Carbon, which has four valence electrons, is present in many biological molecules because of its ability to form four covalent bonds. This property allows carbon to become the backbone of the large molecules, called macromolecules, that are so abundant in living matter. A macromolecule is any molecule with a very large molecular weight. Deoxyribonucleic acid (DNA), ribonucleic acid (RNA), proteins, and polysaccharides are all macromolecules. Macromolecules are types of polymers formed by joining carbon-based monomers together in long, branching chains. A monomer is a single molecule that when combined with other monomers forms a larger molecule. A polymer is a large molecule made of repeating smaller units, or monomers, of similar structure that are bonded together. The chains can fold and interact both intramolecularly and intermolecularly via noncovalent bonds, and this helps give macromolecules specific shapes.It is largely due to carbon's ability to form four covalent bonds that there is such a large variety of biological macromolecules. Consider proteins, for example. There are approximately 10,000 different proteins in a liver cell. Some of these proteins are specialized for liver function, such as proteins involved in the synthesis of bile acid. Other proteins, such as actin, a structural protein, are expressed in every cell in the body. Each protein that occurs in nature has a different and specific function. None of this huge variety would be possible if carbon could not form four covalent bonds.