Respiration – Fermentation (anaerobic): NEET MCQs with Answers

15 free questions on Respiration – Fermentation (anaerobic) for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

Syllabus covered: Fermentation (anaerobic)

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Question 1

Previous-year question
Which of the statements given below about cellular respiration are correct? [Re-NEET 2024]
A. Cellular respiration is the breaking of C-C bonds of complex organic molecules by oxidation.
B. The entire cellular respiration takes place in mitochondria.
C. Fermentation takes place under anaerobic condition in germinating seeds.
D. The fate of pyruvate formed during glycolysis depends on the type of organism also.
E. Water is formed during respiration as a result of O₂ accepting electrons and getting reduced.
  1. A, C, D, E only
  2. A, B, E only
  3. A, B, C, E only
  4. B, C, D, E only
Show answer
Answer: (A) A, C, D, E only
Explanation: Cellular respiration is the breaking of C–C bonds of complex compounds through oxidation (A); fermentation occurs under anaerobic conditions, e.g. in germinating seeds (C); the fate of pyruvate depends on the organism and conditions (D); and water forms when O₂ accepts electrons and protons and is reduced (E). B is wrong because glycolysis occurs in the cytoplasm, so respiration does not take place entirely in mitochondria.

Question 2

Previous-year question
In which one of the following processes is CO₂ NOT released? [AIPMT 2014]
  1. Aerobic respiration in plants
  2. Aerobic respiration in animals
  3. Alcoholic fermentation
  4. Lactate fermentation
Show answer
Answer: (D) Lactate fermentation
Explanation: Lactic acid fermentation involves only the reduction of the three-carbon pyruvate to the three-carbon lactate, so no carbon is lost. Alcoholic fermentation releases CO₂ during the decarboxylation of pyruvate, and aerobic respiration releases it in the link reaction and the Krebs cycle.

Question 3

Previous-year question
The fermentation products of yeast are [AIPMT 1994]
  1. H₂O + CO₂
  2. methyl alcohol + CO₂
  3. methyl alcohol + H₂O
  4. ethyl alcohol + CO₂
Show answer
Answer: (D) ethyl alcohol + CO₂
Explanation: Yeast cells carry out alcoholic fermentation. Glucose is first converted to pyruvic acid by glycolysis; pyruvic decarboxylase then changes pyruvate into acetaldehyde with the release of CO₂, and alcohol dehydrogenase reduces the acetaldehyde to ethyl alcohol. The products are therefore ethanol and carbon dioxide.

Question 4

Previous-year question
Fermentation is the anaerobic production of [AIPMT 1996]
  1. protein and acetic acid
  2. alcohol, lactic acid or similar compounds
  3. ethers and acetones
  4. alcohol and lipoproteins
Show answer
Answer: (B) alcohol, lactic acid or similar compounds
Explanation: Fermentation refers to the anaerobic decomposition of organic compounds by enzymes, yielding alcohols, organic acids (such as lactic acid) and gases. It involves only the partial breakdown of glucose in the absence of oxygen.

Question 5

Previous-year question
The end products of alcoholic fermentation are [AIPMT 1997]
  1. O₂ and C₂H₅OH
  2. CO₂ and acetaldehyde
  3. CO₂ and O₂
  4. CO₂ and C₂H₅OH
Show answer
Answer: (D) CO₂ and C₂H₅OH
Explanation: Fermentation is the partial breakdown of glucose under anaerobic conditions to yield ethyl alcohol. Pyruvate is first decarboxylated by pyruvic decarboxylase, releasing CO₂ and forming acetaldehyde, which is then reduced to ethanol (C₂H₅OH). Thus the end products are CO₂ and ethyl alcohol.

Question 6

Previous-year question
In alcoholic fermentation, [AIPMT 2003]
  1. oxygen is the electron acceptor
  2. triose phosphate is the electron donor while acetaldehyde is the electron acceptor
  3. triose phosphate is the electron donor while pyruvic acid is the electron acceptor
  4. there is no electron donor
Show answer
Answer: (B) triose phosphate is the electron donor while acetaldehyde is the electron acceptor
Explanation: During alcoholic fermentation, triose phosphate acts as the electron donor when it is oxidised in glycolysis. Pyruvate is then decarboxylated to acetaldehyde, and it is acetaldehyde that accepts the electrons (from NADH), being reduced to ethanol by alcohol dehydrogenase.

