LIPIDS AND SECONDARY PLANT PRODUCTS
I. Fats and Oils
A. Background Information.
1. Triglycerides: glycerol plus fatty acids esterified by
single carboxyl group to hydroxyl
2. Fat: Glycerol plus 3 different fatty acids. Solid at
room temperatures.
a. Fatty Acids: even numbers of carbons, especially 16
and 18 carbons
b. Melting point rises with length of fatty acid and
with the extent of saturation
c. Solid fats have saturated fatty acids
3. Oils: Glycerol plus fatty acids (usually 18 carbons)
with one to three double bonds. Liquids at room
temperature.
a. Lower melting points
b. Important plant oils from seeds: cotton, corn,
peanuts, soybean, canola, and coconut
c. Oils from fruits: olive
d. Oils contain fatty acids with 18 carbons
and one to three double bonds
(1) Oleic 18:1, Linoleic 18:2, Linolenic 18:3
4. Most abundant saturated fatty acid is palmitic 16:0
5. Coconut fat: Lauric acid 12:0
B. Distribution and Importance of Fats
1. Fat storage is rare in leaves, stems, and roots
2. Fat storage is high in seeds and fruits
a. Fruits: olive and avocado
3. Angiosperms: fats concentrated in endosperm or
cotyledon of seeds.
4. Gymnosperms: fats stored in female gametophyte of seed.
5. Seeds store fats because fats contain a greater amount
of energy stored per unit volume than carbohydrates.
Carbohydrates also associate with larger volumes of
water than do fats. Thus the size of the seed will
determine the type of storage compound.
a. Large seeds: store carbohydrate
b. Small seeds: store fats
6. Fats stored in specialized bodies in cytosol
a. Oleosomes: lipid bodies or spherosomes
b. Half membrane: polar, hydrophobic surface exposed to
aqueous cytosol. Nonpolar, hydrophobic surface
exposed to fats stored inside.
c. Oleosomes originate from endoplasmic reticulum and
plastids.
C. Formation of Fats
1. Fats are not transported. Synthesized in situ from
sucrose and other translocated sugars.
2. Glycerol comes from glycolysis. Fatty acids are formed
through acetyl-CoA and Malonyl-CoA. Synthesized as 2-
carbon units. Hence the even number of carbons in
fatty acids.
3. Environment: determines the kinds of fatty acids found
in membranes and storage.
a. Temperature
(1) Low Temperature: more linoleic and linolenic
acids
(2) Unsaturation decreases average melting points.
Makes membrane more fluid in cold temperatures.
b. Hypothesis: increased solubility of oxygen under
cold temperatures
(1) Oxygen acts as the hydrogen atom acceptor for the
desaturation process in the endoplasmic reticulum
(2) More oxygen at cool temperatures allows for more
unsaturation in the fatty acids
(3) More unsaturation of fatty acids would allow
membranes to stay more fluid under cold
temperatures and protect them against freezing
damage (rupture due to ice crystal formation).
D. Conversion of fat to sugars: b-oxidation and the
Glyoxylate cycle
1. Occurs in the glyoxysomes
a. Lipases: enzymes used to remove the fatty acids from
glycerol
b. b-oxidation: systematic removal of two carbon units
from the carboxyl end of the fatty acid.
2. One-fourth of the carbon atoms are lost from fatty
acids as CO2, the saving of 3/4 of the carbon atoms is
sufficient for the energy requirements of species with
fat-rich seeds during germination.
II. Waxes, Cutin, and Suberin: Plant Protective Coats
A. Cuticle: slows water loss from all parts of the
herbaceous plant (leaves, stems, flowers, fruits, and
seeds)
1. Protects from loss of water by transpiration
2. Protection against plant pathogens
3. Protection against minor mechanical damage
4. Repellent for water used in agricultural sprays such as
fungicides, herbicides, insecticides, or growth
regulators
a. Sprays are formulated with detergents (surfactants:
SURFace-ACTive-AgeNTS) that reduce the surface
tension of water and allow the spray to spread
across the foliage
5. The cuticle is a heterogeneous mixture of components:
a. Waxes
b. Pectin polysaccharides attached to the cell wall
c. Small amounts of phenolic compounds
6. Cutins and waxes are synthesized by the epidermal cells
and are then secreted onto the plant surface. Waxes
can accumulate in various species specific patterns.
