Explanation

Steps that causes proton gradient to develop during photophosphorylation are as follows (i) protons or hydrogen ions that are produced by the splitting of water accumulates within the lumen of the thylakoids (ii) 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 electrons not to an electron carrier but to an $H^{+}$ carrier. Hence, this molecule removes a proton from stroma, while electrons transporting (iii) The NADP reducetase enzyme is located on the stroma side of the membrane. Along with electron, reduction of $NADP^{+}$ to NADPH $+ H^{+}$ takes place. By this way, the proton removes from stromal side of the thylakoid. Proton gradient is important because it is the break down of this gradient that leads to release of energy. The gradient is broken down due to movement of protons across the membrane to the stroma through the transmembrane channel of the $F_{0}$ of the ATPase. The energy released during the breaking down of proton gradient is used in formation of ATP