What is the function of ADP molecules living in cells?

The function of ADP molecules living in a cell is to locate and taking the energy given off by other processes which are also taking place in the cell. Sometimes this process of taking the energy during oxidation is called "capturing" the energy.

Although more people are familiar with ATP, ADP is closely related to ATP as ADP happens first. ATP has been found to be the product of ADP. These molecules are pretty similar except for the number of phosphates they have. ATP has three phosphate groups while ADP only has two. The reason for this is because the additional group gives it more energy.

The purpose of ADP in any cell is to “capture” energy from other oxidation processes taking place in the cell. The ADP is the product that comes before you get the famous ATP. The biggest difference between the two molecules is the number of phosphate groups. ADP has two phosphate groups, while ATP has three groups total.

That extra grouping gives the ADP more energy. Therefore, it follows that ATP is the energy currency of the cell.
The extra phosphate group is added at various places. It can be added in glycolysis, which happens in the cytoplasm of the cell. It can also be added during the Krebs Cycle or during the electron transport chain that happens alongside the Krebs Cycle.