Photosynthesis and cellular respiration is about as close to a perfect example of a "win-win" relationship as can be found, occuring in nature. Photosynthesis is the process by which plant cells manufacture their own food, glucose, by using carbon dioxide, water, and light energy, usually from the sun. This process creates a simple sugar, glucose, C6H12O6, and oxygen, which is released as a waste product. Cellular respiration in animal cells uses glucose, which is taken from the digested food animals eat, combines it with oxygen from the animal's respiratory system, and produces energy in the form of ATP (adenosine triphosphate), while it releases carbon dioxide and water as waste products.
The two equations are mirror images of each other:
Photosynthesis: CO2 + H2O + light energy ---> C6H12O6 + O2
Cellular Respiration C6H12O^ + O2 ---> CO2 + H2O + energy (ATP)