Metabolism is the sum total of chemical changes that occur in living organisms and which are fundamental to life. Nutrients from the environment are used in two ways by living organisms. They can serve as constituents in the synthesis of components of the organism (assimilation) or they can be oxidatively degraded for the purposes of energy production (dissimilation). All constituents of living organisms are subject to continual processes of breakdown and re-synthesis, normally referred to as "turnover." Turnover is defined as the balance of synthesis and degradation of biomolecules in living organisms. Processes of breakdown are collectively called catabolism and all reactions concerned with synthesis are called anabolism. Amphibolic pathways may serve both functions of degradation and synthesis. Because metabolism is an open system (i.e., having constant inputs and outputs) its multistage processes approach a state of equilibrium that is never quite achieved, at least not while...