NCERT Solutions for Class 11 Biology Chapter 12: Respiration in Plants (NCERT 2026–27)

These Class 11 Biology Chapter 12 solutions cover Respiration in Plants with complete, exam-ready answers to every NCERT exercise question. The chapter explains how cells break the C–C bonds of respiratory substrates by oxidation to release energy and trap it as ATP — through glycolysis, fermentation, the Krebs’ cycle, the electron transport system and oxidative phosphorylation — and ends with the amphibolic nature of respiration and the respiratory quotient.

Class: 11 Subject: Biology Chapter: 12 Title: Respiration in Plants Type: NCERT Exercises (12 questions) Session: 2026–27
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Class 11 Biology Chapter 12 Solutions – Overview

Respiration is the process by which the carbon–carbon (C–C) bonds of complex organic food molecules are broken through oxidation within the cell, releasing energy that is trapped as ATP, the energy currency of the cell. Plants have no special breathing organs; gaseous exchange occurs by simple diffusion through stomata and lenticels because each plant part meets its own modest needs and every living cell lies close to the surface. The favoured respiratory substrate is glucose. Respiration proceeds through glycolysis (cytoplasm) which yields pyruvic acid; the fate of pyruvate then depends on oxygen — under anaerobic conditions it leads to fermentation (lactic acid or alcohol), while in the presence of oxygen it is completely oxidised by aerobic respiration in the mitochondria via the Krebs’ cycle, the electron transport system (ETS) and oxidative phosphorylation. A net gain of up to 38 ATP is possible per glucose. Because the pathway is used for both breakdown and synthesis, it is called an amphibolic pathway, and the ratio of CO2 released to O2 used is the respiratory quotient (RQ).

Key Concepts & Definitions

Cellular respiration: the breaking of C–C bonds of complex organic molecules by oxidation within cells, releasing energy that is trapped as ATP.

Respiratory substrate: the compound oxidised during respiration — usually carbohydrates (glucose), but also fats, proteins and organic acids.

Glycolysis (EMP pathway): partial oxidation of one glucose into two molecules of pyruvic acid in the cytoplasm; given by Embden, Meyerhof and Parnas.

Fermentation: incomplete, anaerobic oxidation of glucose; pyruvate becomes lactic acid (e.g. muscle, some bacteria) or ethanol + CO2 (e.g. yeast).

Aerobic respiration: complete oxidation of substrate in the presence of O2 inside mitochondria, releasing CO2, water and a large amount of energy.

Krebs’ cycle (TCA / citric acid cycle): cyclic oxidation of acetyl CoA in the mitochondrial matrix, releasing CO2 and generating NADH, FADH2 and GTP/ATP.

Electron Transport System (ETS): chain of carriers on the inner mitochondrial membrane that oxidises NADH and FADH2, passing electrons to O2 to form water.

Oxidative phosphorylation: synthesis of ATP using the energy of oxidation–reduction as electrons flow through the ETS to O2.

Amphibolic pathway: a pathway that serves both catabolism (breakdown) and anabolism (synthesis).

Respiratory Quotient (RQ): ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration.

Complete combustion of glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
RQ = (volume of CO2 evolved) ÷ (volume of O2 consumed)

NCERT Exercises — Solutions

1. Differentiate between (a) Respiration and Combustion (b) Glycolysis and Krebs’ cycle (c) Aerobic respiration and Fermentation

ANSWER (a) Respiration vs Combustion:
RespirationCombustion
Slow, step-wise enzyme-controlled oxidation inside living cells.Rapid, uncontrolled non-enzymatic oxidation (burning).
Energy is released gradually and trapped as ATP.Energy is released suddenly, mostly as heat and light.
Occurs at body/cell temperature.Requires high (ignition) temperature.
(b) Glycolysis vs Krebs’ cycle:
GlycolysisKrebs’ cycle
Occurs in the cytoplasm.Occurs in the mitochondrial matrix.
Is anaerobic; does not need O2.Is aerobic in operation (linked to ETS which needs O2).
A linear pathway breaking glucose (6C) into 2 pyruvate (3C).A cyclic pathway oxidising acetyl CoA (2C), releasing CO2.
Net gain of 2 ATP and 2 NADH per glucose; no CO2 released.Per cycle: 3 NADH, 1 FADH2, 1 GTP/ATP and 2 CO2.
(c) Aerobic respiration vs Fermentation:
Aerobic respirationFermentation
Complete oxidation of glucose to CO2 and H2O.Only partial breakdown of glucose.
Requires O2; occurs in mitochondria.Anaerobic; occurs in the cytoplasm.
Large energy yield (up to 38 ATP per glucose).Net gain of only 2 ATP per glucose.
End products: CO2 and water.End products: ethanol + CO2 or lactic acid.

