6 ETC and Chemiosmosis

Aerobic Respiration: The Electron Transport Chain and Chemiosmosis

1. Introduction to the Electron Transport Chain (ETC)

  • The ETC is a critical component in eukaryotic cells located within the inner mitochondrial membrane.

  • It serves as the final destination for oxygen (O2) absorbed through breathing.

  • Functions to transfer electrons from NADH and FADH2 to O2 via four distinct protein complexes.

2. Role of Electron Carriers

  • NADH and FADH2 are the main electron carriers formed during early stages of cellular respiration.

  • They store most of the potential energy from glucose, which is utilized for ATP synthesis.

3. Protein Complexes of the ETC

A. Overview of Complexes

  • Complex I (NADH Dehydrogenase)

  • Complex II (Succinate Dehydrogenase) - the only single peripheral protein complex.

  • Complex III (Cytochrome Complex)

  • Complex IV (Cytochrome Oxidase)

  • Protein complexes arranged by increasing electronegativity; higher electronegativity indicates a stronger attraction for electrons.

4. Electron Flow and Mobile Shuttles

A. Role of Ubiquinone (UQ)

  • UQ is a hydrophobic molecule that transfers electrons from Complex I to Complex II and from Complex II to Complex III.

B. Role of Cytochrome C (Cyt C)

  • Cyt C, located in the intermembrane space, shuttles electrons from Complex III to Complex IV.

5. Mechanism of Electron Transport

  • Complexes I, III, and IV exhibit increasing electronegativity, facilitating electron movement.

  • Electrons are reduced and oxidized across proteins, which powers proton (H+) movement across the membrane.

  • Non-protein groups bound to protein complexes are responsible for attracting electrons, not the proteins themselves.

6. The Role of Oxygen

  • Oxygen, having the highest electronegativity, functions as the final electron acceptor in the ETC.

  • It reacts with electrons from Complex IV and protons from the mitochondrial matrix, yielding water (H2O).

  • This reaction initiates a cascading effect that allows others to fill the electron gap left by NADH and FADH2 oxidation.

7. Proton Gradient and Chemiosmosis

A. Formation of Proton Gradient

  • The electron transport is highly exergonic, generating a proton gradient across the inner mitochondrial membrane.

  • The proton motive force, created by this gradient, serves as potential energy.

B. Chemiosmosis

  • Chemiosmosis is the process utilizing the proton motive force to synthesize ATP via ATP Synthase.

  • ATP Synthase captures H+ ions moving down their gradient, leading to ATP production from ADP and inorganic phosphate.

8. ATP Production Mechanisms

  • Overview of ATP Generation

    • Every NADH can produce up to 3 ATP, while every FADH2 yields about 2 ATP during cellular respiration.

  • Shuttling NADH from glycolysis to the ETC involves either:

    • Glycerol-Phosphate Shuttle: Results in 36 ATP total.

    • Malate-Aspartate Shuttle: More efficient, allows for 38 ATP total.

9. Summary of ATP Yield in Cellular Respiration

  • Total ATP Calculation:

    • From glycolysis: 2 ATP

    • From NADH: 6 ATP (2 NADH x 3 ATP)

    • From citric acid cycle: 18 ATP (6 NADH x 3 ATP)

    • From FADH2: 4 ATP (2 FADH2 x 2 ATP)

    • Grand Total: 38 ATP via malate-aspartate shuttle; 36 ATP via glycerol-phosphate shuttle.

10. Conclusion

  • The Efficient Transfer of electrons and subsequent proton pumping across membranes drive ATP synthesis, showcasing the role of both the ETC and chemiosmosis in cellular respiration.