KARL GAFF / SCIENCE PHOTO LIBRARY KARL GAFF / SCIENCE PHOTO LIBRARY
Polarized light micrograph of urea (CO(NH2)2) crystals. Urea is the end product of the metabolic breakdown of proteins in the mammalian body. During metabolism, the amino groups (NH2) are removed from the amino acids which make up proteins which in turn are converted to ammonia (NH3). The liver manufactures urea by fusing two ammonia molecules (NH3) with a carbon dioxide (CO2) molecule in the urea cycle. The urea is then passed to the kidneys and excreted as urine. Urea is also used in fertilizers as a source of nitrogen and it is the starting product for plastics and pharmaceuticals.
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