5.2.7 Oxidative Phosphorylation

1. Electron Donation

• Reduced NAD (NADH) and Reduced FAD (FADH₂) donate their hydrogen atoms to the ETC.
  • NADH donates electrons to Complex I and is oxidised back to NAD⁺.
  • FADH₂ donates electrons to Complex II and is oxidised back to FAD.
• Each hydrogen atom splits into:
  • Electrons ($e⁻$): Enter the ETC.
  • Protons ($H⁺$): Released into the mitochondrial matrix.

2. Electron Transport Chain

• Electrons are transferred through a series of protein complexes (Complex I to IV) embedded in the inner mitochondrial membrane.
• Each complex has a higher affinity for electrons than the previous one, causing electrons to flow down the chain.
• The energy released at Complexes I, III, and IV is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space.
  • $NADH$: Pumps 10 protons (4 at Complex I, 4 at Complex III, 2 at Complex IV).
  • $FADH₂$: Pumps 6 protons (4 at Complex III, 2 at Complex IV).
• The electrons are eventually transferred to oxygen ($O₂$), the final electron acceptor, which combines with protons to form water ($H₂O$).

3. Proton Gradient Formation

• The pumping of protons creates a proton gradient (electrochemical gradient) across the inner mitochondrial membrane:
  • High proton concentration in the intermembrane space.
  • Low proton concentration in the mitochondrial matrix.

4. ATP Synthesis

• Protons flow back into the mitochondrial matrix through ATP synthase, a specialised protein channel embedded in the membrane.
• This flow of protons is known as the proton motive force and provides energy for ATP synthesis.
• ATP synthase catalyses the reaction: $ADP + Pi → ATP$
• For every 10 protons passing through ATP synthase, approximately 3 ATP molecules are synthesised.

Summary of ATP Production

• 1 NADH contributes enough energy to synthesise 2.5 ATP molecules.
• 1 FADH₂ contributes enough energy to synthesise 1.5 ATP molecules.