Aerobic Respiration: Role of NAD & FAD (CIE A Level Biology)

• The coenzymes NAD and FAD play a critical role in aerobic respiration
• When hydrogen atoms become available at different points during respiration NAD and FAD accept these hydrogen atoms
  • A hydrogen atom consists of a hydrogen ion and an electron

• When the coenzymes gain hydrogen they are ‘reduced’
  • OIL RIG: Oxidation Is Loss, Reduction Is Gain

• Reduced NAD and reduced FAD transfer the hydrogen atoms (hydrogen ions and electrons) from the different stages of respiration to the electron transport chain on the inner mitochondrial membrane, the site where hydrogens are removed from the coenzymes
• When the hydrogen atoms are removed the coenzymes are ‘oxidised’
• Hydrogen ions and electrons are important in the electron transport chain at the end of respiration as they play a role in the synthesis of ATP
  • Electrons from reduced NAD (NADH) and reduced FAD (FADH₂) are given to the electron transport chain
  • Hydrogen ions from reduced NAD (NADH) and reduced FAD (FADH₂) are released when the electrons are lost
  • The electron transport chain drives the movement of these hydrogen ions (protons) across the inner mitochondrial membrane into the intermembrane space, creating a proton gradient (more hydrogen ions in the intermembrane space)
  • The movement of hydrogen ions down the proton gradient, back into the mitochondrial matrix, gives the energy required for ATP synthesis

The Redox Reactions of NAD and FAD

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 × 1 = 2 from Glycolysis
  • 2 × 1 = 2 from the Link Reaction
  • 2 × 3 = 6 from the Krebs cycle

• Reduced FAD:
  • 2 × 1 = 2 from the Krebs cycle