Redox Potential Basics

Standard reduction potentials (E°’ at pH 7) measure a couple’s tendency to gain electrons, with more negative values indicating stronger electron donation by the reduced form. In biological systems like the electron transport chain, electrons flow from more negative to more positive potentials, driving energy production.

Option Analysis

(A) Fumarate/succinate: E°’ = +0.03 V. Succinate weakly donates electrons to fumarate, as seen in Complex II where it enters the chain late. This positive potential means low donation tendency.

(B) NAD⁺/NADH: E°’ = -0.32 V. NADH strongly donates electrons, powering Complex I in oxidative phosphorylation and glycolysis. Its highly negative potential makes it the best donor here.

(C) FAD/FADH₂: E°’ ≈ -0.22 V (free FAD; bound varies to 0 V). FADH₂ donates via Complex II (succinate dehydrogenase), weaker than NADH due to less negative potential and bypassing Complex I.

(D) Pyruvate/lactate: E°’ = -0.19 V. Lactate donates to pyruvate in lactate dehydrogenase reaction under anaerobic conditions, but its potential is less negative than NADH’s.

Redox Potential Comparison Table

NAD+/NADH redox couple exhibits the highest tendency to donate electrons among key biochemical pairs, crucial for understanding electron transport chain (ETC) and CSIR NET Life Sciences preparation. This detailed analysis compares fumarate/succinate, FAD/FADH2, and pyruvate/lactate potentials to identify the strongest donor.

Understanding Redox Couples in Biochemistry

Redox couples drive cellular energy via electron donation, measured by E°’ at pH 7—more negative values signal stronger donation from reduced forms like NADH. In ETC, NADH (-0.32 V) fuels ATP synthesis, while others like fumarate/succinate (+0.03 V) accept electrons.

Detailed Comparison of Options

• Fumarate/succinate (+0.03 V): Positive potential favors fumarate reduction; succinate donates weakly in Complex II.
• NAD+/NADH (-0.32 V): Most negative E°’; NADH donates readily to Complex I, key in glycolysis and TCA cycle.
• FAD/FADH2 (-0.22 V): Intermediate potential; used in acyl-CoA dehydrogenase, enters ETC later.
• Pyruvate/lactate (-0.19 V): Lactate to pyruvate in fermentation; less potent donor than NADH.

This hierarchy explains why NADH yields more ATP (≈2.5) than FADH2 (≈1.5).

CSIR NET Exam Insights

For competitive exams, memorize E°’ values: NAD+/NADH tops donation lists. Practice Nernst equation: E = E°' - (0.059/n) log([Red]/[Ox]) at pH 7.