# Cell Organelles

> The job of each organelle, how plant and animal cells differ, and how organelles work together to make and export a protein.

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

### Story time: A city in a drop
Imagine shrinking down until a single cell in your cheek looks as big as a city. There is a city hall with all the plans (the nucleus). There are power stations burning fuel (mitochondria). There are factories building products (ribosomes), roads to carry them (the endoplasmic reticulum), a post office that packs and labels parcels (the Golgi body) and a recycling centre (lysosomes). A wall with guarded gates surrounds it all (the cell membrane).

Every one of those jobs is happening right now, in each of the roughly 30 trillion cells in your body.

**In short:** Organelles are the small structures inside a cell that each do a job. The nucleus holds the DNA and controls the cell, mitochondria release energy by respiration, and ribosomes make proteins. Plant cells also have a cell wall, chloroplasts and a large vacuole.

## Starting point: what you already know

- Cells are the basic unit of life (see the previous chapter).
- Eukaryotic cells have a nucleus. Prokaryotic cells do not.
- Plants make food by photosynthesis and all living things release energy by respiration.

## The organelles and their jobs

An **organelle** is a small structure inside a cell with a particular job.

| Organelle | Job | City match |
|---|---|---|
| **Nucleus** | Holds DNA, controls the cell. The nucleolus inside makes ribosomes | City hall |
| **Cell membrane** | Controls what enters and leaves | Walls and gates |
| **Cytoplasm** (cytosol) | Jelly-like fluid where many reactions happen | The streets and air |
| **Mitochondria** | Aerobic respiration: release energy from glucose as ATP | Power stations |
| **Ribosomes** | Make proteins | Factories |
| **Rough endoplasmic reticulum (RER)** | Membranes covered in ribosomes. Carries and folds proteins | Factory roads |
| **Smooth endoplasmic reticulum (SER)** | Makes lipids (fats) and breaks down toxins | Chemical plant |
| **Golgi body** (Golgi apparatus) | Modifies, packs and sends out proteins in vesicles | Post office |
| **Vesicles** | Small membrane bags that carry substances | Delivery vans |
| **Lysosomes** | Hold enzymes that break down worn-out parts and invaders | Recycling centre |
| **Cytoskeleton** | Protein fibres that give shape and move things around | Scaffolding and rail lines |

## Only in plant cells

| Structure | Job |
|---|---|
| **Cell wall** (made of cellulose) | Strong outer layer. Supports the cell and stops it bursting |
| **Chloroplasts** | Photosynthesis. Contain green chlorophyll |
| **Large central vacuole** | Stores water and salts. Pushes outward to keep the cell firm (turgid) |

Animal cells may have small vacuoles, and they have **centrioles**, which help in cell division. Most plant cells do not.

*Diagram: Side by side diagrams of an animal cell with a round outline and a plant cell with a box-shaped cell wall, a large vacuole and chloroplasts*
*The plant cell has a thick cell wall (the outer frame), a large vacuole and chloroplasts (green). Both cells have a nucleus, a membrane and mitochondria (orange).*

## Working together: making and exporting a protein

Cells in your pancreas make the hormone insulin and send it into the blood. Follow its path:

1. **Nucleus**: the gene for insulin is copied into a message (mRNA), which leaves through a nuclear pore.
2. **Ribosomes on the RER**: read the message and build the protein.
3. **RER**: the protein is folded and carried along inside the membranes.
4. **Vesicle**: buds off and carries it to the Golgi body.
5. **Golgi body**: finishes and packs the protein into a new vesicle.
6. **Cell membrane**: the vesicle joins the membrane and releases insulin outside the cell (exocytosis).
7. **Mitochondria** supply the energy (ATP) for every step.

### Watch out: "Plant cells do not have mitochondria"
They do. Plants photosynthesise **and** respire. Chloroplasts make glucose, and mitochondria release energy from it, day and night.

**Mind map**

```text
Organelles
  Control
    Nucleus: DNA
    Nucleolus: makes ribosomes
  Energy
    Mitochondria: respiration
    Chloroplasts: photosynthesis
  Making things
    Ribosomes: proteins
    Rough ER: folds and carries
    Smooth ER: lipids
    Golgi: packs and ships
  Other
    Membrane: gatekeeper
    Lysosomes: digest
    Cell wall and vacuole: support
```

### Year 11: Structure fits function
Cells have more of the organelles they need for their job.

- **Muscle cells** and **sperm cells** are packed with **mitochondria** because they use a lot of energy.
- **Pancreas cells** that export enzymes have lots of **RER** and a large **Golgi body**.
- **White blood cells** that eat bacteria have many **lysosomes**.
- **Leaf palisade cells** are full of **chloroplasts** and sit near the top of the leaf where the light is strongest.

The inner membrane of a mitochondrion is folded into **cristae**, and a chloroplast has stacks of membranes called **grana**. Folding gives a much bigger surface area for the reactions that happen on the membranes.

### Year 12: Organelles and the HSC
In Module 5, the path from the nucleus to the ribosome is studied as **transcription** and **translation**. In Module 8, faulty organelles can cause disease. For example, Tay-Sachs disease happens when a lysosome enzyme is missing, so fatty material builds up in nerve cells.

## Quick summary

- The nucleus holds DNA and controls the cell. Mitochondria release energy by respiration. Ribosomes make proteins.
- Proteins for export go: ribosome on RER, to vesicle, to Golgi, to vesicle, to the membrane.
- Plant cells also have a cellulose cell wall, chloroplasts and a large central vacuole.
- Plants have mitochondria too.
- Cells have more of the organelles their job needs.

## Quiz

Try these questions from the chapter quiz.

1. Which organelle releases energy from glucose in aerobic respiration?
   - A. Chloroplast
   - B. Mitochondrion
   - C. Ribosome
   - D. Golgi body
   - Answer: B. Mitochondrion. Mitochondria carry out aerobic respiration and make ATP.

2. Where are proteins made?
   - A. Nucleus
   - B. Lysosomes
   - C. Ribosomes
   - D. Vacuole
   - Answer: C. Ribosomes. Ribosomes join amino acids into proteins.

3. Which organelle packages proteins into vesicles to leave the cell?
   - A. Smooth ER
   - B. Golgi body
   - C. Nucleolus
   - D. Cell wall
   - Answer: B. Golgi body. The Golgi body modifies and packs proteins for export.

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