Page "Amino acid" Paragraph 27

Pyrrolysine trait is restricted to several microbes, and only one organism has both Pyl and Sec.

Of the 22 standard amino acids, 9 are called essential amino acids because the human body cannot synthesize them from other compounds at the level needed for normal growth, so they must be obtained from food.

In addition, cysteine, taurine, tyrosine, and arginine are semiessential amino-acids in children, because the metabolic pathways that synthesize these amino acids are not fully developed.

The amounts required also depend on the age and health of the individual, so it is hard to make general statements about the dietary requirement for some amino acids.