What is urea?
Urea is a nitrogen-containing fertilizer and a chemical compound with the formula CO(NH2)2 (or H2N.CO.NH2). The molecule has two –NH2 groups joined by a C=O or carbonyl functional group. It is also called carbamide. It is very soluble in water but insoluble in ether, with a melting point at 132°C (Walker 1988).
That it is a fertilizer should be the common reply to the question. Synthetic urea is largely used as such in crop agriculture to supplement the essential major element nitrogen (N). According to Soh (2006), the usage of urea as fertilizer accounts for more than 90% of total production.
But there are plenty more as to what is urea.
Urea is a naturally occuring, colorless and odorless nonprotein nitrogenous compound found in urines of humans and most other mammals. It is an organic compound both in accordance with Jöns Jacob Berzelius’ 1807 original definition (Carey 1992) that organic chemistry is the study of compounds produced by living organisms; and based on the current definition that organic compounds are those that contain the element carbon. It is likewise a synthetic or manufactured compound, as in the granular urea fertilizer (46-0-0) which is commercially produced through industrial processing. In other words, urea is a carbon-containing compound that is both naturally and artificially produced.
In the process of digestion of food in humans and other mammals, the proteins yield amino acids which are in turn used in the synthesis of body proteins. But the excess amino acids differ from the carbohydrates and fats in that they cannot be stored. They undergo catabolizm (degradation) which yields ammonia (NH3). A buildup of ammonia in the cells results in toxicity. To remove excess ammonia from the body, it is converted to urea by combining with CO2 in the liver via the Krebs-Henseleit urea cycle. Urea is then transported via the bloodstream to the kidney which filter it and finally excreted mostly with urine (Maynard et al. 1979; Mathews and Van Holde 1990; Bettelheim and March 1998; Wardlaw et al. 2004; Myers 2007).
Excessive concentration of urea is toxic to the tissues. It has to be diluted with enough water to a safe concentration, removed, and excreted. However, the process by which toxic ammonia and urea are removed from an animal body necessitates wastage of water (Maynard et al. 1979). In humans, the presence of urea in the blood indicates kidney disease (Wardlaw et al. 2004).
A summary reaction leading to the formation of urea (CO(NH2)2; modified from Linstromberg and Baumgarten 1987) is given below. The same reaction applies in its commercialized manufacture.
2NH3 + CO2 ------> CO(NH2)2 + H2OBut in birds, land-based reptiles, and insects, the nitrogenous end product is uric acid (C5H4N4O3) which is excreted in nearly solid form. Most fishes and aquatic animals do not find the necessity to convert the ammonia into another product; they excrete it directly via their gills into the water (Maynard et al 1979; Mathews and Van Holde 1990).
According to Bettelheim and March (1998), the average amount of urine that a human excretes is about 1.5 liters per day. About 4 percent thereof is dissolved waste products with urea as the main solute. Other waste products that contain nitrogen are creatine, creatinine, ammonia, and hippuric acid. The typical urine also contains various inorganic ions including PO43-, Ca2+, Mg2+, Cl-, SO42-, and HCO3-.
On the other hand, it seems that not all urea should be considered waste. Hibernating animals can reutilize the urea in the bladder for the synthesis of amino acids. In the entire 6 or 7 months that the black bear hibernates, it does not urinate (Mathews and Van Holde 1990). In ruminant animals on low-protein diets, the urea can be recycled back to the rumen for microbial use (Maynard et al. 1979).
Aside from its common use as fertilizer, there are other more ways of describing what is urea. Here are some:
1. It is an industrial product that is used as feed additive for livestock. Urea is the most common nonprotein nitrogen (NPN) compound used as feed ingredient for ruminant animals. It serves as source of nitrogen for the biosynthesis of protein with the mediation of bacteria and other microorganisms. However, its use as such must consider, among others, its lack of energy and its deficiency in minerals including sulfur. It is readily converted to ammonia in the rumen. When fed in excessive doses, it can result to fatal toxicity due to ammonia accumulation in the rumen which will in turn led to a rise in the level of blood ammonia (Maynard et al. 1979).
2. It is an industrial product used in the manufacture of synthetic resins with various applications such as plastics, adhesives, moldings, laminates, plywood, particleboard, textiles, and coatings (Myers 2007). With formaldehyde, it produces a hard, electrically nonconducting plastic (Linstromberg and Baumgarten 1987).
3. It is an industrial product used in the production of pharmaceutical products. In 1864 the German chemist Adolph von Baeyer first synthesized barbituric acid from diethyl malonate and urea. Certain derivatives of barbituric acid (barbiturates), such as pentothal, pentobarbital, and secobarbital are used as sedatives (tranquilizers) or sleep inducer (Carey 1992; Bettelheim and March 1998; Myers 2007).
4. Other minor uses are as rehydrating lotion, diuretics, deicers, and cold-compresses (Myers 2007).
5. As already mentioned above, the presence of urea in the blood is used as a diagnostic tool to detect kidney disease (Wardlaw et al. 2004)
BETTELHEIM FA, MARCH J. 1998. Introduction to General, Organic & Biochemistry. 5th ed. Orlando, FL: Saunders College Publishing. 809 p.
CAREY FA. 1992. Organic Chemistry. 2nd ed. New York, NY: McGraw-Hill, Inc. 1274 p.
LINSTROMBERG WW, BAUMGARTEN HE. 1987. Organic Chemistry: A brief Course. Lexington, Massachusetts: D.C. Heath and Company. 517 p.
MATHEWS CK, VAN HOLDE KE. 1990. Biochemistry. Redwood City, CA: The Benjamin/Cummings Publishing Co., Inc. p. 686-691.
MAYNARD LA, LOOSLI JK, HINTZ HF, WARNER RG. 1979. Animal Nutrition. 7th ed. New York, NY: McGraw-Hill Book Company. 602 p.
MYERS RL. 2007. The 100 most important chemical compounds : a reference guide. Westport, CT: Greenwood Press. p. 288-290. Retrieved Jan. 6, 2012 from http://www.scribd.com/doc/23712570/29/Chlorophyll.
WALKER PMB (ed.). 1988. Cambridge Dictionary of Science and Technology. New York, NY: Cambridge University Press. p. 940.
WARDLAW GM, HAMPL JS, DiSILVESTRO RA. 2004. Perspectives in Nutrition. 6th ed. New York, NY: McGraw-Hill companies, Inc. 752 p.
(Ben G. Bareja Jan. 2013)
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