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Class 11 Biology Notes Chapter 14 (Respiration in plants) – Biology Book

Philoid

Philoid

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Biology
Detailed Notes with MCQs of Chapter 14: Respiration in Plants. This is a crucial chapter, not just for your Class 11 understanding, but also because concepts related to cellular respiration frequently appear in various government examinations. Pay close attention to the details.

Chapter 14: Respiration in Plants - Detailed Notes

1. Introduction to Respiration:

  • Definition: Respiration is the biochemical process occurring within living cells where complex organic substances (like glucose) are broken down through oxidation to release energy in the form of ATP (Adenosine Triphosphate). Carbon dioxide and water are typically released as byproducts in aerobic respiration.
  • Why Plants Respire: Like all living organisms, plants need energy for various metabolic activities such as absorption, transport, movement, reproduction, and synthesis of organic molecules. This energy comes from the breakdown of food synthesized during photosynthesis or stored food.
  • Gaseous Exchange in Plants:
    • Plants lack specialized respiratory organs like animals.
    • Gaseous exchange occurs primarily through stomata (in leaves and young stems) and lenticels (in older stems and roots).
    • Each living cell in a plant is located quite close to the surface, facilitating direct gas exchange.
    • Loose packing of parenchyma cells in leaves, stems, and roots provides interconnected air spaces.
  • Overall Equation for Aerobic Respiration:
    C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

2. Types of Respiration:

  • Aerobic Respiration: Occurs in the presence of oxygen. Involves complete oxidation of glucose, yielding a large amount of energy (approx. 36-38 ATP). Takes place in the cytoplasm and mitochondria.
  • Anaerobic Respiration (Fermentation): Occurs in the absence of oxygen. Involves incomplete oxidation of glucose, yielding a small amount of energy (Net 2 ATP). Takes place entirely in the cytoplasm.

3. Glycolysis (EMP Pathway - Embden-Meyerhof-Parnas Pathway):

  • Location: Cytoplasm of all living cells (both aerobic and anaerobic organisms).
  • Oxygen Requirement: Does not require oxygen. It's the common pathway for both aerobic and anaerobic respiration.
  • Process:
    • Partial oxidation of one molecule of glucose (6-carbon) into two molecules of pyruvic acid (pyruvate, 3-carbon).
    • It's a 10-step enzymatic process.
    • Key Phases:
      • Energy Investment Phase: 2 ATP molecules are consumed to phosphorylate glucose and fructose-6-phosphate.
      • Energy Payoff Phase: 4 ATP molecules are produced via substrate-level phosphorylation, and 2 molecules of NADH + H⁺ are produced.
  • Net Products of Glycolysis (per glucose molecule):
    • 2 molecules of Pyruvic Acid
    • 2 ATP (4 produced - 2 consumed)
    • 2 NADH + H⁺

4. Fate of Pyruvic Acid:

  • The fate depends on the availability of oxygen and the organism.
    • Aerobic Conditions: Pyruvic acid enters the mitochondria for further oxidation (Link Reaction & Krebs Cycle).
    • Anaerobic Conditions (Fermentation): Pyruvic acid remains in the cytoplasm and is converted into either ethanol or lactic acid.

5. Fermentation (Anaerobic Respiration):

  • Purpose: Regeneration of NAD⁺ from NADH + H⁺ produced during glycolysis, allowing glycolysis to continue producing ATP even without oxygen.
  • Location: Cytoplasm.
  • Types:
    • Alcoholic Fermentation:
      • Occurs in yeast and some plants under anaerobic conditions.
      • Pyruvic acid is first decarboxylated to acetaldehyde, which is then reduced by NADH + H⁺ to ethanol.
      • Enzymes: Pyruvic acid decarboxylase, Alcohol dehydrogenase.
      • Products: Ethanol, CO₂, NAD⁺.
      • Net ATP Gain: 2 ATP (from glycolysis).
    • Lactic Acid Fermentation:
      • Occurs in some bacteria (e.g., Lactobacillus), fungi, and animal muscle cells during strenuous exercise.
      • Pyruvic acid is directly reduced by NADH + H⁺ to lactic acid.
      • Enzyme: Lactate dehydrogenase.
      • Products: Lactic acid, NAD⁺.
      • Net ATP Gain: 2 ATP (from glycolysis).
  • Drawback: Produces very little energy (less than 7% of energy in glucose is released). Products like alcohol or lactic acid can be toxic in high concentrations.

