Proteins are large biomolecules, consisting of one or more long chains of amino acids that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs. Protein can be categorized into two types: complete and incomplete proteins. Complete proteins are those that contain all essential amino acids in sufficient quantity. Incomplete proteins are those that don’t contain all 9 essential amino acids, or don’t have sufficient quantities of them to meet the body’s needs, and must be supplemented with other proteins. There are four levels of protein structure primary, secondary, tertiary, and quaternary. The primary structure of a protein refers to the sequence of amino acids in the polypeptide chain. The primary structure is held together by peptide bonds that are made during the process of protein biosynthesis. The term secondary structure refers to the interaction of the hydrogen bond donor and acceptor residues of the repeating peptide unit. The two most important secondary structures of proteins, the alpha helix and the beta sheet. The tertiary structure will have a single polypeptide chain "backbone" with one or more protein secondary structures, the protein domains. Protein quaternary structure is the number and arrangement of multiple folded protein subunits in a multi-subunit complex.