Prokaryotic vs Eukaryotic Cells
In 1977, microbiologist Carl Woese compared the ribosomal RNA of dozens of organisms and split the living world along a line no microscope had drawn before: cells with a nucleus, and cells without one. That single distinction, a membrane-bound nucleus, separates every living thing on Earth into two fundamental cell architectures. This lesson is where the whole course begins: what those two cell types share, and what sets them apart.
Get oriented
Make a prediction, read the goals and the key terms, then clear the trap before you start.
Before You Read
warm-upYou are shown two cells under a microscope. One is a human cheek cell; one is a bacterium from your mouth.
Before reading: write down two things you would look for to tell which cell is which. What is the single most reliable feature, and why?
Learning Intentions
goalsKnow
- The defining difference between prokaryotic and eukaryotic cells
- Structures common to both cell types
- Typical size ranges and DNA organisation of each
Understand
- Why a membrane-bound nucleus is the key distinguishing feature
- Why "no organelle" does not mean "no function" in prokaryotes
- How microscopy technology shaped what we know about cells
Can Do
- Compare prokaryotic and eukaryotic cells from a variety of sources
- Classify an unknown cell from its described features
- Justify using more than one source when comparing cells
Scan these before reading
vocabMisconceptions To Fix
watch outBuild the first idea
The core of the dot point, then two checks you can answer straight from the card.
Core Content
The Two Fundamental Cell Types
+5 XPOne line divides all cellular life: the membrane-bound nucleus
Every cell that has ever been found belongs to one of two groups, and the dividing line is simply whether the DNA is wrapped inside its own membrane.
A prokaryote (Greek: "before the kernel/nucleus") is a cell with no membrane-bound nucleus and no membrane-bound organelles, its single circular DNA molecule lies free in the cytoplasm in a region called the nucleoid. All bacteria are prokaryotes.
A eukaryote ("true nucleus") is a cell whose DNA is enclosed within a membrane-bound nucleus, and which contains membrane-bound organelles such as mitochondria. Animal, plant, fungal and protist cells are all eukaryotic.
Crucially, both cell types still share the basics of life: a cell membrane, cytoplasm, ribosomes, and DNA. The difference is not what they contain, but how it is organised.
Pause, copy the highlighted definitions into your book before moving on.
The region of a prokaryotic cell where the free circular DNA sits is called the _____.
Odd one out. Three of these are present in every cell, prokaryotic and eukaryotic. Which one is not?
Take it further
The next move, with the diagram to work from and two more checks.
Comparing the Architecture
+5 XPSize, DNA, organelles, feature by feature
We just saw that the nucleus is the dividing line. That raises a question: what else differs once you have, or lack, that nucleus? This card answers it → a feature-by-feature comparison you can reproduce in an exam.
The presence of a nucleus comes bundled with a whole set of other differences in size, DNA shape, and internal organisation.
Text description
- Prokaryotic cell (about 1–5 µm): its DNA lies free in the cytoplasm in the nucleoid (free circular DNA). It has ribosomes but no membrane-bound organelles.
- Eukaryotic cell (about 10–100 µm): its DNA is inside a membrane-bound nucleus, and it has membrane-bound organelles such as mitochondria, as well as ribosomes.
- Both have a cell membrane, cytoplasm, ribosomes and DNA; only the eukaryote has a membrane-bound nucleus and organelles.
- The two cells are not drawn to scale with each other.
Prokaryotes: no nucleus, free circular DNA + plasmids, no membrane-bound organelles, small, 70S ribosomes. Eukaryotes: membrane-bound nucleus, linear chromosomes, membrane-bound organelles, larger, 80S ribosomes. Shared by both: membrane, cytoplasm, ribosomes, DNA.
Pause, copy the highlighted comparison into your book, using the diagram to label each cell.
Which structure is found in BOTH prokaryotic and eukaryotic cells?
Match each feature to the value the comparison gives it.