6. Aerobic Respiration:

  • Occurs in the mitochondria. Involves complete oxidation of pyruvic acid into CO₂ and H₂O, releasing a large amount of energy.
  • Steps:
    • Link Reaction (Pyruvate Oxidation / Gateway Step):
      • Location: Mitochondrial Matrix.
      • Pyruvic acid (3C) is transported into the mitochondria.
      • It undergoes oxidative decarboxylation (removal of CO₂ and oxidation).
      • Forms Acetyl-CoA (2C), CO₂, and NADH + H⁺.
      • Enzyme: Pyruvate dehydrogenase complex.
      • Note: Since glycolysis produces 2 pyruvic acid molecules per glucose, this reaction occurs twice per glucose molecule.
      • Products per glucose (2 Pyruvic acids): 2 Acetyl-CoA, 2 CO₂, 2 NADH + H⁺.
    • Krebs Cycle (Tricarboxylic Acid Cycle - TCA Cycle / Citric Acid Cycle):
      • Location: Mitochondrial Matrix.
      • Acetyl-CoA (2C) enters the cycle by combining with Oxaloacetic acid (OAA - 4C) to form Citric acid (6C).
      • It's a cyclic pathway where Citric acid undergoes a series of reactions, regenerating OAA.
      • Key Events per cycle (per Acetyl-CoA):
        • 2 molecules of CO₂ are released.
        • 3 molecules of NADH + H⁺ are produced.
        • 1 molecule of FADH₂ is produced.
        • 1 molecule of ATP (or GTP via substrate-level phosphorylation) is produced.
      • Products per glucose molecule (2 Acetyl-CoA entering the cycle):
        • 4 CO₂
        • 6 NADH + H⁺
        • 2 FADH₂
        • 2 ATP (or GTP)
    • Electron Transport System (ETS) and Oxidative Phosphorylation:
      • Location: Inner Mitochondrial Membrane.
      • Process:
        • NADH + H⁺ and FADH₂ (produced during glycolysis, link reaction, and Krebs cycle) donate electrons to the ETS.
        • ETS consists of a series of electron carriers (Complex I to IV) arranged in order of increasing redox potential. Key carriers include Flavins (FMN), Iron-Sulphur proteins (Fe-S), Ubiquinone (UQ), and Cytochromes (Cyt b, c₁, c, a, a₃).
        • Electrons pass from one carrier to another, releasing energy.
        • This energy is used to pump protons (H⁺) from the mitochondrial matrix to the intermembrane space, creating a proton gradient (Proton Motive Force - PMF).
        • Oxygen acts as the final/terminal electron acceptor. It accepts electrons and combines with protons (H⁺) to form water (metabolic water).
        • Oxidative Phosphorylation: The proton gradient drives protons back into the matrix through a channel in the ATP Synthase (Complex V / F₀-F₁ particle). This flow of protons provides energy for the synthesis of ATP from ADP and inorganic phosphate (Pi). This is chemiosmosis.
      • ATP Yield from Oxidation:
        • 1 NADH + H⁺ → approx. 3 ATP
        • 1 FADH₂ → approx. 2 ATP

7. The Respiratory Balance Sheet:

  • Calculates the theoretical net gain of ATP for aerobic respiration of one glucose molecule.
  • Assumptions:
    • Sequential, orderly pathway functioning.
    • NADH synthesized in glycolysis enters mitochondria for oxidation.
    • No intermediates are diverted into other pathways.
    • Only glucose is being respired.
  • Calculation:
    • Glycolysis: 2 ATP (substrate-level) + 2 NADH (→ 6 ATP via ETS*) = 8 ATP
    • Link Reaction: 2 NADH (→ 6 ATP via ETS) = 6 ATP
    • Krebs Cycle: 2 ATP/GTP (substrate-level) + 6 NADH (→ 18 ATP via ETS) + 2 FADH₂ (→ 4 ATP via ETS) = 24 ATP
    • Total Theoretical Yield: 8 + 6 + 24 = 38 ATP
    • Note: If NADH from glycolysis uses a shuttle system that yields only 2 ATP per NADH (like the glycerol phosphate shuttle common in some cells), the total yield would be 36 ATP. NCERT often leans towards 38 ATP but acknowledges the variability.

