Part 3 — Fermentation (25 MCQs)

Part 3 — Fermentation (25 MCQs)

1. Fermentation differs from aerobic respiration in that it:
   A. Produces more ATP per glucose
   B. Requires oxygen
   C. Regenerates NAD⁺ from NADH without an electron transport chain
   D. Produces only CO₂ and H₂O as end-products
   Answer: C. Regenerates NAD⁺ from NADH without an electron transport chain
   Explanation:

• A: Fermentation yields much less ATP.

• B: Fermentation is anaerobic.

• C (Correct): Fermentation oxidizes NADH back to NAD⁺ by reducing pyruvate to lactate or acetaldehyde to ethanol.

• D: Fermentation produces organic end-products (lactate, ethanol), not only CO₂ and H₂O.

2. Which enzyme catalyzes the conversion of pyruvate to acetaldehyde in alcoholic fermentation?
   A. Pyruvate decarboxylase
   B. Alcohol dehydrogenase
   C. Lactate dehydrogenase
   D. Acetyl-CoA synthetase
   Answer: A. Pyruvate decarboxylase
   Explanation:

• A (Correct): Pyruvate decarboxylase removes CO₂ from pyruvate to give acetaldehyde.

• B: Alcohol dehydrogenase reduces acetaldehyde to ethanol (next step).

• C: Lactate dehydrogenase reduces pyruvate to lactate in lactic fermentation.

• D: Activates acetate to acetyl-CoA; not in alcoholic fermentation.

3. In lactic acid fermentation, pyruvate is directly converted to:
   A. Ethanol and CO₂
   B. Lactate (lactic acid)
   C. Acetyl-CoA
   D. Oxaloacetate
   Answer: B. Lactate (lactic acid)
   Explanation:

• A: Alcoholic fermentation gives ethanol + CO₂.

• B (Correct): Pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD⁺.

• C/D: Not the products in lactic fermentation.

4. Which of the following is true for fermentation in plants?
   A. Only roots ferment.
   B. It occurs mainly under anaerobic conditions (waterlogged soils).
   C. Fermentation produces more ATP than oxidative phosphorylation.
   D. Fermentation entirely replaces photosynthesis.
   Answer: B. It occurs mainly under anaerobic conditions (waterlogged soils).
   Explanation:

• A: Not only roots; any tissue under low O₂ can.

• B (Correct): Anaerobic conditions promote fermentation to maintain NAD⁺ and some ATP.

• C: False; fermentation yields far less ATP than oxidative phosphorylation.

• D: Incorrect; photosynthesis is separate.

5. Which fermentation pathway is used in yeast?
   A. Lactic acid fermentation only
   B. Alcoholic fermentation producing ethanol and CO₂
   C. Butyric acid fermentation
   D. Propionic acid fermentation
   Answer: B. Alcoholic fermentation producing ethanol and CO₂
   Explanation:

• A: Yeast mainly use alcoholic fermentation.

• B (Correct): Yeasts convert pyruvate → acetaldehyde (CO₂ released) → ethanol, regenerating NAD⁺.

• C/D: Other microbes use those; not typical yeast.

6. Which enzyme reduces acetaldehyde to ethanol in yeast?
   A. Alcohol dehydrogenase
   B. Pyruvate decarboxylase
   C. Lactate oxidase
   D. Pyruvate kinase
   Answer: A. Alcohol dehydrogenase
   Explanation:

• A (Correct): Alcohol dehydrogenase reduces acetaldehyde to ethanol, oxidizing NADH to NAD⁺.

• B: Removes CO₂ from pyruvate to form acetaldehyde (previous step).

• C/D: Incorrect.

7. During anaerobic fermentation, why is NAD⁺ regeneration essential?
   A. NAD⁺ is used in the TCA cycle.
   B. Glycolysis requires NAD⁺ to accept electrons during G3P oxidation.
   C. NAD⁺ is the final electron acceptor.
   D. NAD⁺ directly produces ATP.
   Answer: B. Glycolysis requires NAD⁺ to accept electrons during G3P oxidation.
   Explanation:

• A: TCA requires NAD⁺ too, but under anaerobic conditions TCA is limited.

• B (Correct): Without NAD⁺, glyceraldehyde-3-phosphate dehydrogenase cannot function, halting glycolysis and ATP production.

• C: NAD⁺ is an electron carrier, not final acceptor; in fermentation final acceptor is organic molecule like pyruvate.

• D: NAD⁺ does not directly produce ATP.

8. Which of the following is a typical product of anaerobic respiration (fermentation) in some plants subjected to flooding?
   A. Ethanol and CO₂
   B. High amounts of ATP (like 36 ATP)
   C. Malate and oxaloacetate only
   D. Starch accumulation
   Answer: A. Ethanol and CO₂
   Explanation:

• A (Correct): Many plants under anaerobiosis (waterlogging) produce ethanol and CO₂ via alcoholic fermentation in roots.

