Oxidative Phosphorylation

  • The NADH and FADH2, formed during glycolysis, β-oxidation of fatty acids and the TCA cycle, give up their electrons to reduce molecular O2 to H2O.
  • Electron transfer occurs through a series of protein electron carriers, the final acceptor being O2; the pathway called as the electron transport chain (ETC).
  • The function of ETC is to facilitate the controlled release of free energy that was stored in reduced cofactors during catabolism.
  • Energy is released when electrons are transported from higher energy NADH/FADH2 to lower energy O2.
  • This energy is used to phsophorylate ADP.

Oxidative Phosphorylation

  • There are 3 sites of the chain that can give enough energy for ATP synthes These sites are:
  1. Site I between FMN and Coenzyme Q at enzyme complex I.
  2. Site II between cyt b and cyt C1 at enzyme complex III
  3. Site III between cyt a and cyt a3 at enzyme complex IV
  • Because energy generated by the transfer of electrons through the electron transport chain to O2 is used in the production of ATP, the overall process is known as oxidative phosphorylation.
  • It refers to the coupling of the electron transport in respiratory chain with phosphorylation of ADP to form It is a process by which the energy of biological oxidation is ultimately converted to the chemical energy of ATP.
  • Oxidative phosphorylation is responsible for 90 % of total ATP synthesis in the cell.
  • Oxidative phosphorylation is the process in which ATP is formed as a result of the transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers.

Oxidative Phosphorylation and ETC


The  chemiosmotic theory explains the mechanism of oxidative phosphorylation.

  • It suggests that the transfer of electrons through the electron transport chain causes protons to be translocated (pumped out) from the mitochondrial matrix to the intermembrane space at the three sites of ATP production (i.e. it acts as a proton pump) resulting in an electrochemical potential difference across the inner mitochondrial membrane.
  • The electrical potential difference is due to accumulation of the positively charged hydrogen ions outside the membrane and the chemical potential difference is due to the difference in pH, being more acidic outside the membrane.
  • This electrochemical potential difference drives (forces) ATP synthase to generate ATP from ADP and inorganic phosphate.

Chemical Theory

It suggests that there is a direct chemical coupling of oxidation and phosphorylation through high-energy intermediate compounds. This theory is not accepted, as the postulated high-energy intermediate compounds were never found.

ATP Synthesis

  • ∆Go for transfer of 2 electrons from NADH to O2 is – 220 kJ/mol. This is sufficient to synthesize 7 molecules of ATP (∆Go’ for ATP synthesis is 31 kJ/mol).
  • However, a significant amount of energy is used up to pump H+ out of the mitochondria. Only a third is used for ATP synthesis.
  • The number of ATP generated depends on the site at which the substrate is linked to the respiratory chain:
    • If substrate is linked to the chain through NAD+ , 3 ATP are formed for each molecule oxidized.
    • If substrate is linked to the chain through FAD, 2 ATP are formed.

P/ O Ratio

  • It is the ratio of the number of molecules of ADP converted to ATP to the number of oxygen atoms utilized by respiratory chain.
  • It is a measure to the efficiency of oxidative phosphorylation.
  • It is 3/1 if NADH+H+ is used and 2/1 if FADH2 is used.


  1. http://www.csun.edu/~jm77307/Oxidative%20Phosphorylation.pdf
  2. file:///C:/Users/user/Desktop/Lecture_04.pdf
  3. Smith, C. M., Marks, A. D., Lieberman, M. A., Marks, D. B., & Marks, D. B. (2005). Marks’ basic medical biochemistry: A clinical approach. Philadelphia: Lippincott Williams & Wilkins.
  4. Rodwell, V. W., Botham, K. M., Kennelly, P. J., Weil, P. A., & Bender, D. A. (2015). Harper’s illustrated biochemistry (30th ed.). New York, N.Y.: McGraw-Hill Education LLC.
  5. Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2000). Lehninger principles of biochemistry. New York: Worth Publishers.

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He attended St. Xavier’s College, Maitighar, Kathmandu, Nepal, to complete his Master of Science in Microbiology. He worked as a Lecturer at St. Xavier’s College, Maitighar, Kathmandu, Nepal, from Feb 2015 to June 2019. After teaching microbiology for more than four years, he joined the Central Department of Microbiology, Tribhuvan University, to pursue his Ph.D. in collaboration with Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. He is interested in research on actinobacteria, myxobacteria, and natural products. He has published more than 15 research articles and book chapters in international journals and well-renowned publishers.

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