5.7.5

• Final stage of aerobic respiration

  → oxidative phosphorylation

  • takes place in mitochondria, involving electron carrier proteins, arranged in chains called the electron transport chains, embedded in inner mitochondrial membranes (cristae) + a process called chemiosmosis

  1. reduced NAD + reduced FAD are reoxidised when they deliver their hydrogen atoms to the electron transport chain
  2. the hydrogen atoms released from the reduced coenzymes split into protons + electrons
  3. protons go into solution in the mitochondrial matrix

  
• The electron transport chain

  electrons from the hydrogen atoms pass along the chain of electron carriers

  • each electron carrier protein has an iron ion at its core which can gain an electron, becoming reduced (Fe 2+)
  • the reduced iron ion can then donate the electron to the iron ion in the next electron carrier in the chain, becoming reoxidised (Fe 3+)
  • as electrons pass along the chain, some of the energy is used to pump protons across the inner mitochondrial membrane, into the intermembrane space
• The proton gradient + chemiosmosis

  as protons accumulate in the intermembrane space, a proton gradient forms across the membrane

  • proton gradients generate a chemiosmotic potential which is also known as proton motive force, pmf, source of potential energy

  → ATP is made using the energy of proton motive force
• the proton gradient + chemiosmosis 2

  • protons cannot easily diffuse through lipid bilayer of the mitochondrial membranes, as the outer membrane has a low degree of permeability to protons + the inner membrane is impermeable to protons

  → protons can however diffuse through protein channels associated with ATP synthases that are in the inner membrane

  • as protons diffuse down their protons cause a conformational (shape) change in the ATP synthase that allows ADP + Pi to combine , forming ATP

  → this flow of protons is known as chemiosmosis + it is coupled to the
• the proton gradient + chemiosmosis 3

  formation of ATP

  • the formation of ATP in this way, in the presence of oxygen, is oxidative phosphorylation
  • oxygen is the final electron acceptor; combines with electrons coming off the electron transport chain + with protons, diffusing down the ATP synthase channel, forming water

  → equation : 4H+ + 4e- + O2 →2H2O
• How much ATP is made during oxidative phosphorylation ?

  the reduced coenzymes provide both protons + electrons to the electron transport chain

  • the protons + electrons from the 10 molecules of reduced NAD can theoretically produce 25 molecules of ATP
  • the protons + electrons from the two molecules of reduced FAD

  → oxidative phosphorylation may therefore produce 28 molecules of ATP per molecule of glucose

  
• The total ATP tally per molecule of glucose during aerobic respiration

  table 2

  this theoretical yield is rarely achieved, and the actual yield may be closer to 30 molecules of ATP per molecule of glucose, or even less, because:

  • some ATP is used to actively transport pyruvate into the mitochondria
  • some ATP is used in a shuttle system that transports reduced NAD, made during glycolysis, into mitochondria
  • some protons may leak out through the outer mitochondrial membrane