Study Notes: Aerobic Respiration & Electron Transport Chain

Chapter 10: Chemotrophic Metabolism II - Aerobic Respiration

Overview of Aerobic Respiration

  • Presence of oxygen significantly influences the net energy released during glucose oxidation in the mitochondria of cells.

Learning Objectives

  1. Understand the process of aerobic respiration in mitochondria, especially:
    • Energy-gaining steps of glycolysis
    • Pyruvate oxidation
    • The TCA (Krebs) cycle
    • Electron transport
    • Proton pumping
    • ATP synthesis
  2. Identify the major inputs and outputs of each pathway in terms of:
    • Carbon compounds
    • Electron carriers
    • Energy captured (substrate-level phosphorylation of ADP to ATP or oxidative phosphorylation)

Major Pathways in Aerobic Respiration

  • Glycolysis
    • Splits glucose into two pyruvate molecules.
    • Generates 2 ATP.
  • Citric Acid Cycle (TCA cycle)
    • Oxidizes pyruvate to acetyl CoA.
    • Completely oxidizes incoming carbon to CO2.
    • Conserves energy as reduced coenzyme molecules (NADH and FADH2).
  • Electron Transport
    • Transfers electrons from reduced coenzymes (NADH and FADH2) to O2.
    • Coupled with active transport of protons across the membrane, generating an electrochemical proton gradient.
  • ATP Synthesis
    • Uses the proton gradient to drive ATP synthesis.

Composition and Function of Mitochondrial Membranes

  • Mitochondrial Volume Composition by Cell Type:
    • Skeletal muscle cells: 3-8%
    • Liver cells: 20%
    • Heart muscle cells: 35-40%
  • Mitochondrial membranes house the Electron Transport Chain (ETC) and ATP synthesis machinery, responsible for generating approximately 34 ATP molecules during respiration.

Outputs from Glycolysis and TCA Cycle

  • Energy Outputs:
    • Glycolysis and TCA jointly produce:
    • 4 ATP
    • 10 reduced NADH
    • 2 reduced FADH2
  • Exergonic Reactions:
    • Energy released upon oxidation of reduced coenzymes can be harnessed to drive ATP synthesis.
    • Reaction: ADP + Pi
      ightarrow ATP ext{ (ΔG = 7.3 kcal/mol, endergonic)}
    • Example reactions:
    • NADH + H^+ + rac{1}{2} O2 ightarrow NAD^+ + H2O ext{ (ΔG = -52.4 kcal/mol)}
    • FADH2 + rac{1}{2} O2
      ightarrow FAD + H_2O ext{ (ΔG = -45.9 kcal/mol)}

Oxidation of Coenzymes

  • The oxidation of coenzymes (NADH and FADH2) is a highly exergonic process.
  • Direct electron transfer to oxygen would release energy as heat, hence a multi-step process with various electron carriers is utilized.
  • The Electron Transport Chain (ETC) is embedded in the inner mitochondrial membrane to shuttle electrons from NADH and FADH2 to molecular oxygen.
  • Overall reaction for cellular respiration:
    ext{Glucose} + 6O2 ightarrow 6CO2 + 6H_2O

Mechanism of the Electron Transport Chain

  • The electron transfer occurs spontaneously with a negative ΔG as it moves from electron donors (oxidation) to electron acceptors (reduction).
  • Membrane electron carriers in the ETC are organized into 4 large multiprotein complexes.

Functional Complexes in the ETC

  1. Complex I - NADH-Coenzyme Q oxidoreductase
    • Transfers electrons from NADH to Coenzyme Q.
  2. Complex II - Succinate Dehydrogenase
    • Transfers electrons from succinate (via FADH2) to Coenzyme Q.
  3. Complex III - Cytochrome b/c1 Complex
    • Transfers electrons from Coenzyme Q to cytochrome c.
  4. Complex IV - Cytochrome c oxidase
    • Transfers electrons from cytochrome c to molecular oxygen (terminal oxidase).

Proton Pumping Mechanism

  • Mechanism:
    1. NADH and FADH2 deliver H+ ions and electrons to the ETC.
    2. As NADH reduces specific cofactors, it transfers electrons into the ETC while protons remain in the matrix.
    3. Electrons promote the activation of protein channels to pump H+ ions out of the mitochondrial matrix.
  • Proton Pumping:
    • Complexes I, III, and IV act as sites for proton pumping; Complex II does not pump protons.
    • 10 protons are pumped for each pair of electrons transported through complexes I, III, and IV, establishing an electrochemical gradient.

Biological Significance of the ETC

  • The ETC minimizes energy loss as heat and importantly generates a transmembrane electrochemical proton gradient maximizing ATP generation potential.
  • Represents an example of the functional capabilities of protein membranes.

ATP Synthesis via ATP Synthase

  • ATP synthase utilizes the proton gradient established by the ETC to synthesize ATP.
  • F0F1 ATP Synthase Characteristics:
    • F0: Membrane part that includes a proton channel and a rotor component.
    • F1: Catalytic part of the enzyme that synthesizes ATP.
    • Proton flow through the channel induces a spinning motion, initiating the synthesis of ATP from ADP and phosphate (Pi).
    • Reaction: Each 3 protons channeling through ATP synthase results in the synthesis of 1 ATP.

Experimental Proof of ATP Rotor Movement

  • Studies confirmed the movement of ATP synthase’s rotor using fluorescent tagging of an actin filament, showing that the actual movement rate is approximately 100 revolutions per second without any load.

Energy Transformations in Aerobic Respiration

  1. Chemical to Chemical Energy: (Glycolysis & TCA Cycle)
    • ATP production via substrate-level phosphorylation.
    • Reduced coenzymes produced: NADH and FADH2.
  2. Chemical to Potential Energy: (Electrochemical gradient of protons)
    • Electron transport in ETC.
    • Active transport of protons against the gradient.
  3. Potential Energy to Kinetic Energy:
    • Proton flow through ATP synthase (facilitated diffusion).
  4. Kinetic to Chemical Energy:
    • ATP synthesis through phosphorylation.
  5. Chemical to Thermal Energy:
    • Heat loss during respiration.

ATP Production from Respiration

  • Synthesis process:
    • The F0F1 complex drives ATP synthesis using protons.
    • Rough estimates indicate:
    • 3 ATPs produced per NADH and approx 2 ATPs per FADH2.
  • Overall Reducing Coenzymes Production:
    • Glycolysis: 2 NADH + 2 ATP.
    • TCA Cycle: 8 NADH + 2 FADH2 + 2 ATP (including pyruvate dehydrogenase step).
  • Total ATP from Reduced Coenzymes:
    • From 10 NADH (30 ATP) + 2 FADH2 (4 ATP) = 34 ATP.
  • Total ATP from respiration:
    • 34+2+2=38extATP34 + 2 + 2 = 38 ext{ ATP}.
  • 90% of ATP generated by oxidative phosphorylation.