- Prokaryotic chromosome
- Eukaryotic chromosome
- Prokaryotic ribosome
- Typical prokaryotic cell length
- 70S
- One circular molecule, free in the nucleoid
- About 1 to 5 micrometres
- Several linear molecules, inside the nucleus
Work it yourself
Put the idea on paper in your own book before you read on.
Activity 1
Build a Comparison Table
Pattern A, Structured comparison
In your book, draw a three-column table with the headings Feature, Prokaryotic, Eukaryotic. Complete a row for each of: nucleus, DNA shape, organelles, typical size, ribosome type, and one example organism. Then add a final row titled "Shared features" and list everything the two cell types have in common.
Finish the picture
The last piece of the dot point, then a true or false check across all three cards.
How We Know, Using a Variety of Sources
+5 XPWhy the electron microscope changed the story
We just compared the two cell types confidently. That raises a question: how do scientists actually know prokaryotes have no internal membranes? This card answers it → it depends entirely on the resolving power of the microscope used.
What we can claim about a cell's structure is limited by the tool we use to look at it, better resolution revealed detail that earlier scientists simply could not see.
A light microscope can resolve points about 200 nm apart, enough to see a cell and its nucleus, but not fine internal membranes. The electron microscope resolves to around 0.2 nm, a thousand times finer, and this is what revealed that prokaryotes lack the internal membrane-bound compartments eukaryotes have.
Because any single diagram or micrograph can be schematic, incomplete, or misleading about scale, scientists use a variety of sources, electron micrographs, labelled diagrams, and data tables, and cross-check them. This is an example of evaluating the reliability of secondary sources.
Light microscope resolution ≈ 200 nm (sees cells and nuclei); electron microscope ≈ 0.2 nm (revealed prokaryotes lack internal membranes). Compare cells using a variety of cross-checked sources, because any one source may be schematic or misleading about scale.
Add the resolution figures and the "variety of sources" point to your notes before the check below.
The membrane-bound nucleus is the defining feature that separates eukaryotes from prokaryotes.
Prokaryotic cells contain no DNA because they have no nucleus.
The electron microscope's higher resolution was needed to show prokaryotes lack internal membranes.
See where it goes
Where this shows up outside the classroom, and why the dot point is worth knowing.
Why this difference matters in medicine
The structural gap between prokaryotic and eukaryotic cells is exactly what makes antibiotics possible. Many antibiotics target features unique to bacterial (prokaryotic) cells, the 70S ribosome, or the peptidoglycan cell wall, that human (eukaryotic) cells do not have. This lets the drug kill the bacterium while leaving your own cells unharmed. Understanding the two cell types is therefore not just a classification exercise: it underpins how we treat infection, which you will meet again in Year 12 when you study disease.
Compare the cellular structures of prokaryotic and eukaryotic cells.
Compare asks for similarities and differences, on the same named criteria. A paragraph about prokaryotes followed by a paragraph about eukaryotes is a Describe answer, however accurate each half is.
6 marks over two cell types means about three points of comparison, each stated for both in the same sentence. Choose the criteria before writing.
Both are cells: both are bounded by a cell membrane, both contain cytoplasm, both carry DNA as their genetic material, and both have ribosomes for making proteins. They differ in whether the DNA is enclosed in a nucleus, in whether membrane-bound organelles are present, and in size.
In a eukaryotic cell the DNA is linear, wound around histone proteins, and enclosed within a nuclear membrane that separates it from the cytoplasm. In a prokaryotic cell the DNA is a single circular chromosome lying free in the cytoplasm in a region called the nucleoid, with no surrounding membrane, and there may be additional small circular plasmids.
Eukaryotic cells contain membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi body, which compartmentalise the cell so that incompatible reactions can run at once in separate spaces. Prokaryotic cells have none of these; their reactions occur in the cytoplasm or on the inner face of the cell membrane.
Prokaryotic cells are typically 1 to 5 micrometres across, whereas eukaryotic cells are typically 10 to 100 micrometres, so a eukaryotic cell is roughly ten times wider and around a thousand times greater in volume. Both have ribosomes, but the prokaryotic ribosome is smaller (70S) than the eukaryotic one (80S).
