,CHEM 210 Module 8 Exam | Biochemistry |
Portage Learning | Q & A | 2026 Edition
1. Which of the following best describes the primary function of the urea cycle?
A) To synthesize amino acids from ammonia and α-keto acids
B) To convert toxic ammonia into urea for excretion
C) To produce ATP through substrate-level phosphorylation
D) To generate NADPH for biosynthetic reactions
Correct Answer: To convert toxic ammonia into urea for excretion
Rationale: The urea cycle is a series of enzymatic reactions that converts highly toxic ammonia into urea,
which is then excreted in the urine. This process occurs primarily in the liver and is essential for nitrogen
disposal in mammals. The cycle involves five key reactions and consumes ATP.
2. What is the primary location where the urea cycle occurs in the cell?
A) Cytosol only
B) Mitochondrial matrix only
C) Both cytosol and mitochondrial matrix
D) Nucleus
Correct Answer: Both cytosol and mitochondrial matrix
Rationale: The urea cycle is compartmentalized between the cytosol and the mitochondrial matrix. The
first two reactions (carbamoyl phosphate synthesis and ornithine transcarbamoylase) occur in the
mitochondria, while the remaining reactions occur in the cytosol.
3. In the urea cycle, which molecule is the direct carrier of ammonia into the cycle?
A) Glutamate
B) Aspartate
,C) Carbamoyl phosphate
D) Ornithine
Correct Answer: Carbamoyl phosphate
Rationale: Carbamoyl phosphate is the activated form of ammonia that enters the urea cycle. It is
synthesized from ammonia, bicarbonate, and two ATP molecules by carbamoyl phosphate synthetase I
in the mitochondrial matrix. This molecule then reacts with ornithine to begin the cyclic portion of the
pathway.
4. Which amino acids are directly involved in the urea cycle?
A) Ornithine and aspartate
B) Ornithine and threonine
C) Aspartate and threonine
D) Glutamine and alanine
Correct Answer: Ornithine and aspartate
Rationale: Ornithine and aspartate are the amino acids directly involved in the urea cycle. Ornithine
combines with carbamoyl phosphate to form citrulline, and aspartate provides the second nitrogen
atom for urea formation. Threonine and glutamine are not cyclic participants.
5. How many ATP equivalents are consumed in the synthesis of one molecule of urea?
A) 1
B) 2
C) 3
D) 4
Correct Answer: 4
, Rationale: The urea cycle consumes 4 ATP equivalents per urea molecule synthesized. Two ATP are used
to form carbamoyl phosphate, and one ATP is converted to AMP and pyrophosphate (equivalent to two
ATP) in the argininosuccinate synthetase reaction.
6. True or False: GTP is invested in the urea cycle.
A) True
B) False
Correct Answer: False
Rationale: GTP is not invested in the urea cycle. The cycle uses ATP as the energy source, not GTP. GTP is
produced in the citric acid cycle but does not participate directly in urea synthesis.
7. Which of the following molecules from the urea cycle can enter the citric acid cycle?
A) Ornithine
B) Citrulline
C) Fumarate
D) Arginine
Correct Answer: Fumarate
Rationale: Fumarate is produced when argininosuccinate is cleaved into arginine and fumarate.
Fumarate can enter the citric acid cycle, linking urea cycle function to the TCA cycle and energy
metabolism.
8. What is fumarate converted to so that it can re-enter the mitochondrion?
A) Citrulline
B) Malate
C) Ornithine
D) Aspartate
Portage Learning | Q & A | 2026 Edition
1. Which of the following best describes the primary function of the urea cycle?
A) To synthesize amino acids from ammonia and α-keto acids
B) To convert toxic ammonia into urea for excretion
C) To produce ATP through substrate-level phosphorylation
D) To generate NADPH for biosynthetic reactions
Correct Answer: To convert toxic ammonia into urea for excretion
Rationale: The urea cycle is a series of enzymatic reactions that converts highly toxic ammonia into urea,
which is then excreted in the urine. This process occurs primarily in the liver and is essential for nitrogen
disposal in mammals. The cycle involves five key reactions and consumes ATP.
2. What is the primary location where the urea cycle occurs in the cell?
A) Cytosol only
B) Mitochondrial matrix only
C) Both cytosol and mitochondrial matrix
D) Nucleus
Correct Answer: Both cytosol and mitochondrial matrix
Rationale: The urea cycle is compartmentalized between the cytosol and the mitochondrial matrix. The
first two reactions (carbamoyl phosphate synthesis and ornithine transcarbamoylase) occur in the
mitochondria, while the remaining reactions occur in the cytosol.
3. In the urea cycle, which molecule is the direct carrier of ammonia into the cycle?
A) Glutamate
B) Aspartate
,C) Carbamoyl phosphate
D) Ornithine
Correct Answer: Carbamoyl phosphate
Rationale: Carbamoyl phosphate is the activated form of ammonia that enters the urea cycle. It is
synthesized from ammonia, bicarbonate, and two ATP molecules by carbamoyl phosphate synthetase I
in the mitochondrial matrix. This molecule then reacts with ornithine to begin the cyclic portion of the
pathway.
4. Which amino acids are directly involved in the urea cycle?
A) Ornithine and aspartate
B) Ornithine and threonine
C) Aspartate and threonine
D) Glutamine and alanine
Correct Answer: Ornithine and aspartate
Rationale: Ornithine and aspartate are the amino acids directly involved in the urea cycle. Ornithine
combines with carbamoyl phosphate to form citrulline, and aspartate provides the second nitrogen
atom for urea formation. Threonine and glutamine are not cyclic participants.
5. How many ATP equivalents are consumed in the synthesis of one molecule of urea?
A) 1
B) 2
C) 3
D) 4
Correct Answer: 4
, Rationale: The urea cycle consumes 4 ATP equivalents per urea molecule synthesized. Two ATP are used
to form carbamoyl phosphate, and one ATP is converted to AMP and pyrophosphate (equivalent to two
ATP) in the argininosuccinate synthetase reaction.
6. True or False: GTP is invested in the urea cycle.
A) True
B) False
Correct Answer: False
Rationale: GTP is not invested in the urea cycle. The cycle uses ATP as the energy source, not GTP. GTP is
produced in the citric acid cycle but does not participate directly in urea synthesis.
7. Which of the following molecules from the urea cycle can enter the citric acid cycle?
A) Ornithine
B) Citrulline
C) Fumarate
D) Arginine
Correct Answer: Fumarate
Rationale: Fumarate is produced when argininosuccinate is cleaved into arginine and fumarate.
Fumarate can enter the citric acid cycle, linking urea cycle function to the TCA cycle and energy
metabolism.
8. What is fumarate converted to so that it can re-enter the mitochondrion?
A) Citrulline
B) Malate
C) Ornithine
D) Aspartate
