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Respiration Ultimate Exam
Question 1. Which mitochondrial compartment contains the enzymes of the citric acid (Krebs) cycle? A) Outer membrane B) Intermembrane space C) Matrix D) Cristae surface Answer: C Explanation: The matrix is the innermost compartment of the mitochondrion and houses all enzymes of the Krebs cycle. 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) Produce ATP directly via substrate-level phosphorylation D) Protect mitochondrial DNA Answer: B Explanation: Cristae fold the inner membrane, dramatically expanding its surface area, which is essential for housing the electron-transport chain complexes and ATP synthase. Question 3. Which molecule is the universal energy currency of the cell? A) ADP B) NADH C) ATP D) GTP Answer: C Explanation: Adenosine triphosphate (ATP) stores and transfers energy in cells through the hydrolysis of its high-energy phosphate bonds.
Respiration Ultimate Exam
Question 4. In the ATP-hydrolysis reaction, the bond broken to release energy is between: A) The α- and β-phosphate B) The β- and γ-phosphate C) The ribose and adenine D) The phosphate and oxygen of the inorganic phosphate (Pi) Answer: B Explanation: Hydrolysis of the terminal (β-γ) phosphoanhydride bond releases ~7.3 kcal/mol of free energy. Question 5. Which factor most directly influences the activity of glycolytic enzymes? A) Mitochondrial DNA mutation B) Cytosolic pH C) Ribosomal RNA abundance D) Nuclear membrane permeability Answer: B Explanation: Glycolytic enzymes are sensitive to cytosolic pH; acidic conditions inhibit several key steps, reducing glycolytic flux. Question 6. The enzyme that catalyzes the first committed step of glycolysis is: 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 heavily regulated, making it the primary control point of glycolysis.
Respiration Ultimate Exam
Question 10. Which molecule allosterically activates phosphofructokinase-1? A) ATP B) Citrate C) AMP D) Acetyl-CoA Answer: C Explanation: High levels of AMP signal low cellular energy, activating PFK-1 to accelerate glycolysis. Question 11. Pyruvate dehydrogenase complex (PDC) converts pyruvate into which molecule before it enters the Krebs cycle? A) Oxaloacetate B) Acetyl-CoA C) Lactate D) Malate Answer: B Explanation: PDC decarboxylates pyruvate, attaches CoA, and generates Acetyl-CoA, releasing CO₂ and NADH. Question 12. The conversion of pyruvate to acetyl-CoA by PDC produces: A) One molecule of ATP B) One molecule of NADH C) One molecule of FADH₂ D) One molecule of GTP Answer: B Explanation: PDC reduces NAD⁺ to NADH during the oxidative decarboxylation of pyruvate. Question 13. Which of the following is the immediate product formed when Acetyl-CoA condenses with oxaloacetate?
