What is the function of adp in metabolism?

When mitochondria are respiring and decomposing glucose and producing energy, energy is used to add a phosphate to adp to become atp. The atp then travels throughout the cell and its energy can be used e.g. in ribosomes synthesising protein and it returns to adp and the cycle continues. i.e it is a cyclic way of transfering energy to all parts of a cell from where the energy is produced.