[Campbell Biology P.260] In the experiment with isolated thylakoids, where thylakoids were f... | Practice Question

In the experiment with isolated thylakoids, where thylakoids were first made acidic pH 4 and then transferred to a basic solution pH 8 , leading to ATP synthesis in the dark, what is the crucial condition enabling ATP synthesis, and where would ATP accumulate?

  • A: The presence of light energy; ATP would accumulate inside the thylakoid lumen.
  • B: A proton gradient across the thylakoid membrane; ATP would accumulate in the solution outside the thylakoid.
  • C: The direct input of ATP; ATP would accumulate inside the thylakoid lumen.
  • D: The activity of photosystem II; ATP would accumulate in the solution outside the thylakoid.

Explanation

The experiment mimics chemiosmosis, where a proton gradient across the thylakoid membrane drives ATP synthesis by ATP synthase. By first soaking thylakoids in an acidic low pH solution, their lumen becomes acidic. Transferring them to a basic high pH solution creates a pH gradient high H+ inside, low H+ outside . This gradient provides the proton motive force for ATP synthase to make ATP, which is released into the surrounding solution analogous to the stroma . This process does not require light directly.