Biochemistry MCQs Set 3
51. Which monosaccharide is a component of RNA?
A. Glucose
B. Ribose
C. Deoxyribose
D. Fructose
Correct Answer: B. Ribose
Explanation: RNA (Ribonucleic Acid) contains ribose sugar, which has a hydroxyl group (-OH) on the 2′ carbon. DNA contains deoxyribose, which lacks this oxygen atom.
52. The enzyme that converts glucose to glucose-6-phosphate in liver is:
A. Hexokinase
B. Glucokinase
C. Phosphorylase
D. Isomerase
Correct Answer: B. Glucokinase
Explanation: While hexokinase works in most tissues, the liver specifically uses glucokinase (Hexokinase IV). It has a high K_m for glucose, allowing the liver to process large amounts of glucose only when blood levels are high.
53. Which pathway is active during fasting to maintain blood glucose?
A. Glycolysis
B. Glycogenesis
C. Gluconeogenesis
D. Lipogenesis
Correct Answer: C. Gluconeogenesis
Explanation: Gluconeogenesis (creating new glucose from non-carbohydrate sources) and glycogenolysis (breaking down stored glycogen) are the primary ways animals maintain blood sugar during fasting.
54. Acetyl-CoA cannot be converted into glucose because:
A. It produces ATP
B. The reaction is irreversible
C. Carbon is lost as CO2
D. It inhibits glycolysis
Correct Answer: C. Carbon is lost as CO2
Explanation: In the TCA cycle, two carbons enter as Acetyl-CoA and two are lost as CO2. Consequently, there is no net gain of carbon that can be used for the synthesis of glucose via oxaloacetate.
55. Which enzyme is absent in muscle but present in liver?
A. Hexokinase
B. Pyruvate kinase
C. Glucose-6-phosphatase
D. Aldolase
Correct Answer: C. Glucose-6-phosphatase
Explanation: Muscle lacks Glucose-6-phosphatase; therefore, it cannot release free glucose into the blood. It uses its glycogen stores only for its own energy needs. The liver uses this enzyme to provide glucose for the whole body.
56. The main function of NADPH is:
A. ATP production
B. Oxidative phosphorylation
C. Reductive biosynthesis
D. Electron transport
Correct Answer: C. Reductive biosynthesis
Explanation: NADPH provides the reducing power for anabolic pathways, such as fatty acid and cholesterol synthesis. It is also vital for maintaining antioxidants like glutathione.
57. Which lipid is amphipathic in nature?
A. Triglyceride
B. Cholesterol ester
C. Phospholipid
D. Wax
Correct Answer: C. Phospholipid
Explanation: Phospholipids are amphipathic because they have a hydrophilic (water-loving) phosphate head and hydrophobic (water-fearing) fatty acid tails. This allows them to form cell membranes.
58. Carnitine is required for:
A. Fatty acid synthesis
B. Transport of fatty acids into mitochondria
C. Ketone body utilization
D. Cholesterol synthesis
Correct Answer: B. Transport of fatty acids into mitochondria
Explanation: Long-chain fatty acids cannot cross the inner mitochondrial membrane on their own. Carnitine acts as a shuttle to transport them in so they can undergo beta-oxidation.
59. Which ketone body is quantitatively highest in blood?
A. Acetone
B. Acetoacetate
C. beta-hydroxybutyrate
D. Acetyl-CoA
Correct Answer: C. beta-hydroxybutyrate
Explanation: In animals suffering from ketosis, beta-hydroxybutyrate is the most abundant ketone body in the circulation and is often measured to diagnose subclinical ketosis in dairy cows.
60. Essential amino acids are those which:
A. Are synthesized in liver
B. Are required in large amounts
C. Cannot be synthesized by the body
D. Contain sulfur
Correct Answer: C. Cannot be synthesized by the body
Explanation: Essential amino acids must be provided in the diet because the animal’s body lacks the metabolic pathways to synthesize them in sufficient quantities.
61. Which amino acid is both glucogenic and ketogenic?
A. Leucine
B. Lysine
C. Alanine
D. Isoleucine
Correct Answer: D. Isoleucine
Explanation: Isoleucine, Phenylalanine, Tryptophan, and Tyrosine are both glucogenic (can form glucose) and ketogenic (can form ketone bodies). Leucine and Lysine are purely ketogenic.
