There are 20 different kinds of amino acids. Adult humans can only produce 11 of them, so the other nine must be consumed via diet. The amino acids that humans cannot produce are called essential amino acids because they are needed for the body to function.
The essential amino acids are histidine, threonine, isoleucine, methionine, leucine, lysine, phenylalanine, tryptophan, and valine. Arginine's nature as an essential amino acid in children is debated, some sources considering it essential and some considering it semi-essential. The nonessential amino acids are alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine. All amino acids are used by both plants and animals to build proteins for muscle and chemicals meant for signaling within the body.
Essential amino acids can be obtained by eating plant protein. Plants, unlike animals, are able to produce all their amino acids by themselves. However, not all of the amino acids are always found together in the same plant tissue. Frances Moore Lappe, an influential writer on vegetarian nutrition, recommends that different sources of plant protein be combined to produce what she calls a "complete" protein containing all essential amino acids, but this is not strictly necessary. Of course, protein from animals contains all essential amino acids naturally.