Question 7

Previous-year question
The energy-releasing metabolic process in which a substrate is oxidised without an external electron acceptor is called [AIPMT 2010, 2008]
  1. glycolysis
  2. fermentation
  3. aerobic respiration
  4. photorespiration
Show answer
Answer: (B) fermentation
Explanation: Fermentation occurs when oxygen is lacking and the electron transport chain is unusable; it becomes the cell's primary means of ATP production. The organic substrate itself (or a derivative of it) serves as the terminal electron acceptor, so no external acceptor is required. Aerobic respiration, in contrast, uses O₂ as the final acceptor.
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Question 8

Previous-year question
In which one of the following processes is CO₂ not released? [AIPMT 2014]
  1. Aerobic respiration in plants
  2. Aerobic respiration in animals
  3. Alcoholic fermentation
  4. Lactate fermentation
Show answer
Answer: (D) Lactate fermentation
Explanation: In lactate (lactic acid) fermentation, pyruvate is directly reduced to lactate by lactate dehydrogenase without any decarboxylation, so no CO₂ is released. This anaerobic reaction occurs in some bacteria and in animal muscle cells, and it regenerates NAD⁺ so that glycolysis can continue. Alcoholic fermentation and both forms of aerobic respiration all release CO₂.

Question 9

Previous-year question
What proportion of the energy of glucose is released during lactic acid fermentation? [NEET 2022]
  1. More than 18%
  2. About 10%
  3. Less than 7%
  4. Approximately 15%
Show answer
Answer: (C) Less than 7%
Explanation: In lactic acid fermentation only a small fraction of the energy contained in glucose is released — less than 7 per cent — and not all of even that is captured in the high-energy phosphate bonds of ATP. This is why fermentation is a far less efficient energy-yielding process than aerobic respiration.

Question 10

Previous-year question
During the anaerobic digestion of organic waste, as in the production of biogas, which one of the following is left undegraded? [2003]
  1. Lignin
  2. Cellulose
  3. Hemicellulose
  4. Lipids
Show answer
Answer: (A) Lignin
Explanation: Lignin is a highly cross-linked aromatic polymer that is extremely resistant to microbial attack, and its degradation requires oxygen-dependent enzymes possessed mainly by white-rot fungi. Under the anaerobic conditions of a biogas digester it therefore remains largely intact, while cellulose, hemicellulose and lipids are broken down.

Question 11

Previous-year question
The bacterium Clostridium botulinum, which causes botulism, is [2004]
  1. an obligate aerobe
  2. an obligate anaerobe
  3. a facultative anaerobe
  4. a facultative aerobe
Show answer
Answer: (B) an obligate anaerobe
Explanation: Clostridium botulinum cannot grow in the presence of oxygen and is therefore an obligate anaerobe. This is why botulism is associated with improperly sterilised canned and bottled foods, where the sealed anaerobic interior allows the spores to germinate and produce the neurotoxin.

Question 12

Previous-year question
Life without air would be [1993]
  1. impossible
  2. reductional
  3. anaerobic
  4. free from oxidative damage
Show answer
Answer: (C) anaerobic
Explanation: Life is possible without oxygen: many bacteria and archaea live anaerobically, obtaining energy by fermentation or by using nitrate, sulphate or carbon dioxide as terminal electron acceptors. Such life is described as anaerobic, and obligate anaerobes are in fact killed by oxygen.

Question 13

Previous-year question
The energy-releasing metabolic process in which the substrate is oxidised without an external electron acceptor is called [2008]
  1. aerobic respiration
  2. glycolysis
  3. photorespiration
  4. fermentation
Show answer
Answer: (D) fermentation
Explanation: Fermentation is defined by the absence of any external electron acceptor: the electrons removed from the substrate are passed back to an organic molecule derived from that same substrate, so no oxygen or other outside acceptor is involved and no electron transport chain operates.

Question 14

Previous-year question
The fermentation products of yeast are [1994]
  1. methyl alcohol and water
  2. water and carbon dioxide
  3. ethyl alcohol and carbon dioxide
  4. methyl alcohol and carbon dioxide
Show answer
Answer: (C) ethyl alcohol and carbon dioxide
Explanation: Saccharomyces decarboxylates pyruvate to acetaldehyde by pyruvate decarboxylase and then reduces the acetaldehyde to ethanol by alcohol dehydrogenase, regenerating NAD⁺. Ethanol and carbon dioxide are therefore the products, and it is the carbon dioxide that makes bread rise.

Question 15

Previous-year question
Fermentation is the anaerobic production of [1995]
  1. ethers and acetones
  2. protein and acetic acid
  3. alcohol and lipoproteins
  4. alcohol, lactic acid or similar compounds
Show answer
Answer: (D) alcohol, lactic acid or similar compounds
Explanation: In fermentation the pyruvate formed by glycolysis is reduced to an organic end product — ethanol and carbon dioxide in yeast, lactic acid in Lactobacillus and in vigorously working muscle. The purpose of the step is not to yield energy but to reoxidise NADH so that glycolysis can continue.

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