8. Amphibolic Pathway:

  • The respiratory pathway (especially Krebs cycle) is involved in both catabolism (breakdown) and anabolism (synthesis). Hence, it's better termed an amphibolic pathway.
  • Catabolic Role: Breakdown of carbohydrates, fats (fatty acids → Acetyl CoA; glycerol → PGAL), and proteins (amino acids → pyruvate or Krebs cycle intermediates).
  • Anabolic Role: Respiratory intermediates can be withdrawn and used as precursors for synthesis:
    • Acetyl CoA → Fatty acids, Gibberellins, Carotenoids
    • α-Ketoglutaric acid → Amino acids (e.g., Glutamate)
    • Oxaloacetic acid → Amino acids (e.g., Aspartate), Alkaloids, Pyrimidines
    • Succinyl CoA → Chlorophyll, Cytochromes

9. Respiratory Quotient (RQ):

  • Definition: 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
  • Significance: Depends on the type of respiratory substrate being oxidized.
  • RQ Values:
    • Carbohydrates (e.g., Glucose): RQ = 1 (6 CO₂ / 6 O₂ = 1)
    • Fats (e.g., Tripalmitin): RQ < 1 (e.g., Tripalmitin RQ ≈ 0.7). Fats require more O₂ for complete oxidation than carbohydrates relative to CO₂ produced.
    • Proteins: RQ ≈ 0.9.
    • Organic Acids (e.g., Malic acid): RQ > 1 (e.g., Malic acid RQ = 1.33). They contain more oxygen relative to carbon.
    • Anaerobic Respiration: RQ = Infinite (CO₂ is evolved, but O₂ is not consumed).

Multiple Choice Questions (MCQs):

  1. In which part of the plant cell does glycolysis occur?
    a) Mitochondria
    b) Chloroplast
    c) Cytoplasm
    d) Nucleus

  2. What are the net products of glycolysis from one molecule of glucose?
    a) 2 Pyruvic acid, 2 ATP, 2 FADH₂
    b) 2 Pyruvic acid, 2 ATP, 2 NADH + H⁺
    c) 2 Acetyl CoA, 2 CO₂, 2 ATP
    d) 6 CO₂, 6 H₂O, 38 ATP

  3. During alcoholic fermentation, pyruvic acid is first converted to X and then to Y. Identify X and Y.
    a) X = Lactic acid, Y = Ethanol
    b) X = Acetaldehyde, Y = Ethanol
    c) X = Ethanol, Y = Acetaldehyde
    d) X = Acetyl CoA, Y = Ethanol

  4. The Krebs cycle (TCA cycle) takes place in the:
    a) Cytoplasm
    b) Inner mitochondrial membrane
    c) Intermembrane space of mitochondria
    d) Mitochondrial matrix

  5. Which molecule links glycolysis and the Krebs cycle?
    a) Pyruvic acid
    b) Malic acid
    c) Acetyl-CoA
    d) Citric acid

  6. What is the role of oxygen in aerobic respiration?
    a) It combines with Acetyl-CoA at the start of the Krebs cycle.
    b) It acts as the final electron acceptor in the Electron Transport System.
    c) It is required for the substrate-level phosphorylation in glycolysis.
    d) It directly oxidizes glucose to CO₂ and H₂O.

  7. The synthesis of ATP using the energy from the proton gradient across the inner mitochondrial membrane is called:
    a) Substrate-level phosphorylation
    b) Oxidative phosphorylation (Chemiosmosis)
    c) Photophosphorylation
    d) Fermentation

  8. If the respiratory substrate is a fat (like Tripalmitin), the Respiratory Quotient (RQ) will be:
    a) Equal to 1
    b) Less than 1
    c) Greater than 1
    d) Equal to 0

  9. Which of the following statements correctly describes the respiratory pathway as amphibolic?
    a) It only breaks down complex molecules into simpler ones.
    b) It only synthesizes complex molecules from simpler ones.
    c) It is strictly an anaerobic process.
    d) It involves both breakdown (catabolism) and synthesis (anabolism) of molecules.

  10. How many ATP molecules are produced via substrate-level phosphorylation during one turn of the Krebs cycle?
    a) 1
    b) 2
    c) 3
    d) 4

Answer Key for MCQs:

  1. c) Cytoplasm
  2. b) 2 Pyruvic acid, 2 ATP, 2 NADH + H⁺
  3. b) X = Acetaldehyde, Y = Ethanol
  4. d) Mitochondrial matrix
  5. c) Acetyl-CoA
  6. b) It acts as the final electron acceptor in the Electron Transport System.
  7. b) Oxidative phosphorylation (Chemiosmosis)
  8. b) Less than 1
  9. d) It involves both breakdown (catabolism) and synthesis (anabolism) of molecules.
  10. a) 1 (as GTP, which is equivalent to ATP)

Make sure you understand the location, inputs, outputs, and significance of each stage. Remember the ATP calculations and the concept of RQ. This forms a strong foundation for questions on cellular respiration. Revise these notes thoroughly.

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