The living cells or organisms are composed of a variety of organic and inorganic compounds of different (smaller or larger) molecular size. These are generally involved in the particular chemical reactions occurring inside the cell or body. These reactions are called as biochemical reactions. The sum of all these reactions constitutes metabolism. The reactions participating in the metabolism are termed as metabolic reactions, and all these chemical compounds are called metabolites. Due to the biological origin of metabolites, they are called bio-molecules.
On the basis of vitality for the cell or organism, these metabolites are categorized as primary and secondary metabolites.
The primary metabolites include amino acids, sugars, nucleotides etc., which are essentially vital for the cell. This kind of metabolites are universal, i.e., found in all type of cells or organisms. They are directly involved in normal physiological and biochemical processes of the body.
In contrast, there are other small molecules, specific to the particular type of cells, are called secondary metabolites. These are not vital, and more appropriately, their direct role or exact function is not known yet in many cases. Many of these metabolites are useful for humans, and certain secondary metabolites have ecological importance. For example;
antibiotics, spices, scents, essential oils
drugs like vinblastine, curcumin
polymeric substances like rubber, gums, cellulose
lectins such as concanavalin A
pigments like carotenoids, anthocyanins
alkaloids like morphine, codeine
terpenoids like mono-, diterpenes
toxins like abrin, ricin.
Drugs like vinblastine and vincristine are obtained from plant Catharanthus roseus (Vinca rosea) to treat cancer.
Curcumin, a polyphenolic compound, is isolated from herbaceous perennial plant Curcuma longa (turmeric). It has strong anti-oxidative and anti-inflammatory property, hence being used in various kinds of diseases.