Electron transport chain

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A chain of ferrous proteins located in the inner mitochondrial membrane involved in ATP production in aerobic respiration. Electron transport chains involve a gradual loss of energy as the transport chain progresses. This is often used for other purposes, such as the tranpsort of H+ ions during aerobic respiration [1]


Electron Transport Chain

The electron transport chain is the final destination for NADH and FADH2 produced in the biochemical respiration of glucose. It occurs across the mitochondrial membranes in a series of redox reactions which leads to hydrogen ion accumulation in the intermembranal space. The process relies on this high hydrogen ion concentration causing them to flow down a concentration gradient into the mitochondrial matrix which in consequence provides ATP synthase with energy required for ATP synsthesis by substrate level phosphorylation.(1)

Complex I - NADH Dehydrogenase (2) 

Complex I contains NADH dehydrogenase which allows NADH to deposits two electrons when it arrives at the ETC and reduces ubiquinone (Q). The energy released by this exchange allows complex 1 to pump 4 H+ ions across the inner membrane into the membranal space. (1) 

Complex II - Succinate-CoQ Reductase (2) 

In the same way complex I recieves electrons from NADH, complex II is also the destination of FADH2 produced earlier in the respiratory pathway. Complex I and II are connected by ubiquinone and both will reduce Q (QH2) which due to it's lipid soluble quality will travel to the next complex to deliver the electrons. (1) 

Complex III - CoQ cytochrome C Reductase (2) 

The complex is comprised of cytochrome C and B and is called cytochrome oxidoreductase. Due to its composition the complex contains haem groups which carry electrons by redox reactions of the Iron ion at its centre. The electrons are passed from Q however is only capable of transfering one electron at a time. (1) 

Complex IV -  Cytochrome oxidase (2) 

Within the complex there are 2 heme groups and three copper ions. The cytochromes within the complex hold molecular oxygen between the iron of the heme and copper ions allowing it to be reduced. The reduced oxygen forms bonds with two hydrogen ions forming a molecule of water (1). This is how the water in the simplified respiration equation is formed (3):
C6H12O6 + 6O2  →       6CO2 +      6H2O


From this series of redox reactions, energy has been released at each point and used to pump hydrogen ions into the intermembrane space of the mitochondria. This establishes both a concentration and electorchemical gradient which favours movement of the ions back into the matrix. Howvever as the ions are polar they can only move via facillitated diffusion using ATP synthase. ATP synthase contains an FO portion which is imbedded into the membrane and an F1-ATPase portion which is within the matrix (4). Hydrogen ions move through the channel in the centre of the FO portion to the motor F1-ATPase region which in turn spins transfering mechanical energy. This energy allows phosphorylation of ADP producing 90% of the ATP synthesised in flucose catabolism (1).

  1. Alberts,B et al (2008) Molecular Biology of the Cell 5th Ed. New York: Garland Publishing

1) Biology for Majors I . Electron Transport Chain . https://courses.lumenlearning.com/w,-biology1/chapter/reading-electron-transport-chain/ (accessed 23/10/2018)

2) Surender Rawat. Electron Transport Chain . https://www.slideshare.net/SurenderRawat3/electron-transport-chain-54417067 . (accessed 23/10/2018)

3) BBC Bitesize . Respiration . https://www.bbc.com/bitesize/guides/zwghcj6/revision/2 . (accessed 23/10/2018)

4) Editors of Biology Dictionary . ATP synthase .  https://biologydictionary.net/atp-synthase/ . (accessed 23/10/2018) 

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