NEET Biology Excretory Products And Their Elimination Based Practice Questions & Answers with Viewers Discussion and Detailed Explanation

Answer: (a)

Urea is formed of two molecules of ammonia and one molecule of $CO_2$. During catabolism, proteins break up into amino acids. The amino acids are converted into keto-acids (such as α-keto-glutaric acid) with the removal of ammonia (deamination). The keto-acid enters the Krebs cycle to yield energy. Carbon dioxide is formed during decarboxylation.

Ornithine (amino acid) combines with one molecule of $NH_3$ and $CO_2$ producing citrulline (amino acid) and water. Citrulline combines with another molecule of ammonia and forms arginine (essential amino acid) and water. Arginine is broken into urea and ornithine in the presence of an enzyme arginase and water. Thus the cycle is repeated. This is called urea or ornithine or Krebs'- Henseleit cycle after the names of its discoverers.

Most of the urea is produced in the liver. The liver cells continuously release urea into the blood and kidneys withdraw it from the blood to excrete it in urine.