How are photosynthesis and cellular respiration coupled reactions?
The products of photosynthesis are the reactants for respiration, and the products of respiration are the reactants for photosynthesis.
Photosynthesis is a complex set of enzyme-catalyzed reactions that use carbon dioxide (CO2), water (H2O) and light energy to create glucose (C6H12O6) and oxygen gas (O2).
Cellular respiration is a complex set of enzyme-catalyzed reactions that break apart glucose, making 6 molecules of CO2 along the way. The final step of respiration uses oxygen as a driving force to create energy in the form of ATP. As a result, O2 gas is reduced to H2O.
So, the glucose and O2 created by photosynthesis is used in respiration to make CO2 and water - the materials needed in photosynthesis. In this way, these two reactions are coupled.