Generally, the naturally occurring living plant could be regarded as the most sophisticated and meticulously designed biosynthetic laboratory not only confined to the primary metabolites such as: Amino acids, carbohydrates, terpenes, fatty acids which are mostly consumed as a source of edible food material by human beings, but also for a plethora of secondary metabolites of enormous pharmaceutical significance, for instance: glycosides, flavonoids, alkaloids, essential oils and the like. Interestingly, such naturally found chemical substances which specifically attribute
plant drugs their marked and pronounced therapeutic activities are collectively termed as ‘phytopharmaceuticals’. Therefore, a higher plant is nothing but an intricate solar energised biochemical reactor that is exclusively responsible for the mass production of primary as well as secondary metabolites from air, water, minerals and sunlight - a source of UV radiations.
However, the primary metabolites are more or less widely distributed in nature practically in all organisms that are essentially required for the overall growth as well as physiological development by virtue of their basic cell metabolism. Nevertheless, the secondary metabolites are biosynthetically engineered products solely derived from the primary ones and are confined in their distribution strategically ie; being restricted to a particular taxononic group. These products may be regarded either as various chemical adaptations to environmental stresses or they may be considered as nature’s protective, defensive or offensive chemical entities against the host of microorganism, fungi, insects
and higher herbivorous predators.
Thus, with regard to cellular economic cognizance the secondary products are mostly tedious to form and subsequently accumulate, and hence, invariably show up in the plant kingdom in relatively much lesser amounts in comparison to the primary metabolites. The secondary metabolites are also regarded to be as the waste products of the plant metabolic processes.
The different biosynthetic reactions taking place in the plant cells are based on certain enzymes. In fact, it is the control of enzymatic activity on the plant metabolism which ultimately governs a specific biosynthetic pathway. In general, the enzymatic reactionbs in plants ae reversible. Under the influence of specific enzymes the secondary metabolites are either synthesized or hydrolysed in plants.
The biosynthetic pathways in plants may be duly elucidated and extensively studied by the aid of isotopically labelled precursors. Nowadays, with the advent of ‘tracer technology’, it is a lot easier to introduce isotopes into the anticipated precursors of plant metabolites and employed as specific ‘markers’ in the elaborated biogenetic experiments. It is now quite possible to unfold the mysteries of biosynthetic pathways with the use of radioactive carbon (14C), hydrogen (3H), sulphur (35S) and phosphorus (32P).
The biosynthesis of different categories of glycosides shall be discussed briefly in the sections that follow.

Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar

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