Plants produce terpenes as secondary metabolites to defend against herbivores and pathogens. Terpenes are the largest class of plant secondary metabolites and are formed from polymerization of isoprene units. They include mono-, sesqui-, and diterpenes. Many terpenes have antimicrobial or insecticidal properties and are toxic to herbivores. Some terpenes produced as essential oils in plants can act as feeding deterrents. Terpenes also play roles in plant growth and development. Additionally, some terpene compounds released from forests can increase cloud thickness and reflect sunlight, acting as a natural cooling mechanism for the planet.

1. TERPENES

2. A Natural Mechanism
• In natural habitats plants are surrounded by
number of enemies such as bacteria, viruses, fungi
nematodes, insects and herbivores animals.
• Plants can not avoid themselves by moving away
from these enemies.
• They protect themselves in other ways, e.g.
Cuticle(waxy outer layer)
• Periderm (secondary protective tissue)
• which provide barrier to bacterial and fungal
activity .

3. A Natural Mechanism
• Plants also defend themselves against
herbivores and pathogenic microbes by
producing some defensive chemical compound
called secondary compounds.
• These secondary compounds may serve other
important functions such as;
• Structural support (lignin)
• or
• Pigments (anthocyanins).

4. Metabolites
• Primary metabolites
• Primary metabolites are produce in all plants
and have direct role in growth and development
e.g. amino acids, nucleotides and sugars etc.
• Secondary metabolites
• A diverse array of organic compounds have no
direct function in growth and development and
have restricted distribution in plant kingdom e.g.
limonene, carotinoids and anthocyanins etc.

5. Classes of Secondary
Metabolites
• Three major classes of secondary
metabolites
– 1- Terpenes
– 2- Phenolics
– 3- Nitrogen containing compounds

7. Terpenes and Terpenoids
• Terpenes
• are largest class of secondary metabolite,
some are volatile and insoluble in water,
biosynthesize from acetyle-Co A or glycolytic
intermediate.
• Terpenoids
– are terpene like substances or derivatives of
terpenes or modified terpenes.

8. Nomenclature
• All terpenes are formed by union of five carbon
elements that is iso-pentane
• All basic structure of terpenes are called
isoprene units, because terpenes can
decompose on high temperature to give
isoprenes.
These five carbon units polymerize to give rise to
large number of terpenes.

9. Classification of Terpenes
TYPE OF NUMBER OF ISOPRENE
TERPENE CARBON ATOMS UNITS
Hemiterpene
Monoterpene
Sesquiterpene
Diterpene
Triterpene
Tetraterpene
Rubber
C5
C10
C15
C20
C30
C40
> 500
one
two
three
four
six
eight
> 100
hemi = half di = two
sesqui = one and a half tri = three
tetra = four
NOTE:
Sesterterpene C25 five

10. Representative
Monoterpenes
a-Phellandrene
(eucalyptus)
Menthol
(peppermint)
Citral
(lemon grass)
O
H
OH

11. Representative
Monoterpenes
a-Phellandrene
(eucalyptus)
Menthol
(peppermint)
Citral
(lemon grass)
O
H
OH

12. Representative
Monoterpenes
a-Phellandrene
(eucalyptus)
Menthol
(peppermint)
Citral
(lemon grass)

13. Representative
Sesquiterpenes
a-Selinene
(celery)
H

14. Representative
Sesquiterpenes
a-Selinene
(celery)
H

15. Representative
Sesquiterpenes
a-Selinene
(celery)

16. Representative Diterpenes
Vitamin A
OH

17. Representative Diterpenes
Vitamin A
OH

18. Representative Diterpenes
Vitamin A

19. Biosynthesis of Terpenes
• Terpenes are biosynthesized from primary
metabolites at least in two different ways
1. Mavalonic Acid Pathway
• 2. Methylerythritole Phopsphate (MEP)
Pathway

20. Mevalonic Acid
Pathway
• Three molecules of acetyle-CoA join together to
form a molecule of mevalonic acid.
CH3COH
O
CH3COH
O
CH3COH
O
+ +
HOCCH2CCH2CH2OH
CH3
OH
O
Mevalonic acid

21. C
CH2
CH2
CH3
OH C
CH2
O
OH
OH
mevalonic acid
C
CH2
CH2
CH3
O C
CH2
O
O
O
P H
P P
:B-Enz
2 ATP
ADP
AMP
CH2
H
CH2
CH3
OPP
CH3
H
CH2
CH3
OPP
3,3-dimethylallyl
pyrophosphate
isopentenyl
pyrophosphate
Enz-B: H+
MEVALONIC ACID PATHWAY
The isopentenyl and 3,3-dimethylallyl pyrophosphate intermediates
These five-carbon
intermediates are
responsible for the
formation of all the
terpenes.
DMAPP IPP
Pyrophosphorylation
+ Decarboxylation+dehydration

22. CH2
H
CH2
CH3
OPP
CH3
H
CH2
CH3
OPP
3,3-dimepyrophosphate
thylallyl
isopentenyl
pyrophosphate
Enz-B:
H+
Methylerythritole phopsphate(MEP) pathway
The isopentenyl and 3,3-dimethylallyl pyrophosphate intermediates
DMAPP
IPP
+
These five-carbon
intermediates are
responsible for the
formation of all the
terpenes.

