 Plants remain the most important source of natural drugs, however only
about 10% have been fully studied.
- More than 30% of prescription drugs are natural products.
- More than 60% of anticancer and anti-infective drugs are natural products.
The main sources of drugs are as follows:
1- Natural sources of drugs include:
plants, microorganisms, marine plants, animals, (totally obtained from nature).
2- Semisynthetic substances:
Such as steroidal hormones and
corticosteroids.
3- Synthetic substances:
These are drugs which are manufactured by total synthesis
Why do we need to know the biological activities and / or chemical constituents of
plants?
1- The discovery of new therapeutic agents
2- The disclosing of new sources of economic materials
complex chemical substances
for the synthesis of
3- Isolation of a novel chemical structure often leads the chemist to a successful
synthesis of a series of synthetic compounds which may have some medicinal
value.
Plant constituents may also be known as plant metabolites
Primary and secondary plant metabolites
Plant constituents comprise a wide variety of organic substances that are
formed and accumulated by plants. They include:
I-Primary metabolites which are produced by primary metabolism
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 The primary metabolites such as carbohydrates, proteins, fats, and nucleic
acids, are essential for life and are commonly present in all organisms in
large amounts.
■ Life cannot exist without primary metabolism
■ II-Secondary metabolites , also known as natural products, are those
products of metabolism that are not essential for normal growth,
development or reproduction of an organism. In this sense they are
"secondary".
■
Each plant family, genus, and species produces a characteristic
mix of these secondary metabolites, and they can sometimes be used as
taxonomic characters in classifying plants.
■ Humans use many of these compounds as medicines due to their variable
biological activities.
■
-Secondary metabolites are rare in animals, but are common in:
plantsm fungi and bacteria.
 Secondary metabolites, may be produced in the plant as :
 Defense against predators,
 Detoxifying agents.
 They include most of the pharmacologically active natural plant products.
They are usually produced in small quantity in the plant.

However, some groups of natural products could be assigned to both
divisions e.g. fatty acids and sugars. Most of these compounds are described
as primary metabolites, whilst some representatives are of rare occurrence
and characteristic to certain plant species and are thus closer to secondary
metabolites.
Factors influencing the production of plant secondary metabolites
■
Heredity or genetic composition that induces both qualitative and
quantitative changes.
■ Stage of development , and
■ Environmental changes that result mainly in quantitative variations.
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Biogenesis of secondary plant metabolites
Biogenetic pathways
 The pathways involved in the biosynthesis or biogenesis of the different types
of the secondary plant constituents are dependent on the fundamental
metabolic cycles of the living tissue.
 These are summarized in Scheme (1).
Classification of plant secondary metabolites
1-Phenolic compounds
 These include a wide range of plant substances, which are recognized by
their hydrophilic nature and their common origin from the aromatic
precursor shikimic acid.
 They possess at least one aromatic ring with one or more oxygenation sites.
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 Examples of plant phenolics are the flavonoids and their glycosides, the
phenyl propanoids, anthocyanins, xanthones, tannins and quinones.
 Many of the phenolic compounds are reported to have antioxidant and
anticancer activities.
2-Terpenoids
 These are characterized by being lipophilic in nature.
 They are sub-classified according to the number of their isopentene (or
isoprene, C = 5) building units into:
mono- (C = 10), sesqui- (C = 15), di-(C = 20), tri- (C = 30), tetra (C =
40), and poly (C = ) terpenoids.
 Mono- and sesqui-terpenoids are the main constituents of volatile oils,
carotenoids are example of tetraterpenoids.
 Steroids are derived from isoprene through formation of squalene.
 Terpenes are reported for their antimicrobial, anticancer activities and some
also affect the CNS.
 Example of monoterpene
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
5


3-Organic acids, lipids and related compounds:
They have common acetate precursor.
 Examples of simple organic acids accumulating in plants are citric, malic,
fumaric, oxalic, tartaric, malonic, shikimic, quinic and ascorbic acids.
 Fatty acids occur mainly in esterified forms with glycerol or higher aliphatic
alcohols in the form of fixed oils, fats or waxes.
4-Nitrogen containing compounds
 These are distinguished from other classes by being usually basic.
 They give a positive response to either ninhydrin or Dragendorff's reagents.
 Examples are amines, alkaloids, cyanogenic glycosides, and chlorophylls.
5-Water-soluble carbohydrates and their derivatives:

This group includes the mono- and oligosaccharides and the watersoluble glycosides.
 6-Macromolecules:
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
These can be separated from other plant constituents based on their high
molecular weight such as the nucleic acids, proteins and polysaccharides.
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