Here's why:
* Water splitting: In the light-dependent reactions of photosynthesis, water molecules are split by photosystem II (PSII) using light energy. This process releases electrons, protons (H+), and oxygen.
* Electron transport chain: The released electrons travel through an electron transport chain, moving from PSII to photosystem I (PSI). This movement of electrons generates a proton gradient across the thylakoid membrane, which is used to power ATP synthesis (photophosphorylation).
* NADP+ reduction: The electrons eventually reach PSI and are used to reduce NADP+ to NADPH. NADPH is a crucial reducing agent used in the Calvin cycle, the light-independent reactions of photosynthesis, to convert carbon dioxide into sugars.
Therefore, while water splitting provides the initial electrons, it's NADP+ that acts as the final electron acceptor in noncyclic photophosphorylation.
