13. Respiration and Energy Transfer - part 06 - Electron Transport chain

 

13. Respiration and Energy Transfer - part 06 - Electron Transport chain


Electron Transport chain (Electron transfer system) :

  • Wherever the NADH2 (NADH+H+) and FADH2 are produced during glycolysis, connecting link reaction and Krebs cycle, they are oxidised with the help of various electron carriers and enzymes.
  • These carriers and enzymes are arranged on inner mitochondrial membrane in the form of various complexes as complex I, II, III, VI and V. NADH+H+ is oxidised by NADH dehydrogenase (complex I).
  • It's electrons are transferred to ubiquinone (coenzyme Q CoQ) present on inner membrane of mitochondria. 
  • Reduced ubiquinone is called as ubiqunol. 
  • FADH2 is oxidised by complex II (Succinate dehydrogenase) and these electrons are also transferred to CoQ. 
  • During oxidation of NADH+H+ and FADH2, electrons and protons are released but only electrons are carried forward whereas protons are released into outer chamber of mitochondria.
  • Ubiquinol is oxidised by complex-III (Cytochrome bc1, complex) and it's electrons are transferred to cytochrome C. 
  • Cytochrome C is a small, iron-containing protein, loosely associated with inner membrane. 
  • It acts as a mobile electron carrier, transferring the electrons between complex III and IV.
  • Cytochrome C is oxidised by complex IV or cytochrome C oxidase consisting of cytochrome a and a3. 
  • Electrons are transferred by this complex to the molecular oxygen. This is terminal oxidation. 
  • Reduced molecular oxygen reacts with protons to form water molecule called as metabolic water.
  • Protons necessary for this are channeled from outer chamber of mitochondria into inner chamber by F0 part of oxysome (complex V) present in inner mitochondrial membrane. 
  • This proton channeling by F0 is coupled to catalytic site of F1 which catalyses the synthesis of ATP from ADP and inorganic phosphate. This is oxidative phosphorylation. 
  • As transfer of protons is accompanied with synthesis of ATP, this process is named as 'Chemiosmosis' by Peter Mitchell.
  • Oxidation of one NADH+H+ leads to production of 3 ATP molecules where as oxidation of FADH2 leads to production of 2 ATP molecules. 
  • However the number of ATP produced depends upon the physiological conditions and source of respiratory substrate.

Significance of ETS :

  • The electron transport system (ETS) or terminal oxidation generates major amount of energy in the form of ATP molecules.
  • 34 ATP molecules out of total 38 ATP molecules are produced through ETS.
  • It regenerates oxidized coenzymes such as NAD+ and FAD+ from their reduced forms (NADH+H+ and FADH2) for recycling.
  • It also provides water molecules necessary for Krebs cycle.
  • It releases energy in a stepwise manner to prevent damage of cells.



always remember :

  • Not only glucose but amino acids from protein metabolism and fatty acids from lipid metabolism also participate in Kreb's cycle through acetyl CoA.

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