23. CH2
H
CH2
CH3
OPP
CH3
H
CH2
CH3
OPP
3,3-dimepyrophosphate
thylallyl
isopentenyl
pyrophosphate
Enz-B: H+
DMAPP (isoprene C5)IPP (isoprene C5)

24. Role in Growth and Development
• Certain terpenes have well characterized function
in plant growth and development considered as
primary rather than secondary metabolites.
• e.g.
• 1- Gibberellins ( diterpenes) causes tallness.
• 2- Sterol (triterpenes) essential component of cell
membrane .

25. Role in Growth and
Development
• 3- Red, orange and yellow Caretenoids
(tetraterpenes) assessory photosynthetic pigment
and prevent photo oxidation (UV).
• 4- Abscisic acid (sesquiterpene) aging hormone.
• 5- Polyterpene alcohol; carries of sugars in cell
wall and glycoprotein synthesis.
• 6- Phytol side chain of chlorophyll which help to
anchor certain molecules in membranes.

27. Terpenes Defend against
Herbivores in Many
Plants• Terpenes are toxins and feeding deterrents to
many plantfeeding insects and mammals.
• 1- The monoterpene esters called Pyrethroids
that occur in the leaves and flowers of
Chrysanthemum species show very striking
insecticidal activity.
• 2- In conifers (pine and fir), monoterpenes are
toxic to numerous insects, including bark beetles

28. Volatile Terpenes (Essential
Oils)
• Many plants contain mixtures of volatile
monoterpenes and sesquiterpenes, called
• essential oils.
• Peppermint, lemon, basil,and sage are examples
of plants that contain essential oils.
• Peppermint oil is menthol, Lemon oil is
limonene.
• Essential oils have well-known insect repellent
properties.

29. Volatile Terpenes (Essential
Oils)• Menthol ( Peppermint)
• stimulation of the skin‘s cold receptors which
produce a cooling effect.
• Similar to the capsaicin (hot peppers) which
stimulates heat receptors.
• They does not change the skin's temperature, but
produces the sensation of temperature change.

30. Essential Oil as
Antiherbivores
• They are frequently
found in glandular
hairs that project
outward from the
epidermis and serve to
“advertise” the toxicity
of the plant, repelling
potential herbivores
even before they take
a trial bite.

31. Terpene Antiherbivore
Compounds
• Limonoids
• a group of triterpenes (C30).
• Limonene
• a bitter substance in citrus fruit.

32. Terpene Antiherbivore
Compounds
• Azadirachtin
• a complex limonoid from the neem tree
(Azadirachta indica) .
• It is the most powerful feeding deterrent to some
insects at doses as low as 50 PPB.
• It is insect control commercial agent because of
its low toxicity to
• mammals.

33. Terpene Antiherbivore
Compounds
• Phytoecdysones
• (isolated from common fern)
• a group of plant steroids that have the same
basic structure as insect molting hormones.
•

34. Terpene Antiherbivore
Compounds
• Triterpenes that are active against vertebrate
herbivores include
• cardenolides and saponins.
• Cardenolides are glycosides that taste bitter
and are extremely toxic to higher animals.
• In humans, they have dramatic effects on the
heart muscle through their influence on
Na+/K+-activated ATPases. In carefully
regulated doses, they slow and strengthen the
heartbeat.

35. Terpene Antiherbivore
Compounds
• Saponins
• are steroid and triterpene glycosides having
soaplike properties.
• The toxicity of saponins is a result of their ability
to form complexes with sterols and interfere with
sterol uptake from the digestive system or disrupt
cell membranes after being absorbed into the
bloodstream.

37. Terpenes: Help Cool
Planet

38. Scientists discover cloud-thickening
chemicals in trees that could offer a
new weapon in the fight against
global warming

39. Secondary Organic Aerosol (SOA)
Formation
• Terpenes (Pine forests distinctive smell), react in
the air to form tiny particles called aerosols.
• Natural aerosols has found that they contained
two previously unknown compounds, which are
photooxidation products of isoprene.
• These compounds are hygroscopic and could
impact cloud formation, rainfall and climate
•

40. Global Air Conditioners
• These aerosol particles help turn water
vapour, in the atmosphere, into clouds.
• The aerosol particles double the thickness of
clouds some 1,000 m above the forests, and
reflect an extra 5% more sunlight back into
space.
• Terpenes are released even more abundantly
in warmer conditions causing trees to act as
• “Global Air Conditioners”.