Biochemistry Multiple Choice Questions on “Nitrogen Excretion and the Urea Cycle”.
1. Urea production occurs almost exclusively in ___________
A. Kidneys
B. Liver
C. Blood
D. Urine
Answer: B
Clarification: Urea production occurs almost exclusively in the liver and is the fate of most of the ammonia channeled there. The urea passes into the blood stream and thus to the kidneys and is excreted into the urine.
2. Which is the first amino group entering into urea cycle?
A. carbamoyl phosphate
B. Ornithine
C. Cituilline
D. Argininosuccinate
Answer: A
Clarification: The carbamoyl phosphate, which functions as an activated carbamoyl group donor enters the urea cycle.
3. The first enzyme in the pathway carbamoyl phosphate synthase I, is allosterically activated by ___________
A. N-acetylglutamate
B. Acetyl coA
C. Glutamate
D. Carbamoyl phosphate
Answer: A
Clarification: The first enzyme in the pathway carbamoyl phosphate synthase I, is allosterically activated by N-acetylglutamate.
4. A second amino group is transferred to cituilline from ___________
A. Aspartate
B. Glutamate
C. Alanine
D. Guanine
Answer: A
Clarification: The second amino group enters from aspartate generated in mitochondria by transamination and transported into the cytosol.
5. Nitrogen atoms of urea produced in the urea cycle are derived from ___________
A. Ammonia and aspartic acid
B. Nitrate
C. Nitrite
D. Ammonia
Answer: A
Clarification: One amino group comes from ammonia and another comes from aspartate.
6. What are the products of urea cycle?
A. One molecule of urea, one molecule of ammonia, one molecule of ATP and one molecule of fumaric acid
B. One molecule of urea, one molecule of AMP, two molecules of ADP and one molecule of fumaric acid
C. One molecule of aspartic acid, one molecule of ammonia, one molecule of ATP and one molecule of fumaric acid
D. Two molecules of urea, two molecules of ammonia, one molecule of ATP and one molecule of fumaric acid
Answer: B
Clarification: Products of urea cycle are one molecule of urea, one molecule of AMP, two molecules of ADP and one molecule of fumaric acid.
7. The carbon atom source while producing urea in the urea cycle is ___________
A. CO2
B. Glucose
C. Aspartic acid
D. Arginine
Answer: A
Clarification: CO2 is the carbon atom source while producing urea in the urea cycle.
8. Which of the following is not an essential amino acid?
A. Glycine
B. Leucine
C. Methionine
D. Histidine
Answer: A
Clarification: Only Leucine, methionine and histidine are essential in this case.
9. Urea cycle converts ___________
A. Keto acids into amino acids
B. Amino acids into keto acids
C. Ammonia into a less toxic form
D. Ammonia into a more toxic form
Answer: C
Clarification: A less toxic ammonia form is produced by urea cycle.
10. What are the inputs to one cycle of urea cycle?
A. One molecule of CO2, one molecule of ammonia, three molecules of ATP and one molecule of aspartic acid
B. One molecule of urea, one molecule of AMP, two molecules of ADP and one molecule of fumaric acid
C. One molecule of aspartic acid, one molecule of ammonia, one molecule of ATP and one molecule of fumaric acid
D. Two molecules of urea, two molecules of ammonia, one molecule of ATP and one molecule of fumaric acid
Answer: A
Clarification: One molecule of CO2, one molecule of ammonia, three molecules of ATP and one molecule of aspartic acid are the inputs to one cycle of urea cycle.
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