An alkaloid is any of a class of nitrogen-containing natural product of plant origin that have an alkaline, or basic, chemical nature. Some alkaloid are simple, monocyclic (one-ring) amines (see cyclic compounds), but many are very complex, polycyclic amines.


More than 200 alkoloids are known. They are present in only about 10 to 15 % of all vascular plants. Often found in the dicotyledone group of the angiosperm, or flowering plants, they seldom occur in monocotyledone or in other plant groups, such as gymnosperms. The most actively growing parts of such plants usually contain the highest percentage of the compounds. Among the more familiar alkaloid are aconitine (From monkshood), atropine (from belladonna), codeine, morphine, and papaverine (from opium poppy), nicotine (From tobacco) quinidine and quinine (from cinchona bark), solanine (from potate and tomato), ricinine (from castor bean) and strychnine and brucine (from Nux fomica).


Why certain plants contain alkaloids remains a mystery, although a number of theories have been formulated; that alkaloids are by-product of plant metabolism ; that they are mean of defense of plants against animal and insect attack; or that they are reservoirs for protein synthesis, regulators of growth and reproduction, or detoxifying agents.


Alkaloids are most widely employed for physiological effects, which range from poisonous to sedative to hallucicinogenic. Socrates was killed by the alkaloid coniine, from poison hemlock. The poisons aconitine and strychnine were once used medically but are now generally considered too hazardous .

Morphine and codeine are analgesics, atropine is a pupil dilator, and scopolamine (from henbane) is a sedative. Lysergic acid, an ergot alkaloid, is a starting material in synthesizing lysergic acid diethylamide, a powerful hallucinogen. Another hallucinogenic alkaloid is mescaline (From peyote).


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