Biological Oxidation and Reduction
High-Yield Summary
- Oxidation = loss of electrons; reduction = gain of electrons ('LEO the lion goes GER'). They always occur together as redox reactions, often split into half reactions to track electron flow.
- NAD⁺ accepts 2 electrons + 1 proton → NADH; diffuses between enzymes, delivers electrons to the ETC to drive ATP synthesis.
- FAD accepts 2 electrons + 2 protons → FADH₂; stays tightly bound to its enzyme (a flavoprotein), useful for reactions removing 2 H atoms (e.g., forming a double bond).
- ATP carries energy via phosphate bonds; NADH/FADH₂ carry energy via electrons — two distinct energy-currency systems in the cell.
Key Terms
- Redox reaction
- A reaction involving electron transfer — always pairs an oxidation with a reduction.
- Half reaction
- The oxidation or reduction portion of a redox reaction, written separately to track electrons.
- Flavoprotein
- A protein with FAD tightly bound as a cofactor; FAD does not dissociate after accepting electrons.
NAD⁺ vs. FAD
| Feature | NAD⁺/NADH vs. FAD/FADH₂ |
|---|---|
| Electrons accepted | 2 / 2 |
| Protons accepted | 1 / 2 |
| Binding behavior | Binds enzyme, then dissociates / Stays tightly bound (flavoprotein) |
| Typical use | General reductions (glycolysis, TCA) / Reactions removing 2 H atoms (e.g., double-bond formation) |
Common MCAT Trap
- NAD⁺ dissociates from its enzyme after accepting electrons; FAD stays bound (as part of a flavoprotein) — a frequently tested distinction.
- "LEO the lion goes GER": Loss of Electrons = Oxidation, Gain of Electrons = Reduction — don't invert it under pressure.
- FAD accepting 2 PROTONS (not 1, like NAD⁺) is why it's suited to double-hydrogen-removal reactions.
Quick Recall
What does LEO the lion goes GER stand for?
How many electrons and protons does NAD⁺ accept when reduced?
Does FAD dissociate from its enzyme after accepting electrons?