A polypeptide is a string of amino acids linked together. Amino acids are the basic building blocks of proteins. A single polypeptide chain might make up the entire primary structure of a simple protein; more complex proteins are formed when two or more polypeptides link together.

Proteins consist of amino acids which are characterized by the -CH(NH2)COOH substructure. Nitrogen and two hydrogens comprise the amino group, -NH2, and the acidentity is the carboxyl group, -COOH. Amino acids link to each when the carboxyl group of one molecule reacts with the amino group of another molecule, creating a peptide bond -C(=O)NH- and releasing a molecule of water (H2O). Amino acids are the basic building blocks of enzymes, hormones, proteins, and body tissues.

Apeptide is a compound consisting of 2 or more amino acids. Oligopeptides have 10 or fewer amino acids. Polypeptides and proteins are chains of 10 or more amino acids, but peptides consisting of more than 50 amino acids are classified as proteins.

Many structures of the body are formed from protein. Hair and nails are made of keratins which are long protein chains containing a high percentage (15%-17%) of the amino acid cysteine. Keratins are also components of animal claws, horns, feathers, scales, and hooves. Collagen is the most common protein in the body and comprises approximately 20-30% of all body proteins.

It is found in tendons, ligaments, and many tissues that serve structural or mechanical functions. Collagen consists of amino acid sequences that coil into a triple helical structure to form very strong fibers. Glycine and proline account for about 50% of the amino acids in collagen. Gelatin is produced by boiling collagen for a long time until it becomes water soluble and gummy.