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Quiz: Cellular Respiration and Fermentation

Test your understanding of glycolysis, the Krebs cycle, oxidative phosphorylation, and fermentation with these review questions.

1. In which cellular location does glycolysis take place?

  1. Mitochondrial matrix
  2. Inner mitochondrial membrane
  3. Cytoplasm (cytosol)
  4. Nucleus
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The correct answer is C. Glycolysis is a ten-step enzymatic pathway that occurs entirely in the cytoplasm and does not require a membrane-bound organelle or oxygen. This is why glycolysis can proceed in both aerobic and anaerobic conditions, and even in cells like mature red blood cells that lack mitochondria entirely.

Concept Tested: Glycolysis location

2. What serves as the final electron acceptor in the electron transport chain during aerobic cellular respiration?

  1. Molecular oxygen (O2)
  2. Carbon dioxide (CO2)
  3. NAD+
  4. Pyruvate
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The correct answer is A. At Complex IV, electrons that have traveled through the chain are transferred to molecular oxygen, which combines with protons to form water. This step is essential because it clears electrons from the chain, allowing electron flow and proton pumping to continue; without oxygen, the entire chain backs up and stops functioning.

Concept Tested: Electron transport chain (mitochondria)

3. Why does FADH2 typically yield fewer ATP molecules than NADH during oxidative phosphorylation?

  1. FADH2 delivers electrons directly to Complex IV, skipping most of the chain
  2. FADH2 is oxidized in the cytoplasm, not the mitochondria
  3. FADH2 requires additional ATP to be transported into the matrix
  4. FADH2 donates electrons at Complex II, bypassing the proton-pumping step at Complex I, so fewer protons are translocated per electron pair
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The correct answer is D. NADH delivers its electrons to Complex I, which pumps protons across the inner membrane. FADH2 instead delivers electrons at Complex II, which does not pump protons, so the electrons it carries bypass one of the three proton-pumping complexes. Fewer protons are translocated per FADH2 oxidized, resulting in a smaller contribution to the proton-motive force and fewer ATP synthesized.

Concept Tested: NADH and FADH2 (electron carrier efficiency)

4. What is the primary function of the Krebs cycle in cellular respiration?

  1. To directly generate the majority of the cell's ATP through substrate-level phosphorylation
  2. To extract high-energy electrons from carbon skeletons and load them onto NAD+ and FAD for delivery to the electron transport chain
  3. To synthesize glucose from smaller carbon fragments
  4. To split glucose into two molecules of pyruvate
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The correct answer is B. While the Krebs cycle does produce a small amount of ATP directly through substrate-level phosphorylation, its primary role is to fully oxidize the acetyl group from acetyl-CoA, releasing CO2 and transferring the extracted electrons onto NAD+ and FAD. These electron carriers then deliver electrons to the electron transport chain, where the bulk of ATP is ultimately produced.

Concept Tested: Krebs cycle function

5. Why is fermentation necessary for cells operating under anaerobic conditions?

  1. It produces additional ATP beyond what glycolysis generates
  2. It allows the electron transport chain to continue functioning without oxygen
  3. It regenerates NAD+ from NADH so that glycolysis can continue producing ATP
  4. It converts lactate directly into acetyl-CoA for the Krebs cycle
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The correct answer is C. Without oxygen, the electron transport chain cannot accept more electrons, so NADH accumulates and NAD+ becomes scarce. Since glycolysis requires NAD+ as an electron acceptor at one of its steps, fermentation regenerates NAD+ by transferring electrons from NADH to an organic molecule such as pyruvate or acetaldehyde, allowing glycolysis to keep producing its modest 2 ATP per glucose.

Concept Tested: Fermentation

6. A drug specifically blocks Complex I of the electron transport chain. What is the most likely consequence for the cell?

  1. NADH accumulates, the proton gradient weakens, oxidative phosphorylation declines, and the cell relies more on substrate-level phosphorylation and fermentation
  2. ATP production increases because Complex II compensates fully for the loss of Complex I
  3. Oxygen consumption increases while NADH levels remain unchanged
  4. The Krebs cycle immediately stops running entirely
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The correct answer is A. Blocking Complex I prevents NADH from being oxidized there, causing NADH to accumulate and NAD+ to become scarce. Because Complex I normally contributes substantially to proton pumping, the proton-motive force weakens and ATP synthesis via oxidative phosphorylation declines. The cell compensates by relying more heavily on substrate-level phosphorylation and, if NAD+ becomes limiting enough, fermentation.

Concept Tested: Electron transport chain disruption

7. Considering only substrate-level phosphorylation (excluding oxidative phosphorylation), how much total ATP is produced per glucose molecule across glycolysis and the Krebs cycle combined?

  1. 2 ATP
  2. 6 ATP
  3. 10 ATP
  4. 4 ATP
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The correct answer is D. Glycolysis produces a net 2 ATP by substrate-level phosphorylation, and the Krebs cycle produces 2 ATP (or GTP) by substrate-level phosphorylation across its two turns per glucose. Adding these together gives 4 ATP total from substrate-level phosphorylation, with the remaining ~28 ATP of the ~32 total yield coming from oxidative phosphorylation via the electron transport chain.

Concept Tested: ATP yield of respiration (substrate-level phosphorylation)

8. A runner's leg muscles begin producing lactate during a sprint. What does this indicate, and what physiological purpose does it serve?

  1. Oxygen delivery is insufficient to keep pace with NADH production, so lactic acid fermentation regenerates NAD+ to sustain glycolysis
  2. The muscle cells have run out of glucose entirely
  3. The electron transport chain is operating at maximum efficiency
  4. The muscle cells have switched to alcoholic fermentation
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The correct answer is A. When oxygen delivery cannot keep up with the rate of NADH production during intense exercise, the electron transport chain cannot process electrons fast enough, and NAD+ becomes depleted. Muscle cells switch to lactic acid fermentation, which regenerates NAD+ by reducing pyruvate to lactate, allowing glycolysis to continue supplying ATP even though the electron transport chain is functionally limited.

Concept Tested: Lactic acid fermentation

9. Why does alcoholic fermentation release CO2 while lactic acid fermentation does not?

  1. Lactic acid fermentation releases CO2 because pyruvate is decarboxylated before forming lactate
  2. Alcoholic fermentation decarboxylates pyruvate to acetaldehyde, releasing CO2, before reducing it to ethanol, while lactic acid fermentation directly reduces pyruvate to lactate without any decarboxylation step
  3. Both pathways release identical amounts of CO2 at the same step
  4. Neither pathway releases CO2 because fermentation is anaerobic
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The correct answer is B. In alcoholic fermentation, pyruvate decarboxylase removes a carboxyl group from pyruvate to form acetaldehyde, releasing CO2 as a byproduct before acetaldehyde is reduced to ethanol using electrons from NADH. Lactic acid fermentation, by contrast, transfers electrons from NADH directly to pyruvate, converting it into lactate in a single step with no decarboxylation and therefore no CO2 release.

Concept Tested: Alcoholic vs. lactic acid fermentation

10. A mitochondrion contains an uncoupling protein that allows protons to leak back into the matrix, bypassing ATP synthase entirely. What is the most likely direct effect?

  1. ATP production would increase because more protons would be available to ATP synthase
  2. Electron transport would halt completely, stopping oxygen consumption
  3. ATP synthesis would decrease because fewer protons flow through ATP synthase, with the dissipated proton gradient released as heat instead
  4. The Krebs cycle would run in reverse to compensate for the lost ATP
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The correct answer is C. Chemiosmosis relies on protons flowing through ATP synthase to convert the proton-motive force into ATP. If an uncoupling protein provides an alternative route for protons to re-enter the matrix, fewer protons pass through ATP synthase, reducing ATP output, while the energy of the dissipated gradient is released as heat instead — the mechanism used by brown fat tissue for thermogenesis.

Concept Tested: Chemiosmosis and oxidative phosphorylation