Redox basics — Cells often capture energy by transferring electrons to specialized carriers. What is the term for the gain of electrons by a molecule?

Introduction:
Energy transduction in biology commonly occurs through oxidation–reduction (redox) reactions. This question checks core vocabulary: what do we call it when a molecule gains electrons during cellular processes such as catabolism and respiration?

Given Data / Assumptions:

• Catabolic pathways release energy by oxidizing substrates.
• Captured electrons are accepted by carriers like NAD+ and FAD.
• Terminology must precisely describe electron transfer events.

Concept / Approach:
Reduction is defined as the gain of electrons. In cells, oxidized cofactors (NAD+, FAD) become reduced to NADH and FADH2 when they accept electrons and, often, associated protons. These reduced carriers later donate electrons to the electron transport chain, enabling ATP synthesis through oxidative phosphorylation.

Step-by-Step Solution:

Verification / Alternative check:
Standard redox mnemonics (OIL RIG: Oxidation Is Loss, Reduction Is Gain [of electrons]) confirm the terminology. Biochemical measurements (e.g., NADH absorbance at 340 nm) directly detect the reduced state after electron gain.

Why Other Options Are Wrong:

• Biosynthesis: building complex molecules; may involve reductions but is not the definition.
• Metabolism: overall chemical processes; too broad.
• Catalysis: rate acceleration by enzymes; does not define electron gain.
• Oxidation: electron loss, the opposite of reduction.

Common Pitfalls:
Mixing the colloquial use of “reduction” with its chemical meaning. In redox chemistry, reduction always means gaining electrons, often accompanied by protonation in biological contexts.

Final Answer:
Reduction.