Q.1 What is the primary source of energy for the citric acid cycle?
ATP
NADH
Acetyl‑CoA
Pyruvate
Explanation - Acetyl‑CoA condenses with oxaloacetate to form citrate, initiating the citric acid cycle.
Correct answer is: Acetyl‑CoA
Q.2 Which molecule directly transfers electrons from glycolysis to the electron transport chain?
FADH₂
NADH
ATP
Coenzyme Q
Explanation - NADH, produced in the cytosol, donates electrons to the ETC via the malate‑aspartate shuttle.
Correct answer is: NADH
Q.3 In oxidative phosphorylation, which complex acts as the primary proton pump?
Complex I (NADH dehydrogenase)
Complex II (succinate dehydrogenase)
Complex III (cytochrome bc1)
Complex IV (cytochrome c oxidase)
Explanation - Complex I pumps protons from the matrix into the intermembrane space, creating a proton gradient.
Correct answer is: Complex I (NADH dehydrogenase)
Q.4 What is the main product of oxidative decarboxylation of pyruvate?
Lactate
Acetyl‑CoA
ATP
CO₂
Explanation - Pyruvate dehydrogenase converts pyruvate into Acetyl‑CoA, CO₂, and NADH.
Correct answer is: Acetyl‑CoA
Q.5 Which substrate is used by the pentose phosphate pathway to generate NADPH?
Glucose‑6‑phosphate
Fructose‑1,6‑bisphosphate
Glucose
Sucrose
Explanation - Glucose‑6‑phosphate dehydrogenase oxidizes it to 6‑phosphoglucono‑δ‑lactone, producing NADPH.
Correct answer is: Glucose‑6‑phosphate
Q.6 Which enzyme is responsible for the first step of glycogen synthesis?
Phosphoglucomutase
Glycogen synthase
Glycogen phosphorylase
Glucokinase
Explanation - Glycogen synthase adds UDP‑glucose to the growing glycogen chain.
Correct answer is: Glycogen synthase
Q.7 During anaerobic glycolysis, which molecule is produced to reoxidize NAD⁺?
Pyruvate
Lactate
Acetyl‑CoA
O₂
Explanation - Lactate dehydrogenase converts pyruvate to lactate, regenerating NAD⁺.
Correct answer is: Lactate
Q.8 Which coenzyme carries electrons from the citric acid cycle to the ETC?
Coenzyme A
Coenzyme Q
Coenzyme F₁₀₀
Coenzyme B
Explanation - Coenzyme Q (ubiquinone) accepts electrons from Complexes I and II and transfers them to Complex III.
Correct answer is: Coenzyme Q
Q.9 What is the ATP yield per glucose molecule in aerobic respiration (approximately)?
2 ATP
12 ATP
30‑32 ATP
50 ATP
Explanation - Aerobic respiration yields about 30–32 ATP per glucose, depending on the shuttle system.
Correct answer is: 30‑32 ATP
Q.10 Which process provides the majority of ATP in a well‑oxygenated cell?
Glycolysis
Citric acid cycle
Oxidative phosphorylation
Fermentation
Explanation - Oxidative phosphorylation produces ~90% of ATP in aerobic cells.
Correct answer is: Oxidative phosphorylation
Q.11 What is the role of the malate‑aspartate shuttle?
Transport NAD⁺ across the membrane
Transfer reducing equivalents into mitochondria
Export lactate to the cytosol
Regulate pH in mitochondria
Explanation - It moves electrons from cytosolic NADH into the mitochondrial matrix.
Correct answer is: Transfer reducing equivalents into mitochondria
Q.12 Which amino acid is a key nitrogen donor in transamination reactions?
Alanine
Glutamine
Glutamate
Valine
Explanation - Glutamate transfers its amino group via transaminases to form other amino acids.
Correct answer is: Glutamate
Q.13 Which pathway is responsible for synthesizing nucleotides de novo?
Pentose phosphate pathway
Citric acid cycle
Glycolysis
Fatty acid β‑oxidation
Explanation - It supplies ribose‑5‑phosphate and NADPH for nucleotide biosynthesis.
Correct answer is: Pentose phosphate pathway
Q.14 What is the function of ATP‑citrate lyase?
Produce citrate from acetyl‑CoA
Break down citrate to acetyl‑CoA and oxaloacetate
Transport ATP across membranes
Oxidize NADH to NAD⁺
Explanation - It generates cytosolic Acetyl‑CoA for fatty acid synthesis.
Correct answer is: Break down citrate to acetyl‑CoA and oxaloacetate
Q.15 Which of the following is a major source of reducing equivalents for the NADPH pool?
Citric acid cycle
Pentose phosphate pathway
Glycolysis
Beta‑oxidation
Explanation - Glucose‑6‑phosphate dehydrogenase generates NADPH in the oxidative phase.
Correct answer is: Pentose phosphate pathway
Q.16 In the electron transport chain, which complex has the highest number of subunits?
Complex I
Complex II
Complex III
Complex IV
Explanation - Complex I contains about 45 subunits in mammals.
Correct answer is: Complex I
Q.17 Which enzyme catalyzes the conversion of acetyl‑CoA to malonyl‑CoA in fatty acid synthesis?
Acetyl‑CoA carboxylase
Fatty acid synthase
HMG‑CoA reductase
Malonyl‑CoA decarboxylase
Explanation - It carboxylates acetyl‑CoA, forming the first committed step of fatty acid synthesis.
