The below mentioned article provides an useful note on secondary metabolites in plants.
The organic compounds such as carbohydrates, proteins, fats, membrane lipids, nucleic acids, chlorophylls and hemes are found throughout the plant kingdom and are central to the metabolism of plants. These compounds are known as primary metabolites. Apart from these substances, many plants particularly those of certain genera and families synthesize a number of organic compounds in them which are not in the mainstream of metabolism and appear to have no direct function in growth and development of plants.
These compounds are extremely numerous and chemically diverse in nature and are called as secondary metabolites or secondary plant products or natural products and include such well known substances as alkaloids, terpenes, (including steroids and rubber), tannins, flavonoids etc.
According to Street and Cockburn (1972) “secondary plant products are compounds which have not so far been shown to be involved in primary metabolism; as far as their functions can at present be assessed they are accessory rather than central to the physiology of the plants in which they occur”.
Secondary plant products are discontinuously distributed throughout the plant kingdom.
Although previously unknown, the role of secondary metabolites in plant defenses against herbivores and pathogens is being increasingly recognised now. Some of these are also known to attract animals for pollination and seed dispersal and as agents of plant-plant competition. Their importance in making medicinal drugs, poisons (insecticides), flavours and industrial materials on commercial scale is already well established.
Secondary metabolites or secondary plant products may be classified into three major groups:
(i) Isoprenoid compounds or terpenes e.g., essential oils, steroids, rubber etc.
(ii) Nitrogen containing secondary metabolites e.g., alkaloids, non-protein amino acids etc.
(iii) Phenolic compounds or phenolics e.g., lignin, tannins, flavonoids etc.
Although extremely numerous and chemically diverse, most secondary metabolites have their origin in a relatively few areas of primary metabolism. Isoprenoids or terpenes which show properties of lipids are synthesized almost entirely from acetyl-CoA through mevalonic acid pathway.
Phenolics are aromatic compounds and are synthesised in plants either:
(i) From acetyl- CoA via malonic acid pathway or
(ii) From erythrose-4-phosphate & phosphoenol-pyruvate via shikimic acid pathway.
The nitrogen containing secondary metabolites such as alkaloids are synthesized in plants primarily from amino acids. An overview of the major biosynthetic pathways of secondary metabolites and their interrelationships with primary metabolism in plants is given in Fig. 24.1.