chapter 20

Describe the three stages of the Calvin cycle.

1. the fixation of carbon dioxide by reaction with ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate; 2. the formation of hexose sugars from 3-phosphoglycerate; and 3. the regeneration of ribulose 1,5-bisphosphate.

How many ATP are required by the C4 pathway for each hexose that is produced? A) 48 B) 30 C) 24 D) 18 E) 12

30

3) What is another name for ribulose 1,5-bisphosphate carboxylase/oxygenase? A) rubisco B) ribulooxidase C) ribulocarboxidase D) ribulose bisphosphatase E) None of the answers is correct.

A

7) What metabolic adaptation is employed by plants living in hot dry climates to prevent water loss and decrease photorespiration? A) crassulacean acid metabolism B) cellulose synthesis at night C) C4 pathway D) increased fructose-2,6-bisphosphatase activity E) inhibition of glycolate salvage

A

In photosynthesis, light energy is transformed into ATP and biosynthetic reducing power in the form of _____. A) NADPH B) NADH C) FADH2 D) ADP E) None of the answers is correct.

A

What is the source of carbons for the Calvin cycle? A) glucose D) glyoxylate B) carbon dioxide E) None of the answers is correct. C) glycogen

B

8) Which of the following is a readily mobilized sugar that is commonly used to transport carbohydrates through a plant? A) glucose B) fructose C) sucrose D) amylose E) amylopectin

C

The C4 pathway is necessary in tropical plants because A) at high temperatures, the plants cannot maintain sufficient water levels within the cells. B) tropical plants do not have proper day/light cycles to maintain the balance of CO2 necessary for carbohydrate storage. C) at high temperatures, the oxygenase activity of rubisco is high. D) All of the answers are correct. E) None of the answers is correct.

C

What is the net reaction of the transketolase and transaldolase steps? A) 3C5 2C6 + C3 D) 2C5 2C4 + C2 B) 2C5 C6 + C4 E) None of the answers is correct. C) 3C5 C6 + 3C3

C

9) Which of the following types of organisms obtain their energy from chemical fuels produced by other organisms? A) heterotrophs B) autotrophs C) chemotrophs D) phototrophs E) heterotrophs and chemotrophs

C check this other quizlet says A

4) What pathway ensures that sufficient amounts of CO2 are available to minimize wasteful photorespiration by transporting CO2 from the mesophyll cells to the bundle sheath cells? A) crassulacean acid metabolism B) gluconeogenesis C) C4 pathway D) sucrose synthesis E) cellulose synthesis

C4

2) What intermediate is formed in the oxygenase reaction catalyzed by rubisco? A) glyoxylate B) hydroxypyruvate C) glycerate D) phosphoglycolate E) malate

D

In the Calvin cycle, 3-phosphoglycerate is ultimately converted into which hexose phosphate? A) glucose 1-phosphate D) All of the answers are correct. B) glucose 6-phosphate E) None of the answers is correct. C) fructose 6-phosphate

D

The purpose of the pentose phosphate pathway is to A) generate ATP. D) A and B. B) generate NADPH. E) B and C. C) metabolize 5-carbon sugars.

D

Which cofactor supplies the reducing power for reduction of glutathione by glutathione reductase? A) FMN B) TPP C) FAD D) NADPH E) NADH

D

Which of the following reactions is NOT part of the pentose phosphate pathway? A) C5 +C5 <-> C3C7 B) C4+C5 <-> C3 +C6 C)C3 +C7 <-> C6+ C4 D) C5 +C6 <-> C2+C9 E) All of the answers are correct.

D

Which sugar(s) is (are) converted into ribulose 5-phosphate by a single enzymatic step? A) ribose 5-phosphate D) A and B B) xylulose 5-phosphate E) B and C C) erythrose 4-phosphate

D

10) What prosthetic group is tightly bound to the enzyme transketolase? A) biotin B) tetrahydrofolate C) FAD D) pyridoxal phosphate E) thiamine pyrophosphate

E

5) What thiol-containing molecule in its reduced state is critical to reducing reactive oxygen species? A) NADPH B) folate C) vitamin E D) vitamin C E) glutathione

E

Which of the following tissues have the least active pentose-phosphate pathway? A) adrenal gland B) liver C) adipose tissue D) ovary E) skeletal muscle

E

Which of the following types of organisms can synthesize glucose from carbon dioxide and water? A) heterotrophs B) autotrophs C) chemotrophs D) phototrophs E) autotrophs and phototrophs

E

In the overall equation for the Calvin cycle, three molecules of __________ and two molecules of ____________ are consumed in incorporating a single CO2 molecule into a hexose. A) NADPH; ATP B) ATP; NADPH C) NADPH; Pi D) Pi; NADH E) NADH; ATP

E but its three molecules of ATP and Two molecules of ATP and three molecules of NADH

What is the hexose monophosphate pool?

In the Calvin cycle, two molecules of DHAP are required for the synthesis of a member of the hexose monophosphate pool. This pool consists of glucose 1-phosphate, glucose 6-phosphate and fructose 6-phosphate, all of which are readily interconverted.

What is the energy requirement for the formation of hexose from CO2 in terms of ATP equivalents and NADPH?

It requires six molecules of CO2 to make one hexose phosphate. Three molecules of ATP and two molecules of NADPH are consumed in each round of the Calvin cycle. Thus, a total of 18 equivalents of ATP and 12 NADPH are required to convert six CO2 into a hexose.

The oxidative phase of the pentose phosphate pathway produces __________, which is required for lipid biosynthesis.

