Plant-derived organic compounds may be classified as either primary or secondary metabolites (SMs). Macromolecules such as structural and functional proteins and informational biopolymers such as nucleic acids are generally excluded from this classification. Primary metabolites (PMs) are widely distributed in virtually all organisms in different forms for storage and are also needed for general growth and physiological development, because of their role in primary cell metabolism. Several PMs are being harvested from higher plants since earliest times. Vegetable oils, fatty acids used in soap and detergent making, sucrose, starch, pectin, hydrocolloid gums and cotton are some of these used as industrial raw materials, foods or food additives. Rarely are PMs used as intermediates in the manufacture of semi-synthetic pharmaceutical products.

SMs are biosynthetically derived from primary metabolites, but are more limited in distribution in the plant kingdom, usually being restricted to a particular taxonomic group. Since they are not nutritive and not directly essential for growth, they have no obvious role in the plant primary or mainstream metabolism.

There have been a number of studies to investigate the physicochemical parameters of SM in recent years, and it has been concluded that

• ‘Libraries’, or collections of these substances, tend to afford a higher degree of ‘drug-likeness’ when compared to compounds in either synthetic or combinatorial ‘libraries’
• Produced by living systems, SMs are subject to transport and diffusion at the cellular level. They are thus capable of modulating protein-protein interactions and therefore can affect cellular processes that may be modified in disease states.
• Compared to synthetic compounds, plant SMs have more protonated amine and free hydroxyl functionalities and more single bonds, with a greater number of fused rings containing more chiral centres.
• They also differ from synthetic products in the average number of halogen, nitrogen, oxygen and sulphur atoms, in addition to their steric complexity.
• They play ecologically significant roles in how plants deal with their environment and are therefore important in their ultimate survival. Plant SMs serve as pollinator attractants or represent chemical adaptations to environmental stress, or they may serve as defensive, protective or offensive chemicals against other higher plants.
• As they serve to combat infectious diseases, aid in weed aggressiveness and discourage herbivores and herbivory, they are by definition biologically/physiologically active compounds.
• SMs being metabolically expensive to produce and accumulate, they are present in plants in much smaller quantities than are PMs.
• Unlike PMs they are biosynthesized in specialized cell types and at distinct developmental stages, making their extraction, isolation and purification difficult.
• Due to their powerful biological activity, they are also economically important as pharmaceuticals, flavours, fragrances, pesticides, etc. Steroidal sapogenins and cardioactive glycosides, alkaloids including anti-cancer Catharanthus alkaloids, cocaine, colchicine, opium alkaloids, physostigmine, pilocarpine, quinine, quinidine, reserpine and d-tubocurarine are a few examples.
• Other SMs are being used in limited quantities as pharmacological tools to study various biochemical processes such as phorbol-type diterpenoid esters from croton oil and from lattices of various species of Euphorbia—potent irritant and carcinogens useful in the study of chemical carcinogenesis.