 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 catalyses 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 . 
 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 <a href='null_relcl' title='1'><mark>that</mark></a>an organism cannot synthesize on its own are referred to as essential amino acids . Key enzymes <a href='null_relcl' title='2'><mark>that</mark></a>synthesize certain amino acids <a href='negative_concord' title='3'><mark>are</mark></a> <a href='negative_concord' title='4'><mark>not</mark></a>present in animals — such as aspartokinase , which <a href='uninflect' title='5'><mark>catalyses</mark></a> the first step in the synthesis of lysine , methionine , and threonine from aspartate . If amino acids <a href='drop_aux' title='6'><mark>are</mark></a>present in the environment , microorganisms can conserve energy by taking up the amino acids from their surroundings and downregulating their biosynthetic pathways . 
Most microorganisms and plants can biosynthesize alll 20 standard amino acids , while animals ( including humans ) must obtain some of the amino acids from da diet . The amino acids an organism cannot synthesize on its own r referred to as essential amino acids . Key enzymes synthesize certain amino acids ain't present in animals — such as aspartokinase , which catalyse the first step in the synthesis of lysine , methionine , and threonine from aspartate . If amino acids present in the environment , microorganisms can conserve energy by takin up tdaamino acids from their surroundings and downregulating their biosynthetic pathways .