Question

The end product of the ornithine cycle is
A. Ammonia
B. Uric acid
C. Urea
D. ${ CO }_{ 2 }$

Answer 1

Ornithine cycle is related to the excretion of nitrogenous wastes from our body. This cycle was discovered by a scientist named Hans Krebs. The ornithine cycle is not observed in ammonotelic and uricotelic organisms.

Complete answer:
The ornithine cycle is also known as the urea cycle or Krebs-Henseleit cycle. The end product formed in the urea cycle is urea which is excreted through the urinary system. It takes place in the liver cells.

The steps involved in the urea cycle are as follows:
-Inside the liver cells, one molecule of ${ CO }_{ 2 }$ gets activated by an enzyme known as biotin. It then combined with two ${ NH }_{ 4 }^{ + }$ or ammonium ions with the help of an enzyme known as carbamoyl phosphate synthetase 1 or CPZ-1 to form carbamoyl phosphate. This carbamoyl phosphate formation requires 2ATPs and one molecule of ${ H }_{ 2 }O$ is released.
-Carbamoyl phosphate and ornithine react in the presence of ornithine transcarbamoylase enzyme to form citrulline.
-Citrulline reacts aspartic acid to form argininosuccinate, in this step ATP is required.
-Argininosuccinate is broken into arginine and fumarate by the enzyme argininosuccinate lyase.
-Arginine undergoes cleavage to give urea and ornithine by the action of the arginase enzyme.
-Urea is transported through the blood to the kidneys for excretion.
-Ornithine is recycled for another round of the urea cycle.

So, the correct answer is, “The end product of the ornithine cycle is urea.”

Note: The urea cycle occurs partly in mitochondria and partly in the cytoplasm of liver cells. The steps involved in the formation of citrulline occur inside mitochondria, the citrulline is then transported out to the cytoplasm in exchange for ornithine, where the remaining steps up to ornithine formation occurs. Substances like water and fumarate are also released as a by-product. Three ATPs are required in the process.