2. What are respiratory substrates? Name the most common respiratory substrate.

ANSWER Respiratory substrates are the organic compounds that are oxidised during respiration to release energy. Usually carbohydrates serve this role, but proteins, fats and even organic acids can be used as respiratory substrates in some plants under certain conditions. The most common (favoured) respiratory substrate is glucose. All carbohydrates are generally first converted into glucose before being respired.

3. Give the schematic representation of glycolysis?

ANSWER Glycolysis is a chain of ten enzyme-controlled reactions in the cytoplasm that converts one glucose into two pyruvate. ATP is used at two steps (glucose → glucose-6-phosphate, and fructose-6-phosphate → fructose-1,6-bisphosphate) and produced later, with NADH formed when PGAL is oxidised. A simplified scheme is:
Glucose (6C) −ATP → Glucose-6-phosphate → Fructose-6-phosphate −ATP → Fructose-1,6-bisphosphate (6C)
↓ (split)
2 × Triose phosphate / PGAL (3C) +NADH → 2 × 1,3-bisphosphoglycerate (BPGA)
+ATP → 2 × 3-phosphoglycerate (PGA) → 2 × 2-phosphoglycerate → 2 × phosphoenolpyruvate (PEP)
+ATP → 2 × Pyruvic acid (3C)
Net per glucose: 2 pyruvate, 2 ATP (gain) and 2 NADH + H+. (4 ATP are made and 2 are used, giving a net of 2 ATP.)

4. What are the main steps in aerobic respiration? Where does it take place?

ANSWER Aerobic respiration involves the complete oxidation of the substrate in the presence of oxygen. Its main steps are: (i) Glycolysis — glucose is broken down to pyruvic acid in the cytoplasm. (ii) Oxidative decarboxylation of pyruvate — pyruvate enters the mitochondrion and is converted to acetyl CoA, releasing CO2 and NADH (in the mitochondrial matrix). (iii) Krebs’ (TCA) cycle — acetyl CoA is completely oxidised, releasing CO2 and generating NADH, FADH2 and GTP/ATP (in the matrix). (iv) Electron transport system & oxidative phosphorylation — NADH and FADH2 are oxidised and electrons passed to O2, forming water and synthesising ATP (on the inner mitochondrial membrane). Thus the first step occurs in the cytoplasm, while all the remaining steps of aerobic respiration take place inside the mitochondria of eukaryotic cells.

5. Give the schematic representation of an overall view of Krebs’ cycle.

ANSWER The Krebs’ cycle begins when acetyl CoA (2C) condenses with oxaloacetic acid / OAA (4C) and water to form citric acid (6C), and OAA is regenerated at the end so the cycle continues. An overall scheme is:
Acetyl CoA (2C) + OAA (4C) + H2O → Citric acid (6C) [citrate synthase; CoA released]
Citric acid → Isocitrate → α-ketoglutaric acid (5C) +CO2, +NADH
α-ketoglutaric acid → Succinyl-CoA (4C) +CO2, +NADH
Succinyl-CoA → Succinic acid (4C) +GTP → ATP
Succinic acid → Malic acid (4C) +FADH2
Malic acid → Oxaloacetic acid / OAA (4C) +NADH → (cycle repeats)
Per turn of the cycle: 2 CO2 released, 3 NADH + H+, 1 FADH2 and 1 ATP (via GTP). For one glucose the cycle turns twice.

6. Explain ETS.

ANSWER The Electron Transport System (ETS) is the chain of carriers, located on the inner mitochondrial membrane, through which electrons pass from one carrier to the next, releasing energy that is used to synthesise ATP. It releases and utilises the energy stored in NADH + H+ and FADH2. Electrons from NADH (formed in the matrix) are oxidised by NADH dehydrogenase (Complex I) and transferred to ubiquinone. FADH2 donates electrons to ubiquinone via Complex II. Reduced ubiquinone (ubiquinol) is then oxidised, passing electrons to cytochrome c through the cytochrome bc1 complex (Complex III). Cytochrome c is a mobile carrier that transfers electrons to cytochrome c oxidase (Complex IV), which contains cytochromes a and a3 and two copper centres. As electrons flow from Complex I to IV they are coupled to ATP synthase (Complex V), which makes ATP from ADP and inorganic phosphate. Oxygen acts as the final/terminal hydrogen (electron) acceptor, combining with electrons and protons to form water. Oxidation of one NADH yields about 3 ATP and one FADH2 yields about 2 ATP.

