Prokaryotes and eukaryotes
Two types of cell
Every living thing is built from one of two cell types.
have a nucleus and other membrane-bound organelles. Animals, plants, fungi and protists are all eukaryotes. They can be single-celled or made of many cells, like you.
have no nucleus and no membrane-bound organelles at all. are the example you need to know. Every prokaryote is a single cell.
A useful way to remember it: pro means "before" — prokaryotes came first, before the nucleus evolved.
Key terms
Inside a bacterial cell
A bacterial cell has:
- A — but made of a different material from a plant cell wall, not cellulose.
- A , controlling what enters and leaves.
- , where chemical reactions happen.
- A (the chromosome), free in the cytoplasm rather than in a nucleus.
- , which are smaller than those in eukaryotic cells.
- One or more — small rings of extra DNA.
Many bacteria also have a , a tail used to move, and some have a slime capsule.
Bacteria do not have mitochondria or chloroplasts. They still respire, but the reactions happen in the cytoplasm and at the cell membrane.
Key terms
Three differences you must be able to state
Exam questions often ask for three differences between prokaryotes and eukaryotes. Pick from:
| Prokaryotic | Eukaryotic |
|---|---|
| No nucleus — DNA is a single loop in the cytoplasm | DNA is inside a nucleus, on chromosomes |
| No membrane-bound organelles (no mitochondria or chloroplasts) | Has membrane-bound organelles |
| Has plasmids | No plasmids |
| Much smaller (about 1 µm) | Much larger (about 10–100 µm) |
| Always single-celled | Single-celled or multicellular |
Write differences as a comparison: "Prokaryotic cells have no nucleus, whereas eukaryotic cells do." A statement about only one type of cell will not get the mark.
Key terms
Comparing sizes and using standard form
Sizes are usually given in . There are 1000 µm in 1 mm, and 1000 nm in 1 µm.
- Bacterial cell: roughly 1 µm, or 1 × 10⁻⁶ m
- Animal or plant cell: roughly 10–100 µm, or 1 × 10⁻⁵ m to 1 × 10⁻⁴ m
So a typical eukaryotic cell is about 100 times wider than a bacterium. Because prokaryotic cells are so much smaller, they are described as being orders of magnitude smaller — each order of magnitude is a factor of 10.
Worked example: a bacterium is 2 µm long and an animal cell is 60 µm across. How many times larger is the animal cell? 60 ÷ 2 = 30 times larger. Always convert both measurements to the same unit before dividing.