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Nitrogen occurs in all organisms, primarily in amino acids (and thus proteins), in the nucleic acids (DNA and RNA) and in the energy transfer molecule adenosine triphosphate. The human body contains about 3% by mass of nitrogen, the fourth most abundant element in the body after oxygen, carbon, and hydrogen.
All living things contain organic macromolecules: Lipids, proteins, carbohydrates and nucleic acids. Characteristic for these organic molecules is that they are made up of only a small number of elements: carbon, hydrogen, oxygen, and to smaller amounts nitrogen, phosphorus and sulfur. There are 20 different amino acids that combine to form polypeptides (proteins). 2  The different amino acids are similar in structure: at the center of the molecule is the alpha carbon that is connected to an amino group, a carboxyl group, a hydrogen atom and the R group (the side chain).