Final answer:
Photosystem II (PSII) and Photosystem I (PSI) are involved in the light-dependent reactions of photosynthesis, capturing light energy and converting it to chemical energy. PSII splits water to replace its electrons and passes energized electrons to PSI through the ETC. PSI further reduces NADP+ to NADPH using these electrons.
Step-by-step explanation:
The light-dependent reactions of photosynthesis involve two separate but interconnected photosystems known as Photosystem II (PSII) and Photosystem I (PSI), both of which are located in the thylakoid membranes of chloroplasts. These photosystems are responsible for capturing light energy and converting it into chemical energy. They consist of a light-harvesting complex and a reaction center, where the photochemistry occurs. Photosystem II is characterized by its ability to split water to replace electrons, releasing oxygen as a byproduct, and transfer excited electrons to the electron transport chain (ETC). Photosystem I receives electrons from the ETC and uses them to reduce NADP+ to NADPH, a molecule that will eventually be used in the Calvin cycle.
The functioning of the two photosystems can be seen as a Z-scheme due to the energetic flow of electrons. Both photosystems work simultaneously and are excited by light energy. The critical differences lie in the source of their replacement electrons and in the destination of their energized electrons. PSII gets replacement electrons from water molecules, while PSI gets them from the ETC. The energized electrons from PSII are passed on to PSI through the ETC, and further from PSI to reduce NADP+ to NADPH.