Final answer:
NADPH generated by photosystem I supplies hydrogen and high-energy electrons for synthesizing sugars in the Calvin cycle, with water being the initial electron source in the electron transport chain.
Step-by-step explanation:
NADPH, which is provided by photosystem I (PSI), provides a source of hydrogen and the energetic electrons needed to bond them to carbon atoms. The initial source of electrons for the chloroplast electron transport chain is water. During the light-dependent reactions of photosynthesis, when a photon hits photosystem II (PSII), it splits water molecules to extract electrons. These electrons are passed through the electron transport chain to PSI, where they are re-energized by another photon. This energy is then used to form NADPH by reducing NADP+ with the additional high-energy electrons and a hydrogen ion (H+). NADPH, also known as "hot hydrogen", carries this energy and hydrogen atoms to the Calvin cycle where they are used to help synthesize sugar molecules from carbon dioxide (CO2).