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. $O _2$-- Cyt a of Complex IV -- Cyt c-- Fe-S of Complex III -- Q -- Fe-S of Complex II -- FADH ${ }_2$
B. FADH ${ }_2$-- Fe-S of Complex II -- Fe-S of Complex III -- Q -- Cyt a of Complex IV -Cyt $c-- O _2$
C. FMN of Complex I -- NADH -- Fe-S of Complex III -- Cyt $b$ of Complex III -- Cyt $c$-Cyt $a$ of Complex IV -- $O _2$
D. FADH ${ }_2$-- Fe-S of Complex II -- Q -- Fe-S of Complex III -- Cyt c -- Cyt $a$ of Complex IV -- $O _2$
Answer (2)
The problem requires ordering electron transport compounds from highest to lowest free energy.
The correct order follows the electron transport chain sequence: F A D H 2 -> Complex II (Fe-S) -> Q -> Complex III (Fe-S) -> Cyt c -> Complex IV (Cyt a) -> O 2 .
Option 4, F A D H 2 -- Fe-S of Complex II -- Q -- Fe-S of Complex III -- Cyt c -- Cyt a of Complex IV -- O 2 , matches this order.
Therefore, the correct list is F A D H 2 − − F e − S o f C o m pl e x II − − Q − − F e − S o f C o m pl e x III − − C y t c − − C y t a o f C o m pl e x I V − − O 2 .
Explanation
Understanding the Problem We need to identify the correct order of electron transport compounds from highest to lowest free energy. This order is determined by the sequence of electron carriers in the electron transport chain, where electrons move from carriers with higher free energy to those with lower free energy. The compounds listed are involved in this process, and we need to arrange them according to their position in the chain.
Determining the Correct Order The general order of electron flow in the electron transport chain is as follows:
NADH (or FADH2)
Complex I (or Complex II for FADH2)
Q (Ubiquinone)
Complex III
Cytochrome c
Complex IV
O 2
Within these complexes are various electron carriers such as Fe-S clusters and cytochromes. We need to place the given compounds in this order.
Analyzing the Options Let's analyze the given options:
Option 1: O 2 -- Cyt a of Complex IV -- Cyt c -- Fe-S of Complex III -- Q -- Fe-S of Complex II -- F A D H 2 . This is incorrect because it starts with O 2 (the final electron acceptor) and ends with F A D H 2 (an initial electron donor). The order is reversed.
Option 2: F A D H 2 -- Fe-S of Complex II -- Fe-S of Complex III -- Q -- Cyt a of Complex IV -- Cyt c -- O 2 . This is also incorrect. While it starts with F A D H 2 , the order of Fe-S of Complex II and Fe-S of Complex III is not correct, and Q should come before Fe-S of Complex III.
Option 3: FMN of Complex I -- NADH -- Fe-S of Complex III -- Cyt b of Complex III -- Cyt c - Cyt a of Complex IV -- O 2 . This option includes FMN and NADH at the beginning, which is correct. However, the placement of Fe-S of Complex III is not accurate, as it should come before Cyt b of Complex III and Cyt c .
Option 4: F A D H 2 -- Fe-S of Complex II -- Q -- Fe-S of Complex III -- Cyt c -- Cyt a of Complex IV -- O 2 . This option appears to be the most correct. F A D H 2 donates electrons to Complex II, which contains Fe-S. Then, electrons move to Q, then to Complex III (containing Fe-S), then to Cyt c, then to Complex IV (containing Cyt a), and finally to O 2 .
Conclusion Based on the analysis, the correct order from highest to lowest free energy is:
F A D H 2 -- Fe-S of Complex II -- Q -- Fe-S of Complex III -- Cyt c -- Cyt a of Complex IV -- O 2
Examples
The electron transport chain is similar to a series of waterfalls. At the top of the waterfall, the water (electrons) has high potential energy (free energy). As the water flows down each level of the waterfall (through each complex in the electron transport chain), it loses potential energy, which is used to do work, like powering a turbine. The final destination of the water is the bottom of the waterfall, where it has the lowest potential energy. Similarly, electrons start with high free energy and gradually lose it as they move through the electron transport chain, ultimately reaching oxygen, which has the lowest free energy in this chain.
The correct order of electron transport compounds from highest to lowest free energy is found in Option D: F A D H 2 -- Fe-S of Complex II -- Q -- Fe-S of Complex III -- Cyt c -- Cyt a of Complex IV -- O 2 . This sequence correctly follows the flow of electrons through the electron transport chain. Each component properly reflects the energy transitions as electrons move toward the final electron acceptor, oxygen.
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