Which of the following stages of cellular respiration directly produces the LEAST reduced electron carriers?

Question

BlackTom AI · Accepted Answer

The electron transport chain (ETC) oxidizes reduced electron carriers rather than generating them, so it directly produces the LEAST reduced carriers.

1) Relevant knowledge points and concepts
- Reduced electron carriers in cellular respiration are primarily NADH and FADH2; they are produced during glycolysis, pyruvate oxidation, and the citric acid (Krebs) cycle.
- The ETC accepts electrons from NADH and FADH2, transfers them through protein complexes to O2, and regenerates NAD+ and FAD (i.e., it oxidizes the carriers).
- The question asks which stage "directly produces the LEAST reduced electron carriers" per glucose molecule.

2) Step-by-step reasoning
- Identify where reduced carriers are generated and in what amounts (per one glucose):
  a) Glycolysis: produces 2 NADH.
  b) Pyruvate oxidation (conversion of 2 pyruvates to 2 acetyl-CoA): produces 2 NADH.
  c) Citric acid cycle (two turns per glucose): produces 6 NADH and 2 FADH2.
- Sum for the three producer stages: glycolysis + pyruvate oxidation + citric acid cycle = 2 + 2 + 6 = 10 NADH and 2 FADH2 per glucose (standard accounting).
- Electron transport chain: consumes these reduced carriers (NADH, FADH2) to pump protons and drive ATP synthesis. It does not generate NADH or FADH2 as net products. While some ETC components transiently accept and pass electrons (e.g., ubiquinone/ubiquinol), the ETC’s net effect is oxidation of NADH/FADH2 to NAD+/FAD.

3) Intermediate calculation steps
- Glycolysis: 2 NADH
- Pyruvate oxidation: 2 NADH
- Citric acid cycle: 6 NADH + 2 FADH2
- ETC net production of NADH/FADH2: 0 (net oxidation)

4) Why the correct answer is correct
- Because the ETC’s role is to accept electrons from reduced carriers and oxidize them, it directly produces zero reduced NADH/FADH2—thus the least.

5) Why other options are incorrect
- a. Pyruvate oxidation directly produces NADH (2 per glucose) — so not the least.
- b. Glycolysis directly produces NADH (2 per glucose) — so not the least.
- c. Citric acid cycle produces the most reduced carriers (6 NADH and 2 FADH2 per glucose) — so not the least.

6) Conclusion
- Therefore the stage that directly produces the least reduced electron carriers is d. Electron transport chain, which matches the provided answer.

Which of the following stages of cellular respiration directly produces the LEAST reduced electron carriers?单项选择题

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