How does the reduced carrier made at step six differ in behaviour from NADH?
AIt leaves the mitochondrion before releasing electrons
BIt carries electrons back into the cytoplasm first
CIt hands its electrons straight to oxygen molecules
DIt stays attached to the enzyme and passes electrons directly
Answer & Solution
Correct answer: D. It stays attached to the enzyme and passes electrons directly
1. NADH is soluble and moves away from the enzyme that reduced it.
2. The carrier reduced at step six remains attached to its enzyme instead.
3. From there it transfers the electrons to the electron transport chain directly.
4. This is possible because the enzyme catalysing step six sits inside the inner membrane of the mitochondrion.
5. Only the final complex of the chain hands electrons to oxygen, so no carrier does that on its own.
6. Nothing here leaves the mitochondrion, which rules out both of the export answers.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.3 Oxidation of Pyruvate and the Citric Acid Cycle_
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