A catabolic pathway is a chemical pathway which generates energy by degrading large molecules (such as carbohydrates and fats) into smaller units (such as monosaccharaides and fatty acids) it is often referred to as a degradative process and this process is an oxidative reaction. The energy produced is normally in the form of ATP. In biological systems, catabolic pathways are usually coupled with anabolic pathways as they provide the energy needed for the anabolic reaction to occur. Catabolic pathways are especially important in cellular respiration particularly in glycolysis and the Krebs cycle. Catabolic reactions produce energy, synthesise important monomers and provide reducing power, all of which are then used in anabolic reactions to produce macromolecules that are either functional, structural or for storage.