# Levels of Organisation

> Unicellular, colonial and multicellular organisms, cell specialisation and stem cells, and how cells build tissues, organs and systems.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/levels-of-organisation/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The slime that thinks
On a forest floor in Japan, a yellow slime called *Physarum* spreads out looking for food. Scientists placed oat flakes on a map of Tokyo, one flake for each big city around it. The slime grew between them and made a network of tubes that looked very like the real Tokyo rail system.

For much of its life this slime is one giant cell with millions of nuclei. When food runs out, it forms spores. Living things can be one cell, a group of cells, or trillions of cells working as a team. How they are organised changes what they can do.

**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

### Key idea: The levels of organisation
**cell → tissue → organ → system → organism**

- **Tissue**: a group of similar cells doing the same job (muscle tissue).
- **Organ**: different tissues working together (the stomach has muscle, gland and lining tissue).
- **System**: organs working together (the stomach, intestines and liver in the digestive system).

### Worked example: A leaf is an organ
A leaf has an outer **epidermis** tissue (protection), a **palisade** tissue (photosynthesis), a **spongy** tissue with air spaces (gas exchange) and **vascular** tissue (xylem and phloem for transport). Different tissues working together make the leaf an organ.

**Mind map**

```text
Organisation
  Types of organisms
    Unicellular: one cell
    Colonial: Volvox
    Multicellular: specialised cells
  Specialisation
    Same DNA, different genes on
    Red blood cell, neuron, sperm
    Root hair, palisade cell
  Stem cells
    Embryonic: any type
    Adult: a few types
    Plant meristems
  Levels
    Cell
    Tissue
    Organ
    System
    Organism
```

### Year 11: Why be multicellular?
Large size brings a problem: the **surface area to volume ratio** falls. Cells deep inside a big organism are far from the outside, so diffusion alone cannot supply them. Multicellular organisms solve this with **specialised exchange surfaces** (lungs, gills, leaves, villi) and **transport systems** (blood vessels, xylem and phloem). The next three chapters look at these.

A flatworm is flat and thin, so every cell is close to the surface and diffusion is enough. An earthworm is round and thicker, so it needs blood vessels. Its whole skin is an exchange surface, and it must stay moist.

### Year 12: Specialisation, genes and disease
Differentiation happens because different genes are **expressed** in different cells, which you study in Module 5 as gene expression. Cancer (Module 8) can start when cells lose their specialisation controls and begin dividing without stopping. Stem cell therapies and the ethics of using embryonic stem cells are common HSC discussion questions.

## 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

Try these questions from the chapter quiz.

1. Which organism is unicellular?
   - A. Mushroom
   - B. Amoeba
   - C. Gum tree
   - D. Earthworm
   - Answer: B. Amoeba. An Amoeba is a single cell.

2. *Volvox* is best described as
   - A. unicellular
   - B. colonial
   - C. a plant organ
   - D. a tissue
   - Answer: B. colonial. Volvox is a ball of similar cells living together.

3. 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
   - Answer: B. Cell, tissue, organ, system. Cells make tissues, tissues make organs, organs make systems.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/levels-of-organisation/#quiz