Correct answer is: Acetyl‑CoA carboxylase
Q.18 What is the primary function of the enzyme hexokinase?
Oxidize glucose to produce energy
Phosphorylate glucose to glucose‑6‑phosphate
Break down glycogen
Transport glucose across the plasma membrane
Explanation - Hexokinase locks glucose inside the cell by phosphorylating it.
Correct answer is: Phosphorylate glucose to glucose‑6‑phosphate
Q.19 Which coenzyme is essential for the transfer of the acetyl group from pyruvate to CoA?
NAD⁺
Coenzyme A
FAD
ATP
Explanation - CoA accepts the acetyl group to form Acetyl‑CoA.
Correct answer is: Coenzyme A
Q.20 Which compound is directly oxidized to produce NADH in the glycolytic pathway?
Fructose‑1,6‑bisphosphate
Dihydroxyacetone phosphate
1,3‑Bisphosphoglycerate
Glyceraldehyde‑3‑phosphate
Explanation - Glyceraldehyde‑3‑phosphate dehydrogenase converts it to 3-phosphoglycerate, generating NADH.
Correct answer is: 1,3‑Bisphosphoglycerate
Q.21 Which metabolic pathway is responsible for the detoxification of ammonia in the liver?
Urea cycle
Citric acid cycle
Oxidative phosphorylation
Glycolysis
Explanation - It converts toxic ammonia to urea for excretion.
Correct answer is: Urea cycle
Q.22 Which enzyme is inhibited by citrate, providing feedback regulation in the TCA cycle?
Isocitrate dehydrogenase
α‑Ketoglutarate dehydrogenase
Citrate synthase
Succinate dehydrogenase
Explanation - Citrate accumulation signals that energy demand is low.
Correct answer is: Isocitrate dehydrogenase
Q.23 What is the main function of the enzyme glutamate dehydrogenase?
Synthesize glutamate from α‑ketoglutarate and ammonia
Break down glutamate to α‑ketoglutarate
Transport glutamate across membranes
Regulate intracellular pH
Explanation - It catalyzes the reversible oxidative deamination of glutamate.
Correct answer is: Synthesize glutamate from α‑ketoglutarate and ammonia
Q.24 Which pathway primarily generates ATP in muscle cells during high‑intensity exercise?
Aerobic respiration
Anaerobic glycolysis
Citric acid cycle
Pentose phosphate pathway
Explanation - It rapidly produces ATP without oxygen, but yields only 2 ATP per glucose.
Correct answer is: Anaerobic glycolysis
Q.25 What is the main product of β‑oxidation?
Acetyl‑CoA
Glycerol
Lactate
Pyruvate
Explanation - Each cycle shortens a fatty acyl‑CoA by two carbons, releasing Acetyl‑CoA.
Correct answer is: Acetyl‑CoA
Q.26 Which molecule is directly oxidized during the first step of the electron transport chain?
NADH
FADH₂
Coenzyme Q
Cytochrome c
Explanation - NADH donates electrons to Complex I.
Correct answer is: NADH
Q.27 The enzyme responsible for the phosphorylation of ADP to ATP in mitochondria is:
ATP synthase
Hexokinase
Phosphofructokinase
Pyruvate kinase
Explanation - It utilizes the proton gradient to drive ATP synthesis.
Correct answer is: ATP synthase
Q.28 Which cofactor is essential for the activity of pyruvate dehydrogenase complex?
Thiamine pyrophosphate
Pyridoxal phosphate
Biotin
Coenzyme A
Explanation - TPP is the first cofactor required for the decarboxylation of pyruvate.
Correct answer is: Thiamine pyrophosphate
Q.29 The enzyme that converts acetyl‑CoA to fatty acids in the cytosol is called:
Acetyl‑CoA carboxylase
Acyl‑CoA oxidase
Fatty acid synthase
HMG‑CoA reductase
Explanation - It catalyzes the multi‑step synthesis of palmitate from acetyl‑CoA and malonyl‑CoA.
Correct answer is: Fatty acid synthase
Q.30 Which reaction is responsible for the production of NADPH during photosynthesis?
Calvin cycle
Light‑dependent reactions
Photorespiration
Oxidative phosphorylation
Explanation - They generate NADPH through the photosynthetic electron transport chain.
Correct answer is: Light‑dependent reactions
Q.31 The ratio of NAD⁺ to NADH in mitochondria is typically:
High NAD⁺, low NADH
Low NAD⁺, high NADH
Equal amounts of NAD⁺ and NADH
Only NADH is present
Explanation - A high NAD⁺/NADH ratio indicates a high capacity for oxidative metabolism.
Correct answer is: High NAD⁺, low NADH
Q.32 Which molecule is the primary product of the light‑independent reactions of photosynthesis?
Glucose
ATP
NADPH
O₂
Explanation - The Calvin cycle fixes CO₂ into glucose.
Correct answer is: Glucose
Q.33 In which organelle does fatty acid β‑oxidation take place?
Mitochondria
Peroxisomes
Endoplasmic reticulum
Chloroplasts
Explanation - Mitochondrial β‑oxidation is the main pathway for energy production.
Correct answer is: Mitochondria
Q.34 Which enzyme catalyzes the phosphorylation of glucose to glucose‑6‑phosphate in the first step of glycolysis?
Hexokinase
Phosphofructokinase
Pyruvate kinase
Glycogen phosphorylase
Explanation - It initiates glycolysis by trapping glucose inside the cell.
