ATP is used in cellular respiration as the energy of activation. Even though the enzyme lowers the energy of activation for a substrate considerably, the substrate still needs some energy to activate.

As shown by the diagram on the left, ATP provides energy for a material by giving the material one of its phosphate tails. Two sometimes, but one most of the time. In any event, when the phosphate tail is taken off, the bond for it is broken, thus releasing energy to the substrate. After one of the phosphate is lost, the ATP becomes ADP.
Note:Breaking the phosphate tail of the ATP requires little energy of activation because the ATP is highly unstable.

The substrate eventually releases the phosphate group. However, energy is need to bring the phosphate and ADP together