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7. Cellular Respiration — Practice Questions 3
This chapter explores how cells harvest chemical energy through glycolysis, pyruvate oxidation, the Krebs cycle, oxidative phosphorylation, and fermentation.
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(Multiple Choice — Click to Reveal Answer)
1. Which molecule is the starting substrate for glycolysis?
(A) Glucose
(B) Pyruvate
(C) Acetyl-CoA
(D) Oxygen
Answer
(A) — Glycolysis begins with glucose, a six-carbon sugar.
2. Which product is generated at the end of glycolysis?
(A) Acetyl-CoA
(B) NADPH
(C) Pyruvate
(D) FADH₂
Answer
(C) — Glycolysis produces two molecules of pyruvate.
3. Where in the mitochondrion does the Krebs cycle occur?
(A) Inner membrane
(B) Matrix
(C) Outer membrane
(D) Intermembrane space
Answer
(B) — The Krebs cycle occurs in the mitochondrial matrix.
4. What molecule acts as the final electron acceptor in aerobic respiration?
(A) NAD+
(B) Oxygen
(C) Carbon dioxide
(D) Glucose
Answer
(B) — Oxygen accepts electrons at the end of the electron transport chain, forming water.
5. During which stage is carbon dioxide first released in cellular respiration?
(A) Glycolysis
(B) Oxidation of pyruvate
(C) Krebs cycle
(D) Electron transport chain
Answer
(B) — Carbon dioxide is first released during the oxidation of pyruvate.
6. Which process produces the majority of ATP during cellular respiration?
(A) Glycolysis
(B) Krebs cycle
(C) Oxidative phosphorylation
(D) Fermentation
Answer
(C) — Oxidative phosphorylation produces the largest amount of ATP.
7. What is the role of NAD+ in cellular respiration?
(A) Final electron acceptor
(B) Electron carrier that becomes reduced to NADH
(C) ATP producer
(D) Enzyme in the Krebs cycle
Answer
(B) — NAD+ accepts electrons and becomes NADH, carrying energy to the electron transport chain.
8. In which part of the cell does glycolysis occur?
(A) Mitochondrial matrix
(B) Cytosol
(C) Inner mitochondrial membrane
(D) Chloroplast
Answer
(B) — Glycolysis occurs in the cytosol of the cell.
9. Which process is common to both fermentation and cellular respiration?
(A) Krebs cycle
(B) Electron transport chain
(C) Glycolysis
(D) Oxidative phosphorylation
Answer
(C) — Glycolysis occurs in both fermentation and cellular respiration.
10. What is a major waste product generated during the Krebs cycle?
(A) Oxygen
(B) Water
(C) Carbon dioxide
(D) Ethanol
Answer
(C) — The Krebs cycle releases carbon dioxide as a waste product.
11. Which molecule enters the Krebs cycle after combining with oxaloacetate?
(A) Glucose
(B) Acetyl-CoA
(C) Pyruvate
(D) NADH
Answer
(B) — Acetyl-CoA combines with oxaloacetate to form citrate and enter the Krebs cycle.
12. During lactic acid fermentation, what is regenerated to allow glycolysis to continue?
(A) ATP
(B) Oxygen
(C) FAD
(D) NAD+
Answer
(D) — NAD+ is regenerated during lactic acid fermentation to sustain glycolysis.
13. Which stage of cellular respiration directly produces the most NADH?
(A) Glycolysis
(B) Krebs cycle
(C) Electron transport chain
(D) Fermentation
Answer
(B) — The Krebs cycle produces the majority of NADH used in the electron transport chain.
14. What is the immediate energy source that drives ATP synthesis by ATP synthase?
(A) Flow of electrons
(B) Hydrolysis of glucose
(C) Proton (H⁺) gradient across the inner mitochondrial membrane
(D) Breakdown of oxygen molecules
Answer
(C) — The proton gradient drives protons through ATP synthase, powering ATP production.
15. Which product results when pyruvate is fully oxidized during aerobic respiration?
(A) Glucose
(B) Oxygen
(C) Carbon dioxide
(D) Water
Answer
(C) — Pyruvate is fully oxidized into carbon dioxide during aerobic respiration.
16. What directly donates electrons to the electron transport chain in cellular respiration?
(A) Oxygen
(B) ATP
(C) NADH and FADH₂
(D) Pyruvate
Answer
(C) — NADH and FADH₂ donate electrons to the electron transport chain.
17. What happens to the electrons carried by NADH during oxidative phosphorylation?
(A) They are used to split water.
