Lecture Notes: Mitochondria and the Electron Transport Chain

Overview of Mitochondrial Function

• Eukaryotic cells (yeast to human) contain membrane-bound organelles with specialized functions.
• Mitochondria: Double-membraned organelles essential for energy production.
  • Energy harnessed from reactions at the inner mitochondrial membrane.

Inner Mitochondrial Membrane

• Acts as a barrier to protons (positively charged particles).
• Maintains a proton concentration gradient:
  • Intermembrane space has more protons than the matrix.
• Contains F1F0 ATP synthase to synthesize ATP using the proton gradient.
  • ATP provides energy for most cellular reactions.

Analogy to Power Plants

• Similar to power plants using wind/water/steam to rotate turbines:
  • ATP synthase rotates protein subunits using proton flow.
  • No proton gradient leads to halted rotation and potential cell death.

Electron Transport Chain (ETC)

• Composed of four protein complexes (I-IV):
  • Complexes I, III, IV pump protons from matrix to intermembrane space.
  • Complex II promotes proton pumping but doesn’t directly pump protons.
• Energy for proton pumping from electron transfer through a series of coupled reactions.

Complex I

• Electrons enter via NADH from sugar metabolism.
• Electrons passed through redox centers (clusters with varying electron affinities).
• Energy released with each electron transfer is used to pump protons.
• Final redox center donates electrons to coenzyme Q.

Complex II

• Similar to Complex I:
  • Electrons enter via FADH2 (a prosthetic group not leaving the complex, receiving electrons from succinate).
  • Transfers electrons between redox centers and donates to coenzyme Q.
• Difference: Does not pump protons.

Role of Coenzyme Q and Cytochrome c

• Coenzyme Q receives electrons from complexes I & II:
  • One electron is recyclable in complex III.
  • The other passes to cytochrome c, which carries it to complex IV.

Complex IV

• Ends the electron transport chain:
  • Converts oxygen to water using four electrons.
  • Strengthens proton gradient by incorporating protons into water and pumping additional protons.
• Oxygen as final electron acceptor, crucial for ATP synthesis.

Conclusion

• Inner mitochondrial membrane acts as a giant cellular power plant due to densely packed protein complexes.
• Oxygen is essential as the final electron acceptor to maintain electron transport and ATP synthesis.
• Further videos provide additional insights into mitochondrial functions and reactions.