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Respiration in Plants – Class 11 Biology Chapter 12 Notes & NCERT PDF

Class 11BiologyChapter 12NCERT book: Biology - Class 11

Notes and a simple summary of Chapter 12 of the NCERT Class 11 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 12: Respiration in Plants

Introduction

All living organisms need energy for their activities, and they obtain it by oxidising food. Cellular respiration is the breaking of C–C bonds of complex compounds by oxidation within cells, releasing energy that is trapped as ATP, the energy currency of the cell. The compounds oxidised are respiratory substrates — usually carbohydrates, but also fats, proteins and organic acids.

Key Concepts

12.1 Do Plants Breathe?

Plants need O₂ and give out CO₂, but have no special respiratory organs; they use stomata and lenticels. Reasons they manage without:

  • Each plant part takes care of its own gas exchange; little transport of gases between parts.
  • Plants respire far more slowly than animals; large gas exchange happens only during photosynthesis (when O₂ is released inside cells).
  • Diffusion distances are short — living cells lie close to the surface; woody stems have living cells in thin layers under the bark with lenticels; loosely packed parenchyma provides air spaces.

    • Complete combustion of glucose releases energy mostly as heat:

    C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

    Cells instead oxidise glucose in many small steps, so that some steps release just enough energy to be coupled to ATP synthesis.

    12.2 Glycolysis (EMP pathway)

  • Named from glycos (sugar) + lysis (splitting); scheme given by Gustav Embden, Otto Meyerhof and J. Parnas.
  • Occurs in the cytoplasm of all living organisms; the only respiratory process in anaerobic organisms.
  • Glucose is partially oxidised to two molecules of pyruvic acid through ten enzyme-controlled reactions.
  • In plants, glucose comes from sucrose (split by invertase into glucose + fructose) or from stored carbohydrate. Hexokinase phosphorylates glucose/fructose to glucose-6-phosphate, which isomerises to fructose-6-phosphate.
  • ATP used at two steps: glucose → glucose-6-phosphate; fructose-6-phosphate → fructose-1,6-bisphosphate.
  • Fructose-1,6-bisphosphate splits into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
  • NADH + H⁺ formed when PGAL → 1,3-bisphosphoglycerate (BPGA).
  • ATP made when BPGA → 3-PGA and when PEP → pyruvic acid (substrate-level).
  • Per glucose: 4 ATP made – 2 ATP used = net 2 ATP, plus 2 NADH.

    • 12.3 Fermentation

    Anaerobic, incomplete oxidation of glucose.

  • Alcoholic fermentation (yeast): pyruvic acid → CO₂ + ethanol, by pyruvic acid decarboxylase and alcohol dehydrogenase.
  • Lactic acid fermentation: some bacteria; also muscle cells during exercise when O₂ is inadequate — pyruvic acid reduced to lactic acid by lactate dehydrogenase, using NADH + H⁺ (reoxidised to NAD⁺).
  • Less than 7 per cent of the energy in glucose is released, and not all is trapped as ATP; the products (acid or alcohol) are hazardous. Yeasts poison themselves when alcohol reaches about 13 per cent.
  • Net gain in fermentation: 2 ATP per glucose.

    • 12.4 Aerobic Respiration

    Complete oxidation of organic substances in presence of O₂, releasing CO₂, water and much energy; in eukaryotes it takes place in mitochondria. Crucial events:

  • Complete oxidation of pyruvate by stepwise removal of hydrogen (in the matrix);
  • Passing electrons to O₂ with ATP synthesis (on the inner membrane).

    • Link reaction (oxidative decarboxylation): in the matrix, catalysed by pyruvic dehydrogenase (coenzymes NAD⁺ and Coenzyme A):

    Pyruvic acid + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺

    Two NADH are formed from the two pyruvates of one glucose.

