Part 4 — Aerobic Respiration & Oxidative Decarboxylation of Pyruvate (25 MCQs)

Part 4 — Aerobic Respiration & Oxidative Decarboxylation of Pyruvate (25 MCQs)

1. What is the immediate product when pyruvate enters the mitochondrial matrix for aerobic respiration?
   A. Ethanol
   B. Lactate
   C. Acetyl-CoA and CO₂ (via pyruvate dehydrogenase complex)
   D. Oxaloacetate
   Answer: C. Acetyl-CoA and CO₂ (via PDH complex)
   Explanation:

• A/B: Products of fermentation.

• C (Correct): Pyruvate dehydrogenase converts pyruvate to acetyl-CoA, releasing CO₂ and producing NADH.

• D: Oxaloacetate is a TCA intermediate, not the immediate product.

2. Where does oxidative decarboxylation of pyruvate (pyruvate dehydrogenase complex) occur in plant cells?
   A. Cytosol
   B. Mitochondrial matrix
   C. Chloroplast stroma
   D. Vacuole
   Answer: B. Mitochondrial matrix
   Explanation:

• A: In eukaryotes, PDH is mitochondrial.

• B (Correct): The PDH complex is located in the mitochondrial matrix.

• C/D: Not correct.

3. Which coenzyme is required by pyruvate dehydrogenase complex (PDH)?
   A. Thiamine pyrophosphate (TPP)
   B. Biotin only
   C. NADPH only
   D. FADH₂ only
   Answer: A. Thiamine pyrophosphate (TPP)
   Explanation:

• A (Correct): PDH requires TPP (from vitamin B1), lipoamide, FAD, NAD⁺, and CoA.

• B/C/D: Incomplete; PDH involves multiple cofactors, not just those listed.

4. Which of the following is generated by the oxidative decarboxylation of one pyruvate molecule?
   A. 1 NADH, 1 CO₂, and 1 acetyl-CoA
   B. 2 NADH and 2 CO₂
   C. 1 ATP and 1 FADH₂
   D. 3 NADH and 1 FADH₂
   Answer: A. 1 NADH, 1 CO₂, and 1 acetyl-CoA
   Explanation:

• A (Correct): PDH yields one NADH and one CO₂ per pyruvate and forms acetyl-CoA.

• B/C/D: Incorrect stoichiometry.

5. Which statement about acetyl-CoA is FALSE?
   A. It is the entry substrate for the TCA cycle via condensation with oxaloacetate.
   B. It can arise from carbohydrates, lipids and some amino acids.
   C. It can be used to synthesize lipids when in excess.
   D. It is stored as an energy reserve in the cytosol.
   Answer: D. It is stored as an energy reserve in the cytosol.
   Explanation:

• A/B/C: True.

• D (Correct — false): Acetyl-CoA is a metabolic intermediate, not stored as such; excess acetyl groups may be used to synthesize fatty acids or ketone bodies (in animals).

6. Which regulatory molecule inhibits the pyruvate dehydrogenase complex?
   A. High NAD⁺ concentration
   B. High ADP concentration
   C. High NADH and high Acetyl-CoA levels
   D. Low ATP concentration
   Answer: C. High NADH and high Acetyl-CoA levels
   Explanation:

• A/B/D: These signal low energy and activate PDH.

• C (Correct): NADH and acetyl-CoA indicate abundant reducing power and substrate, inhibit PDH by feedback mechanisms.

7. Pyruvate dehydrogenase deficiency would lead to:
   A. Increased flow through TCA cycle
   B. Accumulation of pyruvate and increased fermentation (lactate/ethanol)
   C. Increased acetyl-CoA supply to TCA
   D. Higher ATP production by oxidative phosphorylation
   Answer: B. Accumulation of pyruvate and increased fermentation (lactate/ethanol)
   Explanation:

• A/C/D: Incorrect.

• B (Correct): Blocked PDH leads to pyruvate build-up and diversion to fermentation pathways.

8. Which of the following links glycolysis to the TCA cycle?
   A. Oxaloacetate formation in cytosol
   B. Conversion of pyruvate to acetyl-CoA by PDH complex
   C. Conversion of citrate to isocitrate
   D. Gluconeogenesis
   Answer: B. Conversion of pyruvate to acetyl-CoA by PDH complex
   Explanation:

• B (Correct): PDH connects pyruvate (glycolysis product) to acetyl-CoA (TCA substrate).

• A/C/D: Not the direct link.

