ē`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 arginine (är`jənĭn), organic compound, one of the 20 amino acids commonly found in animal proteins...... Click the link for more information. . 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.
urea
or carbamideOne of the simplest organic compounds and the first synthesized from inorganic raw materials (see inorganic compound), by Friedrich Wöhler (1800–82) in 1828. It is the diamide of carbonic acid (H2NCONH2; see amide; carbon dioxide). The chief nitrogenous end product of protein breakdown in mammals and some fishes, it occurs not only in urine but also in blood, bile, milk, and perspiration. It is one of the industrial chemicals produced in vast amounts. With its high nitrogen content and low price, it is a major agricultural fertilizer and animal-feed ingredient. It is also used to make urea-formaldehyde plastics (including foamed plastics; see polyurethanes), to synthesize barbiturates, as a stabilizer in explosives, and in adhesives, hydrocarbon processing, and flameproofing.
urea
Urea
A colorless crystalline compound, formula CH4N2O, melting point 132.7°C (270.9°F). Urea is also known as carbamide and carbonyl diamide, and has numerous trade names as well. It is highly soluble in water and is odorless in its purest state, although most samples of even high purity have an ammonia odor. The diamide of carbonic acid, urea has the structure below.
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Urea occurs in nature as the major nitrogen-containing end product of protein metabolism by mammals, which excrete urea in the urine. The adult human body discharges almost 50 g (1.8 oz) of urea daily. Urea was first isolated in 1773 by G. F. Rouelle. By preparing urea from potassium cyanate (KCNO) and ammonium sulfate (NH4SO4) in 1828, F. Wöhler achieved a milestone, the first synthesis of an organic molecule from inorganic starting materials, and thus heralded the modern science of organic chemistry. See Nitrogen, Protein metabolism
Because of its high nitrogen content (46.65% by weight), urea is a popular fertilizer. About three-fourths of the urea produced commercially is used for this purpose. After application to soil, usually as a solution in water, urea gradually undergoes hydrolysis to ammonia (or ammonium ion) and carbonate (or carbon dioxide). Another major use of urea is as an ingredient for the production of urea-formaldehyde resins, extremely effective adhesives used for laminating plywood and in manufacturing particle board, and the basis for such plastics as malamine. See Fertilizer, Urea-formaldehyde resins
Other uses of urea include its utilization in medicine as a diuretic. In the past, it was used to reduce intracranial and intraocular pressure, and as a topical antiseptic. It is still used for these purposes, to some extent, in veterinary medicine and animal husbandry, where it also finds application as a protein feed supplement for cattle and sheep. Urea has been used to brown baked goods such as pretzels. It is a stabilizer for nitrocellulose explosives because of its ability to neutralize the nitric acid that is formed from, and accelerates, the decomposition of the nitrocellulose. Urea was once used for flameproofing fabrics. Mixed with barium hydroxide, urea is applied to limestone monuments to slow erosion by acid rain and acidic pollutants.
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