ADP is like an uncharged battery. ATP is like a charged battery, ready to provide energy to do work in the cell. The charging of ADP into ATP takes place in the mitochondria. Fat and starch are stable (last a long time) whereas ATP goes dead too quick and will need to be recharged.
glucose is a carbon chain molecule (sugar). When the bonds in the chain are broken, energy is released. That energy is used to turn ADP into ATP. Glucose is the "electricity" used to charge the battery.
