Alkaloids are chemical compounds that usually contain a nitrogen atom in a hetercyclic ring and originate from amino acids. Most alkaloids are secondary metabolites and have strong effects, mostly toxic, on organisms. 

Important alkaloids with their wikipedia links: Atropine, cocaine, codeine, galantamine, lycorine, morphinenicotine

Recommended links: Science Daily, Plant Science BlogWikipedia (alkaloid, english), Wikipedia (Alkaloid, german),

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