Question

Of the following lists of electron transport compounds, which one lists them in order from the one containing electrons with the highest free energy to the one containing electrons with the lowest free energy? Note that not all electron transport compounds in the electron transport chain are listed.

A) FMN of Complex I -- Q -- Fe-S of Complex II -- FADH2 -- Fe-S of Complex III -- Cyt c -- Cyt a of Complex IV -- O2
B) FADH2 -- FMN of Complex I -- Fe-S of Complex II -- Q -- Fe-S of Complex III -- Cyt c -- Cyt a of Complex IV -- O2
C) O2 -- Cyt a of Complex IV -- Cyt c -- Fe-S of Complex III -- Q -- Fe-S of Complex II -- FMN of Complex I -- FADH2
D) FADH2 -- FMN of Complex I -- Fe-S of Complex II -- Fe-S of Complex III -- Q -- Cyt a of Complex IV -- Cyt c -- O

Answer 1

B) FADH2 -- FMN of Complex I -- Fe-S of Complex II -- Q -- Fe-S of Complex III -- Cyt c -- Cyt a of Complex IV -- O2

Step-by-step explanation:

FADH2 and NADH give their high energy electrons to the terminal electron acceptor molecular oxygen via an electron transport chain. As the electrons move through electron carriers of the electron transport chain, they lose their free energy. Part of the free energy of the electrons is used to pump the protons from the matrix into the intermembrane space. Therefore, part of the energy of electrons is temporarily stored in the form of a proton concentration gradient.

NADH gives its electrons to FMN of complex I while FADH2 gives its electrons to the Fe-S center of complex II. Both the complexes are oxidized by coenzyme (Q) which in turn reduces Fe-S centers of complex III. Cyt c of complex IV obtains electrons from complex III and passes them to CuA center, to heme "a" to heme "a3-CuB center" and finally to the molecular oxygen.

So, the compounds arranged with respect to the energy content of electrons in descending order are as follows: FADH2 -- FMN of Complex I -- Fe-S of Complex II -- Q -- Fe-S of Complex III -- Cyt c -- Cyt a of Complex IV -- O2.