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

fadh₂ - fe - s of complex ii - q - fe - s of complex iii - cyt c - cyt a of complex iv - o₂

o₂ - cyt a of complex iv - cyt c - fe - s of complex iii - q - fe - s of complex ii - fadh₂

fadh₂ - fe - s of complex ii - fe - s of complex iii - q - cyt a of complex iv - cyt c - o₂

fmn of complex i - nadh - fe - s of complex iii - cyt b of complex iii - cyt c - cyt a of complex iv - o₂

Explanation:

In the electron - transport chain, electrons are passed from carriers with higher free energy (more reducing power) to carriers with lower free energy. $FADH_2$ is an electron donor that enters the electron - transport chain at Complex II. The electrons then pass through the Fe - S centers of Complex II, then to ubiquinone ($Q$), then to the Fe - S centers of Complex III, then to cytochrome $c$ (Cyt $c$), then to cytochrome $a$ of Complex IV, and finally to $O_2$. As electrons move through the chain, their free energy is gradually released.

Let's analyze each option:

• Option 1: $FADH_2$ is a good electron donor (high free energy of electrons). The Fe - S of Complex II accepts electrons from $FADH_2$. $Q$ (ubiquinone) accepts electrons from Complex II (Fe - S). Fe - S of Complex III accepts electrons from $Q$. Cyt $c$ accepts electrons from Complex III (Fe - S). Cyt $a$ of Complex IV accepts electrons from Cyt $c$. $O_2$ is the final electron acceptor (lowest free energy of electrons in this context).
• Option 2: This option has the order reversed. $O_2$ is the final electron acceptor (lowest free energy) and should be at the end, not the beginning.
• Option 3: The order of $Q$ and Fe - S of Complex III is incorrect. Electrons go from Fe - S of Complex II to $Q$, then to Fe - S of Complex III. Also, the order of Cyt $a$ and Cyt $c$ is wrong. Electrons go from Cyt $c$ to Cyt $a$.
• Option 4: $NADH$ has a higher free energy of electrons than $FADH_2$. Also, electrons from NADH enter at Complex I (FMN is part of Complex I). But the order of Complex I (FMN) and NADH is incorrect. NADH is the electron donor for Complex I.

Answer:

A. $FADH_2 -$ Fe - S of Complex II $- Q -$ Fe - S of Complex III $- $Cyt $c -$ Cyt $a$ of Complex IV $- O_2$