7. Distinguish between the following: (a) Aerobic respiration and Anaerobic respiration (b) Glycolysis and Fermentation (c) Glycolysis and Citric acid Cycle

ANSWER (a) Aerobic vs Anaerobic respiration:
Aerobic respirationAnaerobic respiration
Takes place in the presence of O2.Takes place in the absence of O2.
Glucose is completely oxidised to CO2 and H2O.Glucose is only incompletely oxidised.
Yields up to 38 ATP per glucose.Yields only 2 ATP per glucose.
Occurs in cytoplasm and mitochondria.Occurs only in the cytoplasm; end products are alcohol or lactic acid.
(b) Glycolysis vs Fermentation:
GlycolysisFermentation
Conversion of glucose into pyruvic acid through ten reactions.Conversion of pyruvic acid into ethanol + CO2 or lactic acid.
NAD+ is reduced to NADH + H+.NADH + H+ is re-oxidised to NAD+.
Common first step in both aerobic and anaerobic respiration.Follows glycolysis only under anaerobic conditions.
Net gain of 2 ATP.No additional ATP is produced; it regenerates NAD+.
(c) Glycolysis vs Citric acid cycle:
GlycolysisCitric acid (Krebs’) cycle
Occurs in the cytoplasm.Occurs in the mitochondrial matrix.
A linear pathway from glucose to pyruvate.A cyclic pathway oxidising acetyl CoA.
Does not release CO2.Releases CO2 (2 per turn).
Net gain: 2 ATP + 2 NADH per glucose.Per turn: 3 NADH + 1 FADH2 + 1 ATP (GTP).

8. What are the assumptions made during the calculation of net gain of ATP?

ANSWER The theoretical calculation of a net gain of 38 ATP per glucose rests on the following assumptions: (i) There is a sequential, orderly pathway functioning, with one substrate forming the next, and glycolysis, the TCA cycle and the ETS pathway following one after another. (ii) The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation. (iii) None of the intermediates in the pathway are withdrawn to synthesise any other compound. (iv) Only glucose is being respired — no other alternative substrate enters the pathway at any of the intermediary stages. In a real living system these assumptions do not strictly hold, since all pathways operate simultaneously, intermediates are added and withdrawn as needed, and ATP is used as required — so the figure remains theoretical.

9. Discuss “The respiratory pathway is an amphibolic pathway.”

ANSWER Respiration is traditionally viewed as a catabolic (breakdown) process because it degrades substrates to release energy. However, the same respiratory pathway also supplies carbon skeletons for the synthesis (anabolism) of other molecules, so it functions in both directions — hence it is best described as an amphibolic pathway. For example, fats are first broken down to acetyl CoA before entering respiration; but when the organism needs to make fatty acids, acetyl CoA is instead withdrawn from the respiratory pathway. Similarly, proteins are broken to amino acids that, after deamination, enter the pathway (as pyruvate, acetyl CoA or Krebs’ cycle intermediates), and the same intermediates are withdrawn to synthesise amino acids and proteins. Because respiratory intermediates take part both in the breakdown (catabolism) and the building up (anabolism) of fats and proteins, the respiratory pathway is rightly considered amphibolic rather than purely catabolic.

10. Define RQ. What is its value for fats?

ANSWER Respiratory Quotient (RQ) is the ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration.
RQ = (volume of CO2 evolved) ÷ (volume of O2 consumed)
For fats, RQ is less than 1 (because fats need more O2 for oxidation). For the fatty substrate tripalmitin: 2(C51H98O6) + 145O2 → 102CO2 + 98H2O + energy, giving RQ = 102/145 ≈ 0.7. (For carbohydrates RQ = 1.0 and for proteins it is about 0.9.)

11. What is oxidative phosphorylation?

ANSWER Oxidative phosphorylation is the process of synthesising ATP from ADP and inorganic phosphate using the energy released during the transfer of electrons through the electron transport system to oxygen. As NADH and FADH2 are oxidised, electrons move down the ETS on the inner mitochondrial membrane and the energy of these oxidation–reduction reactions is used by ATP synthase (Complex V) to make ATP. It is called “oxidative” phosphorylation because it is the energy of oxidation (not light energy as in photophosphorylation) that drives phosphorylation. Oxygen acts as the final electron acceptor and is reduced to water, driving the whole process by removing hydrogen from the system.

