Adenosine diphosphate (ADP) (Adenosine pyrophosphate (APP)) is an important organic compound in metabolism and is essential to the flow of energy in living cells. A molecule of ADP consists of three important structural components: a sugar backbone attached to a molecule of adenine and two phosphate groups bonded to the 5 carbon atom of ribose. The carbon molecules that make up the ring structure of a sugar can be named in a way that more specifically designates the location of the phosphate and adenosine attachments: The sugar backbone of ADP is known as a pentose sugar and consists of five carbon molecules. The two phosphate groups of ADP are added in series to the 5’ carbon of the sugar backbone, while the adenosine molecule attaches to the 1’ carbon.