amino acid - "the building blocks of proteins, which are linear chains of amino acids. Amino acids can be linked together in varying sequences to form a vast variety of proteins. Twenty-two amino acids are naturally incorporated into polypeptides and are called proteinogenic or standard amino acids. Of these, 20 are encoded by the universal genetic code. Eight standard amino acids are called essential for humans because they cannot be created from other compounds by the human body, and so must be taken in as food" (See Wikipedia)