The biological levels of organization start with the atom, which is the smallest unit of matter. Take two or more of the most fundamental units, atoms, to form molecules. Molecules that are biologically significant are called macromolecules. One example of a macromolecule is deoxyribonucleic acid (DNA). DNA contains the basis for the structure and functions of all living organisms. The succeeding levels of organization of living things are: organelle, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystem, and the biosphere.