Differentiation between Redox Reactions and Redox Couples

The movements of electrons between cellular reductants and oxidants represent another form of energy transfer in cells.

A reductant (or reducing agent) is a substance that loses or donates electrons to another substance; the latter substance is the oxidant (or oxi­dizing agent).

Conversely, an oxidant is a substance that accepts electrons from another substance, the lat­ter being the reductant.

Reactions that involve the movement of electrons between reductant and oxidant are called redox reactions.

Different chemical substances have different poten­tials for donating or accepting electrons. The tend­ency of hydrogen to dissociate

H₂<===> 2H⁺ +2 e^–

thereby releasing electrons, is used as a standard against which the tendencies of other substances to release or accept electrons is measured. The electron donor (e.g., H₂ in the above reaction) and the electron acceptor (e.g., 2 H ⁺ in the above reaction) are called a redox couple or half cell. The tendency of any chemi­cal substance to lose or gain electrons is called the re­dox potential and is measured in volts (V). Measure­ments are made using an electrode that has been standardized against the H₂-2 H ⁺ couple whose re­dox potential is set at 0.0 under standard conditions (pH 0.0, 1M [H⁺], 25°C, and 1 atmosphere pressure).

This potential is noted by the symbol E₀. For biochem­ical reactions, which normally occur at pH 7.0, the re­dox potential of the H₂-2 H ⁺ couple is – 0.421 V; stan­dard redox potentials at pH 7.0 are noted by the symbol The E’₀ values of a number of biologically important redox couples are given in Table 9-4.

Any substance with a more positive E’₀ value than another has the potential for oxidizing that substance (i.e., removing electrons from the substance with the more negative E’₀ value). The greater the difference in redox potentials, the greater the energy changes in­volved.

The change in standard free energy, ∆G⁰‘, is related to E’₀ as follows:

∆G⁰‘ = nf∆E’₀,

Where n is the number of electrons exchanged per molecule, is the Faraday (96,406 J/V), and E’₀ is the difference in redox potential between the more posi­tive and the more negative members of the redox cou­ple. For example, the oxidized form of cytochrome c can oxidize the reduced form of cytochrome b by re­moval of two electrons. The difference between the re­dox potentials of the two is

+ 0.254-(+ 0.030)= + 0.224 V

Therefore,

∆G⁰‘ = – 2 (96,406 J/VX0.224 V)

= – 43.19 kJ (per mole of each cytochrome)

In cells, the energy changes during redox reactions may be coupled to the synthesis of ATP.