Biomolecules contain mainly carbon, which behaves as it always does in organic compounds, forming 4 bonds, usually with a tetrahedral arrangement. (PP 2) The carbon skeleton can be linear, branched, cyclic, or aromatic. Other important elements are H, O, N, P and S. About 30 elements are required by biological systems, including iodine and many metals, though most of these are needed in only tr…