Levels of organization

In evolutionarily advanced multicellular organisms the cell is built up through five levels.

The five levels

The smallest structural and functional unit that exists in a living state is the cell. A group of cells specialised for one definite job is called a tissue. Several tissues together make an organ, and organs grouped together make a system. Systems together build an organism.

cell → tissue → organ → system → organism

Figure to be added

Figure 8.5 — the levels of organization that can be identified in an organism: from the cell through tissue, organ and system to the organism.

Not every organism has all five

A unicellular organism has organization only at the organelle level — there is no tissue and no organ. Tissue- and system-level organization is found only in the evolutionarily advanced organisms, and in those the organs themselves are built to a high standard, shaped as the job they do requires.

Examples — the tongue, the eye, the heart.