12. What is the significance of step-wise release of energy in respiration?

ANSWER If glucose were oxidised in a single step (as in combustion), almost all of its energy would be lost suddenly as heat and could not be used by the cell. Step-wise release is therefore significant because it lets the cell capture energy efficiently instead of wasting it. By oxidising glucose through many small, enzyme-controlled steps, some steps release just enough energy to be coupled to ATP synthesis. This means: (i) energy is liberated gradually and in usable amounts, (ii) a maximum amount is trapped as ATP, (iii) the cell is protected from a damaging sudden rise in temperature, and (iv) the rate of each step can be precisely controlled by enzymes according to the cell’s needs.

Extra Practice Questions

Short Answer Type Questions

Q1. Why do plants not need specialised respiratory organs?

ANSWEREach plant part meets its own gas-exchange needs, the demand for gas exchange is low, and every living cell lies close to the surface so diffusion through stomata and lenticels is enough.

Q2. Where does glycolysis occur, and is it aerobic or anaerobic?

ANSWERGlycolysis occurs in the cytoplasm of the cell and is anaerobic — it does not require oxygen and is present in all living organisms.

Q3. Name the enzymes that catalyse alcoholic fermentation in yeast.

ANSWERPyruvic acid decarboxylase and alcohol dehydrogenase convert pyruvic acid into CO2 and ethanol.

Q4. How many ATP, NADH and FADH2 are formed when 2 pyruvate are completely oxidised in the Krebs’ cycle?

ANSWERFor two turns of the cycle: 8 NADH + H+, 2 FADH2 and 2 ATP (via GTP), along with the release of 4 CO2.

Q5. Why is ATP called the “energy currency” of the cell?

ANSWEREnergy released by oxidation is not used directly but is trapped in ATP, which is broken down whenever and wherever energy is needed for the cell’s activities — so ATP acts like spendable currency.

Long Answer Type Questions

Q1. Describe the fate of pyruvic acid under different conditions.

ANSWERPyruvic acid, the key product of glycolysis, has three possible fates depending on cellular need and oxygen availability. (1) Lactic acid fermentation: in some bacteria and in muscle cells during vigorous exercise when O2 is inadequate, pyruvate is reduced to lactic acid by lactate dehydrogenase, with NADH + H+ re-oxidised to NAD+. (2) Alcoholic fermentation: in yeast under anaerobic conditions, pyruvate is converted to ethanol and CO2 by pyruvic acid decarboxylase and alcohol dehydrogenase. Both fermentations release less than seven per cent of glucose energy with a net gain of only 2 ATP. (3) Aerobic respiration: when O2 is available, pyruvate enters the mitochondrion, is converted to acetyl CoA, and is completely oxidised through the Krebs’ cycle and ETS, releasing CO2, water and a large amount of energy (up to 38 ATP).

Q2. Explain the structure and role of ATP synthase (Complex V) in ATP synthesis.

ANSWERATP synthase, also called Complex V, uses the energy released during electron transport to synthesise ATP. It has two major components. The F1 headpiece is a peripheral membrane protein complex that contains the site for synthesis of ATP from ADP and inorganic phosphate. The F0 component is an integral membrane protein complex that forms the channel through which protons (H+) cross the inner mitochondrial membrane. As electrons flow through the ETS, protons are pumped into the intermembrane space, creating an electrochemical proton gradient. The passage of protons back through the F0 channel, down this gradient, is coupled to the catalytic F1 site to produce ATP. For each ATP produced, 4 H+ pass through F0 from the intermembrane space to the matrix. This is the membrane-linked ATP synthesis explained by the chemiosmotic hypothesis.

Q3. Compare fermentation and aerobic respiration with respect to efficiency and end products.

ANSWERFermentation accounts for only a partial breakdown of glucose, whereas in aerobic respiration glucose is completely degraded to CO2 and H2O. In fermentation there is a net gain of only two molecules of ATP for each glucose degraded to pyruvic acid, while many more molecules of ATP (up to 38) are generated under aerobic conditions, making aerobic respiration far more efficient. In fermentation NADH is oxidised to NAD+ rather slowly, whereas this re-oxidation is very vigorous in aerobic respiration. The end products also differ: fermentation yields ethanol + CO2 (in yeast) or lactic acid (in muscle and some bacteria), and these products are hazardous — either acid or alcohol accumulates — whereas aerobic respiration yields only CO2 and water.

