Plantlet The Blogging Platform of Department of Botany, University of Dhaka
Metabolism: (Gr. metabole = change) the totality of the chemical changes in living cells which involves the buildup and breakdown of chemical compounds.
Metabolites
Primary metabolites: Molecules that are essential for growth and development of an organism.
Secondary metabolites: Molecules that are not essential for growth and development of an organism .
Primary metabolism: biosynthesis, utilization and breakdown of the essential compounds and structural elements of the living organism, such as:
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➢ Sugars and polysaccharides; amino acids, peptides and proteins (including enzymes); fatty acids and nucleotides.
➢ The starting materials are CO2, H2O and NH3.
➢ All organisms possess similar primary metabolic pathways and use similar primary metabolites.
What are the secondary metabolites ?
➢ They are the phytochemicals that do not participate in plant metabolism .
➢ They are not directly needed by the plant as they do not perform any physiological functions .
➢ They may include pharmaceuticals, flavours, fragrances, cosmetics, food additives, feed stocks, antimicrobials .
Why secondary metabolites?
➢ Chemical warfare to protect plants from the attacks by predators, pathogens, or competitors
➢ Attract pollinators or seed dispersal agents
➢ Important for abiotic stresses
➢ Medicine
➢ Industrial additives
➢ According to WHO survey nearly 70 – 80% of total world population depend on herbal drugs
➢ The production of specialty chemical by plants is a multi billion industry.
Types of Secondary Metabolites
| Type | Example | Uses |
| Alkaloids | Caffeine, Codeine, Quinine | Stimulant, Analgesic, Antimalarials |
| Cyanogenic glycoside | Diosgenin | Progesterone |
| Flavonoids | Quercetin, Procyanidins | Antibacterial, Antimicrobial, Anti-inflammatory |
| Phytic acid | – | Antioxidant |
| Gossypol | Hypokalemic paralysis | – |
| Phytoestrogens | Resveratrol | Reduce risk of cardiovascular diseases |
| Carotenoids | α-carotene, β-carotene and lycopene | Contribute to photosynthesi |
| Hormone | Applications |
| Abscisic acid | Inhibits growth in response to change in temperature & light.
Controls closing of stomata in dry conditions. ABA is applied to plants before shipping. |
| Auxins | Stimulate cell elongation, differentiation of xylem & phloem, root initiation.
Suppress growth of lateral buds. Delay leaf senescence. |
| Cytokinins | Promote cell division.
Lateral buds development. Delay senescence. |
| Ethylene | Fruit repening.
Inhibitionof stem elongation. |
| Gibberellins | Synthesis in apical portions of roots & stems. |
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.
Secondary Metabolites
➢ Possibly over 250,000 secondary metabolites in plants .
➢ Classified based on common biosynthetic pathways where a chemical is derived.
➢ Four major classes: ✓Alkaloids, ✓Glycosides, ✓Phenolics, ✓Terpenoids .
1. Alkaloids :
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 .
➢ Class of alkaloids, the vinca alkaloids from Vinca rosea, the Madagascar periwinkle, can also bind to tubulin and inhibit microtubule polymerization.
➢Vinblastine and vincristine are used as potent agents for cancer chemotherapy,
➢ The alkaloid colchicine, a constituent of the swollen, underground stems of the autumn crocus (Colchicum autumnale) and Meadow saffron, inhibits the polymerization of tubulin into microtubules.
2. Phenolics:
• Derived from aromatic amino acids, such as phenylalanine, tyrosine, and trytophan.
• All contain structures derived from phenol .
• Some examples:
Coumarins: antimicrobial agents, feeding deterrents and germination inhibitors.
Lignin: abundant in secondary cell wall, rigid and resistant to extraction or many degradation reagents.
More Phenolics
➢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.
3. Terpenoids :
➢Terpenoids are dimers and polymers of 5 carbon precursors called isoprene units (C5 H8).
➢ Give scent, flavors, colors, medicine.
➢ Three plant hormones are derived from the terpenoid pathway.
➢ Terpenoids often evaporate from plants and contribute to the haze we see on hot sunny days.
4. Glycosides:
• Compounds that contain a carbonhydrate and a noncarbohydrate
• Glucosinolates: found primarily in the mustard family to give the pungent taste.
