Levels of biological organization

From the atom to the biosphere, in order. The full scale of biological organization so you don't get it wrong on the exam.

Direct answer: The levels go from simplest to most complex: atom → molecule → organelle → cell → tissue → organ → organ system → organism → population → community → ecosystem → biome → biosphere. The cell is the basic unit of life.

The levels in order

Atom — e.g. carbon, oxygen.
Molecule — e.g. water, DNA.
Organelle — e.g. mitochondrion.
Cell — basic unit of life.
Tissue — similar cells working together.
Organ — e.g. heart, leaf.
Organ system — e.g. circulatory system.
Organism — a complete living being.
Population — organisms of the same species.
Community — different populations together.
Ecosystem — community + physical environment.
Biome and biosphere — all life on Earth.

Exam trap: the order goes from lower to higher complexity. A typical mistake is skipping the tissue level between cell and organ, or confusing population (one species) with community (several species).

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Frequently asked questions

What is the order of the levels of organization?

Atom, molecule, organelle, cell, tissue, organ, system, organism, population, community, ecosystem, biome and biosphere.

What is the basic unit of life?

The cell.

What is the difference between population and community?

A population is a single species; a community brings together several species.