Cells maintain a complex internal habitat that is highly structured and controlled. They host a never-ending array of metabolic reactions that are required for functions such as nutrient processing, growth, and release of energy. Among the most remarkable features of cell metabolism are enzymes, specialized proteins that perform key roles in most of these reactions. Recently, biochemists clarified the actions of an enzyme called OMP decarboxylase, used to synthesize a molecule that is found universally in the genetic material of all organisms and viruses. They found that in the presence of the OMP enzyme, this reaction happens very rapidly—about 30 times per second. The chances of the reaction happening without the enzyme would take so long (78 million years) that it is essentially not a realistic possibility. It is clear that without this enzyme, life as we know it could never have evolved. This is but one example of the amazing cellular machinery that is ultimately responsible for the tremendous diversity in the earth's life forms. In addition, it shows us that the metabolic activities of cells often share common patterns. With these themes in mind, this chapter will cover some of the major unifying characteristics of metabolism, enzymes, the flow of energy, and the pathways that govern nutrient processing. Having this background will greatly benefit your understanding of future topics such as genetics, drug therapy, disinfection, biotechnology, diseases, and the identification of microorganisms.