Question at position 5 During oxidative phosphorylation, what happens to the NADH and FADH2 that are produced from the citric acid cycle (Krebs cycle)?They each acquire an oxygen atomThey are reduced to NAD+ and FAD, respectivelyThey are oxidized to NAD+ and FAD, respectivelyThey each acquire a hydrogen atom单项选择题
A
They each acquire an oxygen atom
B
They are reduced to NAD+ and FAD, respectively
C
They are oxidized to NAD+ and FAD, respectively
D
They each acquire a hydrogen atom

Question

Question at position 5 During oxidative phosphorylation, what happens to the NADH and FADH2 that are produced from the citric acid cycle (Krebs cycle)?They each acquire an oxygen atomThey are reduced to NAD+ and FAD, respectivelyThey are oxidized to NAD+ and FAD, respectivelyThey each acquire a hydrogen atom单项选择题

A

They each acquire an oxygen atom

B

They are reduced to NAD+ and FAD, respectively

C

They are oxidized to NAD+ and FAD, respectively

D

They each acquire a hydrogen atom

BlackTom AI · Accepted Answer

NADH and FADH2 donate their high-energy electrons to the mitochondrial electron transport chain (ETC) and in doing so are converted back to their oxidized forms, NAD+ and FAD.

1. Relevant concepts
- Oxidation is loss of electrons; reduction is gain of electrons.
- Oxidative phosphorylation uses the ETC and ATP synthase: electrons from NADH and FADH2 travel through protein complexes, driving proton pumping and ATP synthesis; molecular oxygen is the final electron acceptor and is reduced to water.

2. Step-by-step reasoning
- Step 1: NADH produced in the citric acid cycle arrives at Complex I of the ETC and donates two electrons. Donation of electrons means NADH is oxidized to NAD+.
- Step 2: FADH2 (often from succinate dehydrogenase) donates two electrons at Complex II, becoming oxidized to FAD.
- Step 3: Electrons flow from Complexes I/II → III → IV; energy released pumps protons from the matrix to the intermembrane space, creating a proton motive force.
- Step 4: At Complex IV, electrons combine with O2 and protons to form H2O (oxygen is reduced).
- Step 5: Protons flow back through ATP synthase, driving phosphorylation of ADP to ATP.

3. Intermediate “calculations” / conversions (qualitative)
- NADH → loses 2 e− → NAD+ (oxidation).
- FADH2 → loses 2 e− → FAD (oxidation).

4. Why the correct answer is correct
The correct choice states that NADH and FADH2 are oxidized to NAD+ and FAD, respectively. Because they donate electrons to the ETC during oxidative phosphorylation, they undergo oxidation (loss of electrons), regenerating the oxidized cofactors required for continued operation of the citric acid cycle.

5. Why the other options are incorrect
- “They each acquire an oxygen atom”: They do not gain oxygen atoms; oxygen is the final electron acceptor downstream, not something appended to NADH/FADH2.
- “They are reduced to NAD+ and FAD, respectively”: This reverses redox terms—NAD+ and FAD are the oxidized forms; reduction would add electrons, not produce NAD+ or FAD.
- “They each acquire a hydrogen atom”: Donation of electrons (and generally removal of hydride or H+) means they lose hydrogen, not gain it.

6. Conclusion
Therefore, during oxidative phosphorylation NADH and FADH2 are oxidized to NAD+ and FAD, respectively, which matches the provided correct answer.

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