Final answer:
Water (H2O) provides the electrons for the light reactions in photosynthesis, which are then used alongside ATP and NADPH in the Calvin cycle to produce glucose.
Step-by-step explanation:
The electrons for the light reactions in photosynthesis are provided by H2O (water). During the light-dependent reactions, chlorophyll absorbs energy from sunlight, which is then used to split water molecules into oxygen, hydrogen ions, and electrons. Oxygen is released as a byproduct, while the electrons move through the electron transport chain, creating an energy gradient used to form ATP and NADPH. The ATP and NADPH produced are then utilized in the Calvin cycle, which is the light-independent part of photosynthesis where carbon dioxide is fixed into glucose.