Bioenergetics and redox states Which pair represents reduced coenzymes as they participate in cellular metabolism and electron transport?

Introduction / Context:Many metabolic questions test your ability to recognize oxidized versus reduced forms of electron carriers. Reduced coenzymes donate electrons to the respiratory chain or to biosynthetic reactions.

Given Data / Assumptions:

• NADH is the reduced form of NAD+.
• FADH2 is the reduced form of FAD.
• ATP/GTP are phosphoryl transfer molecules, not redox pairs.
• CoA carries acyl groups; ubiquinone/ubiquinol interconvert but only ubiquinol is the reduced form.

Concept / Approach:Identify reduced species: the suffixes “H” or “H2” indicate additional hydride/electrons. These carry reducing equivalents to the electron transport chain, enabling proton pumping and ATP synthesis.

Step-by-Step Solution:Recall redox pairs: NAD+/NADH and FAD/FADH2.Select the option listing both reduced forms.Exclude options listing oxidized forms or unrelated cofactors.

Verification / Alternative check:Stoichiometric coupling in oxidative phosphorylation: oxidation of NADH and FADH2 correlates with proton pumping and ATP generation, confirming their role as reduced carriers.

Why Other Options Are Wrong:NAD+ and FAD: oxidized, not reduced.ATP and GTP: energy currency via phosphate transfer, not electron carriers.CoA and ubiquinone: CoA is acyl-carrier; ubiquinone is the oxidized form (reduced form is ubiquinol).FMN/Cytochrome c option as stated refers to oxidized states or mixed roles, not both reduced carriers.

Common Pitfalls:Equating high-energy phosphates with redox carriers; overlooking that names ending in “-one” are often oxidized (e.g., ubiquinone).

Final Answer:NADH and FADH2.