The big idea: Not every cell carries the same set of organelles (its internal structures).
How cells are sorted
- We sort cells into types — prokaryotic or eukaryotic, and within eukaryotes animal, plant or fungal.
- We do it by asking which organelles each one has and which it lacks.
- A few structures are found in every cell.
- The rest are the distinguishing features that tell the cell types apart.
- Organelle
- A structure inside a cell that has a particular job (for example a nucleus, a mitochondrion or a chloroplast).
- Prokaryotic cell
- A cell with NO nucleus and NO membrane-bound organelles; its DNA is free in the cytoplasm (for example a bacterium).
- Eukaryotic cell
- A cell that keeps its DNA inside a nucleus and contains membrane-bound organelles (for example an animal, plant or fungal cell).
- Membrane-bound organelle
- An organelle wrapped in its own membrane, such as the nucleus, mitochondria or chloroplasts. Only eukaryotes have these.
Found in every cell: Four structures are present in all cells — prokaryotic and eukaryotic alike:
DNA, cytoplasm, a plasma membrane and ribosomes.
Because every cell has them, these four can never be the answer to 'name something that tells two cell types apart'.
The four structures highlighted in blue are found in EVERY cell — prokaryote and eukaryote alike: DNA, cytoplasm, a plasma membrane and ribosomes.
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The first split is prokaryotic vs eukaryotic, and it comes down to one structure: the nucleus.
Prokaryote
- No nucleus
- No membrane-bound organelles
- Its DNA floats free in the cytoplasm
Eukaryote
- Keeps its DNA inside a nucleus
- Packed with membrane-bound organelles such as mitochondria
Prokaryotic vs eukaryotic cells. Both share ribosomes, a plasma membrane, cytoplasm and DNA — but only the eukaryote keeps its DNA inside a nucleus and has membrane-bound organelles.
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| Structure | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | Absent — DNA is free in the cytoplasm | Present — DNA enclosed in a nucleus |
| Membrane-bound organelles (e.g. mitochondria) | Absent | Present |
| Ribosomes | Present (small, 70S) | Present (larger, 80S) |
| Plasma membrane | Present | Present |
| DNA | Present (one circular loop, plus plasmids) | Present (linear, in chromosomes) |
| Typical size | Smaller (about 1–5 µm) | Larger (about 10–100 µm) |
Then: animal vs plant (both eukaryotic): Animal and plant cells are both eukaryotic, so both have a nucleus, mitochondria, ribosomes and a plasma membrane.
Plant cells add three structures that animal cells do not have
- a cellulose cell wall
- chloroplasts (for photosynthesis)
- a large central vacuole
Answering the classic question: So 'name an organelle in plant cells but not animal cells' is answered by chloroplast, large central vacuole or cellulose cell wall.
Animal, plant and fungal cells are all eukaryotic, but only the plant cell has a cellulose cell wall, chloroplasts and a large central vacuole.
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| Organelle / structure | Animal cell | Plant cell |
|---|---|---|
| Cellulose cell wall | Absent | Present |
| Chloroplast | Absent | Present |
| Large central (permanent) vacuole | Absent (small, temporary vacuoles only) | Present |
| Nucleus | Present | Present |
| Mitochondria | Present | Present |
| Ribosomes | Present | Present |
| Plasma membrane | Present | Present |
Only in plant cells
- Cellulose cell wall (rigid, fixed shape)
- Chloroplasts (carry out photosynthesis)
- Large central vacuole (keeps the cell turgid)
Shared by animal AND plant
- Nucleus (holds the DNA)
- Mitochondria (release energy)
- Ribosomes and plasma membrane
Watch the wording: cell wall is not unique to plants: Plants, fungi and many prokaryotes all have a cell wall — but the material differs: cellulose in plants, chitin in fungi, peptidoglycan in most bacteria.
So 'has a cell wall' does not prove a cell is a plant. Chloroplasts and a large central vacuole are the safer plant-only answers, because animals and fungi have neither.
A memory hook: Prokaryote = Plain (no nucleus, no membrane-bound organelles).
Plant = the extra three: wall, chloroplast, vacuole — the photosynthesis-and-support kit that animals skip.
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How this is tested: Short State questions ask for one organelle; Paper 2 asks you to set differences out side by side.
What you may be asked
- A 1-mark State question often asks for one organelle found in plant cells but not animal cells — answer with chloroplast, large central vacuole or cellulose cell wall (one is enough).
- A partner 1-mark State asks for one structure common to both prokaryotic and eukaryotic cells — answer ribosomes, plasma membrane, DNA or cytoplasm.
- On Paper 2 a Distinguish or Compare question wants the differences set out side by side (prokaryote vs eukaryote, or animal vs plant).
IB-style question — distinguish prokaryotic and eukaryotic cells
Distinguish between a prokaryotic cell and a eukaryotic cell. [3]
How to score all three marks
- Nucleus. A prokaryotic cell has no nucleus — its DNA is free in the cytoplasm — whereas a eukaryotic cell keeps its DNA inside a nucleus.
- Membrane-bound organelles. A prokaryotic cell has no membrane-bound organelles, whereas a eukaryotic cell does (for example mitochondria).
- Size. A prokaryotic cell is smaller (about 1–5 µm) than a eukaryotic cell (about 10–100 µm). (Award 1 mark per distinct contrast, max 3. Each point must compare BOTH cell types.)
Final answer
Prokaryote: no nucleus (DNA free in cytoplasm), no membrane-bound organelles, smaller. Eukaryote: DNA in a nucleus, has membrane-bound organelles, larger.
Why this scores full marks: Each point is a paired contrast — it says what the prokaryote does and what the eukaryote does.
A 'distinguish' answer that only describes one cell type ('eukaryotes have a nucleus') does not score — you must spell out the difference.
| Structure | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | Absent — DNA is free in the cytoplasm | Present — DNA enclosed in a nucleus |
| Membrane-bound organelles (e.g. mitochondria) | Absent | Present |
| Ribosomes | Present (small, 70S) | Present (larger, 80S) |
| Plasma membrane | Present | Present |
| DNA | Present (one circular loop, plus plasmids) | Present (linear, in chromosomes) |
| Typical size | Smaller (about 1–5 µm) | Larger (about 10–100 µm) |