Chapter: Respiration in Plants

• Overview

  • Definition of Cellular Respiration
  • Oxidation of C-C bonds of complex molecules
  • Release of Energy (stored as ATP)
  • Difference between Photosynthesis and Respiration

• Plant Respiration vs. Animals

  • No specialized respiratory organs
  • Gas exchange via Stomata and Lenticels
  • O2 utilization and CO2 release
  • Loose packing of parenchyma cells in leaves, stems, and roots

• Glycolysis (EMP Pathway)

  • Site: Cytoplasm of the cell
  • Oxygen Requirement: Aerobic and Anaerobic (common to both)
  • Key Steps:
    • Phosphorylation of Glucose
    • Splitting of Fructose 1,6-bisphosphate into PGAL and DHAP
    • Formation of NADH + H+
    • Substrate-level phosphorylation to form ATP
  • Net Products: $2$ Pyruvic Acid, $2$ ATP, $2$ NADH + H+

• Fermentation (Anaerobic Respiration)

  • Site: Cytoplasm
  • Types:
    • Alcoholic Fermentation: Pyruvate converted to Ethanol and CO2 (e.g., Yeast)
    • Lactic Acid Fermentation: Pyruvate converted to Lactic Acid (e.g., Bacteria, muscles)
  • Drawbacks:
    • Incomplete oxidation of glucose
    • Hazardous end-products
    • Very low net ATP gain ($2$ ATP)

• Aerobic Respiration

  • Site: Mitochondria
  • Phases:
    • Link Reaction (Matrix): Pyruvate to Acetyl CoA + NADH + CO2
    • Tricarboxylic Acid (TCA) Cycle / Krebs Cycle:
      • Occurs in Mitochondrial Matrix
      • Begins with condensation of Acetyl CoA with Oxaloacetate to form Citric Acid
      • Produces NADH, FADH2, and substrate-level ATP
    • Electron Transport System (ETS) & Oxidative Phosphorylation:
      • Occurs on Inner Mitochondrial Membrane
      • Complexes I to IV and ATP Synthase (Complex V)
      • Role of Cytochromes and Ubiquinone
      • Terminal Oxidation: O2 acts as final electron acceptor, forming H2O
      • Proton Gradient drives ATP synthesis via ATP Synthase

• Respiratory Quotient (RQ)

  • Formula: $\text{Volume of } CO_2 \text{ evolved} / \text{Volume of } O_2 \text{ consumed}$
  • Values for Substrates:
    • Carbohydrates: $1.0$
    • Fats/Lipids: Less than $1$ (approx. $0.7$)
    • Proteins: $0.9$
    • Organic acids: More than $1$ (e.g., Malic acid)

• Amphibolic Pathway

  • Role of Respiratory Pathway as both catabolic and anabolic
  • Breakdown of substrates (Catabolism)
  • Withdrawal of intermediates like Acetyl CoA to synthesize fatty acids/chlorophyll (Anabolism)