The synthesis of large molecules from smaller compounds involves the net reduction of carbon, as in the synthesis of glucose from carbon dioxide during photosynthesis, or the synthesis of lipids from carbohydrate by animals. If synthesis and breakdown were the exact opposites of each other, there would be no way for an organism to carry out net synthesis or degradation. This problem is a consequence of the laws of thermodynamics, which state that the free energy change of a given reaction is constant and that favored reactions are characterized by products having lower free energy than reactants. Therefore, if a reaction has a negative free energy in one direction, it will have a positive free energy, that is, be unfavored, in the other direction.
Organisms get around this problem by using different reactions in one or more steps of the overall pathway, depending on whether the reaction is proceeding in a catabolic or anabolic direction. The mammalian liver can both break down glucose for energy and synthesize glucose from food (amino acids, for example). Different reactions and enzymes participate in the catabolic (breakdown) and anabolic (synthetic) directions. The anabolic direction involves the input of energy in the form of ATP hydrolysis. This coupling of ATP hydrolysis to an otherwise unfavored reaction is a general theme in biochemistry.