Question

What is true of urea biosynthesis?
A. Uric acid is starting point
B. Urea is synthesized in lysosomes
C. Urea cycle enzymes are located inside mitochondria
D. Urea is synthesized in kidney

Answer 1

Urea biosynthesis is a cyclic process in which ammonia is converted into urea. This process takes place in ureotelic organs. In this process, the highly toxic element produced as a result of different metabolic activities of the body, ammonia is transformed into urea and excreted from the body.

Complete answer: Urea synthesis or urea cycle is a process to produce urea from ammonia. Urea is synthesized in the body of organisms as part of the urea cycle. This cycle occurs either from the oxidation of amino acids or from ammonia. In this urea synthesis, ammonia donates its amino groups and L-aspartate are transformed into urea, while L-ornithine, citrulline, L-arginosuccinate, and L-arginine act as intermediates.
This method of urea synthesis was observed by Hans Krebs and Kurt Henseleit. It was the first metabolic cycle to be discovered.
This process occurs primarily at the liver which was an accessory excretory organ. Due to the amino acid catabolism, ammonia is produced. As it is toxic and harmful to the body so all animals need a way to excrete this product. During the whole process of synthesis two amino groups convert, one is from ${NH^+}_4$ and another is from aspartate and a carbon atom from ${HCO^−}_3$ to the nontoxic excretion product urea. The required energy comes from the phosphate bonds that are three ATP hydrolysed to two ADP and one AMP.
The urea cycle i.e transformation from ammonia to urea executes in five main steps. At first, ammonia enters into the cycle and the following four are all a part of the cycle itself. Ammonia is converted into carbamoyl phosphate and enters the cycle. The urea cycle consists of four enzymatic reactions, one of them is mitochondrial and the rest three are cytosolic reactions. This reaction is performed by the definite enzyme. As the one reaction of the urea cycle occurs in mitochondria and respective enzymes to this reaction are located inside mitochondria. So, urea cycle enzymes are located inside mitochondria.
Hence the right answer to the question is option C.

As the starting element of urea synthesis is ammonia, and it takes place in the liver so the other options are incorrect.

Note: The toxic ammonia produced in the body during the amino acid catabolism is converted into less toxic urea by which a process that is known as urea biosynthesis. This process takes place in mitochondria in several steps with the help of enzymes present in the mitochondria.