Question:
The urea cycle occurs in which of the following organelles? Choose the most appropriate option.
The urea cycle occurs in which of the following organelles? Choose the most appropriate option.
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Mnemonic: "Urea is Made in Cells." (M-C for Mitochondria and Cytoplasm).
Only the first two steps are mitochondrial. Citrulline is the "Expatriate" that leaves the mitochondria to finish the job in the cytosol.
Only the first two steps are mitochondrial. Citrulline is the "Expatriate" that leaves the mitochondria to finish the job in the cytosol.
Updated On: Aug 21, 2026
- Mitochondria and Cytoplasm
- Mitochondria and Peroxisomes
- Endoplasmic reticulum and Golgi Complex
- Mitochondria, Chloroplast and Peroxisomes
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Question:
The Urea Cycle (also known as the Ornithine cycle) is the primary metabolic pathway for converting toxic ammonia into less toxic urea for excretion. The question asks for the specific cellular locations where the enzymes of this cycle reside.
Step 2: Detailed Explanation:
• Introduction to the Cycle: The urea cycle involves five main enzymatic steps. It is unique because it is "compartmentalized," meaning it spans two different parts of the cell.
• Mitochondrial Phase: The first two reactions occur within the mitochondrial matrix:
- Formation of Carbamoyl Phosphate from $NH_3$ and $CO_2$ (by CPS-I).
- Reaction of Carbamoyl Phosphate with Ornithine to form Citrulline (by OTC).
• Cytoplasmic Phase: Citrulline is then transported out of the mitochondria into the cytosol, where the remaining three reactions occur:
- Citrulline + Aspartate $\rightarrow$ Argininosuccinate.
- Argininosuccinate $\rightarrow$ Fumarate + Arginine.
- Arginine + $H_2O \rightarrow$ Urea + Ornithine.
• Recycling: The Ornithine produced in the final step is transported back into the mitochondria to start the cycle again.
• Significance of Compartmentalization: This separation allows for the tight regulation of ammonia levels and connects nitrogen metabolism to the TCA cycle (via the production of Fumarate).
• Evaluating other options: Chloroplasts and Peroxisomes are involved in photorespiration, not the urea cycle. ER and Golgi are involved in protein synthesis and sorting.
Step 3: Final Answer:
The urea cycle enzymes are distributed between the Mitochondrial matrix (Steps 1-2) and the Cytoplasm (Steps 3-5).
The Urea Cycle (also known as the Ornithine cycle) is the primary metabolic pathway for converting toxic ammonia into less toxic urea for excretion. The question asks for the specific cellular locations where the enzymes of this cycle reside.
Step 2: Detailed Explanation:
• Introduction to the Cycle: The urea cycle involves five main enzymatic steps. It is unique because it is "compartmentalized," meaning it spans two different parts of the cell.
• Mitochondrial Phase: The first two reactions occur within the mitochondrial matrix:
- Formation of Carbamoyl Phosphate from $NH_3$ and $CO_2$ (by CPS-I).
- Reaction of Carbamoyl Phosphate with Ornithine to form Citrulline (by OTC).
• Cytoplasmic Phase: Citrulline is then transported out of the mitochondria into the cytosol, where the remaining three reactions occur:
- Citrulline + Aspartate $\rightarrow$ Argininosuccinate.
- Argininosuccinate $\rightarrow$ Fumarate + Arginine.
- Arginine + $H_2O \rightarrow$ Urea + Ornithine.
• Recycling: The Ornithine produced in the final step is transported back into the mitochondria to start the cycle again.
• Significance of Compartmentalization: This separation allows for the tight regulation of ammonia levels and connects nitrogen metabolism to the TCA cycle (via the production of Fumarate).
• Evaluating other options: Chloroplasts and Peroxisomes are involved in photorespiration, not the urea cycle. ER and Golgi are involved in protein synthesis and sorting.
Step 3: Final Answer:
The urea cycle enzymes are distributed between the Mitochondrial matrix (Steps 1-2) and the Cytoplasm (Steps 3-5).
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