Which nucleotide functions primarily as an energy-storing molecule?

Adenosine triphosphate (ATP) is the primary energy carrier in all living cells. It is composed of an adenosine molecule bonded to three phosphate groups. The key aspect of ATP's function as an energy-storing molecule lies in the high-energy bonds between the phosphate groups. When ATP is hydrolyzed to adenosine diphosphate (ADP) and inorganic phosphate (Pi), a significant amount of energy is released, which can then be utilized by the cell for various metabolic processes, including muscle contraction, protein synthesis, and other biochemical reactions essential for maintaining cellular function.

The other choices refer to different nucleotides that do not serve the same primary role as energy carriers. Guanosine also plays a role in cellular energy transfer but is less prominent than ATP. Uridine and cytidine serve as building blocks for RNA but do not primarily function as energy-storing molecules. Thus, ATP stands out uniquely for its role as the main energy currency of the cell.