AQA A Level Biology

Revision Notes

5.2.7 Oxidative Phosphorylation

Aerobic Respiration: Oxidative Phosphorylation & ATP Synthesis

  • 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 from oxygen
  • The current model for oxidative phosphorylation is the chemiosmotic theory
    • The model states that energy from electrons passed through a chain of proteins in the membrane (the electron transport chain) is used to pump protons (hydrogen ions) up their concentration gradient into the intermembrane space
    • The hydrogens are then allowed to flow by facilitated diffusion through a channel in ATP synthase into the matrix
    • The energy of the hydrogens flowing down their concentration gradient is harnessed (a bit like water flowing through a hydroelectric damn) resulting in the phosphorylation of ADP into ATP by ATP synthase

An outline of oxidative phosphorylation

  • Hydrogen atoms are donated by reduced NAD (NADH) and reduced FAD (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
  • Oxygen acts as the ‘final electron acceptor’ and combines with protons and electrons at the end of the electron transport chain to form water

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
  • The inner membrane of the mitochondria is impermeable to hydrogen ions so these electron carriers are required to pump the protons across the membrane to establish the concentration gradient

ETC Structure, downloadable AS & A Level Biology revision notes

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.

Exam Tip

Examiners often ask why oxygen is so important for aerobic respiration. Oxygen acts as the final electron acceptor. Without oxygen the electron transport chain cannot continue as the electrons have nowhere to go. Without oxygen accepting the electrons (and hydrogens) the reduced coenzymes NADH and FADH2 cannot be oxidised to regenerate NAD and FAD, so they can’t be used in further hydrogen transport.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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