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 essential for various biological processes, including metabolism and muscle contraction. It acts as a signaling molecule, helping to regulate energy levels within cells. The conversion between ADP and ATP is vital for maintaining the energy balance in living organisms, supporting functions from cellular respiration to muscle movement.