Question

Urea synthesis occurs in
A. Kidney
B. Liver
C. Brain
D. Muscles

Answer 1

In terrestrial animals, urea is the main nitrogenous waste after ammonia is metabolized in the body. Such animals in which urea is the main nitrogenous waste are called ureotelic animals. Urea is a water-soluble organic compound formed by the metabolism of proteins. It is excreted through the urine. Urea is produced by the urea cycle involving several steps of conversion.

Complete answer: The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra. Kidneys are the main excretory organs, reddish-brown in color, bean-shaped in structure. The kidneys are situated between the levels of the last thoracic vertebra and third lumbar vertebra i.e., T12 – L3. It is present close to the dorsal inner wall of the abdominal cavity.
The main function of the kidney is the purification of blood. In our body, the formation of urea occurs in the liver. In the liver, ammonia is converted to urea undergoing the ornithine citrulline cycle, commonly called the urea cycle. This cycle has five steps, which are catalyzed by five different enzymes to form urea from nitrogen. The first two reactions take place in mitochondria and the other three reactions occur in the cytosol of the liver cells.
Hence the correct answer is option (B).

Note: Urea is a colorless solid with no odor and is said to be non-toxic. In our body, it serves the purpose of nitrogen elimination. It has wide use in fertilizers to provide nitrogen to the plants. There is an amino acid called L-ornithine which is converted to different intermediates before being regenerated at the end of this urea cycle. The key step is catalyzed by the enzyme ornithine transcarbamylase.