Bond cleavage or scission is the splitting of chemical bonds. In general, there are two classifications for bond cleavage: homolytic and heterolytic, depending on the nature of the process. In homolytic cleavage, or homolysis, the two electrons in a cleaved covalent bond are divided equally between the products. This process is also known as homolytic, or radical fission. In heterolytic cleavage, or heterolysis, the bond breaks in such a fashion that the originally- shared pair of electrons remain with one of the fragments. This process is also known as heterolytic or ionic fission. The term " bond dissociation energy" refers to the amount of energy required to cleave a bond via homolytic cleavage. In biochemistry the process of breaking down large molecules (by splitting their internal bonds) is catabolism. Enzymes which catalyse bond cleavage are known as lyases, unless they operate by hydrolysis or oxidation/reduction, in which case they are known as hydrolases and oxidoreductases respectively.