8.2.5 Oxidative Phosphorylation

Summary of oxidative phosphorylation

• Oxidative phosphorylation is the last stage of aerobic respiration
• It takes place at the inner mitochondrial membrane
• This is the most efficient producer of ATP in the process of aerobic respiration
• It also is the stage that produces water from oxygen
• Oxidative phosphorylation is comprised of the electron transport chain and chemiosmosis
  • During the electron transport chain, electrons are passed along carrier molecules forming an electrochemical gradient
  • Chemiosmosis describes the formation of ATP using this gradient
• Coenzymes NAD⁺ and FAD play a critical role in oxidative phosphorylation by transferring electrons (from hydrogen) from the previous stages of aerobic respiration through a series of carrier molecules

NAD and FAD

• Coenzymes NAD and FAD play a critical role in aerobic respiration by transferring hydrogen through different stages of respiration
• When hydrogen atoms become available at different points during respiration NAD and FAD accept these hydrogen atoms
  • A hydrogen atom consists of a proton (hydrogen ion/H⁺) and an electron (e^-)
• When the coenzymes gain a hydrogen they are ‘reduced’
• They transfer the hydrogen atoms (protons and electrons) from the different stages of respiration to the electron transport chain on the inner mitochondrial membrane, called the cristae (the site where hydrogens are removed from the coenzymes)
• When the hydrogen atoms are removed the coenzymes are ‘oxidised’

The reduction and oxidation of NAD and FAD

Sources of reduced NAD & FAD

• A certain amount of reduced NAD and FAD is produced during the aerobic respiration of a single glucose molecule
• Reduced NAD:
  • 2 x 1 = 2 from Glycolysis
  • 2 x 1 = 2 from the Link Reaction
  • 2 x 3 = 6 from the Krebs cycle
• Reduced FAD:
  • 2 x 1 = 2 from the Krebs cycle

The electron transport chain

• 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 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
• All of the electrons that enter the transport chain come from reduced NAD and reduced FAD molecules produced during the earlier stages of cellular respiration

The importance of protons and electrons

• 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 electron transport chain drives the movement of 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, gives 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.

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

NOS: Paradigm shift; the chemiosmotic theory led to a paradigm shift in the field of bioenergetics

• A paradigm shift is a fundamental change in an approach or already existing assumption
• Some examples of paradigm shifts that have happened in the course of scientific history are:
  • CO₂ emisions - Emissions of CO₂ did not register as a contributing factor to climate change until late into the 20th century
  • Evolution - Charles Darwin's theory of natural selection was a paradigm shift away from the traditional and religious views people held about the development of life on Earth
  • Parasites - Scientists once believed that illnesses were caused by "bad air" called miasma; it wasn't until the 19th century that a paradigm shift in this thinking happened
• The chemiosmotic theory was proposed by Nobel Prize winner Peter Mitchell in 1961
• His idea was a paradigm shift away from the existing theory that the energy for electron transfer was stored as a stable chemical intermediate
• Mitchell’s chemiosmotic hypothesis started a revolution which has led to this paradigm shift in the field of bioenergetics
• The idea was rejected at the time for being too novel and radical
• His theory has shaped understanding of the fundamental mechanisms of:
  • Biological energy conservation
  • Ion and metabolite transport
  • Bacterial motility
  • Organelle structure and biosynthesis
  • Membrane structure and function
  • Homeostasis
  • The evolution of the eukaryote cell

• 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
  • Oxygen is reduced by the electrons, forming 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