Final answer:
The electrons lost from the reaction center of photosystem I are replaced by electrons from photosystem II. These initially come from water molecules that are split in PSII, producing oxygen as a waste product. The chloroplast electron transport chain transfers electrons and pumps protons to generate ATP and NADPH.
Step-by-step explanation:
The electrons lost from the reaction center of photosystem I (PSI) are replaced by electrons from photosystem II (PSII). In the light-dependent reactions of photosynthesis, energy from sunlight is absorbed by pigments in PSII and used to extract electrons from water molecules. These electrons are then passed through the chloroplast electron transport chain (ETC) to PSI, where they can be used to reduce NADP+ to NADPH.
The initial source of these electrons is the water molecules that are split in PSII, which produces oxygen as a byproduct. The ETC is responsible for transferring these electrons from PSII to PSI, and also for pumping protons across the thylakoid membrane to create an electrochemical gradient used by ATP synthase to produce ATP. This intricate flow of electrons and the accompanying proton gradient are essential in the production of the energy carriers ATP and NADPH, which are used in the Calvin cycle to produce glucose from carbon dioxide.