Question

The urea cycle takes place in which of the following compartments of the cell
(A). Cytosol
(B). Endoplasmic reticulum
(C). Mitochondrial matrix
(D). Peroxisomes
Choose the correct answer from the options given below:

• (A) and (D) only.
• (A) and (C) only.
• (C) and (D) only.
• (B) and (D) only.

Hint:

Remember the urea cycle as a journey. It starts inside the mitochondria (matrix) for the first two steps, then the intermediate citrulline travels out to the cytosol for the remaining steps.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The urea cycle is a metabolic pathway that occurs primarily in the liver. Its purpose is to convert toxic ammonia (NH\(_4^+\)), a waste product of amino acid catabolism, into the less toxic compound urea, which can be excreted by the kidneys. The pathway involves multiple steps that are partitioned between different cellular compartments.
Step 2: Detailed Explanation:
The urea cycle is a prime example of metabolic compartmentalization:
Mitochondrial Matrix (C): The cycle begins in the mitochondria. The first two reactions, the formation of carbamoyl phosphate and its reaction with ornithine to form citrulline, occur here.
Cytosol (A): Citrulline is then transported out of the mitochondrion into the cytosol. The remaining three reactions of the cycle, which lead to the production of arginine and finally urea, take place in the cytosol. The ornithine produced in the final step is transported back into the mitochondrion to continue the cycle.
The other organelles listed, the endoplasmic reticulum (B) and peroxisomes (D), are not involved in the urea cycle.
Step 3: Final Answer:
The urea cycle requires enzymes located in both the mitochondrial matrix and the cytosol. Therefore, both (A) and (C) are correct.