They both use energy, they are both reactants to each other, they both depend on each other. The life cycle of photosynthesis and cellular respiration sustains life on Earth.
Photosynthesis makes the glucose that is used in cellular respiration to make ATP. The glucose is then turned back into carbon dioxide, which is used in photosynthesis. While water is broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water.
