Q156 / 200 · NEET 2010 (Screening) · Zoology

The principal nitrogenous excretory compound in humans is synthesised

  • A
    in kidneys but eliminated mostly through liver
  • B
    in kidneys as well as eliminated by kidneys
  • C
    in liver and also eliminated by the same through bile
  • Correct answer
    D
    in the liver, but eliminated mostly through kidneys

Solution

In humans, the principal nitrogenous excretory compound (ie, urea) is synthesized in liver by ornithine cycle. In liver, one molecule of $\mathrm{CO}_2$ is activated by biotin and combines with two molecules of $\mathrm{NH}_3$ in the presence of carbamyl phosphate synthetase enzyme and 2ATP to form carbamyl phosphate and one molecule of $\mathrm{H}_2 \mathrm{O}$ is released. Carbamyl phosphate reacts with ornithine and forms citrulline. Citrulline combines with another molecule of ammonia and form arginine that is broken into urea and ornithine in the presence of an enzyme arginase and water. $$ \begin{array}{r} 2 \mathrm{NH}_3+\mathrm{CO}_2 \stackrel{\text { Arginase }}{\longrightarrow} \mathrm{NH}_2-\mathrm{CO}-\mathrm{NH}_2 \\ +\mathrm{H}_2 \mathrm{O} \end{array} $$ urea is eliminated mostly through kidney as nitrogenous excretory product.

Hence, the correct answer is option: 'D'.

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