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Question 1. Which mitochondrial compartment contains the enzymes of the citric acid cycle? A) Outer membrane B) Intermembrane space C) Matrix D) Cristae surface Answer: C Explanation: The citric acid cycle enzymes are located in the mitochondrial matrix where acetyl‑CoA is oxidized. Question 2. The primary function of the inner mitochondrial membrane’s cristae is to: A) Store calcium ions B) Increase surface area for oxidative phosphorylation C) Protect mitochondrial DNA D) Facilitate protein import Answer: B Explanation: Cristae fold the inner membrane, vastly increasing the area available for the electron transport chain and ATP synthase complexes. Question 3. ATP is produced by substrate‑level phosphorylation during glycolysis at which two steps? A) Hexokinase and phosphoglucose isomerase B) Phosphofructokinase‑1 and aldolase C) Phosphoglycerate kinase and pyruvate kinase D) Glyceraldehyde‑ 3 ‑phosphate dehydrogenase and enolase Answer: C
Explanation: Both phosphoglycerate kinase and pyruvate kinase transfer a phosphate from a high‑energy substrate directly to ADP, forming ATP. Question 4. Which enzyme catalyzes the rate‑limiting step of glycolysis? A) Hexokinase B) Phosphofructokinase‑1 (PFK‑1) C) Pyruvate kinase D) Aldolase Answer: B Explanation: PFK‑1 converts fructose‑ 6 ‑phosphate to fructose‑1,6‑bisphosphate and is highly regulated, making it the glycolytic “valve.” Question 5. During the energy‑investment phase of glycolysis, how many ATP molecules are consumed per glucose molecule? A) 0 B) 1 C) 2 D) 4 Answer: C Explanation: One ATP is used by hexokinase and another by phosphofructokinase‑1, totaling two ATP molecules. Question 6. The net gain of NADH molecules from one molecule of glucose during glycolysis is: A) 0 B) 1 C) 2
C) 2 ATP (or GTP) D) 1 NADPH Answer: D Explanation: NADPH is not generated in the citric acid cycle; the cycle yields NADH, FADH₂, and GTP (or ATP). Question 10. The enzyme that regenerates oxaloacetate from malate is: A) Succinate dehydrogenase B) Malate dehydrogenase C) Aconitase D) Succinyl‑CoA synthetase Answer: B Explanation: Malate dehydrogenase oxidizes malate to oxaloacetate, completing the cycle. Question 11. Which complex of the electron transport chain directly pumps protons from the matrix to the intermembrane space? A) Complex I only B) Complex III only C) Complex IV only D) Complexes I, III, and IV Answer: D Explanation: Complexes I, III, and IV each translocate protons across the inner membrane, establishing the proton motive force. Question 12. Ubiquinone (CoQ) functions in the ETC as a: A) Protein kinase
B) Mobile electron carrier C) Proton pump D) ATP synthase subunit Answer: B Explanation: CoQ shuttles electrons between Complex I (or II) and Complex III while remaining lipid‑soluble within the inner membrane. Question 13. The final electron acceptor in aerobic respiration is: A) NAD⁺ B) FAD C) Oxygen (O₂) D) Water (H₂O) Answer: C Explanation: Oxygen accepts electrons at Complex IV, forming water and allowing continuous electron flow through the chain. Question 14. Chemiosmosis couples the flow of protons back into the matrix with: A) NADH oxidation B) ATP synthesis by ATP synthase C) Pyruvate transport D) Citrate formation Answer: B Explanation: The proton gradient drives the rotary mechanism of ATP synthase, synthesizing ATP from ADP and Pi.