The differences are consistent with two solutions to the same problem: a prokaryotic cell is small and uncompartmentalised, so materials reach every part of it by diffusion alone, while a eukaryotic cell is large enough to need internal membranes both to compartmentalise reactions and to increase the surface available for them.
Prokaryotic and eukaryotic cells are both bounded by a cell membrane, both contain cytoplasm, both carry DNA as their genetic material and both have ribosomes for protein synthesis. They differ in three main respects. In a eukaryotic cell the DNA is linear, wound around histone proteins and enclosed in a nuclear membrane that separates it from the cytoplasm, whereas in a prokaryotic cell it is a single circular chromosome lying free in the cytoplasm in the nucleoid region, often alongside smaller circular plasmids. Eukaryotic cells also contain membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi body, which compartmentalise the cell so incompatible reactions can proceed simultaneously in separate spaces, while prokaryotic cells have none and carry out their reactions in the cytoplasm or on the inner face of the cell membrane. Prokaryotic cells are also much smaller, typically 1 to 5 micrometres against 10 to 100 micrometres, and their ribosomes are smaller at 70S against 80S. These differences are consistent with two solutions to one problem: a prokaryotic cell is small enough for diffusion alone to supply every part of it, while a eukaryotic cell is large enough to require internal membranes both to compartmentalise its reactions and to provide surface area for them.
Explain why prokaryotic cells are limited to a much smaller size than eukaryotic cells.
Explain means cause and effect. Stating that prokaryotes are small restates the question; the marks are for what makes a larger prokaryote unworkable.
5 marks. Establish what a cell must do to stay alive, show why size makes that harder, then say what eukaryotes do instead.
Every cell must take in materials and remove wastes fast enough to keep up with its own metabolism. In a cell with no internal transport system, those materials move by diffusion, and diffusion is only fast over short distances.
As a cell grows, its volume increases faster than its surface area, because volume scales with the cube of the radius and surface area only with the square. A larger cell therefore has proportionally less membrane through which to exchange materials, while having more cytoplasm demanding supply.
A prokaryotic cell has no membrane-bound organelles and no cytoskeleton for directed internal transport, so materials must diffuse across the whole cell from the membrane. Beyond a few micrometres the centre of such a cell could not be supplied fast enough, so the working size is capped at roughly 1 to 5 micrometres.
Eukaryotic cells escape the same limit not by changing the physics but by adding internal membranes: organelles compartmentalise reactions, the endoplasmic reticulum and Golgi body move materials in vesicles rather than by bulk diffusion, and folded internal membranes add exchange surface. Prokaryotic cells are small because diffusion alone must serve them, and eukaryotic cells are large because they do not rely on diffusion alone.
Every cell must take in materials and remove wastes fast enough to sustain its metabolism, and in a cell without an internal transport system those materials move by diffusion, which is only rapid over short distances. As a cell grows its volume increases faster than its surface area, because volume scales with the cube of the radius and surface area only with the square, so a larger cell has proportionally less membrane for exchange while containing more cytoplasm to supply. A prokaryotic cell has no membrane-bound organelles and no cytoskeleton for directed transport, so materials must diffuse across the entire cell from the membrane inward; beyond a few micrometres the centre could not be supplied quickly enough, which caps the working size at roughly 1 to 5 micrometres. Eukaryotic cells escape this limit not by changing the physics but by adding internal membranes: organelles compartmentalise reactions, vesicles move materials directionally rather than by bulk diffusion, and folded internal membranes add exchange surface. Prokaryotic cells are therefore small because diffusion alone must serve them, and eukaryotic cells can be large because they do not depend on diffusion alone.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 12 marks
+5 XPUnderstandBand 4(4 marks) 1. Compare the cellular structures of a prokaryotic cell and a eukaryotic cell. In your answer, identify two structures they share and two ways in which they differ.
