Urea is a waste product formed in the liver during the breakdown of amino acids. It is then transported to the kidneys where it is excreted in urine. The process of urea synthesis takes place in the liver.

Process of Urea Synthesis in the Liver:

1. Formation of Carbamoyl Phosphate: The first step in urea synthesis is the conversion of ammonia and carbon dioxide into carbamoyl phosphate. This reaction is catalyzed by the enzyme carbamoyl phosphate synthetase I (CPSI).

2. Formation of Citrulline: The carbamoyl phosphate then reacts with ornithine to form citrulline. This reaction takes place in the mitochondrial matrix and is catalyzed by the enzyme ornithine transcarbamylase (OTC).

3. Formation of Argininosuccinate: Citrulline is then transported out of the mitochondria and into the cytosol of the liver cell where it reacts with aspartate to form argininosuccinate. This reaction is catalyzed by the enzyme argininosuccinate synthase.

4. Formation of Urea: Argininosuccinate is then cleaved by the enzyme argininosuccinase to form arginine and fumarate. Arginine is then hydrolyzed by the enzyme arginase to form urea and ornithine. Urea is then transported to the kidneys for excretion in urine.

In summary, urea synthesis takes place in the liver and involves a series of enzymatic reactions that convert ammonia and carbon dioxide into urea, which is excreted in urine.