During cellular respiration, glucose is broken down in the cells to release energy in the form of ATP which is used in the body for various metabolic processes and activities. However, approximately 20% of the energy produced is used to form the energy-rich compound, ATP, whereas the rest of the energy is released as heat. ATP is formed in a complex process which involves the activity of many enzymes that use the released energy to combine ADP and a phosphate group. What does ADP stand for?