NCERT Solutions for Class 11 Biology Chapter 12: Respiration in Plants – Free PDF Download

Chapter 12, Respiration in Plants, explains how plant cells break down glucose through glycolysis, fermentation, and aerobic respiration (the Krebs’ cycle and electron transport system) to release energy trapped as ATP. It also covers the respiratory balance sheet, why the respiratory pathway is called amphibolic, and the concept of respiratory quotient (RQ).

Last Updated: September 23, 2026

Exercises

Q1. Differentiate between — (a) Respiration and Combustion: Respiration is a biochemical, enzyme-controlled process…

(a) Respiration and Combustion: Respiration is a biochemical, enzyme-controlled process that occurs inside living cells in a series of slow, step-wise reactions; most of the energy released is trapped in the form of ATP, and only a small part is lost as heat. Combustion, on the other hand, is a non-cellular, physiochemical process in which a substance burns rapidly in a single step in the presence of oxygen; all the energy is released at once as heat and light, and no ATP is formed.

(b) Glycolysis and Krebs’ cycle: Glycolysis is the partial, anaerobic oxidation of one molecule of glucose into two molecules of pyruvic acid; it occurs in the cytoplasm of the cell and does not require oxygen. The Krebs’ cycle (citric acid cycle) is an aerobic process that occurs in the mitochondrial matrix, where acetyl CoA (derived from pyruvate) is completely oxidised to release CO₂, along with NADH and FADH₂, which are later used to generate ATP through the electron transport system.

(c) Aerobic respiration and Fermentation: Aerobic respiration is the complete oxidation of respiratory substrate in the presence of oxygen, yielding CO₂, water, and a large amount of energy (a net gain of about 38 ATP per glucose molecule). Fermentation is an incomplete, anaerobic breakdown of pyruvic acid into ethanol and CO₂ (alcohol fermentation) or lactic acid (lactic acid fermentation); it releases far less energy, yielding a net gain of only 2 ATP per glucose molecule, and produces potentially harmful end products such as acid or alcohol.

Q2. What are respiratory substrates? Name the most common respiratory substrate — Respiratory substrates are the organic compounds that are oxidised during cellular…

Respiratory substrates are the organic compounds that are oxidised during cellular respiration to release energy. Although carbohydrates are the most common and preferred respiratory substrate, proteins, fats, and even organic acids can also be used as respiratory substrates by plants under certain conditions.

Q3. Give the schematic representation of glycolysis — Glycolysis (the EMP pathway, named after Embden, Meyerhof, and Parnas) takes place in…

Glycolysis (the EMP pathway, named after Embden, Meyerhof, and Parnas) takes place in the cytoplasm and converts one molecule of glucose into two molecules of pyruvic acid through a chain of ten enzyme-catalysed reactions:

Glucose → (ATP used) Glucose-6-phosphate → Fructose-6-phosphate → (ATP used) Fructose-1,6-bisphosphate → splits into two triose phosphates (Dihydroxyacetone phosphate and Glyceraldehyde-3-phosphate/PGAL) → 1,3-bisphosphoglycerate (NADH+H⁺ formed) → 3-phosphoglycerate (ATP formed) → 2-phosphoglycerate → Phosphoenolpyruvate → Pyruvic acid (ATP formed).

Overall, glycolysis uses 2 ATP and produces 4 ATP (net gain of 2 ATP) along with 2 molecules of NADH + H⁺ and 2 molecules of pyruvic acid per glucose molecule.

Q4. What are the main steps in aerobic respiration? Where does it take place? — The main steps of aerobic respiration are:

The main steps of aerobic respiration are:

1. Glycolysis – occurs in the cytoplasm; glucose is partially oxidised to pyruvic acid.
2. Oxidative decarboxylation of pyruvate – occurs in the mitochondrial matrix; pyruvate is converted to acetyl CoA with the release of CO₂ and formation of NADH.
3. Krebs’ cycle (Tricarboxylic acid cycle) – occurs in the mitochondrial matrix; acetyl CoA is completely oxidised to CO₂, generating NADH, FADH₂, and ATP (as GTP via substrate-level phosphorylation).
4. Electron Transport System (ETS) and oxidative phosphorylation – occurs on the inner mitochondrial membrane; electrons from NADH and FADH₂ pass through a chain of carriers (Complexes I–IV) to molecular oxygen, and the energy released drives ATP synthesis by ATP synthase (Complex V).

Q5. Give the schematic representation of an overall view of Krebs' cycle — The Krebs' cycle begins in the mitochondrial matrix when acetyl CoA (2C) combines with…

The Krebs’ cycle begins in the mitochondrial matrix when acetyl CoA (2C) combines with oxaloacetic acid (4C) to form citric acid (6C), catalysed by citrate synthase:

Acetyl CoA + OAA → Citric acid → Isocitrate → (CO₂ released, NADH formed) α-Ketoglutaric acid → (CO₂ released, NADH formed) Succinyl CoA → (GTP/ATP formed) Succinic acid → (FADH₂ formed) Fumaric acid → Malic acid → (NADH formed) Oxaloacetic acid (regenerated, cycle continues).

