The elements themselves have distinct properties that influence their roles in biological systems. For example, the ability of a C atom to form strong, stable covalent bonds with C itself and with other major elements (e.g., H, N, O, S, and P) explains why C forms the backbone of a variety of kinetically stable polymers that serve as membranes and other structural biomolecules. In addition, C-C bonds have the highest bond energy of all solid nontransitional elements, which makes C atoms well-suited for their prominent role in energy storage molecules. These biochemical features help to explain why C is often the most abundant element in the dry mass of organisms. Nitrogen is typically the fourth most abundant component of organism dry mass (after C, H, and O). Biological N tends to occur in a reduced state bound to C and H; the resulting amine group (R-NH2) is basic. The combination of this basic amine group with an acid carboxyl group is the peptide bond that links amino acids in proteins. At neutral pH, the amine group is protonated, providing one of the few sources of positive charge in biomolecules. In addition, N-H bonds are polar and readily form H bonds to other polar groups; these bonds are critical for determining the structure of proteins and nucleic acids. Phosphorus is typically the fifth or sixth most abundant element in organisms. It occurs almost exclusively in the oxidized phosphate form (HPO2~) in biological systems. As bound phosphate, it can function in several important ways, including (1) as a link between subunits in large molecular assemblies (e.g., DNA, RNA, and phospholipids), (2) as a carrier of chemical energy (e.g., adenosine 5'-triphosphate, ATP) and substrates (e.g., glucose phosphate), (3) as a signaling mechanism (e.g., cyclic-adenosine monophosphate, c-AMP), and (4) as a component of biominerals (e.g., calcium phosphate). Phosphate serves as an effective link in nucleic acids and a center of mobility in lipids because it readily reacts with attacking reagents. This flexibility is essential to RNA, which must have the capacity to turn over quickly in order to be effective in carrying rapidly reproducible information.
The distinctive chemical properties of C, N, and P suggest major differences in elemental composition among organisms which will be associated with differences in capacities for meeting environmental challenges. However, the connection between elemental composition and biological function becomes much more tangible with a general description of how elements are distributed among biomolecules, cellular components, and tissues.
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