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MCAT Biology Question — Electron Transport Chain

A certain molecule acts by binding to cytochrome oxidase A3, the final enzyme in the electron transport chain. Administration of a large dose of this substance to a human would likely:


A) Lead to death due to an inability of the cell to pass electrons to oxygen, thus stopping aerobic respiration and asphyxiating the cells.
B) Lead to death due to an inadequate supply of ADP to accept a phosphate group at the ATP synthase enzyme.
C) Have no effect as cells would switch which macronutrient they metabolize to circumvent the blocked biochemical pathway.
D) Increase the cell’s ATP production as negative feedback would cause the cell to up-regulate anaerobic pathways.




The electron transport chain works by taking high-energy electrons from molecules like NADH and passing them through a series of enzymes, ultimately ending with oxygen as the final electron acceptor. If this chain is blocked, aerobic respiration would cease, killing the cell. Eukaryotic cells in humans cannot survive using only anaerobic respiration. Thus (A) is the right answer. Several poisons work in the manner described, most notably cyanide.


B: ADP is produced when cells use up ATP to carry out cell functions. Blocking the electron transport chain would have no effect on ADP’s immediate availability.
C: As discussed above, this poison would kill the person.
D: As discussed above, this poison would kill the person.


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