-phosphate.
D) UDP-glucose and fructose.
E) UDP-glucose and UDP-fructose.
22. The major regulator of sucrose biosynthesis in plants is:
A) fructose 2,6-bisphosphate.
B) fructose 1,6-bisphosphate.
C) sucrose.
D) glucose and fructose.
E) glucose 6-phosphate.
23. Which of the following is not true about the biosynthesis of cellulose?
A) UDP-glucose is used as a substrate.
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243
B) Glucose is transported across the membrane via a lipid-linked oligosaccharide intermediate.
C) The extracellular form of cellulose synthase adds up to 15,000 glucose units to one chain.
D) Cellulose polymers line up in an anti-parallel fashion.
E) The reaction catalyzed by cellulose synthase proceeds via inversion of configuration.
24. A precursor in the synthesis of the peptidoglycan of bacterial cell walls is UDP-:
A) galactose.
B) glucose.
C) glucuronic acid.
D) N-acetylglucosamine.
E) penicillin.
25. Penicillin inhibits the synthesis of peptidoglycan:
A) branches.
B) chains.
C) crosslinks.
D) precursors.
E) All of the above
26. Which one of the following reactions, cycles, or pathways is not found in plant systems?
A) The Calvin cycle
B) The gluconeogenesis pathway
C) The glyoxalate cycle
D) The rubisco reaction
E) The urea cycle
27. Which one of the following cellular organelles is not unique to plant cells?
A) Amyloplasts
B) Chloroplasts
C) Glyoxysomes
D) Mitochondria
E) Vacuoles
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244
28. Which one of the following sugar phosphates is not part of the pool of readily interconvertible
metabolites used by the plant cell?
A) Dihydroxyacetone phosphate
B) Fructose 2,6-bisphosphate
C) Glucose 1-phosphate
D) 6-phosphogluconate
E) Xylulose 5-phosphate
29. When glycerol is converted to glucose via gluconeogenesis in germinating seeds, the first glycolytic
intermediate formed is:
A) 1,3-bisphosphoglycerate.
B) dihydroxyacetone phosphate.
C) glycerol 1,3-bisphosphate.
D) glycerol 3-phosphate.
E) ribulose 1,5-bisphosphate.
Short Answer Questions
30. Show the reaction catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco).
31. 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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245
32. 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.
33. Diagram the reaction catalyzed by transketolase when fructose 6-phosphate and glyceraldehyde 3-
phosphate are the substrates.
34. 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.
35. How does glyceraldehyde 3-phosphate formed in the chloroplast stroma by the Calvin cycle reactions
enter the cytosol?
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246
36. Describe how thioredoxin participates in the regulation of several chloroplast enzymes by light.
37. Explain why photorespiration occurs in plant cells carrying out photosynthesis.
38. 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.
39. 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.
40. 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.
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41. Diagram the pathway by which sucrose is synthesized from glucose 6-phosphate; indicate how any
required cofactors are involved.
42. Explain the utility to plants in using sucrose as the transport form of carbon.
43. Describe briefly how the allosteric effector fructose 2,6-bisphosphate (F2,6BP) regulates starch and
sucrose synthesis.
44. Describe the role of fructose 2,6-bisphosphate in the regulation of sucrose biosynthesis in plant cells.
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248
45. Name one similarity and one difference between the biosynthesis of glycogen and the biosynthesis of
cellulose.
46. Describe how plants and some microorganisms can, unlike animals, convert acetyl-CoA derived from
fatty acids into glucose or sucrose.
47. Describe the interconversions between the triose-, pentose- and hexose-phosphate pools in the plant
cell.