B. Suberin:
1. Suberin covers cork cells formed in tree bark by the
crushing action of secondary growth
2. Suberin is formed by many kinds of cells as scar tissue
after wounding
3. Suberin occurs in walls of non-injured root cells as a
Casparian Strip in endodermis and exodermis and in
bundle sheath cells of grasses.
4. Suberin is the protective coating over underground plant
parts.
5. Phenolics bind to the lipid portion of suberin in the
cells wall. Thus suberin is similar to cutin by
having a lipid-polyester component, but differs by
having an abundant phenolic fraction and by having
different kinds of fatty acids.
III. Isoprenoid Compounds
A. General Background:
1. All are composed of a five-carbon unit - isoprene (see
below)
2. Called isoprenoids, terpenoids, or terpenes
a. Terpene: isoprenoids that lack oxygen and are pure
hydrocarbons
b. Isoprenoids: Plant growth regulators (gibberellins
and Abscisic Acid), farnesol (stomatal regulator),
xanthoxin (precursor of Abscisic acid), sterols,
carotenoids, turpentine, rubber, and the phytol tail
of chlorophyll
3. Thousands of isoprenoids have been found in the plant
kingdom
a. Commercial uses are known for some
b. For most isoprenoids, no function in the plant is
presently known
c. Allelochemics: compounds that influence another
species. Many isoprenoids function in this capacity.
d. Alleleopathy: typically considered a special case of
alleleochemy in which there is a negative chemical
interaction between different plant species.
c. Allelochemy against insects and other animal
herbivores is much more prevalent in plants than
is allelopathy
4. Isoprene (C5H8): dimers, trimers, polymers of isoprene
units
a. Isoprene is synthesized from the acetate of
acetyl-CoA.
b. Mevalonic acid pathway
c. Three acetyl-CoA molecules provide the five carbons
for one isoprene unit with the sixth carbon lost as
CO2
B. Sterols
1. Sterols (steroid alcohols): triterpenoids built from 6
isoprene units
2. Most common plant sterols:
a. Sitosterol, Stigmasterol, Campesterol
b. Cholesterol: wide spread in trace amounts in plants
c. Ergosterol: rare in plants but common in some fungi.
Ergosterol is converted by UV radiation of sunshine
to vitamin D2.
d. Antheridiol: sex attractant secreted by female
strains of the aquatic fungus Achlya bisexualis
3. Sterols can exist as glycosides: a sugar (glucose or
manose) is attached to the hydroxyl group of the
sterol as an ester. The hydroxyl group of the sterol
is attached to a fatty acid
a. Free sterols exist in all membranes of all organisms
except bacteria. Important in membrane stability.
b. Sterol glycosides and esters do not exist in
membranes. Currently their function is unknown.
c. Some sterols have alleleochemical activity.
4. Cardiac Glycosides: sterol derivative that cause heart
attacks in vertebrates
a. Used in medicine to strengthen and slow the heartbeat
during heart failure.
b. Coevolution of milkweeds, monarch butterflies, and
blue jays
(1) Milkweeds produce bitter-tasting cardiac
glycosides that protect them against herbivory by
most insects and even cattle. Monarch
butterflies have adapted. Larvae ingest sterol
glycosides that later cause vomiting in Blue Jays
that eat the adult butterflies. Other
butterflies have protective coloration that
makes them look like Monarch butterflies, thus
protecting them from the Blue Jays!
5. Digilanides: (Digitalis) used since prehistoric times
as sources of arrow poisons.
a. Toxins inhibit Na-K ATPases of heart muscle
membranes. Heart failure occurs because of
hypertension or atherosclerosis
b. Digitalis therapy gives a slower and stronger heart
beat. Digitoxin, digoxin, or other derivatives used
for heart disease.
