In a multicellular organism, cells specialize. Cells of the same kind work together, and those groups work together too. Biologists name the levels from small to large.
Key idea
Cell → tissue (similar cells doing one job) → organ (several tissues working together) → organ system (organs working together) → organism.
| Level | Animal example | Plant example |
|---|---|---|
| Cell | A muscle cell | A cell from a leaf |
| Tissue | Muscle tissue | Food-making tissue in a leaf |
| Organ | The stomach | A leaf |
| Organ system | The digestive system | The shoot system (stems and leaves) |
| Organism | A dog | A sunflower |
Why it works
No single kind of cell could do an organ's whole job. The stomach must churn food, make digestive juices and respond to signals, so it has muscle tissue, lining tissue and nerve tissue all working at once. Organs then team up: the stomach can only do its part because the mouth, intestines and liver do theirs.
Worked examples
Worked example: Name the level
Which level is the heart?
- The heart has muscle tissue, nerve tissue and lining tissue working together.
- Several tissues together make an organ.
Answer: An organ.
Worked example: Put in order
Order these from smallest to largest: nervous system, nerve cell, brain, nerve tissue.
- Start with one cell: nerve cell.
- Many nerve cells make nerve tissue.
- The brain is an organ built of nerve tissue and more.
- The brain, spinal cord and nerves form the nervous system.
Answer: Nerve cell → nerve tissue → brain → nervous system.
Common mistake
Placing organs below tissues, or thinking an organ is made of one kind of cell. An organ is built from several tissues. Check: can it do a big job on its own, like pumping or digesting? Then it is at least an organ.