Answer: The fill in the blank can be filled with ATP.
ATP ( adenosine triphosphate) is the energy producing or energy storing molecule. It is also called as the energy currency of the cell. It contains high energy phosphoanhydride bonds ( the bonds between phosphate molecules). When ATP is hydrolysed, it gets converted into ADP ( adenosine diphosphate) and inorganic phosphate. This liberates large amount of energy which can be utilized by the cell for various activities.
Thus, ATP is considered as the energy producing or energy-storing molecule.