Photosynthesis and cellular respiration go hand and hand. Remember the two equations are just a reverse of the opposite equation.
Cellular Respiration-
C₆H₁₂O₆ + 6O₂ (Yeilds or Makes) 6CO₂ + 6H₂O + ATP (Or Energy)
Photosynthesis-
6CO₂ + 6H₂O + ATP (Or Sunlight) (Yeilds or Makes) C₆H₁₂O₆ + 6O₂
When a plant goes through photosynthesis it produces oxygen as a waste product, which you should know is what animals use to breathe, well when animals use oxygen in the process, they also make a waste product which happens to be Carbon Dioxide, which a plant uses to make glucose during photosynthesis, so if we didin't have one we wouldn't have the other.
ANSWER: Cells get their instruction from the formula.