Final answer:
The urea cycle is a set of biochemical reactions that produces urea from ammonium ions to prevent toxicity. It occurs in the liver and involves the exchange of amino acid groups with keto groups. This process is necessary for the Krebs cycle and elimination of ammonium ions.
Step-by-step explanation:
The urea cycle is a set of biochemical reactions that produces urea from ammonium ions in order to prevent a toxic level of ammonium in the body. It occurs primarily in the liver and, to a lesser extent, in the kidney. Prior to the urea cycle, ammonium ions are produced from the breakdown of amino acids. In these reactions, an amine group, or ammonium ion, from the amino acid is exchanged with a keto group on another molecule. This transamination event creates a molecule that is necessary for the Krebs cycle and an ammonium ion that enters into the urea cycle to be eliminated.