Where do we get the reactants of respiration from?

Cellular respiration is the process by which the food (in the form of glucose molecules) is broken down and energy is released. The process can be represented by the following chemical reaction:

`C_6H_12O_6 + 6 O_2 -> 6 CO_2 + 6 H_2O + ATP`

The reactants of cellular respiration are glucose molecules and oxygen. The products are carbon dioxide, water and ATP or adenosine triposphate molecules, which are also the energy molecules. 

These reactants are obtained from the process of photosynthesis, which can be given by the following chemical reaction:

`6CO_2 + 6 H_2O + sunlight -> C_6H_12O_6 + 6 O_2`

Here, carbon dioxide and water react, in presence of sunlight, and generate glucose and oxygen molecules. 

Hence, the two processes, photosynthesis and respiration, are complementary to each other and each one generates products that are used as reactants for the other.

Hope this helps.