The big idea: Every cell on Earth is one of two basic types:
- Prokaryotic — a simple cell with no nucleus. Its DNA floats free in the cytoplasm. All bacteria are prokaryotic. - Eukaryotic — a more complex cell with a true nucleus that holds the DNA, plus other membrane-bound organelles. Animal, plant and fungal cells are eukaryotic.
The single biggest difference to remember: a nucleus, or no nucleus.
Prokaryotic cell
- No nucleus — DNA is free in the cytoplasm
- No membrane-bound organelles
- Small (about 1–5 µm)
- Example: a bacterium
Eukaryotic cell
- Has a nucleus that encloses the DNA
- Has membrane-bound organelles (e.g. mitochondria)
- Larger (about 10–100 µm)
- Examples: animal, plant, fungal cells
A prokaryotic cell (left) has no nucleus — its circular DNA lies free in the cytoplasm — while a eukaryotic cell (right) keeps its DNA in a nucleus and has membrane-bound organelles. Both still share a plasma membrane, cytoplasm, DNA and ribosomes.
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What 'membrane-bound organelle' means: An organelle is a structure inside a cell with its own job.
A membrane-bound organelle is wrapped in its own membrane — like the nucleus or a mitochondrion.
Prokaryotes have none of these; eukaryotes have many.
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The two cell types share the four structures every cell has: a plasma membrane, cytoplasm, DNA and ribosomes. So a comparison is never 'all different' — always name the shared parts too.
The differences come down to how the DNA is organised and whether the cell has membrane-bound compartments.
- Prokaryotic cell
- A cell with no nucleus and no membrane-bound organelles; its DNA lies free in the cytoplasm. All bacteria are prokaryotic.
- Eukaryotic cell
- A cell with a true (membrane-bound) nucleus that encloses the DNA, plus other membrane-bound organelles. Animal, plant and fungal cells are eukaryotic.
- Nucleus
- A membrane-bound organelle that holds the cell's DNA — present in eukaryotes, absent in prokaryotes.
- Histone
- A protein that eukaryotic DNA wraps around to package the long chromosomes. Prokaryotic DNA has no histones — it is 'naked'.
- Plasmid
- A small extra ring of DNA found in many prokaryotes, separate from the main loop of DNA.
- Flagellum
- A long whip-like tail that some prokaryotes spin to move the cell through liquid.
Look closely at the DNA. In a prokaryote the DNA is one circular loop lying naked in the cytoplasm, often with small extra rings called plasmids. In a eukaryote the DNA is in several long, linear chromosomes wound around histone proteins and sealed inside the nucleus.
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | no nucleus (DNA is free in the cytoplasm) | true nucleus (DNA enclosed by a membrane) |
| DNA shape | one circular loop, naked (no histones) | long linear chromosomes wrapped on histones |
| Membrane-bound organelles | none (e.g. no mitochondria) | present (mitochondria, ER, Golgi, etc.) |
| Ribosomes | small (70S) | large (80S) |
| Typical size | small (about 1–5 µm) | larger (about 10–100 µm) |
| Examples | bacteria | animal, plant, fungal & protist cells |
Don't forget what they share: Both prokaryotic and eukaryotic cells have a plasma membrane, cytoplasm, DNA and ribosomes.
A 'compare and contrast' answer must give a similarity as well as a difference — answers that list only differences lose the similarity mark.
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How this is tested: On Paper 1A (multiple choice) you identify a key difference between the two cell types, or pick the structure they share.
On Paper 2 you List, Outline or State differences, or State the function of a prokaryotic structure (flagella, ribosomes). On Paper 1B a 'compare and contrast' asks for a similarity AND a difference.
On Paper 3 a 3-mark Draw can ask for the internal structures of a prokaryotic cell.
IB-style question — compare a bacterium with a liver cell
A scientist examines a bacterium and a human liver cell under a microscope. Compare and contrast the structure of the two cells. Give one similarity and two differences. [3]
Model answer
- Similarity (1 mark). Both cells have a plasma membrane, cytoplasm, DNA and ribosomes — give any one shared structure.
- Difference 1 (1 mark). The liver cell has a nucleus that encloses its DNA; the bacterium has no nucleus — its DNA is free in the cytoplasm.
- Difference 2 (1 mark). The liver cell has membrane-bound organelles (e.g. mitochondria); the bacterium has none. (DNA shape or ribosome size would also score.)
- Answer the command term. State the contrast clearly as paired points — one feature, both cells — so each similarity/difference is unmistakable.
Final answer
Similarity: both have a plasma membrane, cytoplasm, DNA and ribosomes. Differences: the liver cell has a nucleus and membrane-bound organelles; the bacterium has neither (its DNA is a naked loop free in the cytoplasm).
What scores the marks: The marks need paired points: the SAME feature compared in both cells (e.g. 'liver cell has a nucleus; bacterium has no nucleus'). A list of facts about only one cell does not count as a comparison.