Secondary metabolites, also known as natural products, are those products (chemical compounds) of metabolism that are not essential for normal growth, development or reproduction of an organism. In this sense they are "secondary".
The function or importance of these compounds to the organism's development is usually of ecological nature as they are used as defence against predators (herbivores, pathogens etc.), for interspecies competition, and to facilitate the reproductive processes.
Contrary to primary metabolites these compounds are not ubiquitous in the living organisms who produce them nor are they necessarily expressed continuously. Although plants are better known as a source of secondary metabolites, bacteria, fungi and many marine organisms (sponges, tunicates, corals, snails) are very interesting sources, too.
Secondary metabolites can be classified by their chemical structure or physical properties into one or more of the following groups: alkaloids, terpenoids, polyketides, aliphatic, aromatic, and heteroaromatic organic acids, phenols, iridoids , steroids, saponins, peptides, ethereal oils, resins and balsams.
List of most important secondary metabolites of plants: