In photosynthesis, “photo” refers to the capture and utilization of light energy. Photolysis, the splitting of water molecules, releases electrons that excite chlorophyll within Photosystem II, initiating the light-dependent reactions. These reactions generate ATP and NADPH, energy-carrying molecules that fuel the light-independent reactions (Calvin cycle). The Calvin cycle uses ATP and NADPH to convert carbon dioxide into glucose, the primary energy source for plants and the basis of food chains. Thus, “photo” represents the fundamental process of harnessing light energy to drive photosynthesis and sustain life on Earth.