Biopolymers are natural polymers produced by living organisms; in other words, they are polymeric biomolecules derived from cellular or extracellular matter. Biopolymers contain monomeric units that are covalently bonded to form larger structures. There are three main classes of biopolymers, classified according to the monomeric units used and the structure of the biopolymer formed: polynucleotides, polypeptides, and polysaccharides. More specifically, polynucleotides, such as RNA and DNA, are long polymers composed of 13 or more nucleotide monomers. Polypeptides or proteins, are short polymers of amino acids and some major examples include collagen, actin, and fibrin. The last class, polysaccharides,are often linear bonded polymeric carbohydrate structures and some examples include cellulose and alginate. Other examples of biopolymers include rubber, suberin, melanin and lignin.