Nitrogen (N) neighbors carbon in the periodic table and has many similarities in terms of biological chemistry. It too is highly abundant in the universe and is similarly abundant as carbon on Earth. It too is a major element of life and is found in all three phases in ecosystems. Its redox reactions include N-fixation, denitrification, and nitrification. With a valence state of 3, it is not quite as capable as C in forming complex 3-D shapes. The soft tissue of living organisms is generally 2-10% N by dry mass. This relatively large value is due to the fact that molecules needed in large quantities in the cell, such as proteins, nucleic acids, and pigments, contain N.

Chief among the important N-contained biomolecules are proteins, which are about 15% N by mass. Proteins themselves can serve a structural role but, as enzymes, they also drive nearly all the biochemical activities of a cell. Nucleic acids also are about 15% N by mass. In the form of DNA, nucleic acids make up the genetic code, though DNA is not a large fraction of cell mass. Even more important to ecological stoichiometry is the more abundant RNA (especially rRNA), which is critically important to cell growth and thus evolutionary fitness. All organisms require abundant proteins and nucleic acids to live and grow. The lowest N-content in living things is generally found in very metabolically inactive creatures.

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