The Invisible Living World: Beyond Our Naked Eye
Levels of organisation
Climb the ladder from a single cell to a whole living body.
A cell is the basic unit of life, the way a brick is the basic unit of a wall. Bricks make walls, walls make rooms, rooms make a building. A body is built up in steps in the same way.
Think of how a complaint travels in school. You don't go straight to the principal. You tell your teacher, who tells the head of department, who tells the vice-principal, who tells the principal. The body has its own ladder:
| Level | Made of | Example |
|---|---|---|
| cell | — | a muscle cell |
| tissue | a group of similar cells | muscle tissue |
| organ | different tissues organised together | the stomach |
| organ system | several organs working on one major job | the digestive system |
| organism | all the organ systems together | a plant, a dog, you |
These levels of organisation, cell → tissue → organ → organ system → organism, show how simple building blocks come together into a complex living being.
A complex living thing begins life as a single cell, the egg. The egg can divide again and again until it becomes a whole living being of many cells. Such beings are multicellular: all animals, including humans, and all plants.
A surprising fact: the yellow yolk of an ostrich egg is a single cell, the largest known cell in the living world, about 130 mm to 170 mm across. The rest of the egg is not cells: a shell for protection, and the white liquid that feeds the cell as it develops.
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