Microorganisms and Plants can synthesize all of the 20 standard amino acids. Mammals, furthermore, cannot synthesize all 20 and must obtain some of them in their diet.  Those  amino  acids  which  are  supplied  in  the  diet  are  referred  to  as essential,  whereas  the remainder  which can be synthesized  by the organism  are termed nonessential.  This designation refers to the needed of an organism under a particular group of conditions.  In the humans nonessential amino acids are Arg, Gly, Asn, Ala, Cys, Asp, Pro, Glu, Gln, Ser and Tyr, thus the essential ones are Leu, His, Ile, Thr, Lys, Met, Phe, Trp and Val for the structures of the 20 standard amino acids.