6. Synthetic Animal Hormones:
a. Female ovarian hormone progesterone
b. Insect-molting hormones (ecdysones) exist in plants.
Insects rely on these and other plant sterols to form
hormones they need to mature.
c. Triterpenoid saponins: sterols or sterol-like
compounds attached to short chains of sugars. Have
various biological activities in animals. Saponins
can cause foaming in the intestinal tract of cattle.
this can lead to serious bloat in cattle that eat
young alfalfa plants.
d. Estrogens: mammalian steroids including estrone,
estriol, estradiol. It has been question whether
these and related steroids function within the
plant as sex or growth hormones.
e. Brassins or Brassinosteroids: these isoprenoid
derivatives have growth-promoting activity in some
plants. Especially in stem elongation.
Brassinolide: chemically similar to ecdysone.
C. Carotenoids
1. Yellow, orange, or red pigments in colored plastids -
chromoplasts
a. Carotenes: pure hydrocarbons
b. Xanthophylls: contain oxygen
c. Both carotene and xanthophyll contain 40 carbon atoms
derived from eight isoprene units
2. b-Carotene found in carrot roots
a. b-carotene results from cultivation, useful to us
because our livers convert it to vitamin A.
But, b-carotene in the roots of carrots does not
have a known function.
b. b-carotene in mammals also protects against certain
cancers because it acts as an anti-oxidant. Eat
your carrots!
3. Lycopene: the red pigment found in tomato fruits.
4. Lutein: a xanthophyll present in all plants especially
in leaves.
5. Carotenoids found in chloroplasts participate in
photosynthesis. They are important in prevention of
photooxidation of chlorophyll.
6. Carotenoids benefit some plants by attracting
pollinating insects.
7. The Xanthophyll, Violaxanthin, is the metabolic
precursor of the plant hormone: Abscisic Acid.
D. Miscellaneous Isoprenoids and Essential Oils
1. Terpenoids: 10, 15, 20, or 30 carbons
a. 10 or 15 carbons: essential oils
(1) Volatile compounds that contribute to odor.
(2) 71 volatile compounds in orange peels mainly
limonene
(3) Widely used in perfumes.
(4) Contributors to smog and other forms of air
pollution. The Blue Ridge Mountains are blue
(especially in summer) are named because
of atmospheric scattering of blue light by tiny
particles derived from terpenes.
(5) Essential oils attract insects to flowers
(aiding pollination) or to other plant parts on
which insects feed or lay eggs.
(6) Turpentine: one of the best known essential oil
(a) Genus: Pinus
(b) Turpentine: n-heptane, a-pinene, b-pinene,
camphene
(c) Myrcene and limonene: represent important
terpenoids affecting tree-killing bark beetles.
These beetles are highly destructive in the
coniferous forests of North America, causing
millions of dollars of damage annually.
It has been found in the Ponderosa pine that
limonene is an insect repellent, whereas a-
pinene acts as an insect attractant or
aggregation hormone. Trees with high limonene
and low a-pinene contents are rarely attacked by
pine beetles.
b. Essential oils with hydroxyl groups or are
chemically modified in other ways:
(1) Menthol and Menthone: components of mint oils
(2) 1:8 cineole: eucalyptus oils. Performs an
important function in pollination of orchids by
male euglossine bees.
c. Glaucolide A: complex terpenoid derivative. Bitter
principles from family Asteraceae. Bitter principles
repel chewing insects and mammals by their taste.
Glauscolide A from species in the genus Veronia
repel various lepidopterous insects, white-tailed
deer, and cottontail rabbits.
d. Complex mixtures of terpenes containing 0- 30 carbon
atoms comprise the resins of coniferous trees,
and some angiosperms trees of the tropics. These
terpenes are formed in the leaves by specialized
epithelial cells that line the resin ducts. The
terpenes are secreted into the ducts where they
accumulate and protect the trees against insects.
