Rucete ✏ AP Biology In a Nutshell
7. Cellular Respiration — Practice Questions
This chapter covers the pathways by which cells harvest energy from glucose through glycolysis, the Krebs cycle, oxidative phosphorylation, and fermentation.
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(Multiple Choice — Click to Reveal Answer)
1. What is the first metabolic pathway that begins the breakdown of glucose in cellular respiration?
(A) Krebs cycle
(B) Glycolysis
(C) Oxidative phosphorylation
(D) Fermentation
Answer
(B) — Glycolysis is the initial step that breaks down glucose into pyruvate.
2. Where does glycolysis take place within a eukaryotic cell?
(A) Mitochondrial matrix
(B) Inner mitochondrial membrane
(C) Cytosol
(D) Intermembrane space
Answer
(C) — Glycolysis occurs in the cytosol of the cell.
3. What molecule is produced when pyruvate is oxidized before entering the Krebs cycle?
(A) Glucose
(B) Acetyl-CoA
(C) Lactic acid
(D) ATP
Answer
(B) — Pyruvate is converted into Acetyl-CoA during oxidation.
4. In which part of the mitochondrion does the Krebs cycle occur?
(A) Inner membrane
(B) Intermembrane space
(C) Matrix
(D) Cytosol
Answer
(C) — The Krebs cycle takes place in the mitochondrial matrix.
5. Which electron carriers are reduced during the Krebs cycle?
(A) ATP and ADP
(B) NAD+ and FAD
(C) Oxygen and NADH
(D) FADH₂ and oxygen
Answer
(B) — NAD+ and FAD are reduced to NADH and FADH₂ during the Krebs cycle.
6. What is the main purpose of the electron transport chain in cellular respiration?
(A) To split glucose
(B) To generate a proton gradient for ATP production
(C) To create pyruvate
(D) To convert NADH into glucose
Answer
(B) — The electron transport chain creates a proton gradient across the inner mitochondrial membrane, driving ATP synthesis.
7. Which molecule is the final electron acceptor at the end of the electron transport chain?
(A) Carbon dioxide
(B) Water
(C) Oxygen
(D) NAD+
Answer
(C) — Oxygen accepts electrons and combines with protons to form water.
8. How many ATP molecules are produced by glycolysis per molecule of glucose (net gain)?
(A) 2
(B) 4
(C) 6
(D) 8
Answer
(A) — Glycolysis results in a net gain of 2 ATP molecules per glucose.
9. What byproduct is produced during alcoholic fermentation but not during lactic acid fermentation?
(A) Oxygen
(B) Carbon dioxide
(C) Water
(D) Glucose
Answer
(B) — Alcoholic fermentation releases carbon dioxide; lactic acid fermentation does not.
10. Which molecule carries high-energy electrons to the electron transport chain after glycolysis and the Krebs cycle?
(A) ATP
(B) Pyruvate
(C) NADH
(D) Oxygen
Answer
(C) — NADH carries high-energy electrons to the electron transport chain.
11. What process directly generates the most ATP during cellular respiration?
(A) Glycolysis
(B) Krebs cycle
(C) Oxidative phosphorylation
(D) Fermentation
Answer
(C) — Oxidative phosphorylation produces the majority of ATP.
12. What is the role of oxygen in aerobic respiration?
(A) Source of glucose
(B) Electron donor to NAD+
(C) Final electron acceptor
(D) Proton donor to ATP synthase
Answer
(C) — Oxygen is the final electron acceptor at the end of the electron transport chain.
13. Which process occurs whether oxygen is present or absent?
(A) Oxidative phosphorylation
(B) Glycolysis
(C) Krebs cycle
(D) Electron transport
Answer
(B) — Glycolysis occurs in both aerobic and anaerobic conditions.
14. Which molecule must be regenerated during fermentation to keep glycolysis running?
(A) ATP
(B) FADH₂
(C) NAD+
(D) Oxygen
Answer
(C) — NAD+ must be regenerated to allow glycolysis to continue producing ATP.
15. During which stage of cellular respiration is carbon dioxide released?
(A) Glycolysis
(B) Krebs cycle
(C) Oxidative phosphorylation
(D) Chemiosmosis
Answer
(B) — Carbon dioxide is released during the Krebs cycle.
16. What enzyme is responsible for synthesizing ATP during oxidative phosphorylation?
(A) Rubisco
(B) ATP synthase
(C) NADH dehydrogenase
(D) Cytochrome c
Answer
(B) — ATP synthase uses the proton gradient to generate ATP from ADP and phosphate.
