How does the structure of a chloroplast enable it to build up a concentration gradient of protons?

The membrane inside the thylakoid of the chloroplast pumps H + ions from the outside compartment (stroma) to the inside (lumen). This builds the gradient. The electrons are pumped using energy released from a high energy electron which was energized through light absorption. This electron comes from the breakdown of water.

Protons are pumped from the stroma to the interior of the thylakoid. The energy required to pump these protons is supplied by the excited electrons as they pass along the electron transport chain of photosytem II. Both of these mechanisms act to build up a concentration gradient of protons.