Economic and Applied Botany

Medicinal plants and phytochemicals

Medicinal plants are species whose parts or extracts are used to treat disease, and the compounds that produce the therapeutic effect are called phytochemicals. Most plant-derived drugs belong to a small number of chemical classes, and each medicinal species is usually known for one predominant compound. A reliable answer therefore links four things: the compound, its chemical class, the source plant and the use. The same molecule can be both medicinal and toxic, so the dose often decides whether it acts as a drug or as a poison.

Key compounds and their pairings:

• Eugenol — a phenylpropanoid of clove buds (Syzygium aromaticum), applied as a dental analgesic for toothache relief. Ex: oil of clove is placed on a painful tooth to numb the nerve endings.
• Agar — a sulfated galactan from red seaweeds such as Gracilaria, used as a gelling agent in dessert jellies and microbiological media. Ex: agar sets a fruit dessert without animal gelatine.
• Vincristine — a dimeric indole (vinca) alkaloid of Catharanthus roseus, used as an anticancer drug in leukaemia. Ex: a vinca alkaloid is given in a chemotherapy regimen to block mitosis.
• Morphine — an opium alkaloid of the poppy Papaver somniferum, used as a potent analgesic. Ex: morphine relieves severe post-operative pain.
• Caffeine — a purine alkaloid present in cocoa beans and coffee, acting as a central stimulant. Ex: a cup of cocoa drink contains a small amount of caffeine.
• Cocaine — a tropane alkaloid of the coca plant (Erythroxylum coca), used clinically as a local anaesthetic. Ex: cocaine is applied topically during some eye and nasal procedures.
• Atropine — a tropane alkaloid of deadly nightshade (Atropa belladonna) that blocks muscarinic receptors. Ex: atropine dilates the pupil for an eye examination.
• Scopolamine — a tropane alkaloid of henbane (Hyoscyamus niger) with anticholinergic action, used against motion sickness. Ex: a transdermal patch releases scopolamine to prevent nausea on a ship.
• Pilocarpine — an imidazole alkaloid of Pilocarpus jaborandi, a cholinergic drug used for glaucoma and dry mouth. Ex: pilocarpine eye drops lower intraocular pressure.
• Reserpine — an indole alkaloid of Rauwolfia serpentina, formerly used to lower blood pressure. Ex: a Rauwolfia extract was used in early hypertension therapy.
• Glycyrrhizin — a triterpene glycoside (saponin) of liquorice (Glycyrrhiza glabra) roots, valued as a sweetener and anti-inflammatory agent. Ex: liquorice root is added to herbal cough mixtures.
• Quinine — a cinchona alkaloid from the bark of Cinchona calisaya, used against malaria. Ex: quinine tonic water was historically a malaria prophylactic.
• Lawsone — the naphthoquinone dye of henna leaves (Lawsonia inermis). Ex: a paste of henna leaves stains hair and skin orange-brown.
• Picrocrocin — a bitter monoterpene glycoside of saffron stigmas (Crocus sativus), responsible for part of the spice's bitter taste. Ex: dried saffron threads taste bitter when eaten alone.

Headline distinctions: alkaloids contain a basic nitrogen atom and are usually bitter, while glycosides carry a sugar that is released on hydrolysis. Quinine is found in bark and lawsone in leaves, so the plant organ that accumulates the compound is itself a testable fact.

Plant secondary metabolite classes

Primary metabolites such as sugars, amino acids and nucleic acids are needed for growth and reproduction, whereas secondary metabolites are specialised compounds that mediate defence, attraction and stress tolerance. Plant secondary metabolites are grouped into a few major classes, and each class has a characteristic biosynthetic origin and structure.

Major classes and their members: