Photosynthesis and respiration are complementary reactions because cells require the products of photosynthesis to complete the respiration reaction, and likewise require the products of respiration to complete photosynthesis.
The reactants in a chemical reaction are substances that are transformed during the reaction. The reactants in photosynthesis are carbon dioxide and water, and the reactants in cellular respiration are oxygen and glucose.
The products in a chemical reaction are the substances formed during the reaction. Carbon dioxide and water are the products of respiration, and oxygen and glucose are the products of photosynthesis.
During photosynthesis cells convert the reactants carbon dioxide and water into the products sugar (glucose) and oxygen gas.
During respiration, cells covert the reactants oxygen and glucose into the products carbon dioxide and water. In the process, a form of chemical energy called ATP (adenosine triphosphate) is released.
ATP is the form of energy all cells require to perform the functions necessary to life. In other words, during respiration, sugar is converted into the form of chemical energy cells can use and store.