The transfer of the energy in the living organisms from one cell to another occurs via the ATP (Adenosine Triphosphate) molecules. The functional group involved in this transfer of the energy is the phosphate group.
The ADP (Adenosine diphosphate), and ATP (Adenosine Triphosphate) are the molecules involved in the transfer of the energy from one cell to another.
Hence, the answer is 'phosphate'.