Final answer:
Protons flow back into the cell through a process associated with ATP formation.
Step-by-step explanation:
After being kicked out due to the change in retinal shape, protons flow back into the cell through a process associated with ATP formation. This process involves the membrane-spanning protein bacteriorhodopsin, which can acquire protons in the cytosol, change conformation due to the absorption of light, and release protons on the outside of the membrane. The protons then flow into the cell, down their electrochemical gradient.