Atom: The smallest unit of matter.
Molecule: Made up of bonded atoms.
Cell: The basic unit of life.
Tissue: A group of cells working together.
Organ: Structures made of different tissues.
Organism: A living individual.
Population: Groups of the same species in a given area.
Community: Different populations living together.
Ecosystem: A community interacting with its environment.
Biosphere: The global ecological system integrating all living beings and their relationships.