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Every living thing on Earth, from a single bacterium in a drop of pond water to the trillions of cells that make up your own body, is built from cells. The cell is the smallest unit that can carry out all the processes of life, which is why biologists call it the basic unit of life. This lesson opens Topic B1 (Cell-level systems) of OCR Gateway Science A by sorting all cells into two great families — eukaryotic cells and prokaryotic cells — and looking closely at how they are built and how they differ. Getting this distinction secure now makes everything that follows (sub-cellular structures, microscopy, DNA, enzymes and respiration) far easier to understand.
By the end of this lesson you should be able to describe the main features of animal, plant and bacterial cells, explain what eukaryotic and prokaryotic mean, and compare the two cell types confidently — a comparison that comes up again and again on Paper 1.
For most of the history of life on Earth, the only living things were single-celled organisms. The very first cells were prokaryotic — small and simple, with their genetic material floating freely. Much later, larger and more complex eukaryotic cells evolved, with their DNA packaged inside a membrane-bound nucleus and a range of specialised internal compartments called organelles.
The word roots tell the story:
| Term | Greek roots | Meaning |
|---|---|---|
| Eukaryotic | eu (true) + karyon (kernel/nut) | "True nucleus" — DNA enclosed in a nuclear membrane |
| Prokaryotic | pro (before) + karyon (kernel/nut) | "Before the nucleus" — no true nucleus |
So the single most important difference is simply this: eukaryotic cells have a nucleus; prokaryotic cells do not. Hold onto that one idea and the rest of the detail hangs off it neatly.
Exam Tip: OCR loves a question that gives you an unlabelled diagram or a short description and asks you to decide whether a cell is eukaryotic or prokaryotic. The fastest deciding feature is the nucleus — but the size of the cell and the presence of a loop of DNA or plasmids are also strong clues.
Eukaryotic cells make up animals, plants, fungi and protists (a group of mostly single-celled organisms such as Amoeba). They are comparatively large — usually 10–100 micrometres (µm) across — and contain a nucleus plus several other membrane-bound organelles that divide the cell into compartments where different jobs happen.
An animal cell is a useful "baseline" eukaryotic cell because it contains only the organelles that all eukaryotic cells share.
Key:
| # | Structure | Function (in brief) |
|---|---|---|
| 1 | Cell membrane | Controls what enters and leaves the cell |
| 2 | Cytoplasm | Jelly-like site of most chemical reactions |
| 3 | Nucleus | Contains DNA; controls the cell |
| 4 | Mitochondrion | Site of aerobic respiration |
| 5 | Ribosomes | Site of protein synthesis |
You will study each of these structures in detail in the next lesson; here you just need to recognise that an animal cell contains a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Note that ribosomes are present but far too small to draw to scale on a simple diagram — they are still there, dotted through the cytoplasm, doing the vital job of making proteins.
Plant cells are also eukaryotic. They contain everything an animal cell has, plus three extra features that an animal cell never has:
Key:
| # | Structure | Notes |
|---|---|---|
| 1 | Cell wall (cellulose) | Outer layer; supports and strengthens |
| 2 | Cell membrane | Just inside the wall |
| 3 | Chloroplast | Green; contains chlorophyll; photosynthesis |
| 4 | Nucleus | Contains DNA |
| 5 | Permanent vacuole | Large central space holding cell sap |
Exam Tip: A very common mistake is to say plant cells "have a cell wall instead of a cell membrane". This is wrong — plant cells have both. The cell wall sits outside the membrane. Algae also have cellulose cell walls; fungi have cell walls too, but made of chitin, not cellulose.
Prokaryotic cells are bacteria (and a second group called archaea, which you do not need for GCSE). They are much smaller than eukaryotic cells — typically 0.1–5.0 µm across, perhaps a hundredth the volume of a typical animal cell — and crucially they have no nucleus and no membrane-bound organelles.
A prokaryotic (bacterial) cell contains:
Key: 1 cell wall · 2 cell membrane · 3 cytoplasm containing ribosomes · 4 single loop of chromosomal DNA · 5 plasmid · (the brown tail on the right is a flagellum).
Exam Tip: "Plasmid" and "the main DNA loop" are different things. The chromosomal DNA is the single large loop carrying most of the cell's genes; plasmids are small extra rings that bacteria can swap between each other. Both are made of DNA and both sit free in the cytoplasm because there is no nucleus.
This comparison table is the single most exam-important part of the lesson. Learn it so that you can pull out any row in a comparison question.
| Feature | Eukaryotic cell | Prokaryotic cell |
|---|---|---|
| Examples | Animal, plant, fungal, protist | Bacteria |
| Typical size | 10–100 µm | 0.1–5.0 µm |
| Nucleus | Present (membrane-bound) | Absent |
| Genetic material | DNA on chromosomes inside the nucleus | Single loop of DNA free in cytoplasm |
| Plasmids | Not normally present | Often present |
| Mitochondria | Present | Absent |
| Chloroplasts | Present in plant/algal cells | Absent |
| Ribosomes | Present (larger) | Present (smaller) |
| Cell wall | In plants (cellulose), fungi (chitin); none in animals | Present (not cellulose) |
| Cell membrane | Present | Present |
Notice what eukaryotic and prokaryotic cells have in common: both have a cell membrane, cytoplasm, ribosomes and DNA. The differences are all about how the DNA is stored and whether the cell has membrane-bound organelles.
