Complex III

Complex III is the cytochrome bc1 complex or CoQH2-cytochrome c reductase. Complex III possesses a Fe-S protein like complex I. Along with this, a different kind of protein (two molecules) named as cytochrome is also present in complex III. Electrons are transferred from the QH2 to cytochrome C. The transfer of the electrons takes place by this manner: At first, the electrons are transferred from one cytochrome protein to a Fe-S protein and after that, from that Fe-S protein the electrons are again transferred to the second cytochrome protein. The final transfer of the electrons are takes place to a mobile electron carrier cytochrome C out of the complex. The electrons are then carried to complex IV by the cytochrome C complex. Complex III also helps to pump protons from the matrix into the intermembrane space.


Complex IV

Complex IV is cytochrome C oxidase. It is the last complex of the electron transport chain. In this complex, two more cytochrome C are present. The electrons have to pass through those cytochromes. The electrons are then transferred to molecular oxygen by the second cytochrome with the help of a copper ion. The molecular O2 then splits into two oxygen atoms and protons are accepted from the matrix. This leads to the formation of two molecules of water are formed. This complex also helps to pump the protons from the matrix to the intermembrane space.


Another kind of complex can be observed in the electron transport chain, that is, complex V. It is comprised of ATP synthase.


The overall electron transport chain is like this:

Functions of the electron transport chain:

Electron transport chain possesses two important functions0.

  1. Regeneration of the electron carriers
  2. Making of a proton gradient


Questions 1-2


In the final series of cellular respiration that is in the electron transport chain (ETC) the production of NADH and FADH2 takes place during glycolysis. Kreb’s cycle and the oxidation of other catabolic processes would also take place in that time. All of the events lead to a result that is synthesis of ATP. A schematic diagram encapsulates the salient reactions comprising the ETC. This is represented by the below figure. Involvement of five protein complexes could be observed. These are as follows: complex I, complex II, complex III, complex IV and complex V. Along with these complexes coenzyme Q and cytochrome C are also present. The electrons are transferred along with passage of protons from the mitochondrial matrix into the intermembrane space by them. Due to this electron transfer, a transmembrane electrochemical proton gradient is created across the inner mitochondrial membrane. This is ultimately used for driving the protons into the matrix. The energy gets released and is used by the ATP synthase for the production of new ATP molecules.


  1. NADH hydrogenase
  2. NADH dehydrogenase
  3. NADH oxidase
  4. NAD dehydrogenase




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January 18, 2018

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