9. The PDH complex is regulated by phosphorylation (inactivation). Which kinase would be active when energy status is high?
   A. Pyruvate dehydrogenase kinase (PDK) — active when ATP/NADH high
   B. Pyruvate kinase — unrelated
   C. Phosphofructokinase — unrelated
   D. None — PDH is not regulated by phosphorylation
   Answer: A. Pyruvate dehydrogenase kinase (PDK) — active when ATP/NADH high
   Explanation:

• A (Correct): PDK phosphorylates and inactivates PDH when ATP/NADH/acetyl-CoA are abundant.

• B/C: Other enzymes.

• D: Incorrect.

10. Which metal ion is required by pyruvate dehydrogenase for catalytic activity (via cofactors)?
    A. Mg²⁺ (for ATP binding)
    B. Mn²⁺ only
    C. No metal required
    D. Lipoic acid contains sulfur-bound to enzyme, and TPP uses Mg²⁺ as cofactor; Mg²⁺ is required.
    Answer: D. Lipoic acid contains sulfur-bound to enzyme, and TPP uses Mg²⁺ as cofactor; Mg²⁺ is required.
    Explanation:

• A: Mg²⁺ is commonly required for TPP binding and other enzyme activities.

• D (Correct): This choice correctly indicates Mg²⁺ involvement along with lipoic acid as a prosthetic group.

• B/C: Incomplete/incorrect.

11. Which of the following is a product of acetyl-CoA oxidation in the TCA cycle?
    A. Reduced NADH and FADH₂ and CO₂
    B. Glucose and oxygen
    C. Ethanol and lactate
    D. Water only
    Answer: A. Reduced NADH and FADH₂ and CO₂
    Explanation:

• A (Correct): TCA oxidizes acetyl groups to CO₂ and reduces NAD⁺/FAD to supply ETS.

• B/C/D: Incorrect.

12. Pyruvate can be carboxylated to oxaloacetate by which enzyme to replenish TCA intermediates (anaplerotic reaction)?
    A. Pyruvate carboxylase (biotin dependent)
    B. Pyruvate dehydrogenase
    C. Pyruvate decarboxylase
    D. Pyruvate oxidase
    Answer: A. Pyruvate carboxylase (biotin dependent)
    Explanation:

• A (Correct): Pyruvate carboxylase forms OAA from pyruvate (requires biotin), replenishing TCA.

• B/C/D: Different reactions.

13. Which coenzyme is reduced during oxidative decarboxylation of pyruvate?
    A. NAD⁺ to NADH
    B. FAD to FADH₂
    C. NADP⁺ to NADPH
    D. CoA to CoA-SH
    Answer: A. NAD⁺ to NADH
    Explanation:

• A (Correct): PDH reduces NAD⁺ to NADH.

• B/C/D: Not primary redox in PDH.

14. Acetyl-CoA formation is an example of which type of reaction?
    A. Oxidative decarboxylation
    B. Hydrolysis only
    C. Carboxylation
    D. Phosphorylation
    Answer: A. Oxidative decarboxylation
    Explanation:

• A (Correct): Pyruvate loses CO₂ (decarboxylation) and is oxidized; an acetyl group is transferred to CoA.

• B/C/D: Incorrect.

15. Which statement about oxidative decarboxylation of pyruvate in plants is TRUE?
    A. It generates ATP directly.
    B. It produces acetyl-CoA, NADH and CO₂.
    C. It occurs in cytosol.
    D. It is part of glycolysis.
    Answer: B. It produces acetyl-CoA, NADH and CO₂.
    Explanation:

• A: No direct ATP produced.

• B (Correct): PDH yields acetyl-CoA, NADH and CO₂.

• C/D: Incorrect.

16. Which metabolic pathway immediately follows acetyl-CoA formation?
    A. Glycolysis
    B. Tricarboxylic acid (TCA) cycle
    C. Pentose phosphate pathway
    D. Fermentation
    Answer: B. Tricarboxylic acid (TCA) cycle
    Explanation:

• A: Glycolysis precedes, not follows.

• B (Correct): Acetyl-CoA enters the TCA cycle by combining with OAA to form citrate.

• C/D: Not immediate.

17. Which of the following statements about acetyl-CoA is CORRECT?
    A. Acetyl-CoA can be used to synthesise fatty acids when ATP is abundant.
    B. Acetyl-CoA is exported as such to cytosol directly for fatty acid synthesis in plants.
    C. Acetyl-CoA has no role in biosynthesis.
    D. Acetyl-CoA is only produced from lipids.
    Answer: A. Acetyl-CoA can be used to synthesise fatty acids when ATP is abundant.
    Explanation:

• A (Correct): Excess acetyl-CoA and NADPH favor lipogenesis.

