Respiration – Aerobic Respiration (Krebs' Cycle & Electron Transport: NEET MCQs with Answers

15 free questions on Respiration – Aerobic Respiration (Krebs' Cycle & Electron Transport 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: Aerobic Respiration (Krebs' Cycle & Electron Transport Chain) (aerobic)

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Question 1

Previous-year question
Match the ETS complexes in List-I with their names in List-II. [Re-NEET 2024]
List-I:
A. ETS Complex I
B. ETS Complex II
C. ETS Complex III
D. ETS Complex IV
List-II:
I. NADH dehydrogenase
II. Cytochrome bc₁
III. Cytochrome c oxidase
IV. Succinate dehydrogenase
  1. A-IV, B-I, C-III, D-II
  2. A-I, B-IV, C-II, D-III
  3. A-III, B-I, C-IV, D-II
  4. A-I, B-II, C-IV, D-III
Show answer
Answer: (B) A-I, B-IV, C-II, D-III
Explanation: Complex I is NADH dehydrogenase (A-I), complex II is succinate dehydrogenase (B-IV), complex III is the cytochrome bc₁ complex (C-II), and complex IV is cytochrome c oxidase (D-III).

Question 2

Previous-year question
Identify the correct statements about respiration. [NEET (Sawaimadhopur) 2024]
A. Energy of oxidation–reduction is utilised for phosphorylation.
B. Oxygen acts as the final hydrogen acceptor.
C. The photo-oxidation energy is utilised for production of proton gradient required for phosphorylation.
D. The role of oxygen is limited to the terminal stage of the respiration process.
E. Protons cross the outer membrane of mitochondria through the channel formed by an integral membrane protein complex.
  1. A, B, D only
  2. B, C, D, E only
  3. A, C, D only
  4. A, B, C, E only
Show answer
Answer: (A) A, B, D only
Explanation: In the ETS, the energy of oxidation–reduction is used to phosphorylate ADP (A), oxygen acts as the final hydrogen acceptor (B), and its role is limited to the terminal stage (D). C is wrong because the proton gradient in respiration comes from oxidation, not photo-oxidation (that is photosynthesis), and E is wrong because protons pass through the F₀ channel of ATP synthase in the inner, not outer, membrane.

Question 3

Previous-year question
In each turn of the TCA cycle, how many decarboxylation reactions occur? [NEET (Manipur) 2023]
  1. Thrice
  2. Many
  3. Once
  4. Twice
Show answer
Answer: (D) Twice
Explanation: Two decarboxylations occur in each TCA cycle: isocitrate → α-ketoglutarate and α-ketoglutarate → succinyl CoA, releasing two molecules of CO₂. The third CO₂ per pyruvate is released in the link reaction (pyruvate → acetyl CoA), which is not part of the cycle.

Question 4

Previous-year question
In a single turn of the TCA cycle, how many decarboxylation reactions take place, starting from isocitrate? [Re-NEET 2022]
  1. One
  2. Two
  3. Three
  4. Four
Show answer
Answer: (B) Two
Explanation: The TCA cycle has two decarboxylation steps per turn: isocitrate (through oxalosuccinate) → α-ketoglutarate, and α-ketoglutarate → succinyl CoA. Together they release 2 CO₂ in each turn, and the remaining steps release no CO₂.

Question 5

Previous-year question
Which cytochrome serves as the mobile carrier that transfers electrons between complex III and complex IV? [NEET (Overseas) 2022]
  1. Cytochrome a₃
  2. Cytochrome bc₁
  3. Cytochrome c
  4. Cytochrome a
Show answer
Answer: (C) Cytochrome c
Explanation: Cytochrome c is a small protein attached to the outer surface of the inner mitochondrial membrane and acts as a mobile carrier transferring electrons from complex III (cytochrome bc₁) to complex IV (cytochrome c oxidase). Cytochromes a and a₃ are part of complex IV, and bc₁ is complex III itself.

Question 6

Previous-year question
Match the electron transport system complexes in List-I with their components in List-II. [NEET (Overseas) 2022]
List-I:
(a) ETS complex-I
(b) ETS complex-II
(c) ETS complex-III
(d) ETS complex-IV
List-II:
(i) Cyt bc₁
(ii) Cyt a, a₃ and 2 copper centres
(iii) NADH dehydrogenase
(iv) Ubiquinone and FADH dehydrogenase
  1. (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i)
  2. (a)-(iii), (b)-(ii), (c)-(i), (d)-(iv)
  3. (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii)
  4. (a)-(ii), (b)-(i), (c)-(iv), (d)-(iii)
Show answer
Answer: (C) (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii)
Explanation: Complex I is NADH dehydrogenase (a-iii); complex II contains FADH₂-linked succinate dehydrogenase passing electrons to ubiquinone (b-iv); complex III is the cytochrome bc₁ complex (c-i); complex IV, cytochrome c oxidase, contains cytochromes a and a₃ and two copper centres (d-ii).

