Pharmacognosy and Natural Products
What the exam tests
Pharmacognosy and Natural Products tests biosynthetic classes, marker-based standardization, extraction, and the pharmacology of plant-derived drugs. Iranian medicinal plants may appear as examples, not as folklore.
Core rules
- Alkaloids are nitrogenous, usually basic, and often water-insoluble as free bases but soluble as salts: morphine, quinine, atropine, vincristine, colchicine. They are usually extracted by basifying the plant material and partitioning into an organic solvent.
- Terpenoids: monoterpenes from the MEP pathway; sesquiterpenes and triterpenes/steroids from mevalonate. Examples include artemisinin (sesquiterpene lactone), paclitaxel (diterpene), and cardiac glycosides such as digoxin (steroid glycoside).
- Phenolics: shikimate pathway products, including flavonoids, tannins, and coumarins. Tannins bind proteins and can reduce absorption of alkaloids.
- Standardization uses a marker compound, not necessarily the active constituent; specify the plant part, extraction solvent, and assay.
Worked example
A 500 g batch of dried leaf gives 2.5 g of purified alkaloid extract. Extraction yield = 2.5/500 × 100 = 0.50% w/w. If the marker alkaloid is 12% of the extract, the marker content in the leaf is 2.5 g × 0.12 = 0.30 g, or 0.30/500 × 100 = 0.06% w/w. A pharmacopoeial specification of not less than 0.05% would pass; a claim of 0.5% marker would fail because that confuses extract yield with marker content.
Traps
Do not assume the marker is the active drug; do not compare yields from different plant parts or solvents; do not treat essential-oil content as alkaloid content; and remember that digoxin and other cardiac glycosides require narrow therapeutic monitoring.