• B: Acetyl-CoA cannot cross mitochondrial membrane — citrate shuttle often used.

• C/D: Incorrect.

18. Which vitamin deficiency would impair PDH complex activity?
    A. Vitamin C (ascorbate)
    B. Vitamin B1 (thiamine)
    C. Vitamin D
    D. Vitamin A
    Answer: B. Vitamin B1 (thiamine)
    Explanation:

• A/C/D: Not PDH cofactors.

• B (Correct): TPP derived from B1 is an essential cofactor for PDH.

19. Which of the following would most directly reduce acetyl-CoA production from pyruvate?
    A. Activation of pyruvate dehydrogenase phosphatase
    B. Inhibition of pyruvate dehydrogenase complex (PDH)
    C. Increased PDH expression
    D. Increased availability of CoA
    Answer: B. Inhibition of pyruvate dehydrogenase complex (PDH)
    Explanation:

• A/C/D: These would increase PDH activity or substrate availability.

• B (Correct): PDH inhibition reduces conversion of pyruvate to acetyl-CoA.

20. Which of the following is TRUE regarding the fate of acetyl-CoA in plants?
    A. It is used only in the TCA cycle.
    B. It can be used for fatty acid synthesis through citrate export to cytosol.
    C. It is directly converted to glucose.
    D. It is excreted from the cell.
    Answer: B. It can be used for fatty acid synthesis through citrate export to cytosol.
    Explanation:

• A: False — acetyl-CoA also serves as precursor for biosynthesis.

• B (Correct): Citrate shuttle carries acetyl units to cytosol for fatty acid synthesis.

• C: Animals/plants have limited net conversion; glyoxylate cycle in plants/seeds can make carbohydrates from acetyl-CoA.

• D: Not excreted.

21. Which measurement would indicate an increase in oxidative decarboxylation activity?
    A. Increased cellular NADH concentration in mitochondria
    B. Reduced CO₂ release
    C. Decrease in acetyl-CoA levels
    D. Increase in cytosolic ATP only
    Answer: A. Increased cellular NADH concentration in mitochondria
    Explanation:

• A (Correct): PDH produces NADH; increased PDH activity raises mitochondrial NADH.

• B/C/D: Opposite or unrelated.

22. Which compound serves as the acetyl acceptor when acetyl-CoA enters the TCA cycle?
    A. Citrate
    B. Oxaloacetate (OAA)
    C. Succinate
    D. Fumarate
    Answer: B. Oxaloacetate (OAA)
    Explanation:

• A: Citrate is product of condensation.

• B (Correct): Acetyl-CoA condenses with OAA via citrate synthase to form citrate.

• C/D: Later TCA intermediates.

23. Which of the following would be expected if PDH activity increased dramatically?
    A. Lower acetyl-CoA levels
    B. Greater supply of acetyl groups to TCA and increased CO₂ and NADH production
    C. Complete inhibition of glycolysis
    D. Increased fermentation
    Answer: B. Greater supply of acetyl groups to TCA and increased CO₂ and NADH production
    Explanation:

• A: PDH increase would raise acetyl-CoA.

• B (Correct): More PDH activity channels more pyruvate into acetyl-CoA → TCA flux rises → more NADH & CO₂.

• C/D: Not expected.

24. Which enzyme complex performs oxidative decarboxylation of pyruvate?
    A. Pyruvate kinase complex
    B. Pyruvate dehydrogenase complex (E1-E3)
    C. Pyruvate carboxylase complex
    D. Pyruvate decarboxylase (alone)
    Answer: B. Pyruvate dehydrogenase complex (E1–E3)
    Explanation:

• A/C/D: Other enzymes.

• B (Correct): PDH complex (E1: pyruvate decarboxylase, E2: dihydrolipoyl transacetylase, E3: dihydrolipoyl dehydrogenase) executes the multistep reaction.

25. Oxidative decarboxylation of pyruvate results in the net loss of how many carbons per pyruvate molecule?
    A. 0
    B. 1 (as CO₂)
    C. 2
    D. 3
    Answer: B. 1 (as CO₂)
    Explanation:

• A: Incorrect.

• B (Correct): Pyruvate (3C) loses one carbon as CO₂ and forms a 2C acetyl unit (acetyl-CoA).

• C/D: Incorrect.

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