during photosynthesis, is used to break water down into oxygen and hydrogen.; what role do glucose and oxygen play in cellular respiration?; the substances that are present before any chemical reaction are called .; the diagram illustrates photosynthesis. which best describes what is happening in the area marked x?; where in an equation for photosynthesis does oxygen belong?; which stage of the cell cycle involves cell growth and dna replication?; the main sugar that is used as a reactant in cellular respiration is called .; which scientist most likely studied tadpole cells under a microscope?