The light-dependent reactions (photosystems I and II) are the first part of photosynthesis in plant chloroplasts. They take the energy from the sun and convert it to ATP, or an energy source that can be stored for future use. The reactants, or materials that go into the reactions, are water, NADP+, ADP, and phosphate. And of course sunlight. The water produces a source of electrons and protons, the phosphate and ADP are ultimately combined to form ATP, and NADP+ is the ultimate receptor of the electrons. The products, or materials that come out of the reactions, are O2, NADPH, and ATP. In between the reactants and products are an entire cascade of reactions that are constantly cycled over and over to keep producing ATP.