Amphibolic

The term amphibolic (ἀμφίβολος) is used to describe a biochemical pathway that involves both catabolism and anabolism. Catabolism is a degradative phase of metabolism in which large molecules are converted into smaller and simpler molecules, which involves two types of reactions. First, hydrolysis reactions, in which catabolism is the breaking apart of molecules into smaller molecules to release energy. Examples of catabolic reactions are digestion and cellular respiration, where sugars and fats are broken down for energy. Breaking down a protein into amino acids, or a triglyceride into fatty acids, or a disaccharide into monosaccharides are all hydrolysis or catabolic reactions. Second, oxidation reactions involve the removal of hydrogens and electrons from an organic molecule. Anabolism is the biosynthesis phase of metabolism in which smaller simple precursors are converted to large and complex molecules of the cell. Anabolism has two classes of reactions. The first are dehydration synthesis reactions; these involve the joining of smaller molecules together to form larger, more complex molecules. These include the formation of carbohydrates, proteins, lipids and nucleic acids. The second are reduction reactions, in which hydrogens and electrons are added to a molecule. Whenever that is done, molecules gain energy. The term amphibolic was proposed by B. Davis in 1961 to emphasise the dual metabolic role of such pathways. These pathways are considered to be central metabolic pathways which provide, from catabolic sequences, the intermediates which form the substrate of the metabolic processes. All the reactions associated with synthesis of biomolecule converge into the following pathway, viz., glycolysis, the Krebs cycle and the electron transport chain, exist as an amphibolic pathway, meaning that they can function anabolically as well as catabolically. Other important amphibolic pathways are the Embden-Meyerhof pathway, the pentose phosphate pathway and the Entner–Doudoroff pathway.