Originally, it was generally believed that life was not subject to the laws of science the way non-life was. It was thought that only living beings could produce the molecules of life (from other, previously existing biomolecules). Biomonomers and biopolymers are the structural basis of the four main macromolecules classes in biochemistry: carbohydrates, lipids, proteins and nucleic acids or biopolymers. Monomers are smaller micro molecules that are put together to make macromolecules. Biopolymers are those macromolecules that are created when monomers are synthesized together. When they are synthesized, the two molecules undergo a process called catabolization or decomposition. A biomolecule is a chemical compound that naturally occurs in living organisms. Biomolecules consist primarily of carbon and hydrogen, along with nitrogen, oxygen, phosphorus and sulfur. Other elements sometimes are incorporated but these are much less common. Biomolecules are necessary for the existence of all known forms of life. For example, humans possess skin and hair. The main component of hair is keratin, an agglomeration of proteins which are themselves polymers built from amino acids. Amino acids are some of the most important building blocks used in nature, to construct larger molecules. Another type of building block are the nucleotides, each of which consists of three components: a purine or pyrimidine base, a pentose and a phosphate group. These nucleotides, mainly, form the nucleic acids. Besides the polymeric biomolecules, numerous small organic molecules are absorbed or synthesised by living systems