Final answer:
ATP is a nucleotide, which classifies it under nucleic acids among the four major macromolecules in living organisms. It is composed of an adenosine unit and three phosphate groups, with its energy stored in the bonds between the phosphates.
Step-by-step explanation:
Adenosine triphosphate, or ATP, is a complex organic macromolecule that serves as the primary energy carrier within the cell. The structure of ATP consists of an adenosine moiety - a combination of the nitrogenous base adenine and a five-carbon sugar, ribose. Attached to the adenosine are three phosphate groups termed alpha, beta, and gamma phosphates. Among the four major types of macromolecules in living organisms - proteins, carbohydrates, lipids, and nucleic acids, ATP is categorized under nucleic acids since it is a nucleotide similar to those that make up DNA and RNA.
ATP carries energy within the cell, which is immediately available when the bond between its second and third phosphate groups is broken. This reaction, which yields adenosine diphosphate (ADP) and a single phosphate group, is essential for a plethora of cellular functions.