1 mark: two shared structures correctly identified · 2 marks: two valid differences clearly stated · 1 mark: comparison expressed (not just two separate descriptions)
ApplyBand 4(3 marks) 2. An electron micrograph shows a cell 3 µm long with no nuclear envelope, ribosomes throughout the cytoplasm and a single circular DNA molecule. Identify the cell type and justify your answer using two pieces of evidence from the micrograph.
1 mark: correctly identifies the cell as prokaryotic · 2 marks: two pieces of evidence quoted from the micrograph and linked to the identification
EvaluateBand 5–6(5 marks) 3. A student compares prokaryotic and eukaryotic cells using only one labelled textbook diagram. Evaluate the reliability of this approach and explain how using a variety of sources would improve the comparison.
2 marks: identifies specific limitations of a single schematic source · 2 marks: names other source types and what each adds · 1 mark: reaches a justified judgement
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Responses
Q1 (4 marks): Both prokaryotic and eukaryotic cells share a cell membrane, cytoplasm, ribosomes and DNA, any two of these are shared structures. They differ in two key ways: (1) a eukaryotic cell encloses its DNA within a membrane-bound nucleus, whereas a prokaryotic cell has no nucleus and its single circular chromosome lies free in the cytoplasm in the nucleoid region; (2) a eukaryotic cell contains membrane-bound organelles such as mitochondria, whereas a prokaryotic cell has none, its processes occur in the cytoplasm or on the cell membrane. (Other valid differences: linear vs circular chromosomes; larger ~10–100 µm vs smaller ~1–5 µm; 80S vs 70S ribosomes.) A full-mark answer is expressed as a comparison, e.g. "whereas", "in contrast", rather than two separate descriptions.
Q2 (3 marks): The cell is prokaryotic [1]. The absence of a nuclear envelope means the DNA is not enclosed in a membrane-bound nucleus, which is the defining prokaryotic feature [1]. A single circular DNA molecule, and a length of 3 µm (within the typical 1 to 5 µm prokaryotic range rather than the 10 to 100 µm eukaryotic range), both support this [1]. Ribosomes are not evidence either way, because every cell has them.
Q3 (5 marks): A single labelled diagram is a secondary, schematic source, so its reliability is limited [1]. Diagrams are drawn for clarity rather than accuracy: relative sizes are often distorted (a prokaryote and a eukaryote are commonly drawn the same size when the eukaryote is 10 to 100 times longer), and structures may be simplified or omitted [1]. An electron micrograph is primary evidence at high resolution and shows what is actually present, which is how the absence of internal membranes in prokaryotes was established [1]. A data table of measured sizes, chromosome numbers and ribosome types adds quantitative evidence that a drawing cannot supply, and cross-checking sources exposes any disagreement between them [1]. Overall the one-diagram approach is not reliable on its own, and the comparison becomes defensible only when the diagram is checked against micrographs and measured data, which is what the dot point means by using a variety of sources [1].
Retrieve what stuck
Work the spaced-review set first. It mixes this lesson with earlier ones on purpose.
Check what actually stuck
A short retrieval set drawn from this lesson and the ones before it, so the earlier work keeps coming back rather than fading.
Lock it in
bossYou can now compare the two fundamental cell types and justify how we know the difference. Next lesson zooms inside the eukaryotic cell to meet each organelle.
↩ Back to focus-area hubCompare it with where you started
Read your opening prediction again, then mark the lesson complete.
How did your thinking change?
At the start you were asked how you would tell a human cheek cell from a mouth bacterium. The most reliable single feature is the membrane-bound nucleus: the cheek cell (eukaryotic) has one; the bacterium (prokaryotic) does not. Size is a useful clue (the bacterium is far smaller), but size ranges can overlap and depend on magnification, the nucleus is the definitive test.
If your first instinct was "the bacterium has no DNA" or "prokaryotes are too simple to function", revisit Card 1 and the misconception box: prokaryotes have DNA and perform every life process, they simply are not compartmentalised by internal membranes.