(B) They move down the electron transport chain to oxygen.
(C) They synthesize glucose.
(D) They are stored in the mitochondria.
Answer
(B) — Electrons from NADH are transferred through the electron transport chain and eventually accepted by oxygen.
18. Which process regenerates oxaloacetate during cellular respiration?
(A) Glycolysis
(B) Oxidative phosphorylation
(C) Krebs cycle
(D) Fermentation
Answer
(C) — The Krebs cycle regenerates oxaloacetate at the end of each turn to bind with acetyl-CoA again.
19. During which stage is the most direct ATP produced by substrate-level phosphorylation?
(A) Electron transport chain
(B) Glycolysis
(C) Oxidative phosphorylation
(D) Pyruvate oxidation
Answer
(B) — Glycolysis produces ATP by substrate-level phosphorylation, independent of the electron transport chain.
20. In the electron transport chain, energy released by electrons is used to:
(A) Reduce oxygen directly.
(B) Pump protons across the inner mitochondrial membrane.
(C) Convert ATP into ADP.
(D) Break glucose into two pyruvate molecules.
Answer
(B) — Electron energy is used to pump protons, creating a proton gradient across the membrane.
21. In the absence of oxygen, what is the primary purpose of fermentation?
(A) To generate additional ATP(B) To regenerate NAD+ for glycolysis
(C) To store carbon dioxide
(D) To convert ATP into glucose
Answer
(B) — Fermentation regenerates NAD+, allowing glycolysis to continue producing ATP without oxygen.
22. Which molecule is produced at the end of glycolysis and enters the mitochondria for further breakdown if oxygen is available?
(A) Lactate
(B) Pyruvate
(C) NADH
(D) Glucose
Answer
(B) — Pyruvate enters the mitochondria for oxidation into acetyl-CoA in aerobic respiration.
23. How many total NADH molecules are produced per glucose molecule by the end of the Krebs cycle (including glycolysis and pyruvate oxidation)?
(A) 2
(B) 4
(C) 8
(D) 10
Answer
(D) — A total of 10 NADH molecules are generated per glucose by the time the Krebs cycle is completed.
24. Which process results in the direct production of water in cellular respiration?
(A) Krebs cycle
(B) Glycolysis
(C) Oxidative phosphorylation
(D) Fermentation
Answer
(C) — Water is produced when electrons combine with oxygen at the end of oxidative phosphorylation.
25. In aerobic respiration, what is the ultimate fate of the carbon atoms from glucose?
(A) They become part of NADH.
(B) They are incorporated into water molecules.
(C) They are released as carbon dioxide.
(D) They are stored in ATP.
Answer
(C) — Carbon atoms from glucose are fully oxidized and released as carbon dioxide.
26. If the inner mitochondrial membrane became permeable to protons, what would be the immediate consequence for ATP production?
(A) Increased ATP synthesis
(B) Proton gradient disruption, reducing ATP production
(C) Faster electron transport
(D) Increased carbon dioxide production
Answer
(B) — A leaky membrane would dissipate the proton gradient, greatly reducing ATP synthesis by ATP synthase.
27. During cellular respiration, when is the first molecule of CO₂ released?
(A) Glycolysis
(B) Oxidation of pyruvate
(C) Krebs cycle
(D) Electron transport chain
Answer
(B) — CO₂ is first released during the oxidation of pyruvate to acetyl-CoA.
28. What is the primary reason that fats yield more ATP per gram than carbohydrates during aerobic respiration?
(A) They contain more oxygen atoms.
(B) They have more carbon-carbon bonds.
(C) They have more hydrogen atoms and electrons available for ATP generation.
(D) They are easier to transport in cells.
Answer
(C) — Fats contain many hydrogen atoms that donate electrons to the electron transport chain, leading to more ATP production.
29. In the electron transport chain, which complex directly reduces oxygen to form water?
(A) Complex I
(B) Complex II
(C) Complex III
(D) Complex IV
Answer
(D) — Complex IV (cytochrome c oxidase) transfers electrons to oxygen, forming water.
30. Why is FADH₂ considered to contribute less ATP than NADH during oxidative phosphorylation?
(A) It is oxidized later in the electron transport chain.
(B) It carries fewer electrons.
(C) It donates protons instead of electrons.
(D) It is not used in aerobic respiration.
Answer
(A) — FADH₂ donates electrons to a lower energy level complex, resulting in fewer protons pumped and less ATP produced.