Explanation: Pyruvate decarboxylase removes CO₂ from pyruvate, producing acetaldehyde, which is then reduced to ethanol. Question 18. The ATP yield from one molecule of glucose under completely aerobic conditions is approximately: A) 2 B) 12 C) 30‑ 32 D) 48 Answer: C Explanation: Aerobic respiration yields about 30‑32 ATP, accounting for glycolysis, pyruvate oxidation, Krebs cycle, and oxidative phosphorylation. Question 19. Which metabolite acts as an allosteric inhibitor of phosphofructokinase‑1? A) AMP B) Fructose‑2,6‑bisphosphate C) Citrate D) ADP Answer: C Explanation: High citrate levels signal abundant energy and inhibit PFK‑1, slowing glycolysis. Question 20. Beta‑oxidation of fatty acids yields which of the following per two‑carbon unit removed? A) 1 NADH, 1 FADH₂, and 1 Acetyl‑CoA B) 2 NADH, 2 FADH₂, and 2 Acetyl‑CoA C) 1 NADPH, 1 FADH₂, and 1 Acetyl‑CoA
D) No NADH, only ATP Answer: A Explanation: Each cycle of beta‑oxidation produces one NADH, one FADH₂, and one acetyl‑CoA for the citric acid cycle. Question 21. Which mitochondrial carrier protein transports ADP into the matrix and ATP out to the cytosol? A) Phosphate carrier B) ATP/ADP translocase C) Carnitine‑acylcarnitine translocase D) Citrate transporter Answer: B Explanation: The adenine nucleotide translocator (ANT) exchanges ADP⁻³ for ATP³⁻ across the inner membrane. Question 22. The proton‑motive force consists of two components. Which pair correctly describes them? A) Electrical potential (ΔΨ) and pH gradient (ΔpH) B) Temperature gradient and osmotic pressure C) NADH/NAD⁺ ratio and ATP/ADP ratio D) CO₂ concentration and H₂O concentration Answer: A Explanation: ΔΨ (membrane potential) and ΔpH (chemical gradient) together drive ATP synthesis. Question 23. Which of the following enzymes requires thiamine pyrophosphate (TPP) as a cofactor? A) Pyruvate kinase
Question 26. The enzyme citrate synthase catalyzes the formation of citrate from acetyl‑CoA and: A) Malate B) Oxaloacetate C) α‑Ketoglutarate D) Succinate Answer: B Explanation: Citrate synthase condenses acetyl‑CoA with oxaloacetate, initiating the Krebs cycle. Question 27. Which of the following best explains why the electron transport chain is located in the inner mitochondrial membrane rather than the outer membrane? A) The inner membrane has a higher protein‑to‑lipid ratio, allowing efficient proton pumping. B) The outer membrane is too permeable to protons. C) Both A and B. D) Neither A nor B. Answer: C Explanation: The inner membrane’s tightly packed proteins enable proton translocation, while the outer membrane’s porins would dissipate the gradient. Question 28. Which metabolic intermediate can be diverted from the citric acid cycle for fatty acid synthesis? A) Succinate B) Fumarate C) Citrate D) Malate Answer: C
Explanation: Citrate can be exported to the cytosol, cleaved by ATP‑citrate lyase, providing acetyl‑CoA for fatty acid biosynthesis. Question 29. In the context of respiration, feedback inhibition by ATP primarily affects which enzyme? A) Pyruvate kinase B) Isocitrate dehydrogenase C) Hexokinase D) Succinate dehydrogenase Answer: B Explanation: High ATP allosterically inhibits isocitrate dehydrogenase, decreasing the rate of the Krebs cycle. Question 30. Which of the following is a direct product of Complex II (succinate dehydrogenase) activity? A) NADH B) FADH₂ C) ATP D) CO₂ Answer: B Explanation: Complex II oxidizes succinate to fumarate, reducing FAD to FADH₂, which then transfers electrons to ubiquinone. Question 31. The mitochondrial matrix enzyme that converts ADP to ATP using the proton gradient is called: A) Cytochrome c oxidase B) ATP synthase (Complex V)
A) Pyruvate carboxylase B) Pyruvate kinase C) Lactate dehydrogenase D) Malic enzyme Answer: A Explanation: Pyruvate carboxylase uses ATP and CO₂ to carboxylate pyruvate, forming oxaloacetate for gluconeogenesis. Question 35. Which of the following statements about the mitochondrial intermembrane space is correct? A) It contains the enzymes of the citric acid cycle. B) Its pH is similar to the cytosol. C) It accumulates protons pumped by the ETC, creating a gradient. D) It is the site of ATP synthesis. Answer: C Explanation: The ETC pumps protons from the matrix into the intermembrane space, establishing the electrochemical gradient used for chemiosmosis. Question 36. In the context of metabolic regulation, an increase in which molecule would most likely activate phosphofructokinase‑1? A) ATP B) Citrate C) AMP D) H₂O Answer: C Explanation: AMP signals low energy status and allosterically activates PFK‑1, stimulating glycolysis.
