The five levels of organisation
The five levels of organisation
Living organisms are built up in a hierarchy. Each level is made of the level below it:
organelles → cells → tissues → organs → organ systems → organism
You must be able to name the levels in order and give a definition and an example of each.
| Level | Definition | Examples |
|---|---|---|
| Organelle | A structure inside a cell, each with its own function | nucleus, mitochondrion, chloroplast, ribosome, cell membrane, vacuole |
| Cell | The basic building block (unit) of all living organisms | red blood cell, root hair cell, palisade cell, sperm cell |
| Tissue | A group of cells with similar structure and function, working together | muscle, glandular tissue, epithelial tissue, xylem, phloem, mesophyll |
| Organ | A structure made of several different tissues working together to perform a function | heart, stomach, lung, leaf, root, kidney |
| Organ system | A group of organs working together to perform a body function | digestive system, circulatory system, respiratory system, nervous system |
Working up the hierarchy — one worked example
- Organelle: the nucleus inside a muscle cell
- Cell: a muscle cell
- Tissue: muscle tissue — many muscle cells of the same type together
- Organ: the stomach — muscle tissue plus glandular tissue plus epithelial tissue
- System: the digestive system — stomach, small intestine, liver, pancreas
- Organism: the human
And a plant example
- Organelle: a chloroplast in a palisade cell
- Cell: a palisade cell
- Tissue: palisade mesophyll tissue
- Organ: the leaf — mesophyll, epidermis, xylem and phloem together
- System: the shoot system (leaves, stem, flowers)
- Organism: the plant
Common mistake: candidates regularly slip a level, e.g. calling xylem an organ or calling the leaf a tissue. Learn the order and check which level the question is actually asking about.
Examiner tip. Examiners report that a few candidates confused organs with tissues. The single safest thing to fix: a tissue is similar cells; an organ contains several different tissues. If your answer to "what is an organ?" contains the words "similar cells", it is wrong.