Question

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

Answer 1

Urea is also known as carbamide, the primary organic portion of human urine, which is a waste product of many living species. This is because it is at the end of the reaction chain that the amino acids that make up proteins break down.

Complete answer:
Urea is the metabolism 's main nitrogenous waste product and is produced from protein breakdown. It is almost entirely removed from the body by the kidneys in the urine, and the measurement of its concentration, first in the urine and then in the blood, has been used clinically for several years in the assessment of kidney function. Toxic ammonia resulting from protein breakdown is converted to non-toxic urea in a sequence of five enzymatically mediated reactions, collectively known as the 'urea loop'. In addition to ammonia and the five 'urea cycle' enzymes, the presence of bicarbonate, aspartate and energy input in the form of adenosine triphosphate requires the endogenous production of urea.

In liver cells, carbon dioxide and ammonia undergo an ornithine cycle. It occurs in five major phases. The extra proteins in the body are destroyed by the deamination process, where the NH group is eliminated and the liver is turned into ammonium ions. In the presence of the enzyme carbamoyl phosphate synthase, ammonium ions combine with carbon dioxide. Two ATP molecules are absorbed by it. By means of the enzyme ornithine transcarbamoylase, carbamoyl phosphate is converted to citrulline. By means of the enzyme ornithine transcarbamoylase, carbamoyl phosphate is converted to citrulline. A condensation reaction occurs to form argininosuccinate between the amino group of aspartate and the carbonyl group of citrulline. This reaction is ATP based and is catalysed by the synthetase of argininosuccinate. Argininosuccinate undergoes cleavage to form arginine and fumarate by argininosuccinase. Arginine cleaves to form urea and ornithine by arginase.

Hence, the correct answer is (B).

Note: As could be predicted, the rate of production of urea is affected by dietary protein content; low-protein diet is associated with decreased production of urea and increased production of urea is associated with high-protein diet. The kidney's ability to change the reabsorption and secretion of urea as the filtrate passes through the tubule dictates, when required, an significant function for urea in the development of full concentrated urine.