E. Rubber:
1. 3,000 to 6,000 isoprenoid units
2. Over 2.000 plant species form rubber in various amounts.
3. Latex: from the tropical plant Hevea brasiliensis, a
member of the Euphorbiaceae family.
4. Commercial rubber: comes from Castilla elastica.
5. Even Dandelions produce latex!
6. Guayule (Parthenium argentatum) common to Mexico and
southwestern U.S. produces rubber. Guayule was studied
during WWII and was selected in 1978 by U.S. Congress
for development as a natural rubber crop.
7. If you visit Edison's Florida home you can still see
many plants that he imported from around the world that
produce rubber or latex. He was studying them as
alternative rubber sources.
IV. Phenolic Compound and Their relatives
A. General Information:
The functions of most Phenolic compounds are still
unknown. May appear to be by-products of metabolism.
This poor knowledge of ecological and the biochemistry
of plants is one reason that scientists want to study
the tropical rain forests before they are destroyed.
Can you imagine the important metabolic by-products
that may be out there and their possible uses!?
All phenolic compounds have an aromatic ring. This
ring structure makes them more soluble in water and
less soluble in nonpolar organic solvents.
B. Aromatic Amino Acids:
1. Phenylalanine, tyrosine, tryptophan.
a. Produced by way of the Shikimic Acid Pathway.
b. The Shikimic Acid Pathway exists in plants, fungi
and bacteria but not in animals.
c. The Shikimic Acid Pathway is inhibition by
Glyphosate (Roundup), a popular (but expensive)
herbicide. Plants that absorb the herbicide are
injured or killed after one to two weeks, because
they cannot synthesize phenylalanine, tyrosine, and
tryptophane. Since animals do not have the
Shikimic Acid Pathway this herbicide is non-toxic!
C. Miscellaneous simple Phenolics and Related Compounds:
1. Cinnamic, p-coumaric, caffeic, ferulic: derived
from phenylalanine. They are converted into
several derivative besides protein. These are
components of Phytoalexins, coumarins, lignin, and
flavonoids such as anthocyanins.
2. Protocatechuic Acid and Chlorogenic Acid: function
in disease resistance of some plants.
a. Protocatechuic acid prevents smudge in some colored
varieties of onions.
b. Chlorogenic acid may also prevent diseases. It is
widely distributed in different parts of many
plants. Chlorgenic acid is found 13% by weight in
dry coffee. It is not very toxic to humans.
Chlorgenic acid is formed in large amounts in
potato tubers. Upon oxidation of chlorgenic acid
one finds the formation of quinones which are
responsible for the darkening of freshly cut
potato tubers. Chlorogenic acid can also protect
plants against fungal attach since it is readily
metabolized to fungistatic quinones by disease-
resistant plants.
3. Ferrulic Acid: also has a role in plant protection.
Ferrulic Acid is forms part of the phenolic fraction
of suberin.
4. Gallic Acid: converted to gallotannins.
a. Gallotannins inhibit plant growth. Gallotannins are
transferred to vacuoles, where they are unable to
denature cytoplasmic enzymes.
b. Gallotannins are used commercially to tan leather.
Tannins denature proteins by cross-linking them.
This cross-linking also prevents their digestion
by bacteria, thus acting as a preservative.
c. Another function of tannins is to protect plants
against attack by bacteria and fungi, and as a
feeding deterrent against herbivores. The
astringency of tannins inhibits both the digestion
and utilization of foods by herbivores.
5. Coumarins: scopoletin and coumarin:
a. Formed in the shikimic acid pathway from
phenylalanine and cinnamic acid.
b. Coumarin is a volatile compound that is formed
mainly from nonvolatile glucose derivative upon
plant senescence or injury. Alfalfa and sweet
clover, characteristic odor of recently mowed hay.
c. Economic importance:
(1) Dicumarol: anticoagulant responsible for sweet-
clover disease a hemorrhagic or bleeding disease
in ruminant animals that are fed plants that
contain it.
