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

Aerobic respiration: role of NAD and FAD

• 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

  • This is 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

Sources of reduced NAD and 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