In short: Living things are organised in levels: cells form tissues, tissues form organs, organs form systems, and systems form an organism. In multicellular organisms the cells are specialised for different jobs, even though every cell has the same DNA.
Starting point: what you already know
- Cells are the basic unit of life and have organelles with different jobs.
- Your body has systems such as the digestive, circulatory and respiratory systems (Year 8 and 9 science).
- Plants have roots, stems and leaves.
One cell, a colony or many cells
| Type | Description | Examples |
|---|---|---|
| Unicellular | One cell does every job of life | Bacteria, Amoeba, yeast, Paramecium |
| Colonial | Many similar cells live together. Each could survive alone, but some share jobs | Volvox (a ball of green algae cells) |
| Multicellular | Many cells that depend on each other. Cells are specialised | Humans, trees, mushrooms |
A unicellular organism must do everything with one cell, which keeps it small. Multicellular organisms can grow large, live longer and have cells that are very good at one job each. The cost is that each cell can no longer survive on its own.
Cell specialisation
All the cells in your body came from one cell, the fertilised egg. They all carry the same DNA. They become different because each kind of cell switches on different genes. This is called differentiation.
| Specialised cell | Special feature | Why |
|---|---|---|
| Red blood cell | No nucleus, disc shape, full of haemoglobin | More room to carry oxygen, big surface area |
| Nerve cell (neuron) | Very long, with branches | Carries messages over long distances |
| Sperm cell | Tail, many mitochondria | Swims to the egg |
| Root hair cell | Long thin extension | Big surface area to absorb water |
| Palisade cell (leaf) | Packed with chloroplasts | Lots of photosynthesis |
Stem cells
A stem cell has not yet specialised. It can divide many times and turn into other types of cells.
- Embryonic stem cells can become almost any cell type.
- Adult stem cells (for example in bone marrow) can become only a few types, such as blood cells.
- In plants, meristems at the tips of roots and shoots keep making new cells all their life.
Stem cells are used in bone marrow transplants for leukaemia and are being studied to repair damaged nerves and hearts.
From cells to organisms
- Unicellular: one cell
- Colonial: Volvox
- Multicellular: specialised cells
- Same DNA, different genes on
- Red blood cell, neuron, sperm
- Root hair, palisade cell
- Embryonic: any type
- Adult: a few types
- Plant meristems
- Cell
- Tissue
- Organ
- System
- Organism
Quick summary
- Organisms can be unicellular, colonial or multicellular.
- Multicellular organisms have specialised cells. All have the same DNA but switch on different genes.
- Stem cells are unspecialised and can become other cell types.
- Levels: cell, tissue, organ, system, organism.
- Large organisms need exchange surfaces and transport systems because their SA : V is small.
Quiz
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Which organism is unicellular?
- A Mushroom
- B Amoeba
- C Gum tree
- D Earthworm
Show answer
B: Amoeba
An Amoeba is a single cell.
Volvox is best described as
- A unicellular
- B colonial
- C a plant organ
- D a tissue
Show answer
B: colonial
Volvox is a ball of similar cells living together.
What is the correct order of organisation?
- A Tissue, cell, organ, system
- B Cell, tissue, organ, system
- C Organ, cell, tissue, system
- D Cell, organ, tissue, system
Show answer
B: Cell, tissue, organ, system
Cells make tissues, tissues make organs, organs make systems.