The dynamic nature of metabolism results in constant degrading and rebuilding of most cellular materials. For example, proteins exist in a cell for relatively brief times, ranging from minutes to weeks, with most proteins having average life spans of a few days. Structural proteins generally last longer than enzymes, but they too are eventually degraded and synthesized anew. Likewise, other cellular materials are turned over in a similar fashion. This constant turnover of cellular materials keeps the cell in good condition. Molecules that may have been damaged by, for example, being partially oxidized, will sooner or later be degraded and replaced.
When higher animals consume a mixed diet, sufficient quantities of compounds for both biosynthesis and energy supply are available. Carbohydrates yield intermediates of glycolysis and of the phosphogluconate pathway, which in turn yield acetyl coenzyme A (or acetyl-CoA); lipids yield glycolytic intermediates and acetyl coenzyme A; and many amino acids form intermediates of both the TCA cycle and glycolysis. Any intermediate withdrawn for biosynthesis can thus be readily replenished by the catabolism of further nutrients. This situation does not always hold, however. Microorganisms in particular can derive all of their carbon and energy requirements by utilizing a single carbon source. The sole carbon source may be a substance such as a carbohydrate or a fatty acid , or an intermediate of the TCA cycle (or a substance readily converted to one). In both cases, reactions ancillary to those discussed thus far must occur before the carbon source can be utilized.