There is an enzyme aptly called "ATP synthase" which does the job of converting the ADP back into ATP by adding a phosphate. An enzyme is a protein molecule that works as a catalyst to assist in chemical reactions in the body. ATP synthase is a complex enzyme that actually has multiple sections and moving parts. One of the parts bridges the cell membrane, where there is a proton gradient between the inside of the cell and the outside. This charge gradient is an electrochemical energy source. The ATP can cash in on some of this energy by moving protons across the cell membrane and then applying that energy for the uphill reaction to re-energize the ADP into ATP.