Nucleotide diphosphates (NDPs) are molecules composed of a nitrogenous base, a sugar, and two phosphate groups. They play a crucial role in cellular metabolism and energy transfer. When one of the phosphate groups is removed, NDPs can convert into nucleotide monophosphates (NMPs), releasing energy that cells use for various functions. NDPs are essential for the synthesis of nucleic acids, such as DNA and RNA. They also participate in signaling pathways and are involved in the activation of certain enzymes. Common examples of nucleotide diphosphates include ADP (adenosine diphosphate) and GDP (guanosine diphosphate).