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.