Compounds are assigned to classes based on their chemical and structural properties: Synthetic compounds Low molecular weight, non-polymeric compounds produced via either total synthesis or semi-synthetic processes. Small organic molecules Low molecular weight, non-peptidic, biogenic compounds produced by life processes (normally endogenous and of animal origin) and their close analogues. Peptides Synthetic, semi-synthetic and natural short polymers of amino acids linked by peptide bonds (oligopeptides and cyclic peptides). Inorganic compounds Ions and other inorganic compounds. Lipids a broad group of naturally occurring molecules which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules. Amine organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Hormone a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism.