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: Urea Cycle Overview:
The urea cycle, primarily in the liver, transforms toxic ammonia (NH\(_4^+\)) into less harmful urea, which the kidneys excrete. This multi-step pathway is divided across different cell compartments.
Step 2: Cycle Breakdown:
The urea cycle demonstrates metabolic compartmentalization:
Mitochondrial Matrix (C): The cycle starts in the mitochondria with the initial two reactions: carbamoyl phosphate formation and its reaction with ornithine, producing citrulline.
Cytosol (A): Citrulline moves to the cytosol, where the remaining three reactions occur, resulting in arginine and urea. Ornithine, produced in the final step, returns to the mitochondrion.
The endoplasmic reticulum (B) and peroxisomes (D) are not involved in the urea cycle.
Step 3: Conclusion:
The urea cycle utilizes enzymes in both the mitochondrial matrix and the cytosol. Thus, both (A) and (C) are accurate.