Name the main organelles of animal, plant and bacterial cells and say what each one does.
Why it matters
Every sample that reaches a biology or clinical lab is made of cells or comes from them: a blood smear, a bacterial culture, a cell line growing in a flask, a tissue section. When a technician looks down a microscope, runs a cell count or extracts DNA, they are working with the parts of the cell. Knowing what those parts are, and what they do, tells you why a protocol says "lyse the membrane first" or why a stain lights up the nucleus.
The core idea: two kinds of cell
All living cells share four things: a plasma membrane that encloses them, cytoplasm (the watery interior), ribosomes that build proteins, and DNA that holds the instructions. Beyond that, cells come in two broad designs.
- Prokaryotic cells (bacteria and archaea) have no nucleus. Their DNA sits in a region called the nucleoid, usually as one circular chromosome, often with small extra circles called plasmids. Most have a rigid cell wall outside the membrane. They are small, typically 1 to 5 micrometres (µm) long.
- Eukaryotic cells (animals, plants, fungi, protists) keep their DNA inside a nucleus and have many membrane-bound compartments called organelles. They are larger, typically 10 to 100 µm.
A tour of the eukaryotic cell
Think of the cell as a small factory. Each organelle has a job:
| Organelle | Job |
|---|---|
| Nucleus | Stores DNA and controls which genes are used. The nucleolus inside it makes ribosome parts. |
| Ribosomes | Read messenger RNA and join amino acids into proteins. Free in the cytoplasm or attached to the rough ER. |
| Rough endoplasmic reticulum (ER) | Membrane network studded with ribosomes; makes proteins that will be exported or placed in membranes. |
| Smooth ER | Makes lipids and helps detoxify drugs (large in liver cells). |
| Golgi apparatus | Modifies, sorts and packages proteins from the ER into vesicles, the cell's "shipping department". |
| Mitochondria | Release energy from food by aerobic respiration and make ATP. They have their own small DNA. |
| Lysosomes | Bags of digestive enzymes that break down worn-out parts and engulfed material (animal cells). |
| Cytoskeleton | Protein filaments that give shape, move organelles and let cells crawl or divide. |
| Plasma membrane | Controls what enters and leaves (next lesson). |
Plant cells add three features: a cell wall made of cellulose, chloroplasts that capture light for photosynthesis, and a large central vacuole that stores water and keeps the cell firm.
Worked example: following one protein
Antibodies are proteins made by immune B cells (plasma cells) and released into the blood. Here is the route one antibody takes:
- In the nucleus, the antibody gene is copied into messenger RNA.
- The mRNA leaves through a nuclear pore and is read by ribosomes on the rough ER, which feed the new protein chain into the ER.
- The protein folds in the ER and is carried in a vesicle to the Golgi, where sugar groups are added.
- A vesicle from the Golgi fuses with the plasma membrane and releases the antibody outside the cell.
- Mitochondria supply the ATP for all of these steps.
Plasma cells are packed with rough ER for exactly this reason: they are antibody factories. Structure follows function, which is a useful rule throughout biology.
Worked example: why it matters at the bench
A protocol for extracting DNA from cultured human cells starts with a lysis buffer containing a detergent. The detergent dissolves the lipid plasma membrane and the nuclear membrane so the DNA is released. A protocol for bacteria often adds an enzyme such as lysozyme first, because the bacterial cell wall would otherwise protect the cells from the detergent. The difference in the protocol comes straight from the difference in cell structure.
Common mistakes
- Saying bacteria have no DNA organisation. They have no nucleus, but their DNA is organised in the nucleoid.
- Confusing the cell wall and the cell membrane. Plants, fungi and most bacteria have both; animal cells have only the membrane.
- Thinking only plants have mitochondria or only animals do. Plants have both chloroplasts and mitochondria.
- Mixing up sizes. A typical bacterium is around a tenth the length of a typical human cell; viruses (module 3) are smaller still.
Quick recap
- All cells have a membrane, cytoplasm, ribosomes and DNA.
- Prokaryotes have no nucleus or membrane-bound organelles; eukaryotes do.
- Nucleus stores DNA, ribosomes make proteins, ER and Golgi process and ship them, mitochondria make ATP, lysosomes digest.
- Plant cells add a cell wall, chloroplasts and a large vacuole.
- Lab protocols are designed around these structures.