17. In cellular respiration, what is the main purpose of the Krebs cycle?
(A) Produce glucose
(B) Break down water molecules
(C) Harvest electrons for the electron transport chain
(D) Generate oxygen gas
Answer
(C) — The Krebs cycle harvests high-energy electrons for NADH and FADH₂.
18. How many total ATP molecules are produced from one glucose molecule during aerobic respiration (ideal maximum)?
(A) 18
(B) 24
(C) 32
(D) 36
Answer
(C) — Ideally, about 32 ATP molecules are produced per glucose during aerobic respiration.
19. Which molecule enters the Krebs cycle by combining with oxaloacetate?
(A) Pyruvate
(B) Acetyl-CoA
(C) NADH
(D) FADH₂
Answer
(B) — Acetyl-CoA combines with oxaloacetate to form citrate in the Krebs cycle.
20. In what cellular location does the electron transport chain of cellular respiration operate?
(A) Cytosol
(B) Inner mitochondrial membrane
(C) Mitochondrial matrix
(D) Outer mitochondrial membrane
Answer
(B) — The electron transport chain is located on the inner mitochondrial membrane.
21. What happens to pyruvate under anaerobic conditions in muscle cells?
(A) It is oxidized to acetyl-CoA.
(B) It is converted into lactic acid.
(C) It forms glucose.
(D) It enters the electron transport chain.
Answer
(B) — Pyruvate is reduced to lactic acid under anaerobic conditions in muscles.
22. Which pathway regenerates NAD+ during fermentation in muscle cells?
(A) Electron transport chain
(B) Glycolysis
(C) Lactic acid fermentation
(D) Krebs cycle
Answer
(C) — Lactic acid fermentation regenerates NAD+ so glycolysis can continue.
23. What is the role of FAD in cellular respiration?
(A) It carries electrons to the Calvin cycle.
(B) It donates electrons directly to oxygen.
(C) It is reduced to FADH₂ and transports electrons to the electron transport chain.
(D) It synthesizes ATP from ADP.
Answer
(C) — FAD is reduced to FADH₂ and then donates electrons to the electron transport chain.
24. During which process is the majority of NADH generated in cellular respiration?
(A) Glycolysis
(B) Fermentation
(C) Krebs cycle
(D) Electron transport chain
Answer
(C) — The Krebs cycle produces the majority of NADH during cellular respiration.
25. What is the immediate energy source that drives the production of ATP by ATP synthase in cellular respiration?
(A) Flow of electrons along the electron transport chain
(B) Movement of H+ ions down their concentration gradient
(C) Breakdown of glucose into carbon dioxide
(D) Transfer of phosphate groups to ADP
Answer
(B) — The movement of H+ ions down their gradient powers ATP synthase to produce ATP.
26. What would happen to the electron transport chain if oxygen were unavailable?
(A) Electrons would accumulate, halting ATP production.
(B) The Krebs cycle would accelerate.
(C) Glycolysis would immediately produce more ATP.
(D) Protons would continue to pump into the intermembrane space.
Answer
(A) — Without oxygen, the final electron acceptor, electrons back up, and ATP synthesis stops.
27. Which process results in substrate-level phosphorylation during cellular respiration?
(A) Chemiosmosis
(B) Glycolysis and Krebs cycle
(C) Electron transport chain
(D) Lactic acid fermentation
Answer
(B) — Substrate-level phosphorylation occurs in glycolysis and the Krebs cycle.
28. In the absence of oxygen, what is the immediate fate of NADH produced during glycolysis?
(A) It donates electrons to the electron transport chain.
(B) It transfers electrons to pyruvate in fermentation.
(C) It directly produces ATP.
(D) It converts into oxygen.
Answer
(B) — NADH donates electrons to pyruvate during fermentation, regenerating NAD⁺.
29. How many carbon dioxide molecules are produced from one pyruvate molecule during the Krebs cycle?
(A) 1
(B) 2
(C) 3
(D) 4
Answer
(B) — Two CO₂ molecules are released for every pyruvate oxidized in the Krebs cycle.
30. Which electron carrier directly delivers electrons at a lower energy level to the electron transport chain?
(A) NADH
(B) FADH₂
(C) ATP
(D) Oxygen
Answer
(B) — FADH₂ delivers electrons at a lower energy level than NADH in the chain.
31. During oxidative phosphorylation, the energy for the synthesis of ATP is directly obtained from:
(A) Reducing NAD+.
(B) Oxidizing glucose.
(C) A proton gradient across the inner mitochondrial membrane.
(D) Splitting water molecules.
Answer
(C) — A proton gradient powers ATP synthase to form ATP.
