Question

Urea is formed in liver from
A. Ammonia and nitrogen
B. Ammonia and carbon dioxide
C. Ammonia, carbon dioxide, aspartic acid
D. Ammonia and carbon monoxide

Answer 1

Hint: Urea is colourless, odourless and highly soluble in water. It is a common waste product that is produced by many organisms. The human urine contains a heavy load of urea and is non-toxic. The body uses urea mainly for nitrogen excretion and the production of urea mainly takes place in the liver.

Complete Answer:
- Urea is produced in the liver through a series of steps which is known as the urea cycle or ornithine cycle. The proteins and amino acids are broken down in the liver to produce ammonia.
- Ammonia is toxic for the body and thus needs to be removed. So ammonia combines with carbon monoxide resulting in the formation of carbamoyl phosphate. This compound then enters into the cycle.
- It first gets converted to citrulline followed by argininosuccinate. This argininosuccinate is then converted to fumarate and arginine.
- Finally the arginine is cleaved to form urea and ornithine. The ornithine enters back to the cycle whereas the urea is expelled out of the body. After the urea is produced in the liver they are transported to the kidney where they are excreted out along with urine.

So the correct option is D).

Note: Though urea is not toxic to the body but excess accumulation can lead to certain diseases. This is because urea then enters the bloodstream and reaches different organs causing adverse effects. Urea causes irritation to the skin, eye and respiratory tract when contracted externally.