The energy released by removing a phosphate group can be captured and used, for instance, to make chemical reactions occur or muscle fibers move. There is a continual synthesis of ATP in the body, and also a continuous breakdown of ATP to power the various body functions. Thus, the study of energy conversion in living organisms largely consists of the study of ATP synthesis and breakdown. All living cells respire, though a very few, mostly in the Archea, use a different process than described here. In the overall reaction of respiration, a sugar molecule is broken down into water and carbon dioxide, with the production of 36 ATPs.
Sugar consists of carbon, oxygen, and hydrogen.
There are three basic parts to cellular respiration. The first is called glycolysis, the second is called the Krebs cycle, and the third is called electron transport. The Krebs cycle and electron transport occur in the mitochondria parts of eukaryotic cells, while glycolysis occurs outside the mitochondria. At the very end, the hydrogens are combined with oxygen from the air we breathe to make water. More details are in Chapter 5 of your book.