Answer:
A proton gradient is generated by the transport of protons into the thylakoid lumen.
Protons move from the thylakoid lumen to the stroma through ATP synthase, producing ATP.
Step-by-step explanation:
During photosynthesis, the environment is made acidic inside the lumen i.e. H⁺ are pumped into thylakoid lumen from stroma as a result of which more H⁺ are present in the thylakoid lumen as compared to stroma. It happens during light dependent reaction of photosynthesis. The concentration of H⁺ is already higher in lumen and transfer of more and more H⁺ from stroma increases the concentration of H⁺ even more leading to generation of a potential gradient. These H⁺ subsequently tend to move freely from lumen to stroma via "reverse pumps known as ATP synthase". The reason why these are known as reverse pumps is because pumps usually move particles from lower to higher concentration which is an active movement i.e. not natural so such movement requires energy. Naturally particles move from higher to lower concentration gradient until the concentration becomes equal on both the sides but pumps act opposite of this natural process and move particles from lower to higher concentration and utilize energy to do it. But here H⁺ are moving from higher to lower concentration which occurs naturally so ATP synthase rather than using energy tend to generate energy and this free energy is used to generate ATP from ADP & Pi (inorganic phosphate).