Levels of Hierarchy Atom- the most basic complete unit of an element. Molecule- a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction. Organelles- are cells parts that function within a cell. Cells- the basic structural unit of an organism from which living things created. Is one individual cell. Tissues- a group of cells with similar structure that functions together as a unit, but at a lower level t...