 Most microorganisms and plants can <unk> 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 <unk> , which <unk> the first step in the synthesis of <unk> , <unk> , and <unk> from <unk> . If amino acids are present in the environment , microorganisms can conserve energy by taking up the amino acids from their surroundings and <unk> their <unk> pathways . 