Correct answer is: Hexokinase
Q.35 The electron acceptor for Complex IV in the ETC is:
O₂
Cytochrome c
Coenzyme Q
NAD⁺
Explanation - O₂ accepts electrons, forming water in the final step of oxidative phosphorylation.
Correct answer is: O₂
Q.36 Which metabolic pathway is directly involved in the synthesis of cholesterol?
Mevalonate pathway
Pentose phosphate pathway
Glycolysis
Citric acid cycle
Explanation - It uses acetyl‑CoA to produce cholesterol via HMG‑CoA reductase.
Correct answer is: Mevalonate pathway
Q.37 The conversion of lactate back to pyruvate in the liver is catalyzed by:
Lactate dehydrogenase
Pyruvate kinase
Alcohol dehydrogenase
Alcohol oxidase
Explanation - It converts lactate to pyruvate while regenerating NAD⁺.
Correct answer is: Lactate dehydrogenase
Q.38 Which enzyme controls the rate of gluconeogenesis?
Fructose‑1,6‑bisphosphatase
Glyceraldehyde‑3‑phosphate dehydrogenase
Pyruvate kinase
Hexokinase
Explanation - It bypasses phosphofructokinase, allowing glucose synthesis.
Correct answer is: Fructose‑1,6‑bisphosphatase
Q.39 Which of the following best describes the role of Coenzyme Q in the electron transport chain?
It pumps protons across the membrane
It accepts electrons from Complex I and II
It supplies oxygen to the chain
It generates ATP directly
Explanation - Coenzyme Q shuttles electrons between complexes I/II and III.
Correct answer is: It accepts electrons from Complex I and II
Q.40 The enzyme that catalyzes the condensation of oxaloacetate with acetyl‑CoA is:
Citrate synthase
Succinate dehydrogenase
Fumarase
Malate dehydrogenase
Explanation - It initiates the citric acid cycle by forming citrate.
Correct answer is: Citrate synthase
Q.41 Which metabolite is a direct substrate for the synthesis of purine nucleotides?
Ribose‑5‑phosphate
Glucose‑6‑phosphate
Acetyl‑CoA
Fumarate
Explanation - It provides the sugar backbone for purine ribonucleotides.
Correct answer is: Ribose‑5‑phosphate
Q.42 Which enzyme is responsible for the conversion of malate to oxaloacetate?
Malate dehydrogenase
Fumarase
Succinate dehydrogenase
Citrate synthase
Explanation - It oxidizes malate to oxaloacetate, generating NADH.
Correct answer is: Malate dehydrogenase
Q.43 In which compartment does the TCA cycle occur in eukaryotic cells?
Cytoplasm
Mitochondrial matrix
Endoplasmic reticulum
Nucleus
Explanation - All enzymes of the TCA cycle are located in the mitochondrial matrix.
Correct answer is: Mitochondrial matrix
Q.44 Which of the following best explains why lactate can be a fuel for the heart?
It is converted to glucose in the heart
It is directly oxidized to produce ATP
It is stored as glycogen in cardiac tissue
It inhibits fatty acid oxidation
Explanation - Heart mitochondria can oxidize lactate to pyruvate and feed the TCA cycle.
Correct answer is: It is directly oxidized to produce ATP
Q.45 What is the primary coenzyme that carries electrons from Complex II to Complex III?
Coenzyme Q
Cytochrome c
NAD⁺
FAD
Explanation - Coenzyme Q receives electrons from Complex II and delivers them to Complex III.
Correct answer is: Coenzyme Q
Q.46 Which molecule is the primary product of the oxidative decarboxylation of isocitrate?
α‑Ketoglutarate
Citrate
Oxaloacetate
Succinyl‑CoA
Explanation - Isocitrate dehydrogenase converts isocitrate to α‑ketoglutarate, releasing CO₂ and NADH.
Correct answer is: α‑Ketoglutarate
Q.47 The enzyme that catalyzes the transfer of a phosphate group from ATP to pyruvate, producing phosphoenolpyruvate, is:
Enolase
Phosphoglycerate mutase
Phosphoenolpyruvate carboxykinase
Pyruvate kinase
Explanation - Pyruvate kinase converts phosphoenolpyruvate to pyruvate, generating ATP.
Correct answer is: Pyruvate kinase
Q.48 Which of the following statements best describes the role of NADPH in cellular metabolism?
It supplies reducing power for biosynthetic reactions
It directly generates ATP in mitochondria
It acts as a proton shuttle in the ETC
It oxidizes fatty acids
Explanation - NADPH is used in anabolic pathways, such as fatty acid and cholesterol synthesis.
Correct answer is: It supplies reducing power for biosynthetic reactions
Q.49 Which enzyme is the rate‑limiting step in glycolysis?
Hexokinase
Phosphofructokinase‑1
Pyruvate kinase
Glycogen phosphorylase
Explanation - PFK‑1 catalyzes the conversion of fructose‑6‑phosphate to fructose‑1,6‑bisphosphate.
Correct answer is: Phosphofructokinase‑1
Q.50 Which of the following is a direct product of the malic enzyme?
Pyruvate
Acetyl‑CoA
Citrate
Gluconeogenic glucose
Explanation - Malic enzyme decarboxylates malate to Acetyl‑CoA and NADPH.
