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urea

urea (yŏŏē´ə), organic compound that is the principal end product of nitrogen metabolism in most mammals. Urea was the first animal metabolite to be isolated in crystalline form; its crystallization was described in the early 18th cent., and in 1773 it was noted that urea gave off ammonia when heated. This discovery provided a clue to its structure. In 1828 urea also became the first organic compound to be synthesized from inorganic materials (lead or silver cyanate and ammonia); this work was done by German chemist Friedrich Wöhler in 1828. Years of investigation of the biosynthesis of urea culminated in the proposal of the ornithine cycle (sometimes known as the Krebs urea cycle, named for German-born chemist Hans Krebs) in 1932. The proposed cycle has since been amended only in detail. It involves the linking of one molecule of ammonia with one molecule of carbon dioxide to form carbamoyl phosphate which then is added to ornithine resulting in the formation of citrulline. Next the nitrogen-containing amino group from aspartic acid is combined with the citrulline, resulting in the formation of arginine. The addition of a water molecule, arginine is then split into one molecule of urea and one molecule of ornithine, which can now repeat the cycle. In metabolism of proteins and other materials, the ammonia molecule that enters the cycle originates from glutamic acid, but glutamic acid can acquire the group that generates this ammonia from many other amino acids; thus most of the nitrogen in protein can eventually be converted to nitrogen in urea. These reactions have been shown to occur in the liver. Urea is transported in the blood to the kidneys, where it is filtered out; its concentration in urine is about 60 to 70 times as great as that in blood.

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"urea." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

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"urea." The Columbia Encyclopedia, 6th ed.. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/urea

urea

urea (carbamide) A white crystalline water-soluble solid, CO(NH2)2. Urea is the major end product of nitrogen excretion in mammals and other ureotelic animals, being synthesized by the urea cycle. Urea is synthesized industrially from ammonia and carbon dioxide for use in urea–formaldehyde resins and pharmaceuticals, as a source of nonprotein nitrogen for ruminant livestock, and as a nitrogen fertilizer.

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"urea." A Dictionary of Biology. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

"urea." A Dictionary of Biology. . Encyclopedia.com. (August 17, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/urea-0

"urea." A Dictionary of Biology. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/urea-0

urea

urea The end‐product of nitrogen metabolism, excreted in the urine. Chemically it is CO(NH2)2. Synthesized in the liver from ammonia and the amino acid aspartic acid; the major nitrogenous compound in urine, and the major component of the non‐protein nitrogen in blood plasma.

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"urea." A Dictionary of Food and Nutrition. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

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"urea." A Dictionary of Food and Nutrition. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/urea

urea

urea CO(NH2)2; the compound that is formed from carbon dioxide and ammonia via the urea cycle as a result of amino-acid catabolism in ureotelic animals. It is also the final product of purine catabolism in most fish, Amphibia, and freshwater bivalve molluscs (Bivalvia).

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"urea." A Dictionary of Zoology. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

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urea

urea (CO(NH2)2) Organic compound, a white, crystalline solid excreted in urine. Most vertebrates excrete their nitrogen wastes as urea; human urine contains c.25 grams of urea to a litre. Because it is so high in nitrogen, urea is a good fertilizer.

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"urea." World Encyclopedia. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

"urea." World Encyclopedia. . Encyclopedia.com. (August 17, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/urea

"urea." World Encyclopedia. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/urea

urea

u·re·a / yoŏˈrēə/ • n. Biochem. a colorless crystalline compound, CO(NH2)2, that is the main nitrogenous breakdown product of protein metabolism in mammals and is excreted in urine.

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"urea." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

"urea." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (August 17, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/urea-0

"urea." The Oxford Pocket Dictionary of Current English. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/urea-0

urea

urea (yoor-ee-ă) n. the main breakdown product of protein metabolism. It is the chemical form in which unrequired nitrogen is excreted by the body in the urine. Urea is formed in the liver from ammonia and carbon dioxide.

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"urea." A Dictionary of Nursing. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

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"urea." A Dictionary of Nursing. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/urea

urea

urea constituent of urine. XIX. f. F. urée, f. Gr. oûron urine or oureîn urinate.

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"urea." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

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"urea." The Concise Oxford Dictionary of English Etymology. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/urea-1

urea

ureaAchaea, aliyah, Almería, Apia, Bahía, Caesarea, Cassiopeia, Chaldea, Cytherea, Euboea, foreseer, freer, galleria, gynaecea, Iphigenia, Kampuchea, kea, keyer, Latakia, Leah, Lucia, Nicaea, Nicosia, onomatopoeia, Oriya, Pangaea, Pantelleria, pharmacopoeia, pizzeria, ria, rupiah, sangría, seer, sharia, Shia, skier, spiraea (US spirea), Tanzania, taqueria, Tarpeia, Thea, trachea, trattoria, urea •sightseer

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"urea." Oxford Dictionary of Rhymes. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

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"urea." Oxford Dictionary of Rhymes. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/urea