2. Plant Secondary Metabolites
• Plants make a variety of less widely distributed
compounds such as morphine, caffeine, nicotine,
menthol, and rubber. These compounds are the
products of secondary metabolism, which is the
metabolism of chemicals that occurs irregularly or
rarely among plants, and that have no known
general metabolic role in plants.
• Secondary metabolites or secondary compounds
are compounds that are not required for normal
growth and development, and are not made
through metabolic pathways common to all plants.
• Most plants have not been examined for secondary
compounds and new compounds are discovered
almost daily.
3. Plant Secondary Metabolites
• Secondary compounds are grouped into classes
based on similar structures, biosynthetic pathways,
or the kinds of plants that make them. The largest
such classes are the alkaloids, terpenoids, and
phenolics.
• Secondary compounds often occur in combination
with one or more sugars. These combination
molecules are known as glycosides. Usually the
sugar is a glucose, galactose or rhamnose. But
some plants have unique sugars. Apiose sugar is
unique to parsley and its close relatives.
4. Functions of Secondary Compounds
• The most common roles for secondary compounds
in plants are ecological roles that govern
interactions between plants and other organisms.
• Many secondary compounds are brightly colored
pigments like anthocyanin that color flowers red
and blue. These attract pollinators and fruit and
seed dispersers.
• Nicotine and other toxic compounds may protect
the plant from herbivores and microbes.
• Other secondary compounds like rubber and
tetrahydrocannabinil (THC) from cannabis plants
have no known function in plants.
5. Alkaloids
• Alkaloids generally include alkaline substances
that have nitrogen as part of a ring structure.
More than 6500 alkaloids are known and are the
largest class of secondary compounds. They are
very common in certain plant families, especially:
• Fabaceae – peas and beans
• Asteraceae - sunflowers
• Papaveraceae - poppies
• Solanaceae – nightshade, tomato
• Apocynaceae - dogbanes
• Asclepiadaceae - milkweeds
• Rutaceae - citrus
6.
7. Terpenoids
• Terpenoids are dimers and polymers of 5 carbon
precursors called isoprene units (C5 H8).
• Terpenoids often evaporate from plants and
contribute to the haze we see on hot sunny days.
They are expensive to make; they often take 2% of
the carbon fixed in photosynthesis; carbon that
could otherwise be used for sugars.
8.
9. Phenolics
• Compounds that contain a fully unsaturated six
carbon ring linked to an oxygen are called
phenolics.
• Salicylic acid (basic part of aspirin) is a simple
phenol.
• Myristicin is a more complex phenol that provides
the flavor of nutmeg.
• Flavonoids are complex phenolics. They are often
sold in health food stores as supplements to
vitamin C. The most commonly available
flavonoid is rutin from buckwheat.
• Anthocyanins are a type of flavonoid that give
flowers red and blue pigments.
10. More Phenolics
• Some phenolics form polymers.
• Tannins are astringent to the taste. They give
dryness (astringency) to dry wines. They can also
be used to tan leather. They often give water a
tea-colored look. Tannins are common in pines
and oaks.
• Lignin is a major structural component of wood.
The exact structure of lignin is complex and not
known.
11.
12. Minor Secondary Metabolites
• Mustard oil glycosides are nitrogen-sulfur containing
compounds that occur in cabbage, broccoli, horseradish,
watercress and other members of the mustard family
(Brassicaceae). They give the group its characteristic taste
and odor.
• Cyanogenic glycosides occur in several families of plants,
but are especially common in roses (Rosaceae) and peas
(Fabaceae). They are sugar containing compounds that
release cyanide gas when hydrolyzed.
• Cardiac glycosides effect vertebrate heart rate. Especially
common in milkweeds Asclepiadaceae.
• The parsley/carrot family Apiaceae is noted for having
aromatic and poisonous 17 carbon polyacetylenes, though
a few species have alkaloids like Coniium.
18. What is Ethnobotany?
• Ethnobotany is the
study of plants used by
primitive and
aboriginal people. –
John W. Harshberger
1895
19. What is Ethnobotany?
• A better definition is:
Ethnobotany is the study
of the interactions of
plants and people,
including the influence
of plants on human
culture.
Oaxaca, Mexico