31. Which process would cease first if a cell were deprived of oxygen?
(A) Glycolysis
(B) Krebs cycle
(C) Fermentation
(D) ATP hydrolysis
Answer
(B) — Without oxygen, the electron transport chain halts, preventing regeneration of NAD+ and FAD, thus stopping the Krebs cycle.
32. Which enzyme is responsible for synthesizing ATP during chemiosmosis?
(A) Pyruvate dehydrogenase
(B) Cytochrome c
(C) ATP synthase
(D) NADH dehydrogenase
Answer
(C) — ATP synthase uses the proton gradient to synthesize ATP.
33. How many ATP molecules (net) are produced during fermentation per molecule of glucose?
(A) 0
(B) 2
(C) 4
(D) 32
Answer
(B) — Fermentation produces a net gain of 2 ATP molecules, all from glycolysis.
34. Which step of aerobic respiration directly generates the most NADH per glucose molecule?
(A) Glycolysis
(B) Krebs cycle
(C) Electron transport chain
(D) Chemiosmosis
Answer
(B) — The Krebs cycle produces the most NADH molecules per glucose molecule.
35. What would happen if oxygen were suddenly removed from an actively respiring cell?
(A) The electron transport chain would speed up.
(B) ATP production would increase sharply.
(C) The electron transport chain would stop, and NADH would accumulate.
(D) Glycolysis would immediately halt.
Answer
(C) — Without oxygen to accept electrons, the electron transport chain halts, causing NADH accumulation and stopping oxidative phosphorylation.
36. Explain why oxygen is crucial for the electron transport chain in aerobic respiration.
Answer
Oxygen acts as the final electron acceptor, allowing electrons to continue flowing through the chain; without it, the chain stops and ATP production ceases.
37. How does the movement of electrons through the electron transport chain generate a proton gradient?
Answer
As electrons pass through complexes in the chain, energy released is used to pump protons from the mitochondrial matrix into the intermembrane space, creating a gradient.
38. What happens to pyruvate if oxygen is unavailable in animal cells?
Answer
Pyruvate is converted into lactic acid through lactic acid fermentation to regenerate NAD⁺ for glycolysis.
39. Describe the role of acetyl-CoA in the Krebs cycle.
Answer
Acetyl-CoA delivers an acetyl group (two-carbon unit) to the Krebs cycle, where it combines with oxaloacetate to form citrate and continue energy extraction.
40. What is the main advantage of using aerobic respiration over fermentation?
Answer
Aerobic respiration produces significantly more ATP per glucose molecule (~32 ATP) compared to fermentation (only 2 ATP), providing much more efficient energy extraction.
41. How does the proton gradient contribute to ATP production in mitochondria?
Answer
The proton gradient drives protons through ATP synthase, providing the energy needed to convert ADP and inorganic phosphate into ATP.
42. Predict the effect if a mutation impaired the function of ATP synthase.
Answer
ATP production would decrease dramatically because the proton gradient could not be used to synthesize ATP.
43. Why is FADH₂ considered less efficient than NADH in terms of ATP production?
Answer
FADH₂ donates electrons later in the electron transport chain at a lower energy level, resulting in fewer protons pumped and less ATP generated.
44. What role does the mitochondrial matrix play in cellular respiration?
Answer
The mitochondrial matrix is the site where the Krebs cycle occurs and where pyruvate is converted into acetyl-CoA.
45. What would happen to cellular respiration if oxygen levels dropped significantly?
Answer
Electron transport would halt, causing NADH and FADH₂ to accumulate, stopping the Krebs cycle and greatly reducing ATP production.
46. How is water formed during oxidative phosphorylation?
Answer
At the end of the electron transport chain, oxygen combines with electrons and protons to form water as a byproduct.
47. Explain why glycolysis is considered a universal metabolic pathway.
Answer
Glycolysis occurs in nearly all organisms, does not require oxygen, and takes place in the cytosol, suggesting it evolved early in Earth's history.
48. Describe one key difference between aerobic respiration and fermentation.
Answer
Aerobic respiration requires oxygen and produces much more ATP, while fermentation occurs without oxygen and produces minimal ATP.
49. What is the primary source of electrons for the electron transport chain in cellular respiration?
Answer
NADH and FADH₂, which are produced during glycolysis, pyruvate oxidation, and the Krebs cycle, donate electrons to the electron transport chain.
50. Summarize the overall purpose of cellular respiration in a single sentence.
Answer
The main purpose of cellular respiration is to extract energy from organic molecules and store it in the form of ATP to power cellular processes.