How does ATP work?

ATP works after a phosphate group has been transferred from ADP, which is known as phosphorylation. This process occurs whenever a cell needs energy. When a cell needs to store energy, it combines ADP and a phosphate to make ATP. Cells get energy through ATP from respiration, and this can be either aerobic or anaerobic respiration. Adenylate kinase (AK) is the enzyme responsible for catalyzing the phosphoryl transfer between 2 ADP molecules to make ATP.  In addition, it is important to note that ATP is a reversible process, as phosphates can be removed and added when necessary. 

For anaerobic respiration, ATP does not require oxygen and only a small amount of energy is created. Aerobic respiration occurs in the mitochondria, and occurs in three stages; glycolysis, the Krebs cycle, and the cytochrome system. In glycolysis, six-carbon glucose breaks down into two three-carbon pyruvic units. The hydrogens that were removed join with the hydrogen carrier NAD to create NADH2, and the result is an overall addition of 2 ATP molecules. In the Krebs cycle, high energy NADH is produced as well as FADH2 and some ATP. Citric acid becomes converted and electrons are released to NAD. In these reactions, enough energy is released to synthesize an ATP molecule. For every two glucose molecules, two molecules of ATP are created. The cytochrome system is where most of the ATP becomes produced during aerobic respiration. NADH exports hydrogen ions and electrons to the electron transport chain to become oxidized. The electrons provide energy to the proteins in the membrane, which is then used to push hydrogen ions across the membrane. This flow of ions synthesizes approximately 34 ATP molecules.