In thylakoids, protons travel through ATP synthase from the stroma to the thylakoid space. Therefore the catalytic "knobs" of the 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 PSI and PSII
D. on the stroma side of the membrane
E. Built into the center of the thylakoid stack (granum)
Answer: D