The products of one reaction are the reactants of another.
So, cellular respiration consumes sugars and oxygen and creats carbon dioxide, water, and releases energy. Photosynthesis combines energy, carbon dioxide, and water to form sugar and oxygen. The two processes form approximately closed loops except for energy, which has a linear progression from the sun, into plants/sugars, into cellular processes through cellular respiration, and then out into the universe in the form of radiation (all matter emits radiation, the hotter they are, the more radiation they emit). Also, both produce something that the other cannot live without, as demonstrated by Prisley's experiments. They simply could not exist without one another.