For every acetyl CoA that enters, one turn of the cycle produces 3 NADH, 1 FADH₂, 1 ATP (GTP), and releases 2 molecules of CO₂. Since one glucose molecule yields two acetyl CoA molecules, the cycle turns twice per glucose oxidised.

Q6. Explain ETS (Electron Transport System) — The Electron Transport System is a series of electron carriers located on the inner…

The Electron Transport System is a series of electron carriers located on the inner mitochondrial membrane through which electrons removed from NADH and FADH₂ are passed to molecular oxygen, the final electron acceptor, forming water. Electrons from NADH are accepted by Complex I (NADH dehydrogenase) and transferred to ubiquinone; FADH₂ donates electrons to ubiquinone via Complex II. Reduced ubiquinone (ubiquinol) passes electrons to cytochrome c through Complex III (cytochrome bc₁ complex), and cytochrome c carries them to Complex IV (cytochrome c oxidase), which finally transfers electrons to oxygen. As electrons move through Complexes I, III, and IV, protons (H⁺) are pumped from the matrix into the intermembrane space, creating an electrochemical proton gradient. This gradient drives protons back into the matrix through ATP synthase (Complex V), synthesising ATP from ADP and inorganic phosphate — a process called oxidative phosphorylation, explained by the chemiosmotic hypothesis. Oxidation of one NADH yields about 3 ATP, while one FADH₂ yields about 2 ATP.

Q7. Distinguish between the following — (a) Aerobic respiration and Anaerobic respiration: Aerobic respiration occurs in the…

(a) Aerobic respiration and Anaerobic respiration: Aerobic respiration occurs in the presence of oxygen, completely oxidises the respiratory substrate to CO₂ and water, and releases a large amount of energy. Anaerobic respiration (fermentation) occurs in the absence of oxygen, only partially breaks down the substrate, produces organic end products such as ethanol or lactic acid, and releases much less energy.

(b) Glycolysis and Fermentation: Glycolysis is the universal first step of respiration in which glucose is converted to pyruvic acid in the cytoplasm, occurring in all living organisms regardless of oxygen availability. Fermentation is the anaerobic fate of the pyruvic acid formed by glycolysis, in which it is converted to ethanol and CO₂, or to lactic acid, and occurs only in certain prokaryotes and unicellular eukaryotes (or in oxygen-deficient cells like exercising muscle).

(c) Glycolysis and Citric acid Cycle: Glycolysis takes place in the cytoplasm, does not require oxygen, and does not release CO₂. The citric acid cycle (Krebs’ cycle) takes place in the mitochondrial matrix, requires the aerobic environment established by the ETS to keep regenerating NAD⁺ and FAD, and releases CO₂ at multiple steps.

Q8. What are the assumptions made during the calculation of net gain of ATP? — The theoretical calculation of a net gain of about 38 ATP per glucose molecule oxidised…

The theoretical calculation of a net gain of about 38 ATP per glucose molecule oxidised during aerobic respiration rests on several assumptions: (1) there is a sequential, orderly pathway in which glycolysis, the Krebs’ cycle, and the ETS occur strictly one after another; (2) the NADH produced during glycolysis in the cytoplasm is transported into the mitochondria and undergoes oxidative phosphorylation; (3) none of the intermediate compounds of the respiratory pathway are diverted for the synthesis of any other molecule; and (4) only glucose is being respired, with no alternative substrates (such as fats or amino acids) entering the pathway at intermediate stages. In reality, these assumptions rarely hold true, since all pathways function simultaneously and intermediates are constantly withdrawn for biosynthesis.

Q9. Discuss "The respiratory pathway is an amphibolic pathway." — The respiratory pathway is traditionally viewed as catabolic since it breaks down…

The respiratory pathway is traditionally viewed as catabolic since it breaks down glucose to release energy. However, the same intermediates of glycolysis and the Krebs’ cycle — such as pyruvate, acetyl CoA, and various Krebs’ cycle acids — also serve as starting points for the biosynthesis (anabolism) of fatty acids, amino acids, and other compounds. For instance, acetyl CoA is withdrawn from the pathway when the cell needs to synthesise fatty acids, and amino acids can be converted into Krebs’ cycle intermediates through deamination for use in the cycle, or withdrawn from it during protein synthesis. Because the same pathway is used for both the breakdown (catabolism) and synthesis (anabolism) of molecules, the respiratory pathway is best described as an amphibolic pathway rather than a purely catabolic one.

