Question

Which of the following cycles in the liver is mainly responsible for the synthesis of urea?
A. Citrulline cycle
B. Krebs cycle
C. Nitrogen cycle
D. Ornithine cycle

Answer 1

Hint:- The liver is the largest internal organ and is very critical to the human body. It is greatly responsible for a wide range of functions like metabolism, digestion, detoxification, immunity, and vitamin storage among other functions.

Complete Answer:-
The liver metabolizes carbohydrates, lipids, and proteins into biologically useful substances. Protein breakdown or catabolism results in the formation of ammonia. Ammonia is a toxic substance and if retained in the human body may lead to serious health issues. Thus, it is necessary for the body to convert ammonia into a less toxic form i.e. urea. This conversion of ammonia into urea is known as the Urea cycle. The urea cycle is a cyclic pathway and takes place in the liver. Liver cells called hepatocytes serve as the site of the urea cycle. The entire conversion process is highly energy-dependent. So, inside the hepatocyte cell, the cycle starts within the mitochondria first and finally ends in the cytosol. The liver contains all the enzymes required for the synthesis of urea from ammonia. These enzymes are Carbamoyl phosphate synthase (CPS), Ornithine carbamoyltransferase (OCT), argininosuccinate synthase, argininosuccinate lyase, and arginase. Since, a molecule of Urea is built around a key compound Ornithine, which is why the urea cycle is also called the Ornithine cycle.
Thus, the right option is D.

Note:- The urea cycle occurs in ureotelic organisms. After its synthesis in the liver, urea is finally sent via blood to the kidneys for its excretion. The urea cycle is the first metabolic pathway to be discovered. Hans Krebs and Kurt Henseleit in 1932 first studied this metabolic cycle but Ratner and Cohen presented a more detailed study on Urea Cycle.