    12.4.1 Tricarboxylic acid (TCA) / Krebs' cycle (after Hans Krebs):

  • Acetyl group condenses with oxaloacetic acid (OAA) and water to give citric acid (enzyme citrate synthase; CoA released).
  • Citrate → isocitrate → two successive decarboxylations → α-ketoglutaric acid → succinyl-CoA → ... → OAA regenerated.
  • Succinyl-CoA → succinic acid gives one GTP (substrate-level phosphorylation), converted to ATP in a coupled reaction.
  • Per turn: 3 NADH + H⁺ and 1 FADH₂.
  • Needs continuous replenishment of OAA and regeneration of NAD⁺ and FAD⁺.
  • Up to this point, one glucose has yielded CO₂, 8 NADH + H⁺ and 2 FADH₂ (from link reaction and TCA), plus 2 ATP in the TCA cycle.

    • 12.4.2 Electron Transport System (ETS) and oxidative phosphorylation — on the inner mitochondrial membrane:

  • Complex I: NADH dehydrogenase oxidises NADH; electrons go to ubiquinone.
  • Complex II: FADH₂ (from succinate oxidation) passes electrons to ubiquinone.
  • Complex III: cytochrome bc₁ complex passes electrons from ubiquinol to cytochrome c (a small mobile protein on the outer surface of the inner membrane).
  • Complex IV: cytochrome c oxidase (cytochromes a and a₃, two copper centres) passes electrons to O₂.
  • Complex V: ATP synthase — F₁ headpiece (site of ATP synthesis) and F₀ (integral protein forming the proton channel). For each ATP, 4H⁺ pass through F₀ from the intermembrane space to the matrix.
  • 1 NADH → 3 ATP; 1 FADH₂ → 2 ATP.
  • O₂ is the final hydrogen acceptor; its role is only at the terminal stage but it drives the whole process. Because the proton gradient is made using the energy of oxidation–reduction (not light), this is oxidative phosphorylation.

    • 12.5 The Respiratory Balance Sheet

    Assumptions: a sequential pathway (glycolysis → TCA → ETS); glycolytic NADH is transferred into mitochondria and undergoes oxidative phosphorylation; no intermediates are used for synthesis; only glucose is respired. Under these assumptions there is a net gain of 38 ATP per glucose in aerobic respiration. In reality all pathways run simultaneously, so this is a theoretical exercise.

    Fermentation vs aerobic respiration: partial vs complete breakdown; 2 ATP vs many more ATP; NADH reoxidised slowly vs vigorously.

    12.6 Amphibolic Pathway

    Glucose is the favoured substrate. Fats → glycerol (enters as PGAL) and fatty acids (enter as acetyl CoA); proteins → amino acids (after deamination, enter as pyruvate, acetyl CoA or Krebs' cycle intermediates). The same intermediates are withdrawn for synthesis of fatty acids and proteins, so the respiratory pathway is involved in both catabolism and anabolism — an amphibolic pathway.

    12.7 Respiratory Quotient

    RQ = volume of CO₂ evolved ÷ volume of O₂ consumed.

  • Carbohydrates: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, RQ = 1.
  • Fats (tripalmitin): 2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O, RQ = 102/145 ≈ 0.7 (less than 1).
  • Proteins: about 0.9.

    • In living organisms substrates are usually mixed; pure fats or proteins are never the only substrate.

    Diagrams to Know

  • Glycolysis (Figure 12.1): glucose → G-6-P → F-6-P → F-1,6-bisP → DHAP + PGAL → BPGA → 3-PGA → 2-PGA → PEP → pyruvic acid, marking ATP used, ATP made and NADH formed.
  • Krebs' cycle (Figure 12.3): acetyl CoA + OAA → citric acid → α-ketoglutaric acid → succinyl-CoA → succinic acid → ... → OAA, marking 2 CO₂, 3 NADH, 1 FADH₂, 1 GTP.
  • ETS (Figure 12.4): complexes I–IV, ubiquinone, cytochrome c, ATP synthase (complex V).

    • Key-Facts Sheet

  • Glycolysis: cytoplasm; 10 steps; net 2 ATP + 2 NADH per glucose.
  • Link reaction and Krebs' cycle: mitochondrial matrix; ETS: inner membrane.
  • NADH = 3 ATP; FADH₂ = 2 ATP; 4H⁺ per ATP through F₀.
  • Net aerobic yield: 38 ATP per glucose (theoretical).
  • RQ: carbohydrate 1, fat ~0.7, protein ~0.9.

