Every living thing is a sort of imperialist, seeking to transform as much as possible of its environment into itself... I. Cellular Respiration: breaking down sugar in the presence of oxygen (aerobic). Photosynthesis (you recall...) is the process by which CO2 and H2O are used to make sugars and starches. During Cellular Respiration, sugar is broken down to CO2 and H2O, and in the process, ATP is made that can then be used for cellular work. The overall reaction for cellular respiration: (does this reaction look familiar? Overall, it is the reverse reaction of photosynthesis, but chemically, the steps involved are very different.) c6h12o6 + 6o2-----> 6co2 + 6h2o + ~38 atp