Question 37. Which of the following is the immediate electron donor to Complex I? A) FADH₂ B) NADH C) Ubiquinol (QH₂) D) Cytochrome c Answer: B Explanation: NADH transfers its two electrons to Complex I (NADH:ubiquinone oxidoreductase). Question 38. The process by which electrons are transferred from NADH to oxygen through a series of carriers is called: A) Substrate‑level phosphorylation B) Oxidative decarboxylation C) Electron transport chain (ETC) D) Fermentation Answer: C Explanation: The ETC moves electrons stepwise from NADH to O₂, generating a proton gradient. Question 39. Which of the following metabolites is directly produced by the enzyme succinyl‑CoA synthetase? A) GTP (or ATP) B) NADH C) FADH₂ D) CO₂ Answer: A
Answer: A Explanation: Isocitrate dehydrogenase oxidatively decarboxylates isocitrate, yielding NADH and releasing CO₂. Question 43. Which metabolite can be used by both the citric acid cycle and the gluconeogenesis pathway? A) Acetyl‑CoA B) Oxaloacetate C) Succinate D) Malonate Answer: B Explanation: Oxaloacetate is a Krebs cycle intermediate and a key substrate for gluconeogenesis. Question 44. Which of the following best describes the role of the mitochondrial calcium uniporter? A) Export calcium from the matrix. B) Import calcium into the matrix, stimulating dehydrogenases. C) Transport ATP out of the mitochondria. D) Pump protons across the inner membrane. Answer: B Explanation: Increased matrix Ca²⁺ activates several dehydrogenases, enhancing NADH production. Question 45. In the context of respiration, the term “P/O ratio” refers to: A) Phosphate to oxygen ratio in water. B) Number of ATP molecules synthesized per atom of oxygen reduced. C) Number of protons pumped per NADH molecule.
D) Ratio of pyruvate to oxygen consumed. Answer: B Explanation: The P/O ratio quantifies ATP formed per oxygen atom reduced (or per pair of electrons transferred to O₂). Question 46. Which of the following statements about the mitochondrial inner membrane potential (ΔΨ) is correct? A) It is primarily generated by the movement of ATP. B) It is a negative charge inside the matrix relative to the intermembrane space. C) It is irrelevant for ATP synthesis. D) It is created by the influx of sodium ions. Answer: B Explanation: The ETC pumps positive charges (protons) out, leaving the matrix relatively negative, establishing ΔΨ. Question 47. The enzyme that converts acetyl‑CoA to citrate is: A) Aconitase B) Citrate synthase C) Malate dehydrogenase D) α‑Ketoglutarate dehydrogenase Answer: B Explanation: Citrate synthase catalyzes the condensation of acetyl‑CoA with oxaloacetate to form citrate. Question 48. During oxidative phosphorylation, the primary source of the proton gradient is: A) ATP hydrolysis by ATP synthase.
Question 51. Which of the following best explains why the citric acid cycle is considered amphibolic? A) It only produces ATP. B) Its intermediates serve both catabolic and anabolic pathways. C) It occurs in both mitochondria and cytosol. D) It requires both oxygen and carbon dioxide. Answer: B Explanation: Intermediates are drawn off for biosynthesis (e.g., amino acids) while the cycle also oxidizes substrates for energy. Question 52. The enzyme that transfers electrons from FADH₂ to ubiquinone is part of which complex? A) Complex I B) Complex II C) Complex III D) Complex IV Answer: B Explanation: Complex II (succinate dehydrogenase) contains FAD and passes electrons to ubiquinone. Question 53. Which of the following statements about the mitochondrial permeability transition pore (mPTP) is true? A) Its opening enhances ATP production. B) Its prolonged opening can trigger apoptosis. C) It transports NADH across the inner membrane. D) It is a component of Complex IV. Answer: B
Explanation: Uncontrolled mPTP opening collapses the membrane potential, leading to cell death pathways. Question 54. During high‑altitude adaptation, cells increase expression of which enzyme to improve oxygen utilization? A) Hexokinase B) Cytochrome c oxidase (Complex IV) C) Lactate dehydrogenase D) Pyruvate carboxylase Answer: B Explanation: Up‑regulation of Complex IV enhances the capacity of the ETC to reduce available oxygen efficiently. Question 55. Which metabolite directly inhibits the enzyme pyruvate dehydrogenase complex (PDC) through phosphorylation? A) Acetyl‑CoA B) ADP C) NAD⁺ D) FAD Answer: A Explanation: High acetyl‑CoA levels activate pyruvate dehydrogenase kinase, which phosphorylates and inactivates PDC. Question 56. The conversion of succinate to fumarate in the citric acid cycle is coupled to: A) Production of NADH B) Production of FADH₂ C) Direct synthesis of ATP
