The various events that cause the proton gradient across the membrane are :
(a) The splitting of water molecule takes place on the inner side of the membrane, and the protons (H+ ions) that are produced by the splitting of water, accumulate within the lumen of thylakoids.
(b) As electrons move through the photosystems, protons are transported across the membrane. This happens because the primary acceptor of electron which is located towards the outer side of the membrane, transfers its electron carrier to a proton (H+) carrier.
(c) The NADP reductase enzyme is located on the
stroma side of the membrane. Along with electrons, protons are necessary for the reduction of NADP+ to NADPH + H+.