atom
Levels of Organization of Living Things Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms held together by one or more chemical bonds. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by polymerization a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules . An example of a macromolecule is deoxyribonucleic acid DNA Figure 1.15 , which contains the instructions for the structure and functioning of all living organisms.