Final answer:
Water is the initial source of electrons that replace those in the chlorophyll during photosynthesis. These electrons are extracted by sunlight in PSII, travel through the ETC, and ultimately contribute to the formation of ATP and NADPH.
Step-by-step explanation:
The initial source of electrons for the chloroplast electron transport chain (ETC) during photosynthesis is water (H2O). In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons from water. This process is also known as the splitting of water, and it results in the formation of oxygen (O2) and hydrogen ions (H+). The oxygen is eventually released into the atmosphere, and the hydrogen ions contribute to the creation of an electrochemical gradient across the thylakoid membrane.
The extracted electrons then travel through the ETC to photosystem I (PSI), where they are used to reduce NADP+ to NADPH. The high-energy electrons from chlorophyll a in PSII are captured by pheophytin, and as they travel down the ETC, they facilitate the pumping of protons, creating the gradient that powers ATP synthesis through chemiosmosis.