# Cell organelles and their functions

Canonical: https://duckyhelper.com/learn/biology/cell-organelles/
Updated: 2026-10-01

Organelles are the small working parts inside a cell, each with its own job. The nucleus stores DNA, ribosomes build proteins, the endoplasmic reticulum and Golgi apparatus finish and ship them, and mitochondria release energy as ATP. Plant cells also have chloroplasts, a cell wall and a large central vacuole. Bacteria are prokaryotes: they have ribosomes but no nucleus and no membrane-bound organelles.

## Key ideas

There are two kinds of cells. **Prokaryotic** cells (bacteria and archaea) are small and simple: their DNA sits in a region called the nucleoid, with no membrane around it. **Eukaryotic** cells (plants, animals, fungi, protists) keep their DNA inside a nucleus and have many organelles wrapped in membranes.

**Organelles: what they do and where you find them (OpenStax Biology 2e, 4.2 to 4.4)**

| Part | Job | Animal | Plant | Bacteria |
| --- | --- | --- | --- | --- |
| Plasma membrane | Controls what enters and leaves | Yes | Yes | Yes |
| Cytoplasm | Gel-like fluid where many reactions happen | Yes | Yes | Yes |
| Ribosomes | Build proteins | Yes | Yes | Yes |
| Nucleus | Holds DNA and controls the cell | Yes | Yes | No |
| Rough ER | Studded with ribosomes, makes and folds proteins | Yes | Yes | No |
| Smooth ER | Makes lipids, breaks down some toxins, stores calcium | Yes | Yes | No |
| Golgi apparatus | Sorts, tags and packages proteins and lipids | Yes | Yes | No |
| Mitochondria | Cellular respiration: make most of the cell's ATP | Yes | Yes | No |
| Chloroplasts | Photosynthesis | No | Yes | No |
| Lysosomes | Digest worn-out parts and food with enzymes | Yes | No (the vacuole does this job) | No |
| Large central vacuole | Stores water, keeps the cell firm | No | Yes | No |
| Cell wall | Rigid outer layer for support | No | Yes (cellulose) | Yes (most; peptidoglycan) |
| Centrosome | Organizes microtubules for cell division | Yes | No | No |

> **Tip: A way to remember**
>
> Think of a cell as a factory. The nucleus is the main office with the blueprints (DNA). Ribosomes are the workers who build products (proteins). The ER is the assembly line, the Golgi is shipping, lysosomes are recycling, and mitochondria are the power plant.

## Worked examples

**Example 1: follow a protein out of the cell**

Problem: A pancreas cell makes insulin, a protein that leaves the cell. List the parts it passes through, in order.

1. The instructions start as DNA in the **nucleus**, which is copied into mRNA.
2. The mRNA goes to a **ribosome on the rough ER**, which builds the protein into the ER.
3. A small bubble called a **vesicle** carries it to the **Golgi apparatus**, which modifies and packages it.
4. Another vesicle carries it to the **plasma membrane** and releases it outside the cell.

Answer: Nucleus, rough ER ribosome, Golgi apparatus, plasma membrane

**Example 2: identify the cell from its parts**

Problem: Under a microscope, a cell has a cell wall, ribosomes and a plasma membrane, but no nucleus and no chloroplasts. What kind of cell is it?

1. No nucleus means it cannot be a plant or animal cell. It is a prokaryote.
2. A cell wall and ribosomes fit bacteria (see the table).

Answer: A prokaryotic cell, such as a bacterium

**Example 3: which cell needs more mitochondria?**

Problem: A heart muscle cell and a fat storage cell are the same size. Which would you expect to have more mitochondria, and why?

1. Mitochondria make ATP, the cell's energy currency.
2. Heart muscle contracts every second of your life, so it uses a lot of ATP. A fat cell mostly stores energy.

