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HOW DOES SYNTHESIS OF UREA TAKES PLACE?
Urea is the major end product of nitrogen metabolism in humans and mammals. Ammonia, the product of oxidative deamination reactions, is toxic in even small amounts and must be removed from the body. The urea cycle or the ornithine cycle describes the conversion reactions of ammonia into urea. Since these reactions occur in the liver, the urea is then transported to the kidneys where it is excreted. The overall urea formation reaction is:

2 Ammonia + carbon dioxide + 3ATP ---> urea + water + 3 ADP

 One amine group comes from oxidative deamination of glutamic acid while the other amine group comes from aspartic acid. Aspartic acid is regenerated from fumaric acid produced by the urea cycle. The fumaric acid first undergoes reactions through a portion of the citric acid cycle to produce oxaloacetic acid which is then changed by transamination into aspartic acid. In doing so, the liver removes the amino groups from amino acids and uses the amino groups to produce urea. Urea is removed from the body  by the kidneys.


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