World of Microbiology and Immunology | Oxidation-Reduction Reaction

Oxidation-reduction reactions are significant to physiological reactions and biochemical pathways important to microorganisms and immune processes.

The term oxidation was originally used to describe reactions in which an element combines with oxygen. In contrast, reduction meant the removal of oxygen. By the turn of this century, it became apparent that oxidation always seemed to involve the loss of electrons and did not always involve oxygen. In general, oxidation-reduction reactions involve the exchange of electrons between two species.

An oxidation reaction is defined as the loss of electrons, while a reduction reaction is defined as the gain of electrons. The two reactions always occur together and in chemically equivalent quantities. Thus, the number of electrons lost by one species is always equal to the number of electrons gained by another species. The combination of the two reactions is known as a redox reaction. Species that participate in redox reactions are described as either reducing or oxidizing agents. An oxidizing agent is a species that causes the oxidation of another species. The oxidizing agent accomplishes this by accepting electrons in a reaction. A reducing agent causes the reduction of another species by donating electrons to the reaction.

In general, a strong oxidizing agent is a species that has an attraction for electrons and can oxidize another species. The standard voltage reduction of an oxidizing agent is a measure of the strength of the oxidizing agent. The more positive the species' standard reduction potential, the stronger the species is as an oxidizing agent.

In reactions where the reactants and products are not ionic, there is still a transfer of electrons between species. Chemists have devised a way to keep track of electrons during chemical reactions where the charge on the atoms is not readily apparent. Charges on atoms within compounds are assigned oxidation states (or oxidation numbers). An oxidation number is defined by a set of rules that describes how to divide up electrons shared within compounds. Oxidation is defined as an increase in oxidation state, while reduction is defined as a decrease in oxidation state. Because an oxidizing agent accepts electrons from another species, a component atom of the oxidizing agent will decrease in oxidation number during the redox reaction.

There are many examples of oxidation-reduction reactions in the world. Important processes that involve oxidationreduction reactions include combustion reactions that convert energy stored in fuels into thermal energy, the corrosion of metals, and metabolic reactions.

Oxidation-reduction reaction occur in both physical and biological settings (where carbon-containing compounds such as carbohydrates are oxidized). The burning of natural gas is an oxidation-reduction reaction that releases energy [CH ₄(g) + 2O ₂(g) → CO ₂(g) + 2H ₂O(g) + energy]. Redox reactions burn carbohydrates that provide energy [C ₆H ₁₂O ₆(aq) + 6O ₂(g) → 6CO ₂(g) + 6H ₂O(l)]. In both examples, the carbon-containing compound is oxidized, and the oxygen is reduced.

See also Biochemistry