The cost of living is energy and the producers and consumers of energy in the cell are the chemical reactions known collectively as metabolism. Metabolic processes are governed by the same laws of energy as the rest of the universe, so they must be viewed in the light of Gibbs free energy. For the most part, the drivers of changes in Gibbs free energy are changes in concentration of reactants and products but for some reactions, the concentration changes required to run a reaction in the desired direction are not practical. In such cases, cells may use alternative strategies, such as energy coupling reactions (combining an energetically unfavorable reaction with a favorable one, such as the hydrolysis of ATP) to help “drive" the unfavorable reaction. In other cases, cells use alternate pathways around energetically unfavorable reactions.
Depending on your mathematical perspective, life is the sum of the product of the biochemical reactions that occur in cells. The collection of these reactions is known as metabolism. We break the subject into two broad areas: 1) oxidative/reductive metabolism and 2) pathways that involve little oxidation/reduction. This chapter deals with the former.