Question

An essential functional group involved in cellular energy transfer is the _______ group.

Answer 1

An essential functional group involved in cellular energy transfer is the phosphate group. 
It is important because it is charged and can release H+ ions into the solution. This makes the group highly acidic, which is important when it comes to the transfer of energy within a cell. Apart from the phosphate group, hydroxyl, carbonyl, amino, and other are also important.