Correct answer is: Acetyl‑CoA
Q.51 The enzyme that reduces HMG‑CoA to mevalonate is:
HMG‑CoA reductase
Acetyl‑CoA carboxylase
Isopentenyl pyrophosphate isomerase
Farnesyl‑transferase
Explanation - It is the rate‑limiting enzyme in cholesterol synthesis.
Correct answer is: HMG‑CoA reductase
Q.52 Which cofactor is required for the activity of transketolase in the non‑oxidative phase of the pentose phosphate pathway?
TTP
TPP
Coenzyme A
Biotin
Explanation - Transketolase requires thiamine pyrophosphate (TPP) as a cofactor.
Correct answer is: TPP
Q.53 Which metabolite is directly used by the urea cycle to convert ammonia into carbamoyl phosphate?
Phosphate
Ammonium chloride
Carbon dioxide
ATP
Explanation - ATP provides the energy to synthesize carbamoyl phosphate from ammonia and bicarbonate.
Correct answer is: ATP
Q.54 Which of the following best explains why fatty acids are preferred energy sources during fasting?
They are stored in glycogen granules
They yield more ATP per carbon unit
They are readily oxidized to glucose
They are synthesized in the liver during fasting
Explanation - Fatty acids provide 9.5 ATP per acetyl‑CoA via β‑oxidation and TCA cycle.
Correct answer is: They yield more ATP per carbon unit
Q.55 The enzyme that converts acetyl‑CoA and oxaloacetate to citrate is:
Citrate synthase
Isocitrate dehydrogenase
Succinate dehydrogenase
Malate dehydrogenase
Explanation - Citrate synthase catalyzes the first step of the citric acid cycle.
Correct answer is: Citrate synthase
Q.56 In which cellular organelle does gluconeogenesis primarily occur?
Cytoplasm
Mitochondria
Lysosomes
Peroxisomes
Explanation - The cytosolic enzymes of gluconeogenesis convert pyruvate to glucose.
Correct answer is: Cytoplasm
Q.57 Which metabolic pathway is responsible for the synthesis of riboflavin?
Riboflavin pathway
Purine biosynthesis
Urea cycle
TCA cycle
Explanation - Riboflavin is synthesized via a dedicated biosynthetic route.
Correct answer is: Riboflavin pathway
Q.58 Which enzyme catalyzes the phosphorylation of ADP to ATP using the proton gradient?
ATP synthase
Hexokinase
Phosphofructokinase
Pyruvate kinase
Explanation - ATP synthase utilizes the proton motive force to generate ATP.
Correct answer is: ATP synthase
Q.59 Which of the following is a major product of the oxidative phase of the pentose phosphate pathway?
ATP
NADPH
FADH₂
Acetyl‑CoA
Explanation - Glucose‑6‑phosphate dehydrogenase produces NADPH during the oxidative phase.
Correct answer is: NADPH
Q.60 Which enzyme is inhibited by ATP in the regulation of fatty acid synthesis?
Acetyl‑CoA carboxylase
HMG‑CoA reductase
Glycogen synthase
Pyruvate dehydrogenase
Explanation - High ATP levels inhibit acetyl‑CoA carboxylase, reducing fatty acid synthesis.
Correct answer is: Acetyl‑CoA carboxylase
Q.61 The enzyme responsible for the final step of the Calvin cycle is:
Rubisco
Phosphoribulokinase
Glyceraldehyde‑3‑phosphate dehydrogenase
Ribulose‑1,5‑bisphosphate carboxylase/oxygenase
Explanation - Rubisco catalyzes CO₂ fixation during photosynthesis.
Correct answer is: Rubisco
Q.62 Which molecule is directly oxidized by succinate dehydrogenase to produce FADH₂?
Succinate
Fumarate
Malate
Oxaloacetate
Explanation - Succinate dehydrogenase converts succinate to fumarate, generating FADH₂.
Correct answer is: Succinate
Q.63 The enzyme that converts pyruvate to lactate in anaerobic conditions is:
Lactate dehydrogenase
Pyruvate kinase
Phosphofructokinase
Glucose‑6‑phosphate dehydrogenase
Explanation - It regenerates NAD⁺ by reducing pyruvate to lactate.
Correct answer is: Lactate dehydrogenase
Q.64 Which of the following best describes the function of the malate‑aspartate shuttle?
Transfers reducing equivalents into mitochondria
Oxidizes NADH in the cytosol
Produces lactate in muscle cells
Generates ATP directly
Explanation - It moves electrons from cytosolic NADH into the mitochondrial matrix.
Correct answer is: Transfers reducing equivalents into mitochondria
Q.65 Which molecule is a direct product of the breakdown of glycogen in the liver?
Glucose‑6‑phosphate
Glucose‑1‑phosphate
Glucose
Fructose‑6‑phosphate
Explanation - Glycogenolysis ultimately releases free glucose.
Correct answer is: Glucose
Q.66 Which enzyme catalyzes the condensation of two acetyl‑CoA molecules to form acetoacetyl‑CoA?
Acetoacetyl‑CoA thiolase
HMG‑CoA synthase
Acetyl‑CoA carboxylase
Fumarase
Explanation - Acetoacetyl‑CoA thiolase joins two acetyl‑CoA molecules.
Correct answer is: Acetoacetyl‑CoA thiolase
Q.67 The enzyme that catalyzes the conversion of acetyl‑CoA to malonyl‑CoA in fatty acid synthesis is:
Acetyl‑CoA carboxylase
Acyl‑CoA dehydrogenase
HMG‑CoA reductase
Malonyl‑CoA decarboxylase
Explanation - Acetyl‑CoA carboxylase is the first committed step in fatty acid synthesis.
