I hope this answers your question - The purpose of photosynthesis is to convert water and carbon dioxide to glucose. Purpose of cellular respiration is to convert energy stored in glucose to ATP. Sellier respiration is a process of oxidizing food mall cools, most importantly, like glucose, to CO2 and H2O. The energy released is trapped in the form of ATP for you spy all energy consuming activities of cells. A process like this occurs in two phases; glycolysis, then breakdown of glucose to pyruvic acid.