(2) Seed-clover strains have been developed that
contain small amount of coumarin.
d. Scopoletin: a toxic coumarin. Often found in seed
coats. Suspected of preventing germination of
certain seeds and inducing seed dormancy.
Scopoletin is usually leached out of seeds by rain,
thus allowing the seed to only germinate when
abundant moisture is present and not before.
6. Preocenes: cause premature metamorphosis in several
insect species by decreasing the level of insect
juvenile hormone. Sterile adults feeding on plants
high in preocenes have reduced pheromone production.
This is very promising as an insecticide.
V. Phytoalexins, Elicitors, and Plant Disease Protection
A. Phytoalexins: antimicrobial compounds that are more toxic
to fungi than to bacteria
1. Glyceollins: soybean roots
2. Pisatin: pea pods
3. Phaseollin: bean pods
4. Ipomeamarone: sweet potato roots
5. Orchinol: orchid tubers
6. Trifolirhizin: red clover roots.
B. 150 Phytoalexins have been identified, especially in
dicots. Phytoalexins are phenolic phenyl propanoids
which are products of the shikimic acid pathway. Some
are isoprenoid compounds and a few are polyacetylenes.
C. Pathogenic fungi that are successful in attaching plants
do so because they either induce only nontoxic
phytoalexin levels or quickly degrade the phytoalexins
produced by the plant.
D. Viruses and different kinds of compounds can induce
phytoalexin production in plants. Compounds that cause
phytoalexin production are called elicitors. Some
elicitors are polysaccharides that are produced when
pathogenic fungi or bacteria attack plant cell walls.
Other elicitors are the polysaccharides produced from
the degradation of fungal cell walls by plant enzymes
whose production was stimulated by the fungal pathogens.
E. Physical injury and ultraviolet radiation can induce
phytoalexin production by plants.
F. Exogenous elicitors are recognized by receptor proteins in
plant membranes. These receptor proteins signal the
plant to produce phytoalexins.
VI. Lignin:
A. Lignin is a structural material that occurs with
cellulose and other polysaccharides in certain cell walls
of all higher plants. It adds strength to the cell
walls, especially secondary cell walls.
B. Largest amounts of lignin can be found in wood where it
accumulates to a minor extent in the middle lamella, and
primary walls, and to a large extent in the secondary
walls of the xylem elements. Lignin is found between the
cellulose microfibrils, serves to resist compression
forces.
C. Resistance to tension (stretching) is primarily a
function of cellulose.
1. Lignin formation was crucial in the adaptation of
plants to a terrestrial environment.
2. Rigid xylem cell walls are built to conduct sap under
tension over long distances. Lignin is the second
most abundant organic compound on earth next to
cellulose. It makes up 15 to 25 % of dry weight of
woody plants.
D. Lignin protects against attack by pathogens and
consumption by herbivores, other insect and mammals.
VII. Flavonoids:
A. Flavonoids are 15-carbon compounds distributed throughout
the plant kingdom. Flavonoids accumulate in the central
vacuole, but are synthesized outside the vacuole.
B. Anthocyanins, Flavonols, Flavones
1. Anthocyanins: colored pigments that occur in red,
purple, and blue of flowers, fruits, stems, leaves, and
roots. Confined to epidermal cells. Autumn leaf color
is caused by anthocyanin accumulation on bright, cool
days, although yellow or orange carotenoids are the
predominant pigments in autumn leaves of some species.
Rarely found in gymnosperms.
2. Anthocyanidins: removal of the sugar, and are usually
named after the plant from which they were first
obtained
a. Cyanidin: blue cornflower Centaurea cyanus
b. Pelagonidin: bright red geranium Perlargonium
c. Delphinidin: Delphinium blue larkspur
d. Peonidin: reddish peonies
e. Petunidin: purple in petunias
f. Malvidin: mauve, Malvaceae
3. Color of anthocyanins depends on:
a. substituted groups present on the B ring.
(1) Methyl groups cause a reddening effect.
b. association of anthocyanins with flavones, or
flavonols, which cause them to become more blue.
c. association with each other
(1) at high concentrations, can cause either a
reddening or a bluing effect
(2) depends on the anthocyanin and the pH of the
vacuoles in which they accumulate. Reddish in
acidic solution, purple and blue in basic pH.
