REDOX
Summary
REDOX is short for reduction-oxidation: reactions in which electrons move from one species to another. The species that loses electrons is oxidized and the one that gains them is reduced. In metabolism, fuels are oxidized as NAD⁺ and FAD are reduced to NADH and FADH₂.
Detail
Meaning: Redox reactions transfer electrons. Oxidation is loss of electrons and reduction is gain (OIL RIG), and the two happen together. The reducing agent is oxidized; the oxidizing agent is reduced. In metabolism, a more oxidized carbon has more C–O bonds and fewer C–H bonds. Glucose is oxidized to CO₂ while O₂ is reduced to H₂O, with NAD⁺ and FAD carrying electrons as NADH and FADH₂.
Equation: ΔG°' = −nFΔE°', where ΔG°' is the standard free energy change (J/mol), n is the moles of electrons transferred (mol), F is Faraday's constant (96,485 C/mol), and ΔE°' is E°'(acceptor) − E°'(donor) (V).
On the MCAT: Passages give reduction potentials and ask which way electrons flow (toward the more positive E°') or whether a step is spontaneous (a positive ΔE°' gives a negative ΔG). Questions also ask which molecule is oxidized in a pathway step, or how blocking the electron transport chain changes the NADH/NAD⁺ ratio.
Common confusion: NAD⁺ is the oxidizing agent, so it is reduced to NADH when it works. An agent is named for what it does to the other species, not for what happens to itself.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.