An alkaloid is a nitrogen-containing naturally occurring compound, produced by a large variety of organisms, including fungi, plants, animals, and bacteria. Many alkaloids have pharmacological effects on humans and other animals. The name derives from the word alkaline; originally, the term was used to describe any nitrogen-containing base. Alkaloids are part of the group of natural products (also called secondary metabolites) and are chemical derivatives of amino acids or other nitrogen-containing compounds, such as polyamines. Some alkaloids may have a bitter taste. Produced in dedicated biosynthetic pathways consisting of multiple enzyme -catalysed steps, such as prenylation, methylation/demethylation, and various oxidation/reduction reactions, alkaloids are found in plants (e.g. potatoes and tomatoes), animals (e.g. shellfish) and many fungi. Many alkaloids can be purified from crude extracts by acid-base extraction. While many alkaloids are poisonous, some are used medicinally as analgesics (pain relievers) or anaesthetics, particularly morphine and codeine, and for other uses.