Oxidative Phosphorylation (HL) (HL IB Biology)

• The electron transport chain is made up of a series of redox reactions that occur via membrane proteins (also known as electron carriers) embedded into the inner mitochondrial membrane
• The chain is used to transport electrons and move protons (hydrogen ions) across the membrane
  • Electron carriers are positioned close together which allows the electrons to pass from carrier to carrier
  • The cristae of the mitochondria are impermeable to protons so the electron carriers are needed to pump them across the membrane to establish a proton (or electrochemical) concentration gradient that can be used to power oxidative phosphorylation
• Energy is transferred when a pair of electrons is passed to the first carrier in the chain
  • This converts reduced NAD back to NAD
    • The reduced NAD comes from glycolysis, the link reaction and the Krebs cycle
• H⁺ (protons) are created when electrons are removed from hydrogen atoms
  • These protons play a role in generating ATP in the electron transport chain
• As electrons, that are received from reduced NAD (and FAD), are transported along the electron carriers, energy is released in a controlled manner
  • This energy is used to form ATP by adding P_i to ADP
    • 3 ATP molecules are produced for every molecule of reduced NAD
  • This contributes to the total yield of 32 ATP molecules per molecule of glucose oxidised during aerobic respiration
• Oxygen acts as the final electron acceptor in the chain and forms water

• Protons and electrons are important in the electron transport chain as they play a role in the synthesis of ATP
  • Electrons are given to the electron transport chain (from reduced NAD and reduced FAD)
  • Protons (from reduced NAD and reduced FAD) are released when the electrons are lost
  • The carrier proteins pump these protons across the cristae into the intermembrane space, creating a proton gradient (more hydrogen ions in the matrix)
  • Returning the protons down the gradient, back into the mitochondrial matrix, releases the energy required for ATP synthesis

• Movement of electrons through the electron transport chain causes a proton or electrochemical gradient
  • Positively charged protons accumulate in the intermembrane space
  • The movement of protons back into the matrix is then used to power ATP synthesis
• Protons that have built up in the intermembrane space can only pass through the phospholipid bilayer by facilitated diffusion  through a membrane-embedded protein called ATP synthase  
• ATP synthase acts a lot like a water wheel; it is turned by the flow of the protons moving through it, down their electrochemical gradient.
• As ATP synthase turns, it catalyses phosphorylation of ADP, generating ATP
• This process, in which energy from a proton gradient is used to make ATP, is called chemiosmosis.

Chemiosmosis Diagram

Oxidative Phosphorylation, involving the electron transport chain and chemiosmosis, generates a large amount of ATP

• The final link in the electron transport chain is oxygen and is referred to as the final or terminal electron acceptor
  • This is the last acceptor of the electrons and allows for the continued flow of electrons along the chain
  • Oxygen is reduced by the electrons, and when combined with protons from the mitochondrial matrix, it forms water
• If oxygen is not present to accept electrons:
  • Reduced NAD and reduced FAD will not be oxidised to regenerate NAD+ and FAD, so there will be no further hydrogen transport
  • The electron transport chain will stop, and ATP will no longer be produced by chemiosmosis
  • Without enough ATP, cells can’t carry out the reactions they need to function
• The electron transport chain is hugely efficient at generating energy in the cell but relies on an abundance of oxygen

Oxygen is the final electron acceptor and combines with protons to form metabolic water