Levels of Organization

The human body is organized as a series of nested levels, each built from the one below it. A useful way to understand the organism is to climb this ladder.

The ladder

1. Chemical level — atoms (carbon, oxygen, hydrogen, nitrogen, and others) bond into molecules.
2. Molecular level — molecules such as proteins, fats, and DNA.
3. Cellular level — molecules assemble into cells, the smallest living units.
4. Tissue level — groups of similar cells working together.
5. Organ level — two or more tissue types forming a structure with a specific job (the heart, a kidney).
6. Organ system level — organs that cooperate on a shared function (the digestive system).
7. Organism level — all systems working together as one living human.

Why it matters

Each level has properties the level below does not — a feature called emergence. A single heart-muscle cell cannot pump blood; billions of them, arranged as an organ, can. Understanding the human organism means understanding not just the parts, but how each level gives rise to the next.