Amino acids are organic compounds that contain amine (-NH2) and carboxyl (-COOH) functional groups, as well as a side-chain (R group) which confers uniqueness to each amino acid. The main elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen, although other elements can be found in some amino acids. About 500 amino acids are known, but only 20 are coded in the human genome. Amino acids are the monomers which are joined together to make short polymer chains called peptides or longer chains called proteins. Non-protein amino acids play important roles in the formation of biologically important molecules. For example, tryptophan is processed into the neurotransmitter serotonin, while tyrosine (and its precursor phenylalanine) are processed into neurotransmitters dopamine, epinephrine and norepinephrine. When consumed and absorbed by the human body, the standard amino acids are used to make proteins and other molecules or are oxidized to urea and carbon dioxide to be used as a form of energy. The oxidation pathway begins with transamidase removal of the amino group, and this group is then processed through the urea cycle. The other transamidation product is a keto acid that is used for the citric acid cycle. Through the process of gluconeogenesis, some amino acids can also be converted into glucose. Out of the standard 20 amino acids, nine amino acids (His, Ile, Leu, Lys, Met, Phe, Thr, Trp and Val) are considered to be essential amino acids because the human body cannot make them from other molecules in enough amounts needed for normal growth, so they must be obtained from food sources.