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 trace amounts. (PP 3) Biomolecules contain the same types of functional groups as do organic molecules, including hydroxyl groups, amino groups, carbonyl groups, carboxyl groups, etc. (PP 4-5) However, many biomolecules are polyfunctional, containing two or more different functional groups which can influence each other’s reactivity. (PP 6) Biomolecules tend to be larger than typical organic molecules. Small biomolecules have molecular weights over 100, while most biomolecules have molecular weights in the thousands, millions, or even billions. Because of their large size, the majority of biomolecules have specific 3-dimensional shapes. The atoms of a biomolecule are arranged in space in a precise way, and proper arrangement is usually needed for proper function. The 3-dimensional shape is maintained by numerous non-covalent bonds between atoms in the molecule. (PP 7) Because of the weak nature of most noncovalent bonds and because of interactions between the biomolecule and the solvent, the biomolecule’s structure is flexible rather than static.