The answer is the conversion of ADP to ATP using light energy. Photophosphorylation occurs in the light stage of photosynthesis. There are two types of photophosphorylation; cyclic and non-cyclic. In cyclic (which occurs only in bacteria), only photosystem I is utilized in the process while in non-cyclic, both photosystems I and II are used. In non-cyclic photophosphorylation, in addition to ATP, NADH is also synthesized.