Question 7

Previous-year question
Pick out the incorrect statement from the following. [NEET 2021]
  1. During aerobic respiration, role of oxygen is limited to the terminal stage.
  2. In ETC (Electron Transport Chain), one molecule of NADH + H⁺ gives rise to 2 ATP molecules, and one FADH₂ gives rise to 3 ATP molecules.
  3. ATP is synthesized through complex V.
  4. Oxidation-reduction reactions produce proton gradient in respiration.
Show answer
Answer: (B) In ETC (Electron Transport Chain), one molecule of NADH + H⁺ gives rise to 2 ATP molecules, and one FADH₂ gives rise to 3 ATP molecules.
Explanation: Oxidation of one NADH + H⁺ in the ETS yields 3 ATP, while one FADH₂ yields 2 ATP; the statement reverses these values and is therefore incorrect. Oxygen acts only at the terminal stage as the final hydrogen acceptor, ATP is made by complex V (ATP synthase), and the redox reactions of the ETS create the proton gradient.
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Question 8

Previous-year question
Pick out the incorrect statement from the following. [NEET 2018]
  1. Enzymes of TCA cycle are present in mitochondrial matrix.
  2. Glycolysis occurs in cytosol.
  3. Glycolysis operates as long as it is supplied with NAD that can pick up hydrogen atoms.
  4. Oxidative phosphorylation takes place in outer mitochondrial membrane.
Show answer
Answer: (D) Oxidative phosphorylation takes place in outer mitochondrial membrane.
Explanation: The electron transport system and ATP synthase (complex V) are located in the inner mitochondrial membrane, so oxidative phosphorylation occurs there, not in the outer membrane. TCA-cycle enzymes are in the matrix (succinate dehydrogenase being on the inner membrane), glycolysis occurs in the cytoplasm, and it continues only while NAD⁺ is available to accept hydrogen.

Question 9

Previous-year question
What is the role of NAD⁺ in cellular respiration? [NEET 2018]
  1. It functions as an enzyme
  2. It functions as an electron carrier
  3. It is the final electron acceptor for anaerobic respiration
  4. It is a nucleotide source for ATP synthesis
Show answer
Answer: (B) It functions as an electron carrier
Explanation: NAD⁺ is a coenzyme that picks up hydrogen atoms (and hence electrons) removed during glycolysis, the link reaction and the Krebs cycle, and delivers them to the electron transport chain. It is a carrier, not an enzyme, and oxygen is the final acceptor in aerobic respiration.

Question 10

Previous-year question
Which of these statements is INCORRECT? [NEET 2018]
  1. Enzymes of the TCA cycle are present in the mitochondrial matrix
  2. Glycolysis occurs in the cytosol
  3. Oxidative phosphorylation takes place in the outer mitochondrial membrane
  4. Glycolysis operates as long as it is supplied with NAD that can pick up hydrogen atoms
Show answer
Answer: (C) Oxidative phosphorylation takes place in the outer mitochondrial membrane
Explanation: Oxidative phosphorylation takes place on the inner mitochondrial membrane, which carries the electron transport complexes and ATP synthase; the outer membrane is freely permeable and has no such role.

Question 11

Previous-year question
Which statement is wrong for Krebs’ cycle? [NEET 2017]
  1. There is one point in the cycle where FAD⁺ is reduced to FADH₂
  2. During the conversion of succinyl CoA to succinic acid, a molecule of GTP is synthesised
  3. The cycle starts with the condensation of the acetyl group (acetyl CoA) with pyruvic acid to yield citric acid
  4. There are three points in the cycle where NAD⁺ is reduced to NADH + H⁺
Show answer
Answer: (C) The cycle starts with the condensation of the acetyl group (acetyl CoA) with pyruvic acid to yield citric acid
Explanation: The cycle begins with the condensation of acetyl CoA with oxaloacetic acid, not with pyruvic acid. The other three statements are correct: succinate dehydrogenase reduces FAD once, succinyl CoA synthetase makes one GTP, and NAD⁺ is reduced at three points.

Question 12

Previous-year question
NADP⁺ is reduced to NADPH during [AIPMT 1988]
  1. HMP
  2. Calvin cycle
  3. glycolysis
  4. EMP
Show answer
Answer: (A) HMP
Explanation: The pentose phosphate pathway (PPP), also called the hexose monophosphate shunt (HMP) or phosphogluconate pathway, occurs in the cytosol of mammalian cells. In it, glucose is oxidised to CO₂ and water through a series of reactions in which NADP⁺ is reduced to NADPH; complete breakdown of one glucose molecule yields 12 NADPH.

Question 13

Previous-year question
The terminal cytochrome of the respiratory chain, which donates electrons to oxygen, is [AIPMT 1992]
  1. cyt-b
  2. cyt-c
  3. cyt-a₁
  4. cyt-a₃
Show answer
Answer: (D) cyt-a₃
Explanation: The correct sequence of electron carriers in the respiratory chain is cyt-b → cyt-c → cyt-a → cyt-a₃. Cytochrome a₃ is the terminal member; it possesses two copper centres that assist in transferring electrons to molecular oxygen, the final electron acceptor.

Question 14

Previous-year question
Oxidative phosphorylation is the production of [AIPMT 1992]
  1. ATP in photosynthesis
  2. NADPH in photosynthesis
  3. ATP in respiration
  4. NADH in respiration
Show answer
Answer: (C) ATP in respiration
Explanation: Oxidative phosphorylation is the synthesis of ATP from ADP and inorganic phosphate in respiration, using the energy obtained from the oxidation of reduced coenzymes. ATP formation in photosynthesis is instead called photophosphorylation.

Question 15

Previous-year question
The link between glycolysis, the Krebs' cycle and β-oxidation of fatty acids or carbohydrate and fat metabolism is [AIPMT 1992, 1990]
  1. oxaloacetic acid
  2. succinic acid
  3. citric acid
  4. acetyl Co-A
Show answer
Answer: (D) acetyl Co-A
Explanation: Acetyl Co-A is the connecting link. Pyruvic acid from glycolysis undergoes oxidative decarboxylation to form acetyl Co-A, and β-oxidation of fatty acids also yields acetyl Co-A. Both streams therefore converge at this molecule before entering the Krebs' cycle, making it the central hub of carbohydrate and fat oxidation.

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