Redox Potential (also known as reduction potential or oxidation-reduction potential, E° or E′°) is a measure of the tendency of a chemical species to gain or lose electrons (i.e., to be reduced or oxidized) in a redox (reduction-oxidation) reaction.
It indicates how easily a molecule accepts electrons (is reduced) or donates electrons (is oxidized).
Key Concepts
1. Oxidation and Reduction
- Oxidation: Loss of electrons
- Reduction: Gain of electrons
2. Standard Reduction Potential (E°)
- Measured in volts (V) or millivolts (mV)
- Determined under standard conditions: 25°C, 1 M concentration, 1 atm pressure, pH 0
3. Biological Standard Reduction Potential (E′°)
- Measured at pH 7 (biological pH) and 25°C
- More relevant in living systems
Interpreting Redox Potentials
| E′° Value | Meaning | Examples |
|---|---|---|
| Positive E′° | Strong oxidizing agent (readily accepts electrons) | O₂, NAD⁺ |
| Negative E′° | Strong reducing agent (readily donates electrons) | NADH, FADH₂ |
Electrons flow from a molecule with lower (more negative) E′° to one with a higher (more positive) E′°.
Electrons move from strong reducing agents (low E′°) to strong oxidizing agents (high E′°), powering processes like the electron transport chain.
Free Energy and Redox Potential
Redox reactions are linked to free energy change (ΔG°′) by the equation:
ΔG°’ = -nFΔE°
Where:
- ΔG°′ = Standard free energy change (kJ/mol)
- n = Number of electrons transferred
- F = Faraday’s constant (96.5 kJ/V·mol)
- ΔE°′ = Difference in redox potential between electron donor and acceptor
A positive ΔE′° yields a negative ΔG°′, meaning the reaction is spontaneous.
Biological Significance of Redox Potential
1. Cellular Respiration
- Electrons from NADH and FADH₂ are passed through the electron transport chain (ETC).
- They move from carriers with low E′° to high E′°, ultimately to O₂, the final electron acceptor.
- This energy flow drives ATP synthesis via oxidative phosphorylation.
| Carrier | E′° (mV) |
|---|---|
| NAD⁺ / NADH | –320 |
| FAD / FADH₂ | –220 |
| Ubiquinone (Q / QH₂) | +100 |
| Cytochrome c | +220 |
| O₂ / H₂O | +820 |
Electron flow: NADH → Q → Cytochrome c → O₂
Energy released: Used to pump protons and generate ATP
2. Photosynthesis
- Reverse electron flow occurs (electrons from water, low E′°, are excited by light and transferred to NADP⁺).
- Important in light reactions of photosynthesis.
3. Redox Reactions in Metabolism
- Redox potential controls:
- Glycolysis
- TCA (Krebs) cycle
- Fermentation
- Anaerobic respiration
4. Detoxification and Antioxidants
- Enzymes like superoxide dismutase and glutathione reductase manage reactive oxygen species using redox reactions.
Summary Table
| Term | Description |
|---|---|
| Redox Potential (E′°) | Tendency of a compound to accept or donate electrons |
| Unit | Volts (V) or millivolts (mV) |
| E′° > 0 | Strong oxidizing agent (good electron acceptor) |
| E′° < 0 | Strong reducing agent (good electron donor) |
| Electron Flow | Electrons flow from lower E′° to higher E′° |
| Link to ΔG | ΔG = –nFΔE (negative ΔG = spontaneous reaction) |
Examples of Biological Redox Pairs
| Redox Pair | E′° (mV) | Role |
|---|---|---|
| NAD⁺ / NADH | –320 | Electron donor (catabolism) |
| FAD / FADH₂ | –220 | Electron donor (TCA cycle) |
| Pyruvate / Lactate | –190 | Redox in anaerobic glycolysis |
| Fe³⁺ / Fe²⁺ | +770 | Component of cytochromes |
| O₂ / H₂O | +820 | Final electron acceptor in electron transport chain (ETC) |
Key Points
- Redox potential governs how electrons flow in biological systems.
- It is central to energy production (ATP), metabolism, and signaling.
- A higher redox potential = greater ability to accept electrons (oxidant).
- Understanding E′° values helps explain why certain molecules act as energy carriers, like NADH and FADH₂.