Chapter 20 Carbohydrate Biosynthesis in plants (Ed. 6 test bank)

Question 1

The compound that condenses with CO2 in the first reaction of carbon dioxide assimilation is:

Answer 1

ribulose 1,5-bisphosphate

Question 2

Which of these enzymes is not part of the Calvin cycle? A) Aldolase B) Glyceraldehyde 3-phosphate dehydrogenase C) Phosphofructokinase-1 D) Ribulose-5-phosphate kinase E) Transketolase

Answer 2

Phosphofructokinase-1

Question 3

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

Answer 3

Mannose 6-phosphate

Question 4

In the carbon assimilation (“dark”) reactions of photosynthesis, the biosynthesis of 1 mol of hexose from 6 mol of carbon dioxide requires:

Answer 4

12 mol of NADPH and 18 mol of ATP.

Question 5

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.

Answer 5

Phosphorylation by cAMP-dependent protein kinase

Question 6

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.

Answer 6

Are driven ultimately by the energy by the sunlight

Question 7

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.

Answer 7

It produces O2

Question 8

The three subcellular organelles involved in the phosphoglycolate salvage pathway are:

Answer 8

Mitochondrion, peroxisome, and chloroplast

Question 9

In “C4” plants of tropical origin, the first intermediate into which 14CO2 is fixed is:

Answer 9

Oxaloacetate

Question 10

The synthesis of glycogen, starch, and sucrose all:

Answer 10

Use a sugar nucleotide as a substrate

Question 11

The precursors for sucrose biosynthesis are:

Answer 11

UDP-glucose and fructose 6-phosphate

Question 12

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

Answer 12

The urea cycle

Question 13

When glycerol is converted to glucose via gluconeogenesis in germinating seeds, the first glycolytic intermediate formed is:

Answer 13

Dihydroxyacetone phosphate

Question 14

Show the reaction catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco).

Answer 14

Ribulose 1,5-bisphosphate + CO2 –> 2 (3-phosphoglycerate)

Question 15

Draw the structure of 3-phosphoglycerate. Circle the atom(s) that would be labeled first in plants grown in CO2 labeled with radioactive carbon.

Answer 15

Question 16

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.

Answer 16

(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.

Question 17

Explain why photorespiration is necessary for plant cells carrying out photosynthesis.

Answer 17

Because RuBisCo is not particularly specific for the substrate it reacts with RuBP, sometimes it reacts RuBP with O2, which is much more abundant than the proper substrate, CO2. This results in the production of phosphoglycolate which must be processed by photorespiration.

Question 18

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.

Answer 18

The condensation of molecular oxygen with ribulose 1,5-bisphosphate yields 3- phosphoglycerate 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.

Question 19

Diagram the pathway by which sucrose is synthesized from glucose 6-phosphate; indicate how any required cofactors are involved.

Answer 19

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

Question 20

Explain how starch synthase, in contrast to glycogen synthase in animals, can lengthen starch molecules from the reducing end of the polysaccharide chain.

Answer 20

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.