Final answer:
ATP is resynthesized from ADP via the process known as phosphorylation, which can occur through substrate-level phosphorylation or oxidative phosphorylation. ATP hydrolysis is a reversible reaction, and the energy liberated is used by the cell to perform various functions.
Step-by-step explanation:
ATP can be resynthesized from an ADP by the process called phosphorylation. During this biochemical reaction, a high-energy phosphate group is attached to the ADP molecule. This process is critical for storing the energy that cells require for various functions. The recharging of ADP to ATP can occur via several pathways, including substrate-level phosphorylation during glycolysis and the citric acid cycle, and also through oxidative phosphorylation which occurs in the mitochondria. Oxidative phosphorylation is part of the electron transport chain where oxygen is required, hence it being 'oxidative', and ADP is phosphorylated to become ATP as a result of chemiosmosis.
During ATP hydrolysis, energy is released which can then be used by the cell to perform work. However, since the hydrolysis of ATP to ADP is reversible, the reverse process of regenerating ATP from ADP and inorganic phosphate (Pi) also exists and is necessary for sustaining life, as ATP is the principal medium of energy exchange in biological systems. The equation for forming ATP from ADP and Pi is: ADP + Pi + free energy → ATP + H₂O. This phosphorylation of ADP is coupled with energy-release reactions, such as nutrient breakdown or the electron transport chain, to make the endergonic reaction of ATP synthesis favorable.