In thylakoids, protons travel through ATP synthase from the thylakoid space to the stroma. Therefore, the catalytic "knobs" of ATP synthase would be located
A) on the side facing the thylakoid space.
B) on the ATP molecules themselves.
C) on the pigment molecules of photosystem I and photosystem II.
D) on the stromal side of the membrane.
E) built into the center of the thylakoid stack (granum).
Answer: D
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