AQA A Level Biology

Revision Notes

5.2.6 The Role of NAD & FAD

Aerobic Respiration: Role of NAD and FAD

  • 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 a hydrogen they are ‘reduced’
    • OIL RIG: Oxidation Is Loss, Reduction Is Gain
  • They 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 matrix)
    • Movement of hydrogen ions down the proton gradient, back into the mitochondrial matrix, gives the energy required for ATP synthesis

NAD and FAD equations, downloadable AS & A Level Biology revision notes

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

Exam Tip

Note at all stages there is a doubling (2x) of reduced NAD and FAD. This is because one glucose molecule is split in two in glycolysis and so these reactions occur twice per single molecule of glucose.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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