• B: Fermentation produces only a few ATP per glucose.

• C/D: Not typical fermentation products.

9. Which organism(s) use lactic acid fermentation?
   A. Some bacteria and animal muscle cells under anaerobic conditions
   B. Yeasts only
   C. Plants only
   D. All organisms always
   Answer: A. Some bacteria and animal muscle cells under anaerobic conditions
   Explanation:

• A (Correct): Lactic fermentation occurs in many bacteria (e.g., Lactobacillus) and in animal muscles during oxygen deficit. Some plant tissues also may.

• B: Yeast use alcoholic fermentation primarily.

• C: Partial — some plant tissues can.

• D: Not always; depends on organism and conditions.

10. Which statement is TRUE about ethanol produced by fermentation in plants/yeast?
    A. It is fully oxidized to CO₂ within the cytosol.
    B. It diffuses out of cells and can be toxic in high amounts.
    C. It accumulates in plants and is stored as starch.
    D. It donates electrons to NAD⁺.
    Answer: B. It diffuses out of cells and can be toxic in high amounts.
    Explanation:

• A: Ethanol is not oxidized to CO₂ in cytosol; it may be further metabolized in mitochondria if O₂ returns.

• B (Correct): Ethanol is volatile/diffusible and toxic at high concentrations affecting membranes.

• C/D: Incorrect.

11. Fermentation yields per glucose (ATP) approximate:
    A. 36 ATP
    B. 2 ATP
    C. 12 ATP
    D. 0 ATP
    Answer: B. 2 ATP
    Explanation:

• A/C: These are far too high for fermentation.

• B (Correct): Net 2 ATP per glucose via glycolysis; fermentation regenerates NAD⁺ but yields no additional ATP.

• D: False.

12. Which of the following conditions favors alcoholic fermentation in plant tissues?
    A. Well-aerated soil
    B. Waterlogged, anaerobic soil
    C. High light intensity
    D. Low sugar supply
    Answer: B. Waterlogged, anaerobic soil
    Explanation:

• A: Aerobic conditions favor oxidative phosphorylation.

• B (Correct): Anaerobic conditions force tissues to ferment for ATP and NAD⁺ regeneration.

• C/D: Not promoting fermentation.

13. Which enzyme is used to recycle NADH to NAD⁺ in lactic fermentation?
    A. Lactate dehydrogenase
    B. Alcohol dehydrogenase
    C. Malate dehydrogenase
    D. Pyruvate oxidase
    Answer: A. Lactate dehydrogenase
    Explanation:

• A (Correct): Reduces pyruvate to lactate and oxidizes NADH to NAD⁺.

• B: Reduces acetaldehyde to ethanol in alcoholic fermentation.

• C: Part of TCA or malate-aspartate shuttle; not primary in lactic fermentation.

• D: Not relevant.

14. Which of the following is a disadvantage of fermentation in plants?
    A. Rapid ATP production
    B. Accumulation of toxic end-products like ethanol or lactic acid
    C. No need for oxygen
    D. Regeneration of NAD⁺
    Answer: B. Accumulation of toxic end-products like ethanol or lactic acid
    Explanation:

• A/C/D: Advantages or neutral.

• B (Correct): Toxic products can damage cells and reduce viability if prolonged.

15. In yeast fermentation, carbon lost as CO₂ comes from which molecule?
    A. Acetyl-CoA
    B. Pyruvate (decarboxylation to acetaldehyde)
    C. ATP
    D. Oxygen
    Answer: B. Pyruvate (decarboxylation to acetaldehyde)
    Explanation:

• A: Acetyl-CoA forms in aerobic respiration; in fermentation pyruvate is decarboxylated to acetaldehyde producing CO₂.

• B (Correct): Pyruvate decarboxylation releases CO₂ in alcoholic fermentation.

• C/D: Not sources of carbon.

16. Which statement about facultative anaerobes is correct?
    A. They can grow only in the absence of oxygen.
    B. They can switch between aerobic respiration and fermentation.
    C. They never use oxidative phosphorylation.
    D. They only perform photosynthesis.
    Answer: B. They can switch between aerobic respiration and fermentation.
    Explanation:

• A/C/D: Incorrect.

• B (Correct): Facultative anaerobes prefer aerobic respiration when O₂ present (more ATP), switch to fermentation under anaerobiosis.

17. Which plant enzyme is induced in the aleurone layer by gibberellin to help germination under anaerobic conditions?
    A. Alcohol dehydrogenase
    B. α-Amylase (for starch hydrolysis)
    C. Lactate dehydrogenase
    D. Pyruvate oxidase
    Answer: B. α-Amylase (for starch hydrolysis)
    Explanation:

• A/C/D: Stress/anoxia-inducible enzymes can occur, but gibberellin specifically stimulates α-amylase production in aleurone to provide sugars for respiration.

