Final answer:
Energy from ATP is transferred to a reactant in an endergonic reaction through the addition of a phosphate group, a process known as phosphorylation.
Step-by-step explanation:
The energy from ATP (adenosine triphosphate) is transferred to the reactant of an endergonic reaction by the addition of a phosphate group to that molecule. When ATP hydrolyzes into ADP (adenosine diphosphate) and an inorganic phosphate group, the energy released from this exergonic reaction can be coupled with an endergonic reaction to drive it forward. This process is known as phosphorylation, which involves the transfer of a phosphate group from ATP to the molecule, thereby providing the necessary energy for the endergonic reaction to occur.