In both photosystem I and photosystem II, light energy is used to excite electrons. To the confusion of many students, the two photosystems were named in the order of their discovery, and not in the order of their work in the light-dependent reactions. Photosystem II comes first in the sequence of events that moves electrons from water molecules to their destination, NADP+. Hydrogen ions travel with the electrons, and they change NADP+ into NADPH. The process is summarized in this diagram. The diagram shows rays of light striking Photosystem II and Photosystem I. An arrow leads from the splitting of water to Photosystem II , to Photosystem I, and then to the reaction that converts NADP+ to NADPH. Hydrogen ions move across the membrane.

How is the production of NADPH useful for the process of photosynthesis?