Most microorganisms and plants can biosynthesize all 20 standard amino acids , while animals ( including humans ) must obtain some of the amino acids from the diet . The amino acids that an organism cannot synthesize on its own are referred to as essential amino acids . Key enzymes that synthesize certain amino acids are not present in animals — such as aspartokinase , which catalyse the first step in the synthesis of lysine , methionine , and threonine from aspartate . If amino acids are present in the environment , microorganisms can conserve energy by taking up the amino acids from their surroundings and downregulating their biosynthetic pathways .