Pharmacognosy · Short notes

Tannins and Carbohydrate Drugs: Short Notes & Cheat Sheet

Hydrolysable and condensed tannins with their tests, and sugars, starches, gums, mucilages and seaweed polysaccharides used in pharmacy.

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Short notes

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Where pharmaceutical polysaccharides come from

Many exam questions simply ask for the source; group them by origin to remember them.

Tannins

Tannins are polyphenols that precipitate proteins, which makes them astringent.
Hydrolysable tannins (gallotannins, ellagitannins) break down to gallic or ellagic acid and sugar. They give a blue-black colour with ferric chloride. Example: nutgall.
Condensed tannins (proanthocyanidins) are not hydrolysed; heating with acid gives red phlobaphenes. They give a green-black colour with ferric chloride. Examples: catechu, tea, cinnamon.
Pseudotannins (gallic acid, catechins, chlorogenic acid) are small molecules that fail the goldbeater's skin test.
Tests: goldbeater's skin test, gelatin precipitation, ferric chloride colour and lead acetate precipitate.
Uses: astringent for diarrhoea and haemorrhoids. They precipitate alkaloids and heavy metals, and are incompatible with gelatin, alkaloids and iron salts.

Sugars and starches

Sucrose comes from sugarcane and beet; lactose from milk (a common tablet diluent); glucose (dextrose) is used in IV fluids.
Starches are identified microscopically by grain shape: potato (large, oyster-shaped, eccentric hilum), maize (polygonal, central star-shaped hilum), rice (very small, polygonal, compound), wheat (lens-shaped).
Starch turns blue-black with iodine and is used as a diluent, binder and disintegrant.
Cellulose gives absorbent cotton and microcrystalline cellulose.

Gums, mucilages and polysaccharides

Gums are pathological exudates (from injury): acacia (emulsifier; contains an oxidase enzyme, so heat it to destroy the enzyme before use with oxidisable drugs), tragacanth (suspending agent), karaya (bulk laxative, denture adhesive).
Mucilages are normal (physiological) plant products: ispaghula (isabgol; Plantago ovata seed husk, a bulk laxative) and linseed.
Seaweed products: agar (red algae; culture media, laxative), alginates (brown algae; antacid rafts and wound dressings) and carrageenan.
Pectin (citrus peel, apple) is a gelling agent. Guar gum is a seed galactomannan. Xanthan gum is made by the bacterium Xanthomonas campestris.
Dextran (from Leuconostoc bacteria) is a plasma expander. Inulin is used to measure glomerular filtration rate.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Hydrolysable vs condensed tannins

Feature	Hydrolysable · Condensed
Acid hydrolysis	Gallic/ellagic acid + sugar · Red phlobaphenes
Ferric chloride	Blue-black · Green-black
Examples	Nutgall, hamamelis · Catechu, tea, cinnamon
Goldbeater's skin test	Positive · Positive

Carbohydrate drug → source

Drug	Source
Acacia	Acacia senegal (gum exudate)
Tragacanth	Astragalus gummifer
Ispaghula	Plantago ovata seed husk
Agar	Red algae (Gelidium, Gracilaria)
Alginic acid	Brown algae (Laminaria)
Xanthan gum	Xanthomonas campestris
Dextran	Leuconostoc mesenteroides
Pectin	Citrus peel, apple pomace

Memory aids

Gums from wounds, mucilages from normal life: Gums are pathological exudates after injury; mucilages are normal cell products.

Exam traps

Pseudotannins do not pass the goldbeater's skin test.
Acacia contains an oxidase enzyme that must be destroyed by heat before use with easily oxidised drugs.
Agar comes from red algae and alginates from brown algae.
Hydrolysable tannins give blue-black with ferric chloride; condensed tannins give green-black.

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