Respiration Ultimate Exam
A) α-Ketoglutarate B) Citrate C) Succinate D) Malate Answer: B Explanation: The condensation yields citrate, the first intermediate of the Krebs cycle. Question 14. How many molecules of CO₂ are released per turn of the citric acid cycle? A) 0 B) 1 C) 2 D) 3 Answer: C Explanation: One CO₂ is released during isocitrate → α-ketoglutarate and another during α-ketoglutarate → succinyl-CoA, totaling two per cycle. Question 15. For each acetyl-CoA that enters the Krebs cycle, the total number of NADH molecules generated is: A) 1 B) 2 C) 3 D) 4 Answer: C Explanation: Three NAD⁺ are reduced to NADH at the steps catalyzed by isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase. Question 16. The substrate-level phosphorylation step in the Krebs cycle directly produces:
Respiration Ultimate Exam
B) Ubiquinone (CoQ) C) NAD⁺ D) Ferredoxin Answer: B Explanation: Ubiquinone (CoQ) receives electrons from Complex I and II and transports them to Complex III. Question 20. The primary role of Complex III (cytochrome bc₁ complex) is to: A) Pump protons across the inner membrane B) Reduce oxygen directly to water C) Oxidize NADH to NAD⁺ D) Transfer electrons to the outer mitochondrial membrane Answer: A Explanation: Complex III transfers electrons from reduced ubiquinol to cytochrome c while pumping protons into the intermembrane space. Question 21. Which of the following statements about Complex IV (cytochrome c oxidase) is true? A) It receives electrons from NADH directly. B) It pumps protons and reduces O₂ to H₂O. C) It generates FADH₂. D) It is located in the outer mitochondrial membrane. Answer: B Explanation: Complex IV accepts electrons from cytochrome c, pumps protons, and catalyzes the four-electron reduction of O₂ to water. Question 22. Chemiosmosis relies on which of the following gradients? A) Sodium ion gradient only B) Proton (H⁺) gradient across the inner membrane C) Calcium ion gradient across the outer membrane
Respiration Ultimate Exam
D) Chloride ion gradient in the matrix Answer: B Explanation: The electron transport chain creates an electrochemical proton gradient (Δp) that drives ATP synthesis via ATP synthase. Question 23. The ATP synthase (Complex V) is best described as a: A) Sodium-dependent transporter B) Rotary motor enzyme C) Electron carrier protein D) DNA polymerase Answer: B Explanation: ATP synthase has a rotating γ-subunit that couples proton flow to the synthesis of ATP from ADP and Pi. Question 24. Approximately how many ATP molecules are produced from the complete oxidation of one glucose molecule in eukaryotes under optimal conditions? A) 12- 14 B) 18- 20 C) 30- 32 D) 40- 42 Answer: C Explanation: Aerobic respiration yields roughly 30-32 ATP per glucose, accounting for NADH, FADH₂, and substrate-level phosphorylation. Question 25. Which of the following is the final electron acceptor in aerobic respiration? A) NAD⁺ B) FAD C) Oxygen (O₂)
Respiration Ultimate Exam
Explanation: Fermentation enables glycolysis to continue when oxygen is unavailable by regenerating NAD⁺, though ATP yield is low. Question 29. Which of the following metabolic pathways supplies acetyl-CoA directly to the Krebs cycle without passing through glycolysis? A) β-Oxidation of fatty acids B) Pentose phosphate pathway C) Glycogenolysis D) Gluconeogenesis Answer: A Explanation: Fatty acid β-oxidation cleaves acetyl-CoA units that enter the Krebs cycle directly. Question 30. During β-oxidation, each round of the cycle yields: A) 1 NADH, 1 FADH₂, and 1 acetyl-CoA B) 2 NADH, 2 FADH₂, and 2 acetyl-CoA C) 1 NADPH, 1 ATP, and 1 acetyl-CoA D) 1 GTP, 1 NADH, and 1 acetyl-CoA Answer: A Explanation: Each β-oxidation cycle shortens the fatty acid chain by two carbons, producing one NADH, one FADH₂, and one acetyl-CoA. Question 31. Which amino acid catabolism yields fumarate as an entry point into the Krebs cycle? A) Leucine B) Lysine C) Phenylalanine D) Aspartate Answer: D
Respiration Ultimate Exam
Explanation: Aspartate is deaminated to oxaloacetate, which can be interconverted with fumarate via the Krebs cycle. Question 32. High concentrations of which metabolite act as an allosteric inhibitor of phosphofructokinase-1? A) AMP B) ADP C) Citrate D) Fructose-2,6-bisphosphate Answer: C Explanation: Citrate signals abundant biosynthetic precursors and energy, inhibiting PFK-1 to down-regulate glycolysis. Question 33. The “energy charge” of a cell is defined by the ratio of: A) ATP : ADP + AMP B) NADH : NAD⁺ C) ATP : ADP D) FADH₂ : FAD Answer: A Explanation: Energy charge = ([ATP] + 0.5[ADP]) / ([ATP] + [ADP] + [AMP]), reflecting cellular energy status. Question 34. Which of the following statements about the malate-aspartate shuttle is correct? A) It transfers NADPH from cytosol to mitochondria. B) It transports electrons from cytosolic NADH into the mitochondrial matrix. C) It directly moves NADH across the inner membrane. D) It operates only in plant cells. Answer: B Explanation: The malate-aspartate shuttle uses malate to carry reducing equivalents from cytosolic NADH into the matrix where NAD⁺ is regenerated.