Correct answer is: Acetyl‑CoA carboxylase
Q.68 Which coenzyme is required for the activity of pyruvate dehydrogenase complex?
Thiamine pyrophosphate
Pyridoxal phosphate
Biotin
Coenzyme A
Explanation - TPP is the first cofactor in the decarboxylation of pyruvate.
Correct answer is: Thiamine pyrophosphate
Q.69 The enzyme that catalyzes the conversion of α‑ketoglutarate to succinyl‑CoA is:
α‑Ketoglutarate dehydrogenase
Succinate dehydrogenase
Malate dehydrogenase
Citrate synthase
Explanation - It catalyzes the oxidative decarboxylation to produce succinyl‑CoA.
Correct answer is: α‑Ketoglutarate dehydrogenase
Q.70 Which of the following is the most abundant electron donor to the electron transport chain?
NADH
FADH₂
Coenzyme Q
Cytochrome c
Explanation - NADH contributes more electrons to the ETC than FADH₂.
Correct answer is: NADH
Q.71 The enzyme responsible for the conversion of malate to oxaloacetate is:
Malate dehydrogenase
Fumarase
Succinate dehydrogenase
Citrate synthase
Explanation - It oxidizes malate, producing NADH and oxaloacetate.
Correct answer is: Malate dehydrogenase
Q.72 Which enzyme is the rate‑limiting step in the synthesis of fatty acids?
Acetyl‑CoA carboxylase
Fatty acid synthase
HMG‑CoA reductase
Sphingomyelin synthase
Explanation - It catalyzes the formation of malonyl‑CoA, the committed step.
Correct answer is: Acetyl‑CoA carboxylase
Q.73 Which of the following best describes the role of NAD⁺ in glycolysis?
It serves as a substrate for ATP synthase
It accepts electrons from glyceraldehyde‑3‑phosphate
It donates a phosphate group to ADP
It is converted to lactate
Explanation - NAD⁺ is reduced to NADH during the glyceraldehyde‑3‑phosphate dehydrogenase step.
Correct answer is: It accepts electrons from glyceraldehyde‑3‑phosphate
Q.74 Which enzyme catalyzes the oxidation of fumarate to malate?
Fumarase
Succinate dehydrogenase
Citrate synthase
Malate dehydrogenase
Explanation - Fumarase catalyzes the hydration of fumarate to malate.
Correct answer is: Fumarase
Q.75 The enzyme that converts glucose‑6‑phosphate to 6‑phosphoglucono‑δ‑lactone in the pentose phosphate pathway is:
Glucose‑6‑phosphate dehydrogenase
6‑Phosphogluconate dehydrogenase
Phosphoglucomutase
Hexokinase
Explanation - It initiates the oxidative phase, producing NADPH.
Correct answer is: Glucose‑6‑phosphate dehydrogenase
Q.76 Which metabolite is the primary end product of aerobic respiration?
Lactate
Acetyl‑CoA
CO₂
Water
Explanation - Aerobic respiration oxidizes glucose completely, releasing CO₂.
Correct answer is: CO₂
Q.77 Which enzyme converts citrate to isocitrate in the TCA cycle?
Aconitase
Isocitrate dehydrogenase
Succinate dehydrogenase
Malate dehydrogenase
Explanation - Aconitase catalyzes the reversible hydration of citrate to isocitrate.
Correct answer is: Aconitase
Q.78 The enzyme that catalyzes the phosphorylation of ADP to ATP during oxidative phosphorylation is:
ATP synthase
Hexokinase
Phosphofructokinase
Pyruvate kinase
Explanation - It synthesizes ATP using the proton gradient.
Correct answer is: ATP synthase
Q.79 Which of the following statements best describes the role of Coenzyme Q?
It pumps protons across the membrane
It shuttles electrons between Complex I/II and III
It accepts oxygen in the ETC
It directly generates ATP
Explanation - Coenzyme Q (ubiquinone) transfers electrons to Complex III.
Correct answer is: It shuttles electrons between Complex I/II and III
Q.80 Which enzyme catalyzes the conversion of acetyl‑CoA to acetoacetate during ketogenesis?
HMG‑CoA synthase
HMG‑CoA lyase
Acetoacetyl‑CoA thiolase
Acetyl‑CoA carboxylase
Explanation - It forms HMG‑CoA, the precursor for ketone bodies.
Correct answer is: HMG‑CoA synthase
Q.81 The enzyme that converts pyruvate to oxaloacetate during gluconeogenesis is:
Phosphoenolpyruvate carboxykinase
Pyruvate carboxylase
Fructose‑1,6‑bisphosphatase
Glycerol‑3‑phosphate dehydrogenase
Explanation - It catalyzes the carboxylation of pyruvate to oxaloacetate.
Correct answer is: Pyruvate carboxylase
Q.82 Which molecule is the end product of the citric acid cycle?
Acetyl‑CoA
Oxaloacetate
Citrate
Succinate
Explanation - Oxaloacetate is regenerated to start another turn of the cycle.
Correct answer is: Oxaloacetate
Q.83 Which enzyme is the primary regulator of glycolysis under high energy conditions?
Hexokinase
Phosphofructokinase‑1
Pyruvate kinase
Glycogen synthase
Explanation - PFK‑1 is inhibited by ATP and citrate, slowing glycolysis.
