Final answer:
Phosphorylation occurs when a phosphate group is added to ADP to form ATP, an energy-storing reaction that is necessary for cellular activities.
Step-by-step explanation:
When energy from a compound containing phosphate reacts with ADP (adenosine diphosphate) to form ATP (adenosine triphosphate), a process called phosphorylation occurs. This is the addition of a phosphate group to ADP, and it is an energy-consuming reaction. In the formation of ATP, the bond between the newly added phosphate group and ADP stores potential energy.
This energy is released when ATP is hydrolyzed back to ADP and inorganic phosphate (Pi). The hydrolysis of ATP to form ADP can be represented by the chemical equation: ATP + H₂O → ADP + Pi + energy. The reaction liberates energy that was stored in the phosphoanhydride bonds, which is significant for various cellular activities.