The big idea: Cells come in a huge range of shapes and sizes, but every single living cell — bacterial, plant, animal, fungal — shares the same four basic structures.
The four shared structures and their jobs
- DNA — stores the instructions.
- Cytoplasm — holds a place for reactions.
- Plasma membrane — controls the cell's boundary.
- Ribosomes — make proteins.
They are shared because every cell must do these same essential jobs.
- DNA
- The genetic material — the instructions for building and running the cell.
- Cytoplasm
- The watery jelly inside the cell where chemical reactions happen.
- Plasma membrane
- The thin outer boundary that controls what enters and leaves the cell.
- Ribosomes
- Tiny structures that build proteins by joining amino acids together.
However different cells look, the same four structures are in EVERY one (highlighted in blue): DNA, cytoplasm, the plasma membrane and ribosomes — shown here on both a prokaryotic and a eukaryotic cell.
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An easy way to remember: Think D-C-M-R: DNA, Cytoplasm, Membrane, Ribosomes.
If a structure is NOT one of these four, it is not guaranteed to be in every cell.
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Each of the four shared structures does a job that no cell can live without.
What a cell would lose
- Without DNA a cell has no instructions.
- Without cytoplasm there is no place for reactions.
- Without a plasma membrane the cell cannot stay separate from its surroundings.
- Without ribosomes the cell cannot build proteins.
- Organelle
- A specialised structure inside a cell that has a particular function — for example a ribosome, nucleus or mitochondrion.
- Ribosome
- The organelle (found in ALL cells) that builds proteins. The plasma membrane also counts as an organelle; the cytoplasm does not.
- Prokaryotic cell
- A cell with NO nucleus and no membrane-bound organelles (for example a bacterium).
- Eukaryotic cell
- A cell WITH a nucleus and membrane-bound organelles (for example plant, animal and fungal cells).
Careful — 'common to all cells' is exact: A nucleus is NOT common to all cells — prokaryotes have no nucleus; their DNA floats free in the cytoplasm.
A cell wall, mitochondria and chloroplasts are also NOT universal — they are missing from many cell types.
Only DNA, cytoplasm, plasma membrane and ribosomes are in every cell.
| Feature | In prokaryotic cells? | In eukaryotic cells? |
|---|---|---|
| DNA (genetic material) | yes | yes |
| Cytoplasm | yes | yes |
| Plasma membrane | yes | yes |
| Ribosomes | yes | yes |
Where is DNA actually kept?: Every cell contains DNA, but it is not always in the same place.
Where the DNA sits
- In eukaryotic cells (animal, plant, fungal) most DNA sits inside the nucleus — so the nucleus is the usual answer to 'name an organelle that contains DNA'.
- But the nucleus is not the only DNA-containing organelle. Mitochondria carry their own small loop of DNA, and in plant cells the chloroplasts do too.
- In prokaryotic cells there is no nucleus at all: the DNA lies free in the cytoplasm in a region called the nucleoid.
- Nucleus
- The membrane-bound organelle in eukaryotic cells that holds most of the cell's DNA. A DNA base pair could be located here.
- Mitochondrion
- An organelle that releases energy and also carries its own small loop of DNA — so a DNA base pair can be located inside a mitochondrion too.
- Chloroplast
- The organelle in plant (and algal) cells where photosynthesis happens; it also contains its own DNA, so a DNA base pair could be located here.
- Nucleoid
- Not an organelle — the region of a prokaryotic cell's cytoplasm where its free DNA is found. Prokaryotes have no nucleus.
| Cell type | Where the DNA is | Organelle(s) that contain DNA |
|---|---|---|
| Animal / fungal (eukaryotic) | inside the nucleus and inside mitochondria | nucleus, mitochondria |
| Plant (eukaryotic) | inside the nucleus, mitochondria and chloroplasts | nucleus, mitochondria, chloroplasts |
| Prokaryotic (bacteria) | free in the cytoplasm (nucleoid region) | none — there is no nucleus |
Naming an organelle that holds DNA: If a question asks you to name an organelle in which a DNA base pair could be located, the safe answer is the nucleus.
A mitochondrion (or a chloroplast in a plant cell) is also accepted, because each of these organelles contains its own DNA.
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The big idea: Each shared structure is there for a reason: a cell needs instructions, a watery place for reactions, a boundary, and a way to make proteins.
The points to remember
- DNA is the genetic material: it holds the instructions for making the cell's proteins.
- The cytoplasm is made mostly of water, with enzymes dissolved in it: the site of the cell's metabolic reactions.
- The plasma membrane is made of lipids (a double layer) with proteins: it controls what enters and leaves.
- Ribosomes carry out protein synthesis: they make enzymes and the other proteins the cell needs.
- A cytoskeleton of protein fibres gives the cell its shape and moves things around inside it.
Remember it as: DNA instructs, cytoplasm hosts, the membrane controls, ribosomes build.
Real example: a gut bacterium and a liver cell: The bacterium Escherichia coli and a human liver cell look nothing alike, yet both have DNA, a watery cytoplasm, a plasma membrane and ribosomes doing the same jobs.
Water is the medium for life, so a cytoplasm that is mostly water can hold the dissolved substances the reactions need.
Give the role, not just the name: When asked for the roles of these structures, say what each one does. Adding a structure that not every cell has, such as a nucleus, cell wall or mitochondrion, costs a mark.
How this is tested: This sub-topic is asked as a multiple-choice identify question and as a short State / List question.
Where it comes up
- Paper 1A (multiple choice): which structure is found in all cells, in all three domains of life, or in both prokaryotes and eukaryotes — the answer is almost always ribosomes (a shared structure that is also an organelle, like the plasma membrane).
- Paper 1B / Paper 2: a short State / List question can ask you to name the structures common to all cells, so learn all four.
Exam-style question — name the universal structures
A bacterium, a human liver cell and a yeast cell look very different from one another. State the four structures that all three of these cells must contain. [2]
How to score both marks
- Recall what 'common to all cells' means. It means the structure is present in every cell type — prokaryotic and eukaryotic alike.
- List the four universal structures: DNA, cytoplasm, plasma membrane and ribosomes. (1 mark for any two correct; 2 marks for all four.)
- Answer the command term — state the four: all three cells must contain DNA, cytoplasm, a plasma membrane and ribosomes.
Final answer
DNA, cytoplasm, a plasma membrane and ribosomes — these four structures are present in every cell, whether prokaryotic or eukaryotic.
The structures you should name: Make sure your answer has all four: DNA, cytoplasm, plasma membrane and ribosomes.
Do not write 'nucleus' or 'cell wall' — those are not present in every cell.
- DNA
- The genetic material — the instructions for building and running the cell.
- Cytoplasm
- The watery jelly inside the cell where chemical reactions happen.
- Plasma membrane
- The thin outer boundary that controls what enters and leaves the cell.
- Ribosomes
- Tiny structures that build proteins by joining amino acids together.
IB-style questionDescribe[4 marks]
A soil bacterium and a human white blood cell are very different cells. Describe the roles of the components that both of these cells have.
Model answer plan
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