Respiration Ultimate Exam
Question 38. During oxidative phosphorylation, the P/O ratio refers to: A) Phosphate to oxygen atoms in water B) Number of ATP molecules synthesized per atom of oxygen reduced C) Number of protons pumped per NADH oxidized D) Ratio of pyruvate to oxaloacetate formed Answer: B Explanation: P/O ratio indicates how many ATP are generated per atom of oxygen (½ O₂) that accepts electrons. Question 39. Which enzyme deficiency leads to a buildup of pyruvate and lactate, causing lactic acidosis? A) Pyruvate kinase B) Pyruvate dehydrogenase complex C) Lactate dehydrogenase D) Hexokinase Answer: B Explanation: Impaired pyruvate dehydrogenase prevents conversion of pyruvate to acetyl-CoA, shunting pyruvate to lactate. Question 40. In muscle cells during intense exercise, the primary source of ATP after the first few seconds is: A) Oxidative phosphorylation B) Creatine phosphate system C) Glycogenolysis feeding glycolysis D) β-Oxidation of fatty acids Answer: C Explanation: Rapid glycogen breakdown supplies glucose for glycolysis, providing ATP quickly during high-intensity activity.
Respiration Ultimate Exam
Question 41. Which of the following best describes the role of Coenzyme Q (ubiquinone) in the ETC? A) It pumps protons across the inner membrane. B) It transfers electrons between Complex I/II and Complex III. C) It directly reduces oxygen to water. D) It synthesizes ATP from ADP. Answer: B Explanation: CoQ is a mobile lipid-soluble carrier that shuttles electrons from Complex I and II to Complex III. Question 42. The “link reaction” refers to the conversion of pyruvate to: A) Oxaloacetate B) Acetyl-CoA C) Lactate D) Malate Answer: B Explanation: The link reaction (pyruvate oxidation) produces Acetyl-CoA, CO₂, and NADH before entry into the Krebs cycle. Question 43. Which of the following is a true statement about the regulation of the Krebs cycle? A) High ATP levels activate isocitrate dehydrogenase. B) High NAD⁺ concentrations inhibit α-ketoglutarate dehydrogenase. C) Acetyl-CoA accumulation stimulates citrate synthase. D) Citrate acts as an allosteric inhibitor of phosphofructokinase-1. Answer: C Explanation: Elevated Acetyl-CoA signals substrate availability and stimulates citrate synthase, enhancing cycle flux.
Respiration Ultimate Exam
Question 47. Which of the following statements about FADH₂ is correct? A) It donates electrons to Complex I. B) It yields more ATP per molecule than NADH. C) It enters the ETC at Complex II. D) It is produced only during glycolysis. Answer: C Explanation: FADH₂ transfers electrons to Complex II (succinate dehydrogenase), bypassing Complex I, and yields slightly fewer ATP than NADH. Question 48. The primary purpose of the glycerol- 3 - phosphate shuttle is to: A) Transfer cytosolic NADH electrons to ubiquinone in the inner membrane. B) Export ATP from mitochondria to the cytosol. C) Generate NADPH for biosynthetic reactions. D) Import pyruvate into the mitochondrial matrix. Answer: A Explanation: The glycerol- 3 - phosphate shuttle oxidizes cytosolic NADH and passes electrons to FAD in the inner membrane, feeding them into the ETC via ubiquinone. Question 49. Which metabolite is directly produced by the enzyme pyruvate carboxylase? A) Lactate B) Oxaloacetate C) Acetyl-CoA D) Malate Answer: B Explanation: Pyruvate carboxylase catalyzes ATP-dependent carboxylation of pyruvate to oxaloacetate, a key anaplerotic reaction. Question 50. The term “substrate-level phosphorylation” refers to: A) Phosphorylation of substrates by ATP synthase.