32. What is the major purpose of fermentation when oxygen is absent?
(A) Produce maximum ATP
(B) Regenerate NAD+ for glycolysis
(C) Fix carbon dioxide into organic molecules
(D) Store energy in the form of glucose
Answer
(B) — Fermentation regenerates NAD+ to allow glycolysis to continue producing ATP.
33. Which stage of cellular respiration produces the least amount of ATP directly?
(A) Glycolysis
(B) Krebs cycle
(C) Oxidative phosphorylation
(D) Fermentation
Answer
(B) — The Krebs cycle produces only 1 ATP (by substrate-level phosphorylation) per turn.
34. If a mutation damaged ATP synthase, what immediate effect would occur during cellular respiration?
(A) Electrons would accumulate in the transport chain.
(B) No proton gradient would form.
(C) No ATP would be synthesized through chemiosmosis.
(D) Glycolysis would halt immediately.
Answer
(C) — ATP synthase is required for ATP production via chemiosmosis.
35. Why is the inner mitochondrial membrane highly folded into cristae?
(A) To trap oxygen inside the mitochondrion.
(B) To reduce surface area.
(C) To increase surface area for electron transport chains and ATP synthesis.
(D) To concentrate glucose molecules.
Answer
(C) — The folds maximize surface area, enhancing the mitochondrion’s ability to produce ATP efficiently.
36. Explain why oxygen is essential for efficient ATP production in cellular respiration.
Answer
Oxygen serves as the final electron acceptor in the electron transport chain, allowing electrons to flow and enabling proton pumping, which drives ATP synthesis.
37. How does fermentation allow glycolysis to continue in the absence of oxygen?
Answer
Fermentation regenerates NAD+ by transferring electrons from NADH to pyruvate or derivatives, allowing glycolysis to continue producing ATP anaerobically.
38. Compare the ATP yields from aerobic respiration and fermentation for one glucose molecule.
Answer
Aerobic respiration produces about 32 ATP per glucose, whereas fermentation yields only 2 ATP per glucose molecule (from glycolysis alone).
39. What happens to the carbon atoms of glucose during cellular respiration?
Answer
The carbon atoms are released as carbon dioxide during the oxidation of pyruvate and in the Krebs cycle.
40. Describe the role of the proton gradient in oxidative phosphorylation.
Answer
The proton gradient provides the energy that drives protons back through ATP synthase, powering the production of ATP from ADP and phosphate.
41. Why is FADH₂ considered to contribute less to ATP production than NADH?
Answer
FADH₂ donates electrons to the electron transport chain at a lower energy level, resulting in fewer protons pumped and less ATP synthesized compared to NADH.
42. Predict the impact on a cell if the inner mitochondrial membrane became permeable to protons.
Answer
If protons leaked across the membrane, the proton gradient would dissipate, preventing ATP synthesis by chemiosmosis and drastically reducing energy production.
43. How does substrate-level phosphorylation differ from oxidative phosphorylation?
Answer
Substrate-level phosphorylation transfers a phosphate group directly from a substrate to ADP, while oxidative phosphorylation uses the energy from a proton gradient.
44. What would happen to cellular respiration if NAD+ were not regenerated?
Answer
Without NAD+ regeneration, glycolysis and the Krebs cycle would halt because no carrier would be available to accept electrons during redox reactions.
45. Explain the significance of acetyl-CoA in cellular respiration.
Answer
Acetyl-CoA delivers the acetyl group to the Krebs cycle, where it combines with oxaloacetate to continue energy extraction from glucose derivatives.
46. Why does glycolysis occur in both aerobic and anaerobic organisms?
Answer
Glycolysis does not require oxygen and can generate ATP independently, making it a universal pathway for energy production across different organisms.
47. Describe how the movement of electrons through the electron transport chain leads to ATP production.
Answer
Electron movement releases energy that pumps protons into the intermembrane space, creating a gradient that powers ATP synthase to produce ATP.
48. How does the structure of mitochondria support its role in cellular respiration?
Answer
The highly folded inner membrane (cristae) provides a large surface area for electron transport chains and ATP synthase, maximizing ATP production efficiency.
49. What is the relationship between glycolysis and fermentation?
Answer
Fermentation follows glycolysis under anaerobic conditions to regenerate NAD+, allowing glycolysis to continue producing a small amount of ATP.
50. Why is cellular respiration considered more efficient than fermentation?
Answer
Cellular respiration fully oxidizes glucose, extracting much more energy (up to ~32 ATP), whereas fermentation yields only 2 ATP by glycolysis alone.