Levels of Biological Organization

Atom: The fundamental unit of matter consisting of protons, neutrons, and electrons; example: Carbon (C).
Molecule: Formed when two or more atoms bond together; example: Water (H2O).
Organelle: Specialized structures within a cell that perform distinct functions; example: Vacuole.
Cell: The basic unit of life, capable of performing all life processes; example: White blood cell.
Tissue: A collection of similar cells grouped together to perform a specific function; example: Muscle tissue (not specified).
Organ: A structure composed of different tissues working cooperatively to fulfill a specific role; example: Heart.
Organ System: A group of organs that collaborate to perform a major biological function; example: Digestive System.
Organism: An individual living entity that exhibits all characteristics of life; example: Dog.
Population: A group of organisms of the same species situated in a particular area at the same time; example: A herd of Caribou.
Community: The assemblage of various organisms interacting within the same environment; example: White-tailed deer, oak trees, and gray squirrels in a forest.
Ecosystem: Encompasses all living (biotic) entities and their non-living (abiotic) environment; example: Algae, sunfish, frogs, and the water's salinity and temperature.
Biome: A large ecological area characterized by specific climatic conditions and distinctive plant and animal species; example: Tundra.
Biosphere: The global sum of all ecosystems, representing the zone of life on Earth; example: Earth itself.

Levels of Biological Organization Sequence

From smallest to largest: Atom → Molecule → Organelle → Cell → Tissue → Organ → Organ System → Organism → Population → Community → Ecosystem → Biome → Biosphere.
From largest to smallest: Biosphere → Biome → Ecosystem → Community → Population → Organism → Organ System → Organ → Tissue → Cell → Organelle → Molecule → Atom.