Final answer:
To break the last phosphate bond in ATP, water (H₂O) must be added through hydrolysis, resulting in the formation of ADP and an inorganic phosphate group (Pi) and the release of energy.
Step-by-step explanation:
When breaking the last phosphate bond in ATP, a molecule of water (H₂O) must be added. This process, known as hydrolysis, results in the formation of adenosine diphosphate (ADP) and an inorganic phosphate group (Pi). Hydrolysis of ATP releases a significant amount of energy that can be used to power various cellular reactions and processes.