Correct answer is: Phosphofructokinase‑1
Q.84 What is the function of the enzyme succinyl‑CoA synthetase?
Oxidize succinyl‑CoA to succinate
Generate GTP or ATP from succinyl‑CoA
Convert oxaloacetate to malate
Reduce NAD⁺ to NADH
Explanation - It catalyzes substrate‑level phosphorylation in the TCA cycle.
Correct answer is: Generate GTP or ATP from succinyl‑CoA
Q.85 Which enzyme converts 3‑phosphoglycerate to 2‑phosphoglycerate in glycolysis?
Phosphoglycerate mutase
Phosphoglycerate kinase
Glyceraldehyde‑3‑phosphate dehydrogenase
Enolase
Explanation - It catalyzes the shift of the phosphate group.
Correct answer is: Phosphoglycerate mutase
Q.86 Which of the following best describes the role of glutamate in amino acid metabolism?
It directly participates in the TCA cycle
It transfers amino groups between amino acids
It is the final product of catabolism
It is only present in plants
Explanation - Glutamate acts as an amino group donor via transamination reactions.
Correct answer is: It transfers amino groups between amino acids
Q.87 Which coenzyme is essential for fatty acid synthesis?
Coenzyme A
NADPH
FAD
ATP
Explanation - Coenzyme A carries acyl groups during fatty acid synthesis.
Correct answer is: Coenzyme A
Q.88 Which enzyme is responsible for converting acetyl‑CoA and HMG‑CoA to mevalonate?
HMG‑CoA reductase
Acetyl‑CoA carboxylase
Acetoacetyl‑CoA thiolase
Farnesyl‑transferase
Explanation - It catalyzes the rate‑limiting step of cholesterol synthesis.
Correct answer is: HMG‑CoA reductase
Q.89 In the electron transport chain, which complex directly receives electrons from NADH?
Complex I
Complex II
Complex III
Complex IV
Explanation - Complex I oxidizes NADH and pumps protons.
Correct answer is: Complex I
Q.90 The enzyme that catalyzes the conversion of acetyl‑CoA to malonyl‑CoA in fatty acid synthesis is:
Acetyl‑CoA carboxylase
Acyl‑CoA dehydrogenase
HMG‑CoA reductase
Malonyl‑CoA decarboxylase
Explanation - It is the first committed step in fatty acid synthesis.
Correct answer is: Acetyl‑CoA carboxylase
Q.91 Which molecule is the primary product of the citric acid cycle that feeds the electron transport chain?
CO₂
NADH
FADH₂
ATP
Explanation - NADH generated in the TCA cycle donates electrons to the ETC.
Correct answer is: NADH
Q.92 The enzyme that catalyzes the conversion of fumarate to malate is:
Fumarase
Succinate dehydrogenase
Citrate synthase
Malate dehydrogenase
Explanation - Fumarase hydrates fumarate to malate in the TCA cycle.
Correct answer is: Fumarase
Q.93 Which of the following best describes the role of ATP in glycolysis?
It is a substrate for phosphofructokinase
It is directly oxidized by glyceraldehyde‑3‑phosphate
It is produced by pyruvate kinase
It is required for the conversion of glucose to glucose‑6‑phosphate
Explanation - Hexokinase uses ATP to phosphorylate glucose.
Correct answer is: It is required for the conversion of glucose to glucose‑6‑phosphate
Q.94 The enzyme that converts acetyl‑CoA to HMG‑CoA in cholesterol synthesis is:
HMG‑CoA synthase
Acetyl‑CoA carboxylase
Acyl‑CoA oxidase
HMG‑CoA reductase
Explanation - It condenses acetyl‑CoA molecules to form HMG‑CoA.
Correct answer is: HMG‑CoA synthase
Q.95 Which metabolite is directly oxidized to produce FADH₂ during the TCA cycle?
Succinate
Fumarate
Malate
Oxaloacetate
Explanation - Succinate dehydrogenase converts succinate to fumarate, generating FADH₂.
Correct answer is: Succinate
Q.96 Which enzyme catalyzes the conversion of glucose‑6‑phosphate to fructose‑6‑phosphate?
Phosphoglucomutase
Hexokinase
Phosphofructokinase
Glycogen phosphorylase
Explanation - It rearranges the phosphate group to produce fructose‑6‑phosphate.
Correct answer is: Phosphoglucomutase
Q.97 Which of the following best describes the function of the enzyme pyruvate kinase?
Phosphorylates glucose
Converts pyruvate to lactate
Phosphorylates ADP to ATP in glycolysis
Catalyzes the first step of the TCA cycle
Explanation - Pyruvate kinase catalyzes the final substrate‑level phosphorylation step.
Correct answer is: Phosphorylates ADP to ATP in glycolysis
Q.98 The enzyme that catalyzes the conversion of acetyl‑CoA to succinyl‑CoA is:
α‑Ketoglutarate dehydrogenase
Citrate synthase
Succinate dehydrogenase
Malate dehydrogenase
Explanation - It catalyzes the oxidative decarboxylation of α‑ketoglutarate.
Correct answer is: α‑Ketoglutarate dehydrogenase
Q.99 The enzyme that converts lactate back to pyruvate in the liver is:
Lactate dehydrogenase
Pyruvate dehydrogenase
Glycogen phosphorylase
Enolase
Explanation - It oxidizes lactate to pyruvate, regenerating NAD⁺.
Correct answer is: Lactate dehydrogenase
Q.100 Which cofactor is required for the activity of the enzyme transketolase in the non‑oxidative PPP?
