Electron transport is coupled to ATP synthesis in mitochondria, in chloroplasts, and in thermophilic bacterium Methanococcus. Which of the following is likely to affect the coupling of the electron transport chain to ATP synthesis in all of these systems?

A) A potent inhibitor of cytochrome c oxidase
B) The removal of oxygen
C) The absence of light
D) An ADP analog that inhibits ATP synthase

I know that D is the correct answer but why? Could you please make it very detailed?

The correct answer is option D.

Explanation:

Each of the chemiosmotic coupling systems comprises a proton gradient, which is utilized by the enzyme ATP synthase to combine with ADP and phosphorylate it. Thus, any component that obstructs the binding of ATP with the synthase or that prevents the formation of the proton gradient will surely influence all the chemiosmotic systems.  

On the other hand, the element like oxygen and cytochrome c are needed only for aerobic bacteria and for mitochondria, they have no function in methanococcus, and light is needed only for the photosynthetic bacteria and for chloroplasts, and thus, have no role for methanococcus.  

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