The previous chapters have dealt with the principles of thermodynamics
and the involvement of protons in acid/base reactions. In this chapter the
focus is on reduction/oxidation reactions, or redox reactions. Redox reac-
tions are significant for many fundamental biological processes involving
the transfer of electrons from one species to another. The Nernst equa-
tion provides a thermodynamic description of these reactions with the
oxidation/reduction midpoint potential being a useful parameter, char-
acteristic of a redox-active species. Redox reactions lead to the presence of
charged species. The properties of ions in solution can be highly depend-
ent upon the properties of the solvent molecules. The interactions of
ions with the molecules of the solution are described in terms of their
Gibbs energy of formation and a new parameter, the activity coefficient.
The critical role of such processes in biological systems is illustrated for
two different biological applications, the chemiosmotic hypothesis and
respiration.
Oxidation/reduction reactions
The transfer of an electron from one charged species to another species
is central to many biological reactions. Proteins can carry electrons and
usually possess cofactors that can be oxidized or reduced. The process of
electron transfer is usually broken into two reactions, called half reactions,
that are not real reactions but are used for bookkeeping. For example,
consider a reaction in which a molecule X accepts an electron, producing
a reduced species, X−:
X +e−→X− (6.1)