In Plant Cells and Animal Cells you compared two kinds of cell that both have a nucleus. Most of the living things on Earth, by number, have cells with no nucleus at all: the bacteria and the archaea.
Key idea
Prokaryotic cells (bacteria and archaea) have no nucleus and no membrane-bound organelles. Eukaryotic cells (plants, animals, fungi and protists) have both. All cells have a cell membrane, cytoplasm, ribosomes and DNA.
| Prokaryotic cell | Eukaryotic cell | |
|---|---|---|
| Who | Bacteria, archaea | Animals, plants, fungi, protists |
| Nucleus | No: DNA lies in a region called the nucleoid | Yes: DNA inside a nuclear envelope |
| DNA | Usually one circular chromosome, often with small rings called plasmids | Several linear chromosomes |
| Membrane-bound organelles | None | Mitochondria, ER, Golgi, and in plants chloroplasts |
| Ribosomes | Yes (smaller) | Yes (larger) |
| Typical size | About 1–5 µm | About 10–100 µm |
Why it works
"Pro-karyote" means "before the kernel" (the nucleus); "eu-karyote" means "true kernel." Every cell must store instructions (DNA), read them to build proteins (ribosomes), and keep its inside separate from the outside (a membrane around the cytoplasm). Those four features are the minimum kit for life, so every cell has them. Membrane-bound organelles divide a large eukaryotic cell into compartments, so different jobs can run side by side.
Worked example: Classify a cell
Under an electron microscope, a cell 2 µm long has a cell wall, ribosomes and a loop of DNA, but no nucleus and no mitochondria. What kind of cell is it?
- No nucleus and no membrane-bound organelles: prokaryotic.
- The small size (2 µm) fits, too.
- A cell wall alone does not decide it: plants and fungi have walls, and so do most bacteria.
Answer: A prokaryotic cell (a bacterium or an archaeon)
Worked example: How much bigger?
A typical animal cell is 20 µm across and a bacterium 2 µm long. How many bacteria would fit end to end across the animal cell?
- Divide the lengths: $20 \div 2 = 10$.
- About 10 bacteria end to end.
Answer: About $10$
Common mistake
Thinking prokaryotes have no DNA or no ribosomes because they have no nucleus. A nucleus is just a membrane around the DNA. Prokaryotes have DNA, lying in the cytoplasm, and they have ribosomes like every cell; they could not make proteins without them. Quick check: if it is alive and made of cells, it has DNA, ribosomes, cytoplasm and a membrane.