The term Alkaloid‘ or Pfanzenlkalien‘ was coined by the German pharmacist Carl F.W. Meissner in 1819. 

Since  they are  basic  or  Alkali  like,  therefore  the  name  coined  was  ―Alkaloids.  

Alkaloids  are  naturally occurring  organic substances,  predominantly found  in  plant  sources  including  marine algae  and  rarely in animals (e.g. in the toxic secretions of fire ants, ladybugs and toads). 

They occur mostly in seed-bearing plants mainly in berries, bark, fruits, roots and leaves.

Definition: These are the organic products of natural or synthetic origin which are basic in nature containing one or more nitrogen atoms, normally of heterocyclic nature, and possess specific with specific physiological actions on human or animal body, when used in small quantities.

They are Present in various part of plants like: 

Aerial parts: ex: Ephedra ephedrine, 

Entire plant: Vinca- Vincristine, vinblastine, 

Leaves- Tea, Caffeine, 

Root- Rauwolfia- Reserpine, 

Bark- Cinchona- quinine, 

Seed: Nuxvomica- struchnine,

Fruit- Black pepper- piperine, 

Latex: Opium- Morphine,codeine etc.

Uses: Analgesic, Antimalarial, Antispasmodic, hypertension, mental disorders, anticancerous

Distribution:  They  are  found  abundantly  in  angiosperms  i.e.  higher  plants.  Alkaloids  are  found primarily in plants and are especially common in certain families of flowering plants.

Families  rich  in  alkaloids  :  They  are  abundantly  found  in  various  families  like  Apocynaceae (Rauwolfia) Rubiaceae (cinchona), Solanaceae (belladona), Papaveracea (opium), Berberidaceae (podoplyllum)

Opium poppy (Papaver somniferum) and the ergot fungus (Claviceps) each contain about 30 different types of alkaloids. 

Certain plant families are particularly rich in alkaloids; all plants of the poppy family (Papaveraceae) are thought to contain them, for example. The Ranunculaceae (buttercups), Solanaceae (nightshades), and Amaryllidaceae (amaryllis) are other prominent alkaloid-containing families. 

A few alkaloids have been found in animal species, such as the New World beaver (Castor canadensis) and poison-dart frogs (Phyllobates). Ergot and a few other fungi also produce them. 

The function of alkaloids in plants is not yet understood. 

It has been suggested that they are simply waste products of plants’ metabolic processes, but evidence suggests that they may serve specific biological functions. 

In some plants, the concentration of alkaloids increases just prior to seed formation and then drops off when the seed is ripe, suggesting that alkaloids may play a role in this process. 

Alkaloids may also protect some plants from destruction by certain insect species.

The chemical structures of alkaloids are extremely variable. 

Generally, an alkaloid contains at least one nitrogen atom in an amine-type structure—i.e., one derived from ammonia by replacing hydrogen atoms with hydrogen-carbon groups called hydrocarbons. 

This or another nitrogen atom can be active as a base in acid-base reactions. 

The name alkaloid (“alkali-like”) was originally applied to the substances because, like the inorganic alkalis, they react with acids to form salts. 

Most alkaloids have one or more of their nitrogen atoms as part of a ring of atoms, frequently called a cyclic system. 

Alkaloid names generally end in the suffix -ine, a reference to their chemical classification as amines. In their pure form most alkaloids are colourless, nonvolatile, crystalline solids. They also tend to have a bitter taste.

The  first  alkaloid  to  be isolated and crystallized was the potent active constituent of the opium poppy, morphine, in about 1804. 

Alkaloids are often classified on the basis of their chemical structure. 

For example, those alkaloids that contain a ring system called indole are known as indole alkaloids. 

On this basis, the principal classes of alkaloids are the pyrrolidines, pyridines, tropanes, pyrrolizidines, isoquinolines, indoles, quinolines, and the terpenoids and steroids. 

Alternatively, alkaloids can be classified according to the biological system in which they occur. 

For example, the opium alkaloids occur in the opium poppy (Papaver somniferum). 

This dual classification system actually produces little confusion because there is a rough correlation between the chemical types of alkaloids and their biological distribution.

The medicinal properties of alkaloids are quite diverse. 

Morphine is a powerful narcotic used for the relief of pain, though its addictive properties limit its usefulness. 

Codeine, the methyl ether derivative of morphine found in the opium poppy, is an excellent analgesic that is relatively nonaddictive. 

Certain alkaloids act as cardiac or respiratory stimulants. 

Quinidine, which is obtained from plants of the genus Cinchona,  is  used  to  treat  arrhythmias,  or  irregular  rhythms  of  the heartbeat.  

Many  alkaloids  affect respiration, but in a complicated manner such that severe respiratory depression may follow stimulation. 

The  drug  lobeline  (from  Lobelia  inflata)  is  safer  in  this  respect  and  is  therefore  clinically  useful. Ergonovine (from the fungus Claviceps purpurea) and ephedrine (from Ephedra species) act as blood- vessel constrictors. Ergonovine is used to reduce uterine hemorrhage after childbirth, and ephedrine is used to relieve the discomfort of common colds, sinusitis, hay fever, and bronchial asthma.

Many  alkaloids  possess  local  anesthetic  properties,  though  clinically  they  are  seldom  used  for  this purpose. 

Cocaine (from Erythroxylum coca) is a very potent local anesthetic. 

Quinine (from Cinchona species) is a powerful antimalarial agent that was formerly the drug of choice for treating that disease, though it has been largely replaced by less toxic and more effective synthetic drugs. 

The alkaloid tubocurarine is the active ingredient in the South American arrow poison, curare (obtained from Chondrodendron tomentosum), and is used as a muscle relaxant in surgery. 

Two alkaloids, vincristine and vinblastine  (from  Catharanthus  roseus,  formerly  Vinca  rosea),  are  widely  used  as  chemotherapeutic agents in the treatment of many types of cancer.

Nicotine obtained from the tobacco plant (Nicotiana tabacum) is the principal alkaloid and chief addictive ingredient of the tobacco smoked in cigarettes, cigars, and pipes. Some alkaloids are illicit drugs and poisons. 

These include the hallucinogenic drugs mescaline (from Lophophora species) and psilocybin (from Psilocybe mexicana). Synthetic derivatives of the alkaloids morphine and lysergic acid (from Claviceps purpurea) produce heroin and LSD, respectively. 

The alkaloid coniine is the active component of the poison hemlock (Conium maculatum). Strychnine (from Strychnos species) is another powerful poison

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