Final answer:
Water is the initial source of electrons for the plant's electron transport chain during photosynthesis, where it is split to provide electrons to the ETC, leading to the production of NADPH.
Step-by-step explanation:
The initial source of electrons that supplies the plant's electron transport chain during photosynthesis is water (H2O). When light energy is absorbed by the photosystem II (PSII) in the chloroplast, it causes water molecules to split in a process called photolysis. This splitting of water releases electrons, protons (H+ ions), and oxygen. The electrons are then transferred through the electron transport chain (ETC) to photosystem I (PSI), which ultimately reduces NADP+ to the energy-carrying molecule NADPH. NADPH, along with ATP generated by the flow of protons across the thylakoid membrane, is used in the light-dependent reactions of photosynthesis to synthesize sugars from carbon dioxide (CO2).