The sum of all of the chemical reactions that take place in an organism is called metabolism. Most of that carbon, nitrogen, and energy ends up in molecules that are common to all cells and are required for the proper functioning of cells and organisms. These molecules, e.g., lipids, proteins, nucleic acids, and carbohydrates, are called primary metabolites. Unlike animals, however, most plants divert a significant proportion of assimilated carbon and energy to the synthesis of organic molecules that may have no obvious role in normal cell function. These molecules are known as secondary metabolites.
The distinction between primary and secondary metabolites is not always easily made. At the biosynthetic level, primary and secondary metabolites share many of the same intermediates and are derived from the same core metabolic pathways. Secondary metabolites generally, but not always, occur in relatively low quantities and their production may be widespread or restricted to particular families, genera, or even species. They were known, however, to have significant economic and medicinal value and were thus of more than a passing interest to natural products chemists. In recent years, however, it has become increasingly evident that many natural products do have significant ecological functions, such as protection against microbial or insect attack.
Secondary metabolites
For many years the adaptive significance of most secondary metabolites was unknown. These compounds were thought to be simply functionless end products of metabolism, or metabolic wastes. Today we know that many secondary metabolites have important ecological functions in plants:
They protect plants against being eaten by herbivores and against being infected by microbial pathogens.
They serve as attractants (odor, color, taste) for pollinators and seed-dispersing animals.
They function as agents of plant-plant competition and plant-microbe symbioses.
The ability of plants to compete and survive is therefore profoundly affected by the ecological functions of their secondary metabolites.