Adenosine Diphosphate (ADP) is a nucleotide that plays a crucial role in cellular energy transfer. It consists of an adenosine molecule attached to two phosphate groups. When a cell requires energy, ADP can be converted into Adenosine Triphosphate (ATP) by adding an additional phosphate group, a process that occurs during cellular respiration. ADP is also involved in various biochemical processes, including signal transduction and metabolism. It acts as a signaling molecule, helping to regulate energy balance within cells. The conversion between ADP and ATP is vital for sustaining life, as ATP serves as the primary energy currency in biological systems.