Redox basics in metabolism: By definition in biochemical redox reactions, reduction corresponds to which change at the level of electrons or protons?

Introduction / Context:
Oxidation–reduction (redox) reactions drive cellular energy metabolism. Correctly identifying which process is “reduction” is crucial for understanding electron transport chains, cofactor cycling (NAD+/NADH), and anabolic pathways.

Given Data / Assumptions:

• We compare options describing changes in electrons or protons.
• Standard biochemical definitions apply.
• No specific reaction equation is required.

Concept / Approach:
Reduction is gain of electrons (OIL RIG mnemonic: Oxidation Is Loss, Reduction Is Gain of electrons). In many biochemical contexts, reduction is coupled to protonation (addition of H) but the defining feature is electron gain, not proton gain per se. Conversely, oxidation is loss of electrons, frequently associated with dehydrogenation.

Step-by-Step Solution:
Recall the redox mnemonic OIL RIG. Map “reduction” to electron gain, independent of proton changes. Exclude options describing electron loss or proton-only changes. Select “Gain of electrons.”

Verification / Alternative check:
Cofactor chemistry: NAD+ + 2e− + H+ → NADH is a reduction (electron gain) of NAD+; this supports the definition.

Why Other Options Are Wrong:
Electron loss is oxidation; proton gain/loss may accompany redox but does not define it; oxygen addition usually correlates with oxidation, not reduction.

Common Pitfalls:
Equating hydrogen addition with reduction in all cases; while often true, the core definition remains electron gain.

Final Answer:
Gain of electrons.