Answer: The heart muscle cell, because it uses far more ATP

## Common mistakes and how to fix them

- **Saying plants do not have mitochondria.** They do. Plants make sugar in chloroplasts and then break it down for ATP in mitochondria, just like animals.
- **Saying bacteria have no ribosomes.** Every cell needs proteins, so every cell has ribosomes. Bacteria lack a nucleus and membrane-bound organelles.
- **Mixing up cell wall and cell membrane.** Every cell has a membrane. Plants, fungi and most bacteria also have a wall outside it.
- **Calling the nucleus the brain.** It is closer to a library of instructions. The DNA is read when proteins are needed.

**Practice questions**

1. Which organelle makes most of a cell's ATP?
   A. Ribosome
   B. Mitochondrion
   C. Golgi apparatus
   D. Nucleus

   Answer: Mitochondrion. Mitochondria carry out the later stages of cellular respiration, where most ATP is made.

2. Which part is found in plant cells but not in animal cells?
   A. Mitochondria
   B. Ribosomes
   C. Chloroplasts
   D. Plasma membrane

   Answer: Chloroplasts. Chloroplasts carry out photosynthesis. Both plant and animal cells have mitochondria, ribosomes and a plasma membrane.

3. Which part do bacteria have?
   A. Nucleus
   B. Ribosomes
   C. Mitochondria
   D. Golgi apparatus

   Answer: Ribosomes. Bacteria are prokaryotes with no membrane-bound organelles, but they still need ribosomes to build proteins.

4. Which organelle sorts, tags and packages proteins for shipping?
   A. Smooth ER
   B. Lysosome
   C. Golgi apparatus
   D. Vacuole

   Answer: Golgi apparatus. Proteins from the rough ER travel to the Golgi apparatus, which modifies them and sends them on in vesicles.

5. A cell has a nucleus, mitochondria, a cell wall, chloroplasts and a large central vacuole. What is it?
   A. Animal cell
   B. Plant cell
   C. Bacterium

   Answer: Plant cell. A nucleus rules out bacteria. A cell wall, chloroplasts and a large central vacuole together point to a plant cell.

## Frequently asked questions

### Why do mitochondria and chloroplasts have their own DNA?

The endosymbiotic theory says both came from bacteria that were taken in by an early cell and lived on inside it. They still have their own circular DNA and ribosomes, and they divide on their own, much like bacteria. OpenStax covers the evidence in its section on eukaryotic origins.

### What is the difference between rough and smooth ER?

Rough ER has ribosomes on its surface, so it looks bumpy and makes proteins, especially ones that will be shipped out or put in membranes. Smooth ER has no ribosomes and makes lipids, breaks down some toxins, and stores calcium in muscle cells.

### Do I need to know every organelle for a test?

Most high school tests focus on the nucleus, ribosomes, ER, Golgi, mitochondria, chloroplasts, vacuoles, lysosomes, cell membrane and cell wall, and on comparing plant, animal and bacterial cells. AP Biology adds how the endomembrane system works together and why cells are small.

## Sources

- [OpenStax Biology 2e, 4.2 Prokaryotic Cells](https://openstax.org/books/biology-2e/pages/4-2-prokaryotic-cells), accessed 2026-10-01
- [OpenStax Biology 2e, 4.3 Eukaryotic Cells](https://openstax.org/books/biology-2e/pages/4-3-eukaryotic-cells), accessed 2026-10-01
- [OpenStax Biology 2e, 4.4 The Endomembrane System and Proteins](https://openstax.org/books/biology-2e/pages/4-4-the-endomembrane-system-and-proteins), accessed 2026-10-01

## Related

- [Cellular respiration](https://duckyhelper.com/learn/biology/cellular-respiration/)
- [Photosynthesis](https://duckyhelper.com/learn/biology/photosynthesis/)
- [Transcription and translation](https://duckyhelper.com/learn/biology/transcription-and-translation/)
- [Biology study guides](https://duckyhelper.com/learn/biology/)

## Try asking Ducky

- "Look at my labeled cell diagram. Did I mix up any organelles?"
- "Quiz me on plant versus animal cells until I get five right in a row."
- "Explain the endomembrane system with a picture I can follow."

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