Conversion of ammonia to urea occurs during

Deamination of amino acids results in the production of ammonium (NH4+). Ammonium is an extremely toxic base and its accumulation in the body would quickly be fatal. However, the liver contains a system of carrier molecules and enzymes which quickly converts the ammonia (and carbon dioxide) into urea. This is called the urea cycle. Ornithine is one of the substrate and is the reason why it is called ornithine cycle.