In humans, there are nine essential amino acids. An essential amino acid is one that cannot be manufactured in the body; it must come from the food humans eat. The other 11 amino acids are considered to be nonessential.
The nine essential amino acids for humans are histidine, leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Other organisms have different essential amino acids. Nonessential acids do not need to come from food because they can be made by the body. Only 20 amino acids are used by all life forms; however, 500 different amino acids exist on Earth.
Proteins come from a string of monomers, or building blocks, called amino acids. These amino acids combine with one another to produce the large number of proteins used for various structural and functional needs in the body.