Electron Carriers—Identify the Reduced Form Which of the following is the reduced form of a temporary electron carrier molecule used in cellular redox reactions?

Introduction / Context:
Biological redox reactions transfer electrons using carrier molecules that cycle between oxidized and reduced states. Recognizing the reduced versus oxidized forms is essential for tracking electron flow through metabolic pathways such as the TCA cycle and oxidative phosphorylation, and in photosynthesis and respiration.

Given Data / Assumptions:

• Flavin adenine dinucleotide (FAD) ⇄ FADH2.
• Nicotinamide adenine dinucleotide phosphate (NADP+) ⇄ NADPH.
• ATP is a high-energy phosphate carrier, not primarily an electron carrier.

Concept / Approach:
The reduced form carries additional electrons and (often) protons. For FAD, the reduced state is FADH2; for the NADP system it is NADPH (not listed among options). NADP+ and NAD+ are oxidized forms. CO2 is fully oxidized carbon and not an electron carrier, and ATP stores energy in phosphoanhydride bonds rather than as reducing power.

Step-by-Step Solution:

Verification / Alternative check:
In oxidative phosphorylation, FADH2 donates electrons to Complex II (succinate dehydrogenase), confirming its reduced-carrier status.

Why Other Options Are Wrong:

• ATP: energy currency, not a redox carrier.
• NADP+ and NAD+: oxidized forms lacking the hydride.
• CO2: terminal oxidized product of carbon catabolism.

Common Pitfalls:
Mixing up ATP's role with that of electron carriers; conflating NADP+ with NADPH.

Final Answer:
FADH2