NADPH

Which coenzyme is required by glyceraldehyde-3-phosphate dehydrogenase in chloroplasts to convert 3-phosphoglycerate into glyceraldehyde 3-phosphate? A) NADH D) NADP+ B) NADPH E) thiamine pyrophosphate C) NAD+

NADPH

What is photorespiration? Why is it wasteful?

Normal photosynthesis produces O2 and fixes CO2 into carbohydrate. Photorespiration occurs when O2 is used and CO2 is produced. It is a result of oxygen combining with ribulose 1,5-bisphosphate instead of CO2. Phosphoglycolate is produced instead of one of the 3-phosphoglycerates. Phosphoglycolate is not a versatile metabolite, and a salvage pathway recovers three carbons from two glyoxylates. The fourth carbon is released as CO2. The pathway is wasteful because there is no ATP produced, nor is any other molecule produced that can serve as an energy source.

Which enzyme is cited as the most abundant enzyme in the biosphere? Why is this so?

Rubisco (for 1,5-bisphosphate carboxylase/oxygenase) makes up 20% of the chloroplast total protein and is probably the most common protein in the biosphere. This is partially due to the slow catalytic activity of the enzyme, which means larger amounts of rubisco are necessary to carry out the reaction at sufficient levels.

Why is the chemistry of the "dark reactions" referred to as a cycle and not a pathway?

The Calvin cycle begins with the addition of CO2 to ribulose 1,5-bisphosphate and the last step of the process produces ribulose 1,5-bisphosphate. The starting compound is regenerated and acts catalytically. Thus, it is considered a cycle and not a pathway.

How can a deficiency of glucose 6-phosphate confer a physiological advantage?

The deficiency appears to be protective against malaria. The pentose phosphate pathway provides reduced glutathione and other molecules that the malarial parasite requires for optimum growth. Thus, the deficiency of these products acts as a protection mechanism against malaria and is observed in greater frequency among populations from regions where malaria is endemic.

How is the pentose phosphate pathway regulated?

The level of NADP+ is very important in regulating the pathway. Low levels of NADP+ inhibit the oxidation of glucose-6-phosphate because NADP+ is required as the electron acceptor, and NADPH competes with NADP+ in the binding active site of the enzyme. The nonoxidative phase of the pathway is primarily controlled by the availability of the substrates.

Describe the role of thioredoxin in regulating the Calvin cycle.

The light reactions of photosynthesis result in the reduction of ferredoxin, one of the electron carriers of photosynthesis. Ferredoxin-thioredoxin reductase catalyzes the reduction of sulfhydryl groups of thioredoxin by reduced ferredoxin. The reduced thrioredoxin then transfers electrons to the component enzymes of the Calvin cycle that contain regulatory disulfide bonds. The inactive enzymes are activated by the reduction of the disulfide bonds. The enzymes are converted to the inactive form by spontaneous oxidation by oxygen, a product of photosynthesis. In this manner, reducing potential generated by the light reactions activates the Calvin cycle.

Several physiological modes are possible for the metabolic need for NADPH, ribose 5-phosphate, and ATP. In one scenario, such as found in adipose tissue, much more NADPH is required than ribose 5-phosphate. How is this maintained?

Under these conditions, NADPH is formed by the oxidative phase of the pentose phosphate pathway. Then, the ribose 5-phosphate is recycled via a series of reactions, including some from the glycolysis/gluconeogenesis pathway so that, ultimately, three molecules of G6P are converted into 12 NADPH, six CO2, and two G6P molecules. Thus, the net reaction is conversion of G6P to CO2 and NADPH.

Why are components of the Calvin cycle called the "dark reactions"?

Unlike photosynthetic reactions, the reactions of the Calvin cycle do not depend upon light in order to proceed. They require ATP and NADPH produced in the "light reactions."

The KM of glucose-6-phosphate dehydrogenase for NADP+ is _____________ the KM for NAD+. A) a thousand times greater than D) ten times lower than B) ten times greater than E) a thousand times lower than C) equivalent to

a thousand times lower than

The binding site of Mg2+ to rubisco requires the formation of a ________________ group between lysine 201 and CO2.

carbamate

6) Which of the following enzymes is involved in both the Calvin cycle and the pentose phosphate pathway? A) glucose-6-phosphate dehydrogenase B) fructose-1,6-bisphosphatase C) 3-phosphoglycerate kinase D) transaldolase E) triose phosphate isomerase

everything i find is transketolase but you know TRANSALDOLASE

Hemolytic anemia is caused by a deficiency of the enzyme ______________________.

glucose 6 phosphate dehydrogenase

In photorespiration, the oxidation of glycolate to glyoxylate occurs in the __________ .

peroxisomes

In C4 plants, carbon dioxide is added to ____________________ to form oxaloacetate, which is reduced to malate, which carries CO2 to the bundle-sheath cells.

phosphoenolpyruvate

Which form of thioredoxin activates certain Calvin cycle enzymes? A) reduced B) oxidized C) dimeric D) phosphorylated E) dephosphorylated

reduced

When NADPH is abundant, the nonoxidative phase of the pentose phosphate pathway may still be used to produce _______________.

ribose-5-phosphate

The first step of the Calvin cycle involves the addition of CO2 to _______________________ to produce two molecules of 3-phosphoglycerate.

ribulose-1,5-bisphosphate

A _______________ intermediate is formed between the carbonyl group of the ketose substrate and the ε-amino group of a lysine residue of transaldolase.

schiff base

The enzymes that catalyze the dark reactions of photosynthesis are located in the ___________ of the chloroplasts

stroma

The enzyme transketolase transfers a ________-carbon fragment from a ketose to an aldose.

two