How does adenosine triphosphate (ATP) become adenosine diphosphate (ADP)?

Hint: ATP was discovered by Karl Lohmann in 1929. Its formation and breakdown by union and separation of high energy was described in 1941 by Fritz Lipmann. ATP is commonly known as the energy currency of the cell.

Complete answer:
-The energy releasing process in a cell involves oxidation and reduction reactions. During oxidation of respiratory substrates within a cell the energy contained in them is released in a series of step by step reactions controlled by respiratory enzymes. All the energy contained in a respiratory substrate is not released into the cell, it is trapped as chemical energy in the form of chemical bonds of organic phosphates for use when needed. These energy rich compounds are called adenosine triphosphate or ATP.
-ATP carries energy in the two terminal phosphate bonds. The formation of these bonds require a large amount of energy because they are formed against the force of repulsion between similarly charged radicals P and P. Hence these bonds are known as the high energy. The high energy is unstable and readily breaks down by addition of water, releasing a correspondingly large amount of energy and free inorganic phosphates. The removal of the terminal phosphate group of ATP by the hydrolysis is ADP and the separation of the terminal phosphate group of ADP by another hydrolysis gives AMP. Equal amounts of usable energy released per mole of ATP or ATP hydrolysis.
-The energy released by ATP is used by cells in their chemical reaction and is called biologically useful energy.

Note: Cellular respiration does primarily serve to create high energy chemical bonds in energy carriers. ATP is mobile and transports the chemical energy to the energy consuming processes anywhere within the cell. But the ATP molecules do not pass from one cell into another.