Cellular respiration and photosynthesis are two sides of the same coin because these are complementary processes and feed other. Photosynthesis is the process by which sun's light energy is converted into chemical energy of glucose molecule (food). This reaction also makes use of carbon dioxide and water and generates oxygen. Chemically this process can be represented as:
`6 CO_2 + 6 H_2O + sunlight -> C_6H_12O_6 + 6 O_2`
Cellular respiration is the process by which the food is broken down into carbon dioxide and water and consumes oxygen. This process generates the energy that is required to run all the life functions. Chemically this process is given as:
`C_6H_12O_6 + 6 O_2 -> 6 CO_2 + 6H_2O + ATP`
This process is the exact reverse of photosynthesis. Alternatively, we can say that products generated in one process is the reactants for the other. Hence, these processes are complementary and are different sides of the same coin.
Hope this helps.