The primary source of power for photosynthesis is light energy from the sun. Plants have specialized molecules called chlorophylls that are embedded in their chloroplasts. When light strikes the chlorophylls, the energy is absorbed and used to drive the chemical reactions that power photosynthesis. These reactions split water molecules, releasing oxygen into the atmosphere and generating hydrogen protons and electrons. The high-energy electrons are then used to convert carbon dioxide, taken in from the atmosphere, into sugar molecules during the second stage of photosynthesis, known as the Calvin cycle.