62. Negative nitrogen balance occurs during:
A. Growth
B. Pregnancy
C. Starvation
D. Recovery from illness
Correct Answer: C. Starvation
Explanation: A negative nitrogen balance occurs when nitrogen excretion exceeds intake. This happens during starvation, fever, or severe tissue injury where the body breaks down its own muscle protein for energy.
63. All enzymes are proteins except:
A. Pepsin
B. Trypsin
C. Ribozymes
D. Amylase
Correct Answer: C. Ribozymes
Explanation: Ribozymes are RNA molecules that have catalytic activity. All other traditional enzymes are composed of amino acids (proteins).
64. Which factor does NOT affect enzyme activity?
A. Temperature
B. pH
C. Enzyme concentration
D. Atmospheric pressure
Correct Answer: D. Atmospheric pressure
Explanation: Enzymes are highly sensitive to temperature and pH (which can denature them) and concentration levels. Standard biological atmospheric pressure changes have negligible effects on kinetic activity.
65. Non-competitive inhibition decreases:
A. K_m only
B. V_{max} only
C. Both K_m and V_{max}
D. Substrate concentration
Correct Answer: B. V_{max} only
Explanation: In non-competitive inhibition, the inhibitor binds to a site other than the active site.21 Increasing substrate cannot overcome this inhibition, so the 22$V_{max}$ decreases, but the affinity (23$K_m$) remains the same.24
66. Which vitamin acts as a coenzyme in carboxylation reactions?
A. Vitamin A
B. Vitamin $B_{12}$
C. Biotin
D. Vitamin D
Correct Answer: C. Biotin
Explanation: Biotin (Vitamin B_7) is the essential cofactor for carboxylase enzymes, such as those used in gluconeogenesis and fatty acid synthesis.
67. Night blindness is caused by deficiency of:
A. Vitamin D
B. Vitamin E
C. Vitamin A
D. Vitamin K
Correct Answer: C. Vitamin A
Explanation: Vitamin A is a precursor to rhodopsin, the pigment in the retina’s rod cells. A deficiency leads to night blindness (nyctalopia).
68. Selenium deficiency is associated with:
A. Milk fever
B. White muscle disease
C. Rickets
D. Anemia
Correct Answer: B. White muscle disease
Explanation: White muscle disease (enzootic muscular dystrophy) occurs in calves and lambs due to a deficiency of Selenium or Vitamin E, which results in oxidative damage to muscle tissues.
69. Which mineral is essential for hemoglobin synthesis?
A. Copper
B. Zinc
C. Iron
D. Iodine
Correct Answer: C. Iron
Explanation: Iron Fe2+ is the central atom in the heme group of hemoglobin, where it binds to oxygen for transport in the blood.
70. Steroid hormones act by:
A. Activating membrane receptors
B. Using cAMP as second messenger
C. Binding to intracellular receptors
D. Activating ion channels
Correct Answer: C. Binding to intracellular receptors
Explanation: Being lipid-soluble, steroids cross the cell membrane and bind to receptors in the cytoplasm or nucleus to directly influence gene transcription.
71. Insulin is secreted by:
A. Alpha cells
B. Beta cells
C. Delta cells
D. Acinar cells
Correct Answer: B. Beta cells
Explanation: The Beta cells of the Islets of Langerhans in the pancreas produce and secrete insulin in response to high blood glucose.
72. DNA replication is described as:
A. Conservative
B. Dispersive
C. Semi-conservative
D. Random
Correct Answer: C. Semi-conservative
Explanation: Semi-conservative means that each new DNA molecule consists of one original “parent” strand and one newly synthesized “daughter” strand.
73. Codon is present on:
A. DNA
B. tRNA
C. rRNA
D. mRNA
Correct Answer: D. mRNA
Explanation: A codon is a sequence of three nucleotides on the mRNA that corresponds to a specific amino acid. The tRNA carries the matching anti-codon.
74. Start codon for protein synthesis is:
A. UAA
B. UAG
C. AUG
D. UGA
Correct Answer: C. AUG
Explanation: AUG is the universal start codon that codes for the amino acid Methionine. UAA, UAG, and UGA are stop codons.
75. Which compound directly enters the TCA cycle?
A. Pyruvate
B. Acetyl-CoA
C. Lactate
D. Glucose
Correct Answer: B. Acetyl-CoA
Explanation: Acetyl-CoA is the common entry point for the TCA cycle. Pyruvate must first be converted into Acetyl-CoA by the pyruvate dehydrogenase complex before it can enter the cycle.