(3) Epidermal cells containing delphinidin increases
from a pH of 5.5 to 6.6 during aging. Reddish
purple to purplish. Wide variation the hues of
flowers.
4. Functions in flowers as an attractant for birds and bees
that carry pollen from one plant to another, aiding in
pollination.
5. Anthocyanins may play a role in disease resistance.
5. Anthocyanins and flavonoids are of interest to plant
geneticists because it is possible to draw
correlations between morphological differences of
closely related species in a particular genus with the
types of flavonoids they contain.
C. Flavones and flavonols are yellowish or ivory-colored
pigments. Flavones and flavonols absorb ultraviolet
wavelengths. The ultraviolet radiation is visible to
bees and other insects involved in pollination.
Absorption of UV radiation, also acts as a protection
against long-wave UV rays. Flavones and flavonols are
widely distributed in leaves where they act as feeding
deterrents.
1. Light (especially blue wavelengths) promotes formation
of flavonoids. Reddest apples are found on the sunny
side of the tree.
2. Nutritional status of a plant affects production of
anthocyanins. Deficiencies of nitrogen, phosphorous,
or sulfur leads to accumulations of anthocyanins
3. Low temperatures increase anthocyanin formation.
D. Isoflavonoids: functions of are mostly unknown, but they
may have possible Alleleochemic abilities.
1. Rotenone: isoflavonoid from the root of derris (Derris
elliptica). This is an insecticide that resembles
animal estrogens.
2. Estradiol, certain plant Isoflavonoids cause
infertility in female livestock, especially sheep.
3. Subterranean clover has high levels of isoflavones.
Clover disease of sheep in the 1960's in western
Australia caused a decline in fertility. Isoflavones
were suspected to be a factor controlling rodent
populations in certain regions. Their infertility
effects do not seem to deter grazing animals.
VIII. Betalains
A. Red pigment of beets is a betacyanin
B. Red and yellow betalain pigments thought to be related to
the anthocyanins
1 Contain nitrogen
2 Yellow betaxanthins
C. Restricted to 10 plant families: Caryophyllales which lack
anthocyanins
D. Colors: flowers, fruits, yellow orange re and violet
1 Vegetative organs
2 Synthesis is promoted by light
3 Betanin: red beet roots
E. Role in pollination comparable to that of anthocyanins in
other species seems likely.
F. Protection against pathogens is another possible function
IX. Alkaloids
A. Aromatic nitrogenous compound. Many are slightly basic.
Most are white crystalline compounds slightly water-
soluble.
B. Dramatic physiological or psychological activity in
humans
C. More than 3,000 alkaloids have been found in some 4,000
species of plants
1. herbaceous dicots
2. few monocots and gymnosperms possess alkaloids
D. Examples:
Morphine: 1805 opium poppy Papaver somniferum
Nicotine: tobacco
Cocaine: Erythroxylon coca
Quinine: cuprea bark
Caffeine: coffee beans and tea leaves
Strychnine: seeds of Strychnos nuxvomica
Theobromine: cocoa beans
Atropine: black nightshade (Atropa beleadonna)
Colchicine: Colchicum byzantinum
Mescaline: hallucinogenic and euphoric drug from
flowering heads of cactus Lophophora williamsii
Lycoctonin: Delphinium barbeyi
E. Most alkaloids are synthesized only in plant shoots.
Nicotine is produced only in the roots of tobacco.
F. Physiological roles of alkaloids in plants is unknown.
1 No important metabolic function found.
2 Merely by -products?
3 Confer some protection to plants.
4 Avoided by grazing animals and leaf-feeding insects.
5 Danaid butterflies as substrates for synthesis of their
courtship pheromones.
6 Larkspur is not avoided by cattle. But lycoctonine in
Larkspur accounts for more cattle deaths in the U.S.
than any toxin in any other poisonous plant.