Question

The conversion of protein waste, the ammonia into urea occurs in
a. Kidneys
b. Lungs
c. Intestine
d. Liver

Answer 1

The ammonia produced in the body is toxic and is converted into urea by a process called the urea cycle. Once the urea is formed, it enters into the bloodstream, filtered by kidneys, and ultimately excreted in the form of urine.

Complete answer:
Urea is a nitrogen compound that is non-proteinaceous and is synthesized in the liver. The non-essential amino acids, that is, the amino acid which is of no use to our body are deaminated (it is the process of removal of amine group) by oxidase (an enzyme), to produce ammonia (NH3). As this ammonia is toxic for the body it is converted into urea (it is comparatively non-toxic and can be easily excreted by the kidney) by the process of the urea cycle which occurs in the liver. The urea cycle (it is also known as ornithine cycle or Krebs Henseleit cycle) is a five step process which occurs in the mitochondria (only the first step) and then in the cytoplasm of liver cells (last four steps).

The urea cycle first occurs in mitochondria. In the first step, carbamoyl phosphate is produced by a combination of carbon dioxide (CO2) ammonia (NH3). This step occurs in the presence of two ATP (adenosine triphosphate) molecules and enzyme carbamoyl phosphate synthetase I.

Once, the carbamoyl phosphate is formed, the carbamoyl group present in it, combines with the ornithine (an amino acid) to yield citrulline. This step occurs in the cytoplasm of the liver cell in the presence of enzyme ornithine transcarbamoylase.

In the third step, citrulline along with ATP (adenosine triphosphate) is catalyzed to form a citrulline-AMP intermediate in the presence of enzyme argininosuccinate synthetase. Then, the intermediate formed reacts with the amino group which it obtains from the aspartate to produce argininosuccinate.

In the fourth step, the enzyme named argininosuccinate lyase breaks down argininosuccinate into two compounds, that is, fumarate and arginine.

In the fifth step, arginine upon hydrolysis yields urea and ornithine (amino acid).
Hence, Option (D) is the correct answer.

Note: The ammonia produced due to the deamination of amino acid by oxidase is converted into urea by the process of the urea cycle in cells of a different organ whereas the reabsorption and secretion of the synthesized urea take place in the cells of a different organ.