• B (Correct): α-Amylase mobilizes starch; supports energy needs during germination.

18. Which fermentation product may be used by other organisms as a carbon source?
    A. Ethanol
    B. Lactate
    C. Both A and B
    D. None of the above
    Answer: C. Both A and B
    Explanation:

• A/B: Both ethanol and lactate can be consumed by other microbes or plant tissues when oxygen returns (they can be oxidized).

• C (Correct): Both are used by other organisms.

• D: Incorrect.

19. Which factor does NOT influence the rate of fermentation?
    A. Availability of sugars
    B. Presence of oxygen (inverse effect)
    C. Temperature and pH
    D. Amount of chlorophyll in the tissue
    Answer: D. Amount of chlorophyll in the tissue
    Explanation:

• A/B/C: All influence fermentation rate.

• D (Correct): Chlorophyll amount doesn’t directly affect fermentation (unless influencing photosynthesis and sugar supply indirectly).

20. Which of the following best describes “homolactic fermentation”?
    A. Production of ethanol and CO₂ only
    B. Production of lactic acid as main product from glucose
    C. Production of multiple acids including butyric acid
    D. Production of propionic acid only
    Answer: B. Production of lactic acid as main product from glucose
    Explanation:

• A: Alcoholic fermentation.

• B (Correct): Homolactic fermentation yields mainly lactic acid (e.g., Lactobacillus).

• C/D: Different fermentation types.

21. Which of these is NOT regenerated as part of fermentation?
    A. NAD⁺
    B. ATP (net from glycolysis)
    C. FADH₂
    D. Pyruvate converted to organic end-products
    Answer: C. FADH₂
    Explanation:

• A: NAD⁺ regeneration is a key purpose.

• B: Glycolysis produces net ATP used by cell.

• C (Correct): Fermentation doesn’t typically produce or reoxidize FADH₂ as in oxidative phosphorylation.

• D: Pyruvate is converted to fermentation products.

22. Which of the following statements about alcoholic fermentation in plants is correct?
    A. It produces lactic acid only.
    B. It releases CO₂ and helps plants survive temporary anaerobiosis.
    C. It requires mitochondria function fully.
    D. It produces oxygen as a product.
    Answer: B. It releases CO₂ and helps plants survive temporary anaerobiosis.
    Explanation:

• A: Incorrect.

• B (Correct): Alcoholic fermentation produces ethanol and CO₂, aiding NAD⁺ regeneration and short-term survival.

• C: Does not require full mitochondrial ETS.

• D: Incorrect.

23. Which reaction regenerates NAD⁺ in alcoholic fermentation?
    A. Pyruvate → acetyl-CoA
    B. Acetaldehyde → ethanol (via alcohol dehydrogenase)
    C. Pyruvate → lactate
    D. Glucose → glucose-6-phosphate
    Answer: B. Acetaldehyde → ethanol (via alcohol dehydrogenase)
    Explanation:

• A: Aerobic route; does not regenerate NAD⁺ in cytosol.

• B (Correct): Alcohol dehydrogenase uses NADH to reduce acetaldehyde to ethanol, regenerating NAD⁺.

• C: This is lactic fermentation, also regenerating NAD⁺, but the option states pyruvate → lactate (that would also be correct if asked generally). Given the alcoholic pathway, B is best answer.

• D: Not relevant.

24. Which of the following would accumulate if alcoholic fermentation were blocked after pyruvate decarboxylase but before alcohol dehydrogenase?
    A. Ethanol
    B. Acetaldehyde
    C. Lactic acid
    D. Acetyl-CoA
    Answer: B. Acetaldehyde
    Explanation:

• A: Ethanol would not form because alcohol dehydrogenase step is blocked.

• B (Correct): Acetaldehyde would accumulate (product of pyruvate decarboxylase).

• C: Lactic acid forms via lactate dehydrogenase if pathway changes.

• D: Acetyl-CoA is not product in alcoholic fermentation.

25. Which of the following best describes the energy yield in fermentation vs. aerobic respiration?
    A. Fermentation yields similar ATP per glucose as aerobic respiration.
    B. Fermentation yields much less ATP because it only uses substrate-level phosphorylation.
    C. Fermentation yields more ATP than aerobic respiration.
    D. Fermentation yields no ATP.
    Answer: B. Fermentation yields much less ATP because it only uses substrate-level phosphorylation.
    Explanation:

• A/C/D: Incorrect.

• B (Correct): Fermentation yields only the ATP from glycolysis (net 2 ATP) whereas aerobic respiration yields many more via oxidative phosphorylation.

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