    • Exam Focus

    Exercises ask you to differentiate respiration vs combustion, glycolysis vs Krebs' cycle, aerobic vs fermentation; state the assumptions of the balance sheet; discuss the respiratory pathway as amphibolic; define RQ and give its value for fats; explain oxidative phosphorylation; and list the steps of the ETS. Practise counting ATP, NADH and FADH₂ per glucose at every stage.

    Key Terms

  • Respiration (श्वसन)
  • Respiratory substrate (श्वसन क्रियाधार)
  • Glycolysis (ग्लाइकोलिसिस / शर्करा-अपघटन)
  • Fermentation (किण्वन)
  • Pyruvic acid (पाइरुविक अम्ल)
  • Mitochondrion (माइटोकॉन्ड्रिया / सूत्रकणिका)
  • Tricarboxylic acid cycle (ट्राइकार्बोक्सिलिक अम्ल चक्र)
  • Electron transport system (इलेक्ट्रॉन परिवहन तंत्र)
  • Oxidative phosphorylation (ऑक्सीकारी फॉस्फोरिलीकरण)
  • Amphibolic pathway (उभयधर्मी पथ)
  • Respiratory quotient (श्वसन गुणांक)
  • Decarboxylation (विकार्बोक्सिलीकरण)

    • Common Mistakes

  • Placing glycolysis in the mitochondria — it occurs in the cytoplasm.
  • Forgetting that glycolysis uses 2 ATP, so its net yield is 2, not 4.
  • Saying oxygen is used in the Krebs' cycle — O₂ acts only at the end of the ETS as the final hydrogen acceptor.
  • Treating 38 ATP as a measured value — it is a theoretical figure based on stated assumptions.
  • Writing RQ as O₂/CO₂ — it is CO₂ evolved divided by O₂ consumed.
  • Calling respiration only catabolic — it is amphibolic.

    • 💡 Key Learning Points

    • ✓Explain why plants can manage gas exchange without special respiratory organs
    • ✓Trace glycolysis step by step and count its ATP and NADH yield
    • ✓Compare alcoholic and lactic acid fermentation with aerobic respiration
    • ✓Follow pyruvate through the link reaction, Krebs' cycle and the electron transport system
    • ✓Calculate the respiratory balance sheet and RQ values for different substrates

    👨‍🏫 Teaching Tips

    • →Show yeast fermentation of sugar solution in a flask with a balloon to collect CO₂
    • →Give each student one glycolysis step on a card and build the pathway on the wall in order
    • →Use a 'currency' table on the board: tally ATP, NADH and FADH₂ at each stage per glucose
    • →Model the ETS as a bucket brigade passing electrons down to oxygen
    • →Demonstrate respiration in germinating seeds with a KOH trap and a respirometer
    • →Calculate RQ for tripalmitin together (102/145) to show why fats give values below 1

    📋 Assessment Questions

    1. Why is glycolysis considered common to aerobic and anaerobic respiration?
    2. State where the link reaction, Krebs' cycle and ETS occur in a cell.
    3. How many ATP are produced from one NADH and one FADH₂ in the ETS?
    4. List the assumptions behind the figure of 38 ATP per glucose.
    5. Why is the respiratory pathway called amphibolic?

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    Keep learning

    All chapters in Biology - Class 11

    1. 1. The Living World
    2. 2. Biological Classification
    3. 3. Plant Kingdom
    4. 4. Animal Kingdom
    5. 5. Morphology of Flowering Plants
    6. 6. Anatomy of Flowering Plants
    7. 7. Structural Organisation in Animals
    8. 8. Cell: The Unit of Life
    9. 9. Biomolecules
    10. 10. Cell Cycle and Cell Division
    11. 11. Photosynthesis in Higher Plants
    12. 12. Respiration in Plants
    13. 13. Plant Growth and Development
    14. 14. Breathing and Exchange of Gases
    15. 15. Body Fluids and Circulation
    16. 16. Excretory Products and their Elimination
    17. 17. Locomotion and Movement
    18. 18. Neural Control and Coordination
    19. 19. Chemical Coordination and Integration