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Respiration in Plants - Notes | Class 11 | Part 4: Aerobic Respiration

Aerobic Respiration

Aerobic Respiration

  • Aerobic respiration is the complete oxidation of organic substances in the presence of oxygen, releasing CO₂, water, and energy. It occurs in mitochondria.

  • For this, pyruvate (the final product of glycolysis) is transported from the cytoplasm into the mitochondria.

  • The crucial events in aerobic respiration are:

    • Complete oxidation of pyruvate by stepwise removal of all hydrogen atoms, producing three CO₂ molecules. This takes place in the matrix of mitochondria.
    • Passing of electrons removed as part of H-atoms to molecular O₂ with simultaneous synthesis of ATP. This occurs on the inner membrane of mitochondria.

  • Pyruvate (pyruvic acid) enters the mitochondrial matrix and undergoes oxidative decarboxylation in the presence of pyruvic dehydrogenase. This requires coenzymes NAD⁺ and Coenzyme A.

  • During this process, two NADH molecules are produced from two pyruvic acid molecules.

Oxidative Decarboxylation

  • Acetyl CoA then enters the tricarboxylic acid (TCA) cycle.


Tricarboxylic Acid Cycle (Krebs’ Cycle or Citric Acid Cycle)

The TCA cycle was first elucidated by Hans Krebs.

Steps:

  1. Condensation of the acetyl group with oxaloacetic acid (OAA) and water to form citric acid in the presence of the enzyme citrate synthase. A CoA molecule is released.
  2. Citrate is isomerized to isocitrate.
  3. Decarboxylation of isocitrate to α-ketoglutaric acid.
  4. Decarboxylation of α-ketoglutaric acid to succinyl-CoA.
  5. Conversion of succinyl-CoA to succinic acid, synthesizing a GTP molecule (substrate-level phosphorylation). In a coupled reaction, GTP is converted to GDP with simultaneous synthesis of ATP from ADP.
  6. Oxidation of succinate to fumarate and then to malate.
  7. Oxidation of malate to OAA.
Krebs’ Cycle

  • At three points in the TCA cycle, NAD⁺ is reduced to NADH + H⁺. At one point, FAD⁺ is reduced to FADH₂.

  • Continued oxidation of acetyl CoA via the TCA cycle requires replenishment of OAA and regeneration of NAD⁺ and FAD⁺ from NADH and FADH₂.

    • Summary equation of Krebs’ cycle:
Krebs’ Cycle Equation

  • Thus, one glucose molecule is broken down to produce 6 CO₂, 8 NADH + H⁺, 2 FADH₂, and 2 ATP.


Electron Transport System (ETS) & Oxidative Phosphorylation

  • The electron transport system (ETS) is a metabolic pathway in the inner mitochondrial membrane where electrons pass from one carrier to another.

  • This releases and utilizes energy stored in NADH + H⁺ and FADH₂ (formed during the TCA cycle) through oxidation.

  • Electrons are passed to O₂ to form H₂O.

  • Electrons from NADH are oxidized by NADH dehydrogenase (complex I).

  • Electrons are then transferred to ubiquinone (UQ) within the inner membrane. Ubiquinone also receives reducing equivalents via FADH₂ (complex II) generated during succinate oxidation in the citric acid cycle.

  • The reduced ubiquinone (ubiquinol or UQH₂) is oxidized, transferring electrons to cytochrome c via the cytochrome bc₁ complex (complex III). Cytochrome c, a small protein on the outer surface of the inner membrane, acts as a mobile electron carrier between complexes III and IV.

  • Complex IV (cytochrome c oxidase) contains cytochromes a & a₃ and two copper centers.

  • Electrons passing from one carrier to another via complexes I to IV are coupled to ATP synthase (complex V) for ATP production.

Electron Transport System

  • The number of ATP molecules produced depends on the electron donor:

    • Oxidation of 1 NADH → 3 ATP

    Oxidation of 1 FADH₂ → 2 ATP

  • In aerobic respiration, oxygen’s role is limited to the terminal stage, but it is vital as it drives the process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor.

  • In respiration, the energy of oxidation-reduction is used for phosphorylation, a process called oxidative phosphorylation. This differs from photophosphorylation, where light energy creates a proton gradient for phosphorylation.

  • Energy released during the ETS is used to synthesize ATP by ATP synthase (complex V).

  • ATP synthase has two major components:

    • F₁ headpiece (peripheral membrane protein complex): Site for ATP synthesis from ADP and inorganic phosphate.
    • F₀ (integral membrane protein complex): Forms a channel for protons to cross the inner membrane, coupling proton movement to the catalytic site of F₁ for ATP production.
Diagrammatic presentation of ATP synthesis in mitochondria

  • For each ATP produced, 2 H⁺ pass through F₀ from the inter-membrane space to the matrix down the electrochemical proton gradient.

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