"Cellular respiration is like a change machine: you're turning sugars into ATP so it will be a usable form of energy. If you go to a coin operated laundromat, they all seem to run on quarters for some reason. So you might say, "I don't have any quarters but I want to wash my clothes and I have a $10 bill." You put your $10 bill into the change machine and you get 40 quarters and now you could use the coin operated washers and dryers. All the chemical reactions in living things run off of these quarters (ATP). They don't run off $10 bills (sugars/fats/proteins)."