Secondary plant metabolites, physiological effects

There are other small biomolecules, specific to certain types of cells or organisms. For example, vascular plants contain, in addition to the universal set, small molecules called secondary metabolites, which play a role specific to plant life. These metabolites include compounds that give plants their characteristic scents, and compounds such as morphine, quinine, nicotine, and caffeine that are valued for their physiological effects on humans but used for other purposes by plants. The entire collection of small molecules in a given cell has been called that cell s metabolome, in parallel with the term genome (defined earlier and expanded on in... 

As we have discussed earlier that alkaloids are responsible for physiological effects in man or animals. The physiological effects are due to secondary metabolites arising from bio-chemical pathways operating in the plant cell. Alkaloids constitute the largest group of secondary chemical constituents. Basic structures of alkaloids are depicted below ... 

Numerous microorganisms, plants, and animals produce chemicals that may be toxic to other organisms. The diversity of the structures and of the biological activities of these chemicals has been of considerable interest to researchers in numerous scientific disciplines, from synthetic organic chemistry to therapeutics. Most of these substances are not obviously associated with physiologically important functions and are, therefore, classified as secondary metabolites. Toxins, in general, are both proteinaceous and nonproteinaceous materials that exert deleterious physiological effects at relatively low doses. With the exception of venoms, the functions of many of these materials are not well defined. Many are suspected to be involved in defense mechanisms. 

The term essential oil does not reflect its role in the plant s functioning and metabolism. Plant volatile oils funcbon as secondary metabolites. (A metabolite is a compound produced by the plant s metabolism.) Primary metabolites are the compounds needed for the plant to live and include the food substances produced in photosynthesis. The secondary metabolites vary widely in chemical structure and serve a variety of purposes within the plant. They include protective, survival and reproductive roles. However, they are responsible for giving a plant its aroma and flavour and have significant physiological and psychological effects on animals and people. 

Consumption of diets rich in fruits is associated with a lowered risk of cancer and cancer mortahty [1], Antioxidants in fruits contribute to their protective effect as anticarcinogens. These protective effects are attributed chiefly by ubiquitous groups of plant secondary metabolites such as phenolic compounds in addition to antioxidant vitamins C and E. Polyphenols or phenolic compounds are a widespread group of ph3dochemicals that are shown to possess different physiological activities associated with their chemical structures. In plants, phenolic compounds are generally involved in protection or defense... 

Studies evaluating the potential for plant secondary metabolites to alter disease risk through effects on relevant cells and tissues other than those associated with the oral cavity and the gastrointestinal tract should assess the impact of the known metabolites present in vivo rather than the chemical forms that exist only in foods. Furthermore, the pharmacokinetic characteristics of the metabolites should be taken into consideration and efforts made to establish their biological activities at relevant physiologic concentrations. 

---