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 a phosphate group, a process that occurs during cellular respiration. ADP is essential for various biological processes, including muscle contraction and nerve impulse transmission. It acts as a signaling molecule, helping to regulate energy metabolism within cells. The conversion between ADP and ATP is vital for maintaining the energy balance in living organisms.