Oxidative Phosphorylation (College Board AP Biology)

• Oxidative phosphorylation is the last stage of aerobic respiration
  • It takes place at the inner mitochondrial membrane
  • It results in the production of many molecules of ATP and the production of water using oxygen
• The current model used to explain ATP synthesis in oxidative phosphorylation is the chemiosmotic theory

An Outline of Oxidative Phosphorylation

• Hydrogen atoms are donated by NADH and reduced FADH₂ from the Krebs Cycle
• Hydrogen atoms split into protons (H⁺ ions) and electrons
• The high-energy electrons enter the electron transport chain and release energy as they move through the electron transport chain
• The released energy is used to transport protons across the inner mitochondrial membrane from the matrix into the intermembrane space
• A concentration gradient of protons is established between the intermembrane space and the matrix
• The protons return to the matrix via facilitated diffusion through the channel protein, ATP synthase
• The movement of protons down their concentration gradient provides energy for ATP synthesis, through chemiosmosis
• Oxygen acts as the 'final electron acceptor' and combines with protons and electrons at the end of the electron transport chain to form water
• A small amount of the proton gradient formed during oxidative phosphorylation does not go towards phosphorylation of ADP
  • Instead, it is diverted (decoupled) from ATP synthesis and goes directly to generate heat
  • Endotherms (endothermic organisms) can use this release of heat energy to regulate their body temperatures

The Electron Transport Chain

• The electron transport chain is made up of a series of membrane proteins / electron carriers
• They are positioned close together which allows the electrons to pass from carrier to carrier
• These electron carriers are required to pump the protons across the impermeable membrane to establish the concentration gradient

Contrasts to Related Electron Transport Chains

• Electron transport chains are found in chloroplasts, mitochondria and prokaryotic plasma membranes, however, there are some differences in the mechanisms:
  
  • Whilst oxygen is the terminal electron acceptor in aerobic respiration in eukaryotes and aerobic prokaryotes, anaerobic prokaryotes, other molecules are used (eg. nitrate NO₃^-)
    
    • In photosynthesis, NADP+ is the terminal electron acceptor
  • In photosynthesis, the flow of protons back through membrane-bound ATP synthase is called photophosphorylation (compared to oxidative phosphorylation in respiration)

Oxidative Phosphorylation via the Chemiosmotic Theory Diagram

Oxidative phosphorylation via the chemiosmotic theory occurs on the inner mitochondrial membrane and requires NADH and FADH₂ from the Krebs Cycle. It produces water and many molecules of ATP.