Individual proteins are made up of amino acids joined together in chains of various lengths. There are 20 naturally occurring amino acids. Monopeptides are proteins made from single amino acids. Dipeptides are proteins made from two amino acids. Tripeptides are composed of three amino acids.
Polypeptides are proteins made from many amino acids.
At least nine of the amino acids cannot be manufactured by the body and must be supplied by diet. These are called essential amino acids. The essential amino acids are leucine, isoleucine, lysine, phenylalanine, threonine, tryptophan, valine, methionine, and histidine. The remaining amino acids are called non-essential because they can be manufactured in the body as long as there is available nitrogen. The non-essential amino acids are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, and glutamine, glycine, proline, serine, and tyrosine.