What molecule acts as a source of electrons during light reactions?

During the light reactions of photosynthesis, water (H2O) plays a crucial role as the source of electrons. When light energy is absorbed by chlorophyll, it excites electrons and initiates the process of photolysis, where water molecules are split into oxygen, protons, and electrons. The free electrons generated from this splitting process are essential for the electron transport chain, contributing to the synthesis of ATP and NADPH, which are vital for the subsequent Calvin cycle.

Oxygen is a byproduct of this reaction, released into the atmosphere, while carbon dioxide is not directly involved in the light reactions but rather in the Calvin cycle. NADPH, on the other hand, is produced during the light reactions as it acts as an electron carrier, but it does not serve as a source of electrons itself. Thus, water is identified as the primary electron donor in the light reactions, making it the correct answer.