• Transfers energy from electrons that move down it thru electron carriers that alternate between reduced and oxidized → released energy used to establish an electrochemical gradient of protons (H+) across the inner mitochondrial membrane                             
    • Redox reactions needed for proton gradient
    • Heat allows for hibernation; less ATP made but release heat better
    • Carriers = proteins that act as primary electron acceptors
  • Oxygen is ultimate electron acceptor with most affinity→ process needs oxygen
    • Oxygen accepts electrons & protons → forms water
  • NADH donates 2 electrons → oxidized, reverts back to NAD+, and goes back to glycolysis/krebs to do it all again
  • If NADH couldn’t donate electrons → no glycolysis/Krebs/ETC; oxygen can’t accept electrons (not consumed); ATP levels drop


  • “Mechanism of ATP generation that occurs when energy is stored in the form of a proton concentration gradient across a membrane”

                Couples electron transport and energy release to move protons down their energy gradient and through ATP synthase

  1. Krebs Cycle produces NADH and FADH2 in matrix
  2. Electrons are removed from NADH and FADH2
  • Removed by protein complexes in the inner membrane → electrons move along ETC from one protein complex to another
  1. H+ ions (protons) are transported from the matrix to intermembrane space
  • Electrons lose energy as they move down their electrochemical gradient thru redox reactions and complexes → complexes capture released energy to pump H+
  1. A proton gradient (proton motive force) and electrical gradient (voltage) is established across the inner membrane: represents potential energy
  • As H+ are transferred, the concentration of H+ increases (pH decreases) in the intermembrane space and decreases in the matrix (pH increases).
  1. ATP synthase allows protons to flow back into the matrix (down gradient)
  • The flow of protons back through ATP synthase by chemiosmosis generates energy for synthase to phosphorylate ADP to ATP
    • Oxidative phosphorylation in cellular respiration
    • Photophosphorylation in photosynthesis

The Proton Gradient

  • Proton gradient = pH gradient = electrochemical (voltage) gradient = stored/potential energy
  • Proton Motive Force: force exerted on protons by H+ gradient

The ATP Synthase enzyme

  • Channel protein that translocates the hydrogen ions from high concentration to low concentration
    • Cellular Respiration: From intermembrane to matrix
    • Photosynthesis: from thylakoid lumen to stroma
  • Protons move onto binding sites on the rotor, making it spin in a way that catalyzes ATP
    • The active sites in the enzyme create 34 ATP

Role of Oxygen and Anaerobic Respiration

  • Last electron acceptor in ETC; accepts electrons and protons to form water
  • Without oxygen…
    • No electron acceptor so electrons can’t pass thru and no proton gradient established
    • NADH and FADH cannot transfer electrons and H+ → NAD+ and FAD not made → stops the citric acid cycle and glycolysis → no new ATP made → death
  • Anaerobic respiration used by prokaryotes in low oxygen; sulfate or nitrate as final e-acceptor