Structural unit, functional unit
The cell theory calls the cell two kinds of unit — a structural one and a functional one. Those are not the same claim, so take them one at a time.
The structural unit
Looking at the levels of cellular organisation in living things, the answer to "what is the smallest unit a living body is built up from?" is the cell. Separate off anything smaller and it is no longer alive.
The functional unit
The various jobs of a living body are also carried out by cells. For each job there are cells shaped to suit it.
| Cell | Job it carries out |
|---|---|
| Red blood cells | Transport of oxygen in humans |
| Nerve cells | Conduction of impulses |
The cell
A cell is the smallest biological unit shaped to carry out some job — a function. That is what makes the cell the functional unit of life as well as its structural unit.
"Unit" is the right word twice over
A brick is the structural unit of a wall — but a brick does nothing. A cell is more than that: it is both the piece the body is built from and the piece that does the body's work.