OCR expects you to handle the relative sizes of cells, including using standard form and orders of magnitude. The units run:
1 mm=1000 μm=1000000 nm
A typical animal cell might be about 20 μm across, while a typical bacterium is about 2 μm across.
An animal cell is 20 μm in diameter. A bacterial cell is 2 μm in diameter. How many times larger is the diameter of the animal cell?
Divide the larger by the smaller:
2 μm20 μm=10
Answer: the animal cell's diameter is 10 times (101, one order of magnitude) larger.
Common error: trying to compare a value in μm with one in mm without converting first. Always get both measurements into the same unit before dividing.
Express a bacterial length of 2 μm in millimetres, in standard form.
Since 1 μm=0.001 mm=1×10−3 mm:
2 μm=2×10−3 mm=0.002 mm
Answer: 2×10−3 mm.
It is natural to wonder why cells are so tiny, and why a large organism is not just made of one big cell. The answer is about getting substances in and out. A cell must absorb the oxygen and nutrients it needs and remove the waste it produces, and these all pass across its surface (the cell membrane). As a cell gets bigger, its volume grows faster than its surface area, so a very large cell would not have enough surface to supply its large volume. Keeping cells small gives each one a large surface area relative to its volume, so exchange of substances is fast enough to keep the cell alive. This surface-area-to-volume idea is developed fully in Topic B2, but it explains why life is built from many small cells rather than a few enormous ones.
It also explains why bacterial cells can do without the internal compartments of eukaryotic cells. Because a bacterium is so small, substances can diffuse across it quickly and reach everywhere they are needed, so it does not need organelles such as mitochondria to organise its chemistry. A large eukaryotic cell, by contrast, benefits from dividing its work between membrane-bound compartments — keeping respiration in the mitochondria, photosynthesis in the chloroplasts and DNA in the nucleus — so that different reactions can be controlled separately and do not interfere with one another.
It is easy to come away from a first look at cells thinking they are all much the same, but in fact cells are enormously varied. Within the eukaryotes, an oak tree, a mushroom, a human and a single-celled Amoeba are all built from eukaryotic cells, yet their cells differ in shape, size and contents. Plant and algal cells carry chloroplasts and cellulose walls that animal cells lack; fungal cells have walls of chitin and never have chloroplasts; protist cells include some of the largest and most elaborate single cells known. Among the prokaryotes, bacteria come in many shapes — spheres, rods and spirals — and live almost everywhere on Earth, from boiling springs to the inside of your gut. Despite this variety, every one of these cells obeys the same fundamental split: it is either eukaryotic (with a nucleus) or prokaryotic (without one). That single organising idea is what makes the eukaryote/prokaryote distinction so useful, and why it opens this topic.
Question (6 marks): Describe the differences between a typical animal cell and a typical bacterial cell.
Mid-band response: "An animal cell has a nucleus but a bacterial cell does not. The animal cell is bigger than the bacterial cell. A bacterial cell has its DNA in a loop in the cytoplasm. Both cells have a cell membrane, cytoplasm and ribosomes."
Examiner-style commentary: This earns marks for the nucleus difference, relative size and the DNA loop, and sensibly notes the shared features. It is held back by vagueness ("bigger") and by missing the organelle differences. To climb a band, quantify the size difference and bring in mitochondria and plasmids.
Stronger response: "An animal cell is eukaryotic and a bacterial cell is prokaryotic. The animal cell has a nucleus containing its DNA, whereas the bacterial cell has no nucleus — its DNA is a single loop free in the cytoplasm, and it may also have small rings of DNA called plasmids. The animal cell is much larger, around 20 μm compared with about 2 μm. The animal cell contains mitochondria; the bacterial cell does not. Both cells have a cell membrane, cytoplasm and ribosomes."
Examiner-style commentary: This is a clear, well-organised comparison that uses the key terms (eukaryotic/prokaryotic), quantifies the size difference and contrasts an organelle (mitochondria) and the DNA arrangement. To reach the top band, the comparison should be made point-by-point and note that the bacterial ribosomes are smaller.
Top-band response: "A typical animal cell is eukaryotic: its genetic material is held as chromosomes inside a membrane-bound nucleus, and it contains membrane-bound organelles including mitochondria for aerobic respiration. A bacterial cell is prokaryotic: it has no nucleus, so its genetic material is a single circular loop of DNA lying free in the cytoplasm, often accompanied by plasmids, and it has no membrane-bound organelles such as mitochondria. The two also differ greatly in size — an animal cell is roughly 20 μm across whereas a bacterium is roughly 2 μm, about one order of magnitude smaller. Bacterial ribosomes are smaller than those of the animal cell. Both cell types do, however, share a cell membrane, cytoplasm, ribosomes and DNA."
Examiner-style commentary: Full marks. The answer is structured as clear point-by-point contrasts, uses precise terminology, quantifies the size difference with an order-of-magnitude statement, distinguishes chromosomal DNA from plasmids and finishes by stating the shared features — exactly the balance of difference and similarity examiners reward.
This content is aligned with OCR Gateway Science A GCSE Biology (J247), Topic B1 Cell-level systems (cell structures; eukaryotes and prokaryotes). Refer to the official OCR specification document for the exact wording.