Review the overall equation of Photosynthesis. Photosynthesis occurs in two main steps. During the first step, the light reactions, sunlight and water enter as starting substrates and is released as a final end product. Then the ATP and NADPH energy obtained from the light reactions is used to run the Calvin cycle. In the second step, the Calvin cycle, the starting substrate is converted into usable sugar end products like glucose. carbon dioxide; oxygen oxygen; hydrogen sulfide hydrogen sulfide; carbon dioxide oxygen; carbon dioxide .