Q10. Define RQ. What is its value for fats? — The Respiratory Quotient (RQ), or respiratory ratio, is defined as the ratio of the…

The Respiratory Quotient (RQ), or respiratory ratio, is defined as the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration. When carbohydrates are used as the respiratory substrate, the RQ equals 1, since equal volumes of CO₂ and O₂ are exchanged. When fats are used as the respiratory substrate, the RQ is less than 1 (approximately 0.7 for a fat such as tripalmitin), because fats contain proportionally less oxygen and more hydrogen and carbon, requiring relatively more O₂ for complete oxidation than CO₂ produced. (When proteins are the substrate, the RQ is approximately 0.9.)

Q11. What is oxidative phosphorylation? — Oxidative phosphorylation is the process by which ATP is synthesised using the energy…

Oxidative phosphorylation is the process by which ATP is synthesised using the energy released from the transfer of electrons (derived from NADH and FADH₂) through the electron transport chain to molecular oxygen. As electrons move through the carriers on the inner mitochondrial membrane, protons are pumped across the membrane, creating an electrochemical proton gradient. The flow of these protons back into the matrix through the F₀–F₁ ATP synthase complex provides the energy to phosphorylate ADP into ATP. It is termed “oxidative” because the process is driven by the oxidation-reduction (redox) reactions of the electron transport chain, unlike photophosphorylation, which is driven by light energy.

Q12. What is the significance of step-wise release of energy in respiration? — If glucose were oxidised in a single step, as in combustion, almost all the energy would…

If glucose were oxidised in a single step, as in combustion, almost all the energy would be released instantly as heat, and the cell would not be able to capture and use it efficiently. By breaking down glucose through a series of small, enzyme-controlled steps (glycolysis, Krebs’ cycle, and the ETS), the energy is released in manageable packets, many of which are just large enough to be coupled with the synthesis of ATP. This step-wise release therefore allows the cell to trap the maximum possible amount of energy as usable chemical energy (ATP) rather than losing it as heat, making cellular respiration a highly efficient energy-extraction process.

Class 11 Biology Chapter 12 – Notes and Extra Questions

This chapter’s official NCERT exercise set (2023 rationalised syllabus, 2026-27 session) contains 12 numbered questions — with Q1 and Q7 each carrying three “differentiate/distinguish” sub-parts, effectively covering 18 distinct comparison pairs. While the chapter itself was renumbered from Chapter 14 (in older, pre-rationalisation editions) to Chapter 12 in the current structure, the exercise questions themselves are largely unchanged from earlier editions — this chapter’s core question bank (differentiate-type comparisons, ETS, amphibolic pathway, RQ, ATP-calculation assumptions) has been retained through rationalisation, unlike several other chapters that saw numerical or diagram-heavy questions trimmed. For exams, focus on being able to sketch the glycolysis pathway and Krebs’ cycle from memory, clearly explain the role of NAD⁺/FAD as electron carriers, and be ready to justify RQ values (1 for carbohydrates, ~0.7 for fats, ~0.9 for proteins) rather than just memorising them.

Frequently Asked Questions

How many exercise questions are there in Class 11 Biology Chapter 12, Respiration in Plants?

The current NCERT textbook (2023 rationalised syllabus) lists 12 exercise questions at the end of the chapter. Question 1 and Question 7 each contain three “differentiate between” sub-parts, so students effectively need to prepare answers for 18 comparison pairs in addition to the other 10 standalone questions.

Why is the respiratory pathway called an amphibolic pathway instead of a catabolic pathway?

Respiration is usually thought of as catabolic because it breaks down glucose to release energy. But the intermediates formed along the way — such as acetyl CoA and various Krebs’ cycle acids — are also used as starting materials for building (anabolism) fatty acids, amino acids, and other biomolecules. Since the same pathway supports both breakdown and synthesis, it is called an amphibolic pathway.

What is the net ATP yield from one molecule of glucose during aerobic respiration versus fermentation?

Under the theoretical assumptions given in the NCERT textbook, complete aerobic respiration of one glucose molecule yields a net gain of about 38 ATP molecules. In contrast, fermentation (anaerobic breakdown) yields a net gain of only 2 ATP molecules per glucose, since the substrate is only partially oxidised and most of the energy remains locked in the end product (ethanol or lactic acid).

Why does the respiratory quotient (RQ) differ for carbohydrates, fats, and proteins?

RQ depends on the relative proportion of carbon, hydrogen, and oxygen in the respiratory substrate. Carbohydrates already contain oxygen and hydrogen in the same ratio as water, so their complete oxidation gives an RQ of exactly 1. Fats contain a much larger proportion of hydrogen relative to oxygen, so more O₂ must be consumed to fully oxidise them compared to the CO₂ produced, giving an RQ of about 0.7. Proteins fall in between, with an RQ of approximately 0.9.

Written by Satish

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