Removing a phosphate group from the triphosphate tail in a molecule of ATP releases energy because it involves the breaking of a phosphoanhydride bond. This bond connects two phosphate groups and contains a large amount of potential energy stored within it. When the bond is broken, this energy is released and becomes available for cellular processes.

The process of breaking the phosphoanhydride bond in ATP is catalyzed by enzymes known as ATPases. These enzymes facilitate the transfer of the terminal phosphate group from ATP to another molecule, such as water, ADP (adenosine diphosphate), or a specific enzyme substrate. As a result of this transfer, the energy released from the broken bond is harnessed and can be utilized by the cell.

The energy released from ATP hydrolysis is essential for driving various biological processes, including muscle contraction, active transport of molecules across cell membranes, synthesis of macromolecules, and many more. Without this energy source, cellular activities would come to a halt.

In summary, removing a phosphate group from ATP releases energy due to the breaking of the phosphoanhydride bond, providing the necessary energy for various cellular functions.