Respiration Ultimate Exam
B) Direct transfer of a phosphate group from a high-energy substrate to ADP. C) Phosphorylation of enzymes by kinases. D) The use of a proton gradient to generate ATP. Answer: B Explanation: Substrate-level phosphorylation occurs when a phosphorylated intermediate donates its phosphate directly to ADP, forming ATP (e.g., in glycolysis and the Krebs cycle). Question 51. Which of the following best describes the relationship between glycolysis and the citric acid cycle? A) Both occur in the mitochondrial matrix. B) Glycolysis produces pyruvate, which is converted to acetyl-CoA before entering the cycle. C) The citric acid cycle generates glucose for glycolysis. D) Glycolysis and the citric acid cycle share the same enzymes. Answer: B Explanation: Glycolysis yields pyruvate in the cytosol; pyruvate is transported into mitochondria and converted to acetyl-CoA, which then enters the Krebs cycle. Question 52. Which molecule is the primary allosteric activator of pyruvate kinase in the liver? A) ATP B) Acetyl-CoA C) Fructose-2,6-bisphosphate D) Citrate Answer: C Explanation: Fructose-2,6-bisphosphate strongly activates pyruvate kinase, favoring glycolysis in liver cells. Question 53. In the context of cellular respiration, the term “anaplerotic” refers to:
Respiration Ultimate Exam
B) Calcium (Ca²⁺) C) Proton (H⁺) D) Potassium (K⁺) Answer: C Explanation: The proton motive force (Δp) drives the rotation of ATP synthase’s c-ring, synthesizing ATP. Question 57. The enzyme that converts isocitrate to α-ketoglutarate also produces which reduced cofactor? A) NADPH B) NADH C) FADH₂ D) FMNH₂ Answer: B Explanation: Isocitrate dehydrogenase catalyzes oxidative decarboxylation, reducing NAD⁺ to NADH. Question 58. Which metabolic condition would most strongly increase the activity of the enzyme pyruvate dehydrogenase? A) High acetyl-CoA/CoA ratio B) High NADH/NAD⁺ ratio C) High Ca²⁺ concentration in the mitochondrial matrix D) High ATP/ADP ratio Answer: C Explanation: Calcium ions activate pyruvate dehydrogenase phosphatase, which dephosphorylates and activates the PDC. Question 59. During anaerobic glycolysis in skeletal muscle, the accumulation of which metabolite causes the sensation of “burn”? A) Lactate B) Acetyl-CoA
Respiration Ultimate Exam
C) Pyruvate D) ATP Answer: A Explanation: Lactate accumulation lowers pH, stimulating pain receptors and the feeling of burning during intense exercise. Question 60. Which of the following statements about the mitochondrial DNA (mtDNA) is true? A) It encodes all proteins of the electron transport chain. B) It is inherited maternally in most organisms. C) It contains introns like nuclear DNA. D) It is replicated in the cytosol. Answer: B Explanation: mtDNA is typically transmitted from the mother to offspring and encodes a limited set of proteins, rRNAs, and tRNAs. Question 61. Which coenzyme is tightly bound to the inner membrane protein succinate dehydrogenase? A) NAD⁺ B) FAD C) Coenzyme Q D) Biotin Answer: B Explanation: Succinate dehydrogenase contains a covalently attached flavin adenine dinucleotide (FAD) that accepts electrons from succinate. Question 62. The term “proton-motive force” (PMF) is composed of two components. Which are they? A) Electrical potential (Δψ) and temperature gradient (ΔT) B) Chemical gradient (ΔpH) and electrical potential (Δψ) C) Osmotic pressure and ΔpH
