Answer
When energy is required , ADP takes one phosphate and ATP is formed.
Explanation
ADP (Adenosine diphosphate) is different from ATP (adenosine triphosphate) in the number of phosphate groups in its structure. These are the source of energy for the cells and the presence of phosphate groups in these compounds indicate the strength of energy. ADP has two phosphate groups while ATP has three. When the cell requires energy ADP gains one phosphate and ATP is formed which is responsible for energy production. Then again ATP is broken down and forms ADP and Pi (inorganic phosphate) release.
