biology : class11 Payoff Phase : Location, Steps, Products, ATP Yield : Glycolysis MCQ Questions and Answers
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Payoff Phase
MCQ questions and answers.
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MCQ Questions
1. In the payoff phase of glycolysis, which event occurs during the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate?
A.
Oxidation of glyceraldehyde-3-phosphate is coupled to reduction of NAD+ and addition of inorganic phosphate
B.
Reduction of glyceraldehyde-3-phosphate is coupled to oxidation of NADH and release of phosphate
C.
Phosphorylation of glyceraldehyde-3-phosphate uses ATP and produces NADH
D.
Dehydration of glyceraldehyde-3-phosphate produces phosphoenolpyruvate and NAD+
Correct Answer:
Oxidation of glyceraldehyde-3-phosphate is coupled to reduction of NAD+ and addition of inorganic phosphate
Explanation:
In the first payoff-phase reaction, glyceraldehyde-3-phosphate is oxidized. NAD+ accepts electrons to form NADH, while inorganic phosphate is incorporated, producing 1,3-bisphosphoglycerate.
2. Why does one molecule of glucose generate two molecules of NADH during the payoff phase of glycolysis?
A.
The glucose molecule is phosphorylated twice by ATP-dependent reactions
B.
Glucose is split into two three-carbon molecules, each yielding one glyceraldehyde-3-phosphate molecule
C.
Two NAD+ molecules are reduced during the conversion of phosphoenolpyruvate to pyruvate
D.
Each pyruvate molecule is oxidized twice before glycolysis ends
Correct Answer:
Glucose is split into two three-carbon molecules, each yielding one glyceraldehyde-3-phosphate molecule
Explanation:
After the cleavage phase, one glucose produces two glyceraldehyde-3-phosphate molecules. Each undergoes oxidation in the payoff phase and reduces one NAD+ to NADH, giving two NADH per glucose.
3. Which payoff-phase reaction directly produces ATP by substrate-level phosphorylation and uses 1,3-bisphosphoglycerate as the phosphate donor?
A.
Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate
B.
1,3-bisphosphoglycerate to 3-phosphoglycerate
C.
3-phosphoglycerate to 2-phosphoglycerate
D.
Phosphoenolpyruvate to pyruvate
Correct Answer:
1,3-bisphosphoglycerate to 3-phosphoglycerate
Explanation:
Phosphoglycerate kinase transfers a phosphate group from the high-energy 1-position of 1,3-bisphosphoglycerate to ADP, producing ATP and 3-phosphoglycerate.
4. For each glyceraldehyde-3-phosphate molecule entering the payoff phase, how many ATP molecules are formed directly by substrate-level phosphorylation?
A.
One
B.
Two
C.
Three
D.
Four
Correct Answer:
Two
Explanation:
Each glyceraldehyde-3-phosphate molecule gives rise to one ATP at the phosphoglycerate kinase step and one ATP at the pyruvate kinase step. Therefore, two ATP are formed per glyceraldehyde-3-phosphate.
5. Which sequence correctly represents the carbon intermediates of the payoff phase after 1,3-bisphosphoglycerate is formed?
A.
3-phosphoglycerate → 2-phosphoglycerate → phosphoenolpyruvate → pyruvate
B.
2-phosphoglycerate → 3-phosphoglycerate → phosphoenolpyruvate → pyruvate
C.
3-phosphoglycerate → phosphoenolpyruvate → 2-phosphoglycerate → pyruvate
D.
Phosphoenolpyruvate → 3-phosphoglycerate → 2-phosphoglycerate → pyruvate
Explanation:
After ATP formation from 1,3-bisphosphoglycerate, the pathway proceeds through 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate, and finally pyruvate.
6. Which payoff-phase reaction involves dehydration and creates the high phosphoryl-transfer potential of phosphoenolpyruvate?
A.
1,3-bisphosphoglycerate to 3-phosphoglycerate
B.
3-phosphoglycerate to 2-phosphoglycerate
C.
2-phosphoglycerate to phosphoenolpyruvate
D.
Phosphoenolpyruvate to pyruvate
Correct Answer:
2-phosphoglycerate to phosphoenolpyruvate
Explanation:
Enolase removes water from 2-phosphoglycerate, producing phosphoenolpyruvate. The resulting enol phosphate has high phosphoryl-transfer potential and supports ATP formation in the final payoff-phase step.
7. If phosphoglycerate kinase is inhibited after glyceraldehyde-3-phosphate oxidation has proceeded, which immediate combination would be expected to accumulate and decrease?
A.
1,3-bisphosphoglycerate would accumulate, while ATP production at that step would decrease
B.
3-phosphoglycerate would accumulate, while NADH production would decrease
C.
Phosphoenolpyruvate would accumulate, while pyruvate formation would decrease
D.
Glyceraldehyde-3-phosphate would accumulate, while NAD+ consumption would decrease
Correct Answer:
1,3-bisphosphoglycerate would accumulate, while ATP production at that step would decrease
Explanation:
Phosphoglycerate kinase converts 1,3-bisphosphoglycerate into 3-phosphoglycerate while forming ATP. Inhibition therefore causes the substrate to accumulate and blocks ATP generation at that reaction.
8. What is the gross ATP yield from the payoff phase of glycolysis per molecule of glucose, assuming both glyceraldehyde-3-phosphate molecules proceed through the complete phase?
A.
Two ATP
B.
Three ATP
C.
Four ATP
D.
Six ATP
Correct Answer:
Four ATP
Explanation:
Each of the two glyceraldehyde-3-phosphate molecules produces two ATP in the payoff phase: one through phosphoglycerate kinase and one through pyruvate kinase. Thus, the gross yield is four ATP per glucose.
9. Which statement best distinguishes the two substrate-level phosphorylation steps of the payoff phase?
A.
The first uses 1,3-bisphosphoglycerate as phosphate donor, whereas the second uses phosphoenolpyruvate
B.
The first uses ATP as phosphate donor, whereas the second uses NADH
C.
The first occurs before NADH formation, whereas the second occurs before glyceraldehyde-3-phosphate oxidation
D.
The first produces pyruvate, whereas the second produces 3-phosphoglycerate
Correct Answer:
The first uses 1,3-bisphosphoglycerate as phosphate donor, whereas the second uses phosphoenolpyruvate
Explanation:
ATP is formed first when 1,3-bisphosphoglycerate donates phosphate to ADP and again when phosphoenolpyruvate donates phosphate to ADP. These are the two substrate-level phosphorylation reactions of the payoff phase.
10. At the end of the payoff phase, what is the net combination of products generated from one glucose molecule when only glycolytic accounting is considered?
A.
Two pyruvate, two NADH, four ATP, and no net ATP investment
B.
Two pyruvate, two NADH, four ATP gross, and two ATP net for glycolysis
C.
One pyruvate, one NADH, two ATP gross, and one ATP net
D.
Two pyruvate, four NADH, two ATP gross, and four ATP net
Correct Answer:
Two pyruvate, two NADH, four ATP gross, and two ATP net for glycolysis
Explanation:
The payoff phase produces two pyruvate, two NADH, and four ATP per glucose. Since two ATP were consumed earlier in glycolysis, the overall net ATP yield of glycolysis is two ATP.
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