Levels of Organisation in Multicellular Organisms
Senior 1 · Biology
Levels of Organisation in Multicellular Organisms
Lesson 5 of 5
25 min
Imagine a camera that starts on a single muscle cell in the wall of Mutesi's stomach and slowly zooms out. First you see the cell alone; then thousands like it lying side by side in a sheet; then the whole stomach, where that sheet is wrapped in other layers; then the stomach joined to the oesophagus, the intestines and the liver; and at last Mutesi herself, walking to school. Each step is a new level of organisation, and each level is built from the one before.
Definition
Tissue: a group of similar cells that work together to carry out the same job. Example: muscle tissue, which contracts.
Organ: a structure made of several different tissues working together to carry out one or more jobs. Example: the stomach; a leaf.
Organ system: a group of organs working together to carry out a major job for the body. Example: the digestive system.
Organism: a complete living thing, made of organ systems working together (or, in unicellular organisms, of one cell).
Figure 1: The levels of organisation in an animal and a plant.
Below the cell are its organelles, and beyond the organism are populations, communities and ecosystems, which you will study in S2. In this unit we stay between the cell and the organism.
Animals are built from just four main types of tissue:
| Animal tissue | Job | Example |
|---|---|---|
| Epithelial tissue | covers surfaces and lines tubes | skin; the ciliated lining of the windpipe |
| Connective tissue | supports, joins and transports | bone, cartilage, fat, blood |
| Muscle tissue | contracts to cause movement | the muscles of the arm; the wall of the stomach |
| Nervous tissue | carries electrical impulses | the brain, spinal cord and nerves |
Plants have tissues too: the epidermis covers the plant, the palisade and spongy mesophyll photosynthesise, and xylem and phloem carry water and food.
| Organ system | Main organs | Main job |
|---|---|---|
| Digestive | mouth, oesophagus, stomach, intestines, liver | breaks down food and absorbs it |
| Gas exchange (respiratory) | nose, trachea, bronchi, lungs | takes in oxygen and removes carbon dioxide (unit 8) |
| Circulatory | heart, blood vessels | transports substances round the body |
| Nervous | brain, spinal cord, nerves | detects changes and coordinates the body |
| Skeletal and muscular | bones, joints, muscles | support, protection and movement (unit 10) |
| Excretory | kidneys, bladder | removes waste from the blood |
| Reproductive | testes or ovaries and their ducts | produces gametes and new life (unit 13) |
Warning
An organ is made of several tissues, and blood is a tissue. Students often call the stomach "muscle tissue", but the stomach is an organ: it has muscle tissue, epithelial tissue lining it, connective tissue, blood and nerves. Blood is sometimes mistaken for an organ; it is a connective tissue, made of cells in a liquid (plasma).
Worked example 1: putting levels in order
Arrange from the simplest to the most complex: digestive system, muscle cell, Keza, stomach, muscle tissue.
Worked example 2: which level?
| Structure | Level | Reason |
|---|---|---|
| root hair cell | cell | a single specialised cell |
| xylem | tissue | many similar vessels doing one job |
| blood | tissue | cells of several kinds in plasma, doing the job of transport |
| heart | organ | made of muscle, connective, nervous and epithelial tissues |
| root system | organ system | all the roots of a plant working together |
Rule: ask "Is it one cell? Many similar cells? Several tissues? Several organs?"
Medicine in Rwanda works at every level. A laboratory technician checks a blood smear for malaria parasites, working at the level of cells. A pathologist looks at a thin slice of tissue from a biopsy. A radiographer takes an X-ray of an organ such as the lungs. A doctor examines how a whole system, such as breathing, is working, and treats the patient: the organism.
Biological Levels in Biology: The World Tour, Amoeba Sisters. The first half goes from cell to organism, as in Figure 1; the second half climbs on to populations and ecosystems, which you will meet in S2.
Tip
Memory hook: Cell, Tissue, Organ, System, Organism: "Clever Teachers Organise Sensible Outings."
Practice: levels of organisation
Key points
Further reading for this unit
Sources and credits
Figure 1, the tables, the worked examples and the practice questions are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
