NCERT Class 11 Science — Chapter 2: Respiration in Plants

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Assertion (A): Oxygen plays a vital role in the early stages of the Krebs cycle. Reason (R): Oxygen acts as the final hydrogen acceptor in the electron transport system.

Official previous-year question

Verified 1 Aug 2026.

Options

ABoth A and R are true and R is the correct explanation of A
BBoth A and R are true but R is NOT the correct explanation of A
CA is true but R is false
DA is false but R is true

Did you get this right?

Solution

The correct option is

A is false but R is true

Explanation

Aerobic cellular respiration consists of distinct stages: Glycolysis, the Krebs cycle (Citric Acid Cycle), and the Electron Transport System (ETS). Understanding where oxygen is specifically utilized is central to distinguishing these phases.

Statement-wise Analysis:

Assertion (A): Incorrect. Oxygen does not play a direct role in the Krebs cycle, nor is it used in the early stages of this pathway. The Krebs cycle involves the enzymatic breakdown of Acetyl-CoA into carbon dioxide within the mitochondrial matrix. While the cycle is aerobic (meaning it stops in the absence of oxygen because electron carriers cannot be recycled), oxygen itself is not a reactant in any of the chemical reactions constituting the cycle.
Reason (R): Correct. Oxygen acts as the final hydrogen (electron) acceptor in the Electron Transport System (ETS). Located in the inner mitochondrial membrane, the ETS transfers electrons through a series of carriers. At the very end of this chain (Complex IV), molecular oxygen accepts electrons and combines with protons (H⁺) to form metabolic water.

Key Takeaway:
Oxygen is the terminal electron acceptor in aerobic respiration and is directly consumed only at the very end of the Electron Transport System; it does not participate in the reactions of the Krebs cycle.

Your note

More from Chapter 2: Respiration in Plants

Assertion (A): Plants do not present great demands for gas exchange compared to animals. Reason (R): Each plant part takes care of its own gas-exchange needs, resulting in massive transport of gases from one part to another.

hard

Match the enzyme with its specific action: List I | List II A. Hexokinase | 1. Converts Pyruvate to Acetyl CoA B. Invertase | 2. Reduces Pyruvate to Lactic acid C. Lactate dehydrogenase | 3. Splits Sucrose into Glucose and Fructose D. Pyruvate dehydrogenase | 4. Phosphorylates Glucose Select the correct answer using the code given below

medium

Match the respiratory substrate/enzyme with its entry or action: List I | List II A. Fatty acids | 1. Degrade proteins B. Glycerol | 2. Enter pathway as Acetyl CoA C. Amino acids | 3. Enter pathway as PGAL D. Proteases | 4. Enter at various stages after deamination Select the correct answer using the code given below

medium

Match the respiration context with its feature: List I | List II A. Yeast | 1. Site of Glycolysis B. Muscle cells (during heavy exercise) | 2. Site of Krebs’ cycle C. Mitochondria | 3. Produces Ethanol and CO₂ D. Cytoplasm | 4. Produces Lactic acid Select the correct answer using the code given below

hard

With reference to gas exchange in plants, consider the following statements: 1. Individual organs rely entirely on transport from other parts for gaseous needs. 2. Plant metabolism does not demand large-scale convective transport of gases. 3. Intercellular air spaces in parenchyma reduce diffusion distances. 4. In woody stems, living tissues occur in thin layers beneath the bark with openings that aid exchange. Which of the statements given above are correct?

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The correct option is

A is false but R is true

Explanation

Statement-wise Analysis:

Assertion (A): Incorrect. Oxygen does not play a direct role in the Krebs cycle, nor is it used in the early stages of this pathway. The Krebs cycle involves the enzymatic breakdown of Acetyl-CoA into carbon dioxide within the mitochondrial matrix. While the cycle is aerobic (meaning it stops in the absence of oxygen because electron carriers cannot be recycled), oxygen itself is not a reactant in any of the chemical reactions constituting the cycle.
Reason (R): Correct. Oxygen acts as the final hydrogen (electron) acceptor in the Electron Transport System (ETS). Located in the inner mitochondrial membrane, the ETS transfers electrons through a series of carriers. At the very end of this chain (Complex IV), molecular oxygen accepts electrons and combines with protons (H⁺) to form metabolic water.