Electron Transport Chain

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•  Transfer of electrons between carriers in the electron transport chain in the membrane of the cristae is

    coupled to proton pumping

The final stage of aerobic respiration is the electron transport chain, which is located on the inner mitochondrial membrane 

  • The inner membrane is arranged into folds (cristae), which increases the surface area available for the transport chain

The electron transport chain releases the energy stored within the reduced hydrogen carriers in order to synthesise ATP

  • This is called oxidative phosphorylation, as the energy to synthesise ATP is derived from the oxidation of hydrogen carriers

Oxidative phosphorylation occurs over a number of distinct steps:

  • Proton pumps create an electrochemical gradient (proton motive force)
  • ATP synthase uses the subsequent diffusion of protons (chemiosmosis) to synthesise ATP
  • Oxygen accepts electrons and protons to form water

Step 1:  Generating a Proton Motive Force

  • The hydrogen carriers (NADH and FADH2) are oxidised and release high energy electrons and protons
  • The electrons are transferred to the electron transport chain, which consists of several transmembrane carrier proteins
  • As electrons pass through the chain, they lose energy – which is used by the chain to pump protons (H+ ions) from the matrix
  • The accumulation of H+ ions within the intermembrane space creates an electrochemical gradient (or a proton motive force)


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•  In chemiosmosis protons diffuse through ATP synthase to generate ATP


Step Two:  ATP Synthesis via Chemiosmosis

  • The proton motive force will cause H+ ions to move down their electrochemical gradient and diffuse back into matrix
  • This diffusion of protons is called chemiosmosis and is facilitated by the transmembrane enzyme ATP synthase
  • As the H+ ions move through ATP synthase they trigger the molecular rotation of the enzyme, synthesising ATP


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•  Oxygen is needed to bind with the free protons to maintain the hydrogen gradient, resulting in the formation

    of water


Step Three:  Reduction of Oxygen

  • In order for the electron transport chain to continue functioning, the de-energised electrons must be removed
  • Oxygen acts as the final electron acceptor, removing the de-energised electrons to prevent the chain from becoming blocked
  • Oxygen also binds with free protons in the matrix to form water – removing matrix protons maintains the hydrogen gradient
  • In the absence of oxygen, hydrogen carriers cannot transfer energised electrons to the chain and ATP production is halted


Summary:  Oxidative Phosphorylation

  • Hydrogen carriers donate high energy electrons to the electron transport chain (located on the cristae)
  • As the electrons move through the chain they lose energy, which is transferred to the electron carriers within the chain
  • The electron carriers use this energy to pump hydrogen ions from the matrix and into the intermembrane space
  • The accumulation of H+ ions in the intermembrane space creates an electrochemical gradient (or a proton motive force)
  • H+ ions return to the matrix via the transmembrane enzyme ATP synthase (this diffusion of ions is called chemiosmosis)
  • As the ions pass through ATP synthase they trigger a phosphorylation reaction which produces ATP (from ADP + Pi)
  • The de-energised electrons are removed from the chain by oxygen, allowing new high energy electrons to enter the chain
  • Oxygen also binds matrix protons to form water – this maintains the hydrogen gradient by removing H+ ions from the matrix

Overview of Oxidative Phosphorylation