Answer: ADP ( adenosine diphosphate) is a lower energy molecule, which is converted to ATP ( adenosine triphosphate) by the addition of one high energy phosphate group.
This is an endergonic process as it requires energy.
ATP thus formed in this reaction is a high energy molecule as it stores large amount of energy, which is present in the phosphoanhydride bond ( formed between phosphate).
The reaction can be written as-
ADP + Pi → ATP
