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Which one of the following electron acceptors is present in the respiratory chain?
(a)Cytochrome f
(b)Cytochrome ${ a }_{ 3 }$
(c)Plastocyanin
(d)Ferredoxin.

Answer
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Hint: Respiratory chain is a complex multienzyme chain, which transports energized hydrogen or electrons from a donor molecule to molecular oxygen. The components of cytochrome include several hydrogens and electrons.

Complete answer:
The major components of the Respiratory chain consist of several hydrogen and electrons. Cytochromes are red or brown iron-containing haemato proteins, which serve as the electron transporting enzymes in cellular respiration and photosynthesis. All about 50 odd kinds of cytochromes have so far been identified. They fall under three main classes, namely a,b and c. Five members of these usually exist in a chain, called cytochrome series or respiratory assembly. They are arranged in the order cytochromes b-${ c }_{ 1 }$-c-a-${ a }_{ 3 }$. Only the last member of the chain, namely, ${ a }_{ 3 }$ can donate electrons directly to molecular oxygen. So it alone is directly oxidized by oxygen. Therefore, it is referred to as cytochrome oxidase. Just as UQ, Cyt. c etc. Also is a mobile carrier of electrons. Cytochrome ${ a }_{ 3 }$ is a component of complex IV of the respiratory chain. It delivers electrons to the terminal electron acceptor oxygen.

Additional information:
Plastocyanin is the soluble one-electron carrier. They carry electrons between two photosystems in plants.
Cytochrome f carry electrons from reduced quinine to plastocyanin in cytochrome ${ b }_{ 6 }f$ complex of plants.
Ferredoxin carries the electrons from P700 to ${ NADP }^{ + }$
So, the correct answer is option (b) ‘cytochromes ${ a }_{ 3 }$’.

Note: Complex IV consists of cytochromes a and ${ a }_{ 3 }$, and its prosthetic groups include genes and the copper ions ${ Cu }_{ A }$ and ${ Cu }_{ B }$.
These copper ions are very vital in the transfer of electrons to ${ O }_{ 2 }$ without generating incompletely reduced, highly reactive, and seriously harmful intermediates, such as ${ H }_{ 2 }O$ or hydroxyl free radicals.
Electrons flow from Cyt c to ${ O }_{ 2 }$ along complex IV causes a net movement of protons from the intermediate space. Thus complex IV functions as a proton pump that generates a proton-motive force.