Photosynthesis and cellular respiration are complementary biochemical reactions. Photosynthesis requires the products of respiration, while respiration requires the products of photosynthesis. Together these reactions allow cells to make and store energy and help regulate atmospheric concentrations of carbon dioxide and oxygen.
The oxygen produced by plants during photosynthesis is what humans and animals inhale for the blood to transport to the cells for respiration. The carbon dioxide produced during respiration is released from the body and absorbed by plants to help provide the energy they need for growth and development. This is the never ending cycle that sustains life on earth.