How does a proton gradient act like a battery during photosynthesis?

In photosynthesis, the proton gradient acts like a battery by storing potential energy in the form of an electrochemical charge across the thylakoid membrane.1

During the light reactions, electron flow drives the movement of hydrogen ions (protons) into the inner thylakoid space. Because the membrane is impermeable to these ions, a higher concentration of protons builds up inside the thylakoid than outside, establishing both a chemical concentration gradient and an electrical charge.1

This stored energy functions similarly to a charged battery. An enzyme complex known as ATP synthase (or the coupling factor) acts as a conduit, allowing protons to flow back across the membrane. As protons move through this channel, the released energy is harnessed to catalyze the phosphorylation of adenosine diphosphate (ADP) into ATP, effectively converting the stored electrochemical potential into chemical bond energy. This mechanism is a central component of chemiosmotic theory.123