TPP
Biotin
Coenzyme A
Pyridoxal phosphate
Explanation - Transketolase needs thiamine pyrophosphate (TPP) as a cofactor.
Correct answer is: TPP
Q.101 What is the main product of fatty acid β‑oxidation in mitochondria?
Acetyl‑CoA
Glycine
Lactate
Malate
Explanation - Each cycle produces Acetyl‑CoA, which enters the TCA cycle.
Correct answer is: Acetyl‑CoA
Q.102 Which enzyme is the rate‑limiting step in the synthesis of purine nucleotides?
PRPP synthetase
GMP synthase
IMP dehydrogenase
Aminoimidazole‑carboxamide ribonucleotide formyltransferase
Explanation - It generates PRPP, the ribose phosphate donor for purine synthesis.
Correct answer is: PRPP synthetase
Q.103 The enzyme that converts oxaloacetate to malate is:
Malate dehydrogenase
Citrate synthase
Fumarase
Succinate dehydrogenase
Explanation - It reduces oxaloacetate to malate, generating NADH.
Correct answer is: Malate dehydrogenase
Q.104 Which of the following best describes the function of the enzyme glucose‑6‑phosphate dehydrogenase?
It generates ATP via substrate‑level phosphorylation
It oxidizes glucose‑6‑phosphate to produce NADPH
It catalyzes the final step of glycolysis
It phosphorylates fructose‑6‑phosphate
Explanation - It catalyzes the first step of the oxidative PPP, generating NADPH.
Correct answer is: It oxidizes glucose‑6‑phosphate to produce NADPH
Q.105 Which of the following is a major source of ATP in the brain?
Lactate
Pyruvate
Glucose
Acetyl‑CoA
Explanation - Glucose is the primary fuel for cerebral oxidative metabolism.
Correct answer is: Glucose
Q.106 In which pathway is ribose‑5‑phosphate produced?
Citric acid cycle
Pentose phosphate pathway
Glycolysis
Beta‑oxidation
Explanation - The PPP generates ribose‑5‑phosphate for nucleotide synthesis.
Correct answer is: Pentose phosphate pathway
Q.107 Which enzyme is the most energy‑efficient for ATP production in mitochondria?
Complex I
Complex II
Complex III
Complex IV
Explanation - Complex I pumps protons and thus contributes most to the proton gradient.
Correct answer is: Complex I
Q.108 The enzyme that catalyzes the conversion of pyruvate to acetyl‑CoA is:
Pyruvate dehydrogenase
Pyruvate kinase
Pyruvate carboxylase
Glyceraldehyde‑3‑phosphate dehydrogenase
Explanation - It oxidatively decarboxylates pyruvate to Acetyl‑CoA.
Correct answer is: Pyruvate dehydrogenase
Q.109 Which of the following best describes the role of citrate in the regulation of glycolysis?
It activates phosphofructokinase‑1
It inhibits phosphofructokinase‑1
It directly generates ATP
It is converted to lactate
Explanation - Citrate allosterically inhibits PFK‑1, reducing glycolytic flux.
Correct answer is: It inhibits phosphofructokinase‑1
Q.110 Which enzyme converts pyruvate to oxaloacetate during gluconeogenesis?
Pyruvate carboxylase
Phosphoenolpyruvate carboxykinase
Fructose‑1,6‑bisphosphatase
Glucose‑6‑phosphatase
Explanation - It catalyzes the ATP‑dependent carboxylation of pyruvate.
Correct answer is: Pyruvate carboxylase
Q.111 Which metabolite is the end product of anaerobic glycolysis in muscle cells?
Pyruvate
Lactate
Acetyl‑CoA
CO₂
Explanation - Anaerobic glycolysis converts pyruvate to lactate to regenerate NAD⁺.
Correct answer is: Lactate
Q.112 In the urea cycle, which enzyme catalyzes the condensation of carbamoyl phosphate and ornithine to citrulline?
Citrulline synthase
Ornithine transcarbamylase
Arginase
Carbamoyl phosphate synthetase I
Explanation - It combines ornithine with carbamoyl phosphate to form citrulline.
Correct answer is: Ornithine transcarbamylase
Q.113 Which of the following best describes the function of the enzyme succinate dehydrogenase?
Oxidizes succinate to fumarate and reduces FAD
Reduces fumarate to malate
Oxidizes malate to oxaloacetate
Reduces citrate to isocitrate
Explanation - Succinate dehydrogenase is part of both the TCA cycle and the ETC.
Correct answer is: Oxidizes succinate to fumarate and reduces FAD
Q.114 Which coenzyme is required for fatty acid chain elongation?
Coenzyme A
NADPH
ATP
FAD
Explanation - CoA carries growing acyl chains during fatty acid synthesis.
Correct answer is: Coenzyme A
Q.115 The enzyme that converts acetyl‑CoA to acetyl‑CoA in the cytosol is:
Acetyl‑CoA synthetase
Acyl‑CoA oxidase
HMG‑CoA synthase
Acetyl‑CoA carboxylase
Explanation - It generates Acetyl‑CoA from acetate in the cytosol.
Correct answer is: Acetyl‑CoA synthetase
Q.116 Which enzyme catalyzes the formation of glucose from gluconeogenic precursors?
Glucose‑6‑phosphatase
Fructose‑1,6‑bisphosphatase
Glycogen phosphorylase
Phosphofructokinase
Explanation - It removes the phosphate from glucose‑6‑phosphate to release free glucose.
