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 Any intermediate withdrawn for biosynthesis can thus be readily replenished by the catabolism of further nutrients.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.
An exergonic process is one in which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process.  Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs free energy change is negative (∆ G < 0). "Exergonic" (from the prefix exo-, derived for the Greek word ἔξω exō , "outside" and the suffix -ergonic, derived from the Greek word ἔργον ergon , " work ") means "releasing energy in the form of work". In thermodynamics, work is defined as the energy moving from the system (the internal region) to the surroundings (the external region) during a given process.