MCQs & Assertion–Reason

1. Glycolysis takes place in the:

(a) mitochondrial matrix    (b) cytoplasm    (c) inner mitochondrial membrane    (d) chloroplast

2. The end product of glycolysis is:

(a) acetyl CoA    (b) lactic acid    (c) pyruvic acid    (d) citric acid

3. The net gain of ATP from one molecule of glucose in glycolysis is:

(a) 2    (b) 4    (c) 36    (d) 38

4. In alcoholic fermentation by yeast, pyruvic acid is converted into:

(a) lactic acid    (b) ethanol and CO2    (c) acetyl CoA    (d) malic acid

5. The Krebs’ cycle begins with the condensation of acetyl CoA and:

(a) citric acid    (b) succinic acid    (c) oxaloacetic acid    (d) α-ketoglutaric acid

6. Oxidation of one molecule of FADH2 in the ETS produces:

(a) 1 ATP    (b) 2 ATP    (c) 3 ATP    (d) 4 ATP

7. The final acceptor of electrons (hydrogen) in the ETS is:

(a) NAD+    (b) FAD+    (c) oxygen    (d) cytochrome c

8. The total net gain of ATP during aerobic respiration of one glucose molecule is:

(a) 2    (b) 8    (c) 30    (d) 38

9. The respiratory quotient (RQ) of carbohydrates being completely oxidised is:

(a) 0.7    (b) 0.9    (c) 1.0    (d) greater than 1

10. Gaseous exchange in plants takes place mainly through:

(a) lungs    (b) gills    (c) stomata and lenticels    (d) xylem vessels

Answer key: 1-(b), 2-(c), 3-(a), 4-(b), 5-(c), 6-(b), 7-(c), 8-(d), 9-(c), 10-(c).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: ATP is called the energy currency of the cell.

Reason: Energy released in respiration is trapped as ATP and broken down whenever and wherever energy is needed.

A-R 2. Assertion: Glycolysis does not require oxygen.

Reason: Glycolysis occurs in the cytoplasm and is present in all living organisms.

A-R 3. Assertion: The respiratory pathway is an amphibolic pathway.

Reason: It functions only in the breakdown of organic substrates and never in their synthesis.

A-R 4. Assertion: The RQ of fats is less than one.

Reason: Oxidation of fats consumes more oxygen relative to the carbon dioxide released.

A-R 5. Assertion: Energy in respiration is released step-wise rather than in a single step.

Reason: Step-wise oxidation lets some steps release just enough energy to be coupled to ATP synthesis.

Answer key: 1-(A), 2-(B), 3-(D), 4-(A), 5-(A).

Common Mistakes to Avoid

Watch out for these

  • Confusing the net ATP of glycolysis (2) with the gross ATP (4) — remember 2 ATP are spent first.
  • Saying glycolysis releases CO2 — it does not; CO2 is released only during pyruvate oxidation and the Krebs’ cycle.
  • Writing that NADH gives 2 ATP and FADH2 gives 3 — it is the reverse (NADH → 3 ATP, FADH2 → 2 ATP).
  • Calling 38 ATP a fixed real value — it is a theoretical maximum based on assumptions that do not hold in living cells.
  • Treating respiration as purely catabolic — it is amphibolic because intermediates are also used for synthesis.
  • Stating that pure fats or pure proteins are normally used as respiratory substrates — in living organisms substrates are usually mixed.

Exam Tips

How to score full marks in this chapter

Memorise the location of each stage (glycolysis → cytoplasm; link reaction and Krebs’ cycle → matrix; ETS and oxidative phosphorylation → inner membrane) and the yield per stage. For “differentiate” questions, always answer in a two-column table with at least 3–4 clear points. When asked to give schematic representations, draw clean flow arrows showing where ATP/NADH/FADH2 are used or produced and where CO2 is released. Quote exact figures — RQ = 1.0 (carbohydrate), 0.7 (fat, tripalmitin), about 0.9 (protein), and a net 38 ATP per glucose — and state the assumptions when explaining the balance sheet.

Frequently Asked Questions

What is Class 11 Biology Chapter 12 Respiration in Plants about?

Chapter 12 explains how cells release energy by oxidising respiratory substrates and trapping it as ATP — through glycolysis, fermentation, the Krebs’ cycle, the electron transport system and oxidative phosphorylation — and covers the respiratory balance sheet, the amphibolic nature of respiration and the respiratory quotient.

How many ATP are produced during aerobic respiration of one glucose molecule?

There can be a theoretical net gain of 38 ATP per molecule of glucose during aerobic respiration, based on assumptions that do not strictly hold in a living system. Fermentation gives a net of only 2 ATP.

What is the respiratory quotient (RQ) and its value for fats?

RQ is the ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration. It is 1.0 for carbohydrates, about 0.9 for proteins, and less than 1 for fats — about 0.7 for tripalmitin.

Are these Class 11 Biology Chapter 12 solutions free?

Yes. All ClearStudy NCERT Solutions for Class 11 Biology are free and follow the official NCERT textbook for the 2026–27 session.

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