Correct answer is: Glucose‑6‑phosphatase
Q.117 Which of the following best explains the function of the enzyme pyruvate carboxylase?
It converts pyruvate to acetyl‑CoA
It converts pyruvate to oxaloacetate
It converts pyruvate to lactate
It converts pyruvate to glucose
Explanation - Pyruvate carboxylase is the anaplerotic enzyme replenishing TCA cycle intermediates.
Correct answer is: It converts pyruvate to oxaloacetate
Q.118 Which enzyme is responsible for the decarboxylation of isocitrate in the citric acid cycle?
Isocitrate dehydrogenase
Citrate synthase
Aconitase
Succinate dehydrogenase
Explanation - It converts isocitrate to α‑ketoglutarate, releasing CO₂ and NADH.
Correct answer is: Isocitrate dehydrogenase
Q.119 Which enzyme converts 6‑phosphogluconate to ribulose‑5‑phosphate?
6‑Phosphogluconate dehydrogenase
Glucose‑6‑phosphate dehydrogenase
Phosphoglucomutase
Enolase
Explanation - It completes the oxidative phase of the PPP, generating NADPH.
Correct answer is: 6‑Phosphogluconate dehydrogenase
Q.120 The enzyme that converts oxaloacetate to phosphoenolpyruvate during gluconeogenesis is:
Phosphoenolpyruvate carboxykinase
Fructose‑1,6‑bisphosphatase
Glucose‑6‑phosphatase
Pyruvate kinase
Explanation - It catalyzes the GTP‑dependent conversion of oxaloacetate to PEP.
Correct answer is: Phosphoenolpyruvate carboxykinase
Q.121 Which of the following best describes the role of the enzyme phosphofructokinase‑1?
It phosphorylates glucose
It catalyzes the reversible conversion of fructose‑6‑phosphate to fructose‑1,6‑bisphosphate
It converts pyruvate to lactate
It converts acetyl‑CoA to malonyl‑CoA
Explanation - PFK‑1 is the key regulatory enzyme of glycolysis.
Correct answer is: It catalyzes the reversible conversion of fructose‑6‑phosphate to fructose‑1,6‑bisphosphate
Q.122 Which enzyme is the first step in the breakdown of glycogen to glucose‑1‑phosphate?
Glycogen phosphorylase
Glycogen synthase
Phosphoglucomutase
Hexokinase
Explanation - It releases glucose‑1‑phosphate from glycogen.
Correct answer is: Glycogen phosphorylase
Q.123 Which coenzyme is essential for the dehydrogenation of 2‑oxoglutarate in the TCA cycle?
NAD⁺
FAD
Coenzyme A
TPP
Explanation - α‑Ketoglutarate dehydrogenase uses NAD⁺ to produce NADH.
Correct answer is: NAD⁺
Q.124 Which enzyme catalyzes the conversion of acetyl‑CoA to acetyl‑phosphatidate?
Acyl‑CoA synthetase
Acetyl‑CoA carboxylase
Acyl‑CoA dehydrogenase
Acetyl‑phosphate synthase
Explanation - It carboxylates acetyl‑CoA to generate malonyl‑CoA for fatty acid synthesis.
Correct answer is: Acetyl‑CoA carboxylase
Q.125 The enzyme that converts pyruvate to oxaloacetate during gluconeogenesis requires which cofactor?
ATP
GTP
NADH
Coenzyme A
Explanation - Pyruvate carboxylase uses ATP to carboxylate pyruvate.
Correct answer is: ATP
Q.126 Which enzyme is responsible for the formation of the first two carbons of fatty acid chains?
Acetyl‑CoA carboxylase
Acetyl‑CoA synthetase
Fatty acid synthase
HMG‑CoA synthase
Explanation - It generates malonyl‑CoA, the extender unit in fatty acid synthesis.
Correct answer is: Acetyl‑CoA carboxylase
Q.127 Which molecule directly receives electrons from Complex IV in the ETC?
Cytochrome c
Coenzyme Q
ATP synthase
Complex I
Explanation - Complex IV transfers electrons to cytochrome c, which then passes them to Complex III.
Correct answer is: Cytochrome c
Q.128 In the TCA cycle, which intermediate is the most abundant?
Citrate
α‑Ketoglutarate
Succinyl‑CoA
Oxaloacetate
Explanation - Citrate accumulates when the cycle is active and can exit for fatty acid synthesis.
Correct answer is: Citrate
Q.129 The enzyme that converts glutamine to glutamate is:
Glutaminase
Glutamine synthetase
Glutamate dehydrogenase
Glutamine amidotransferase
Explanation - Glutaminase hydrolyzes glutamine to glutamate, releasing ammonia.
Correct answer is: Glutaminase
Q.130 Which enzyme is the most important for the synthesis of NADPH during the oxidative PPP?
Glucose‑6‑phosphate dehydrogenase
6‑Phosphogluconate dehydrogenase
Transketolase
Transaldolase
Explanation - It is the rate‑determining step in NADPH production.
Correct answer is: Glucose‑6‑phosphate dehydrogenase
Q.131 Which of the following is a direct product of the oxidative decarboxylation of isocitrate?
α‑Ketoglutarate
Citrate
Oxaloacetate
Succinate
Explanation - Isocitrate dehydrogenase converts isocitrate to α‑ketoglutarate.
Correct answer is: α‑Ketoglutarate