Which of the following is the correct sequence of electron acceptors in the electron transport chain?
  1. Cytochrome a,a3,b,c
  2. Cytochrome c,b,a,a3
  3. Cytochrome b,c,a3,a
  4. Cytochrome b,c,a,a3


The correct option is D Cytochrome b,c,a,a3
The correct sequence of electron acceptors in the electron transport chain is cytochrome b,c,a,a3. During the electron transport chain, there is transfer of electrons from NADH+H+ and FADH2 to oxygen by a series of electron carriers. The energy of these electrons is utilised to synthesise ATP.

The electron transport chain is in the inner mitochondrial membrane and it has four complexes that accept and carry electrons from NADH+H+ and FADH2. These molecules get oxidised to release electrons which pass through the ETC (electron transport chain) located on the inner mitochondrial membrane. 

The electrons released due to the oxidation of NADH+H+ moves in the following sequence: Complex I Complex III complex IV. 

The electrons released due to oxidation of FADH2 moves in the following sequence: Complex II complex III complex IV.  

The electrons released by the oxidation of NADH+H+ are transferred to complex I, and then get further transported to ubiquinone located within the inner membrane. Ubiquinone also receives the electrons released by the oxidation of FADH2 through complex II. Ubiquinone is a mobile electron carrier. The ubiquinone is converted to ubiquinol and electrons are then passed on to cytochrome b and cytochrome c1 of complex III. From complex III, electrons are transported to cytochrome c. Cytochrome c is a small protein attached to the outer surface of the inner membrane of mitochondria. It acts as a mobile carrier for the transfer of electrons between complex III and IV. In Complex IV, the electrons pass through cytochromes a and a3. Oxygen is the final acceptor of the electrons in the ETC. It accepts electrons and picks up protons to form water. 

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