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Look at anything that is alive — a blade of grass, a slice of mould on old bread, the tip of your own finger — and you are looking at something built from cells. A cell is the smallest structure that can carry out all the reactions of life on its own, which is why it is called the basic unit of life. This lesson begins Topic B1 (Cell-level systems) of your OCR Gateway Combined Science course by dividing every cell that has ever existed into just two groups: eukaryotic cells and prokaryotic cells. It is a simple split, but a powerful one, and getting it clear now will make the rest of the topic — organelles, microscopy, DNA, enzymes and respiration — much easier to follow.
By the end of this lesson you should be able to describe the main features of animal, plant and bacterial cells, say what the words eukaryotic and prokaryotic mean, and compare the two cell types with confidence. That comparison is one of the most reliable question types you will meet in the biology part of your combined-science papers.
This lesson builds AO1 (recall of cell structures and what eukaryotic and prokaryotic mean) and AO2 (applying that knowledge to compare unfamiliar cells and to decide which type a described cell is).
Life began with cells that were small and simple. These first cells were prokaryotic: their genetic material was not shut away anywhere in particular but drifted freely inside them. Much later, larger and more complicated eukaryotic cells appeared. In a eukaryotic cell the DNA is packed inside a nucleus surrounded by its own membrane, and the cell is divided up into several small working compartments called organelles.
The names themselves tell you the key difference, because they come from Greek words:
| 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 one idea to fix in your mind above all others is this: eukaryotic cells have a nucleus, and prokaryotic cells do not. Almost every other difference between the two follows on from that single fact.
Exam Tip: A very common question shows you a diagram or a short description and asks whether the cell is eukaryotic or prokaryotic. The quickest thing to check is whether there is a nucleus. If there is no nucleus, look for the other prokaryotic clues — a much smaller size, a loop of DNA, and small extra rings of DNA called plasmids.
Eukaryotic cells make up animals, plants, fungi and protists (a group that includes many single-celled organisms, such as Amoeba). They are fairly large — usually somewhere between 10 and 100 micrometres (µm) across — and each one contains a nucleus together with several other membrane-bound organelles. These organelles split the cell into compartments, so that different jobs can be carried out in different places without getting in each other's way.
An animal cell is a good starting point because it contains only the organelles shared by every eukaryotic cell — nothing extra.
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 |
The next lesson looks at each of these structures in detail; for now you simply need to be able to recognise them. An animal cell has a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. The ribosomes are far too small to draw at the same scale as the rest of the cell, but they are still there, scattered through the cytoplasm, quietly building the cell's proteins.
Plant cells are eukaryotic too. A plant cell contains everything an animal cell has, plus three extra features that no animal cell ever 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: Here is a very common misconception to avoid: students often write that plant cells have a cell wall "instead of" a cell membrane. That is wrong. Plant cells have both — the cell wall sits on the outside of the membrane. (Algae also have cellulose walls; fungi have walls too, but made of chitin, not cellulose.)
Prokaryotic cells are bacteria. (There is a second prokaryotic group called archaea, but you do not need it for combined science.) They are far smaller than eukaryotic cells — typically only 0.1 to 5.0 µm across, perhaps a hundredth of the volume of a typical animal cell — and, crucially, they have no nucleus and no membrane-bound organelles at all.
Inside a bacterial cell you will find:
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: Do not confuse the main DNA loop with a plasmid. The chromosomal DNA is the one large loop that carries most of the cell's genes; plasmids are the small extra rings that bacteria can even pass between one another. Both are DNA, and both lie free in the cytoplasm because there is no nucleus to contain them.
This table is the part of the lesson most likely to earn you marks, so learn it well enough to pull out any single row when a comparison question asks for it.
| 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 |
It is just as worth noticing what the two cell types have in common. Both have a cell membrane, cytoplasm, ribosomes and DNA. The real differences are all about how the DNA is stored and whether the cell is divided into membrane-bound compartments.
You are expected to handle the relative sizes of cells, and to use standard form where it helps. The units line up like this:
1 mm=1000 μm=1000000 nm
As a rough guide, a typical animal cell is about 20 μm across, whereas a typical bacterium is only 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 measurement by the smaller one:
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 put both measurements into the same unit before you divide.
Write a bacterial length of 2 μm in millimetres, using 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 a fair question to ask why cells are so tiny, and why a big organism such as you is not simply built from one huge cell. The answer is about supplying the cell. Every cell has to take in the oxygen and nutrients it needs and get rid of its waste, and all of that has to pass across its surface — the cell membrane. The trouble is that as a cell gets bigger, its volume grows faster than its surface area. A very large cell would therefore not have enough surface to serve its large volume. Keeping cells small gives each one a large surface area compared with its volume, so substances can be exchanged fast enough to keep it alive. You will meet this surface-area-to-volume idea again in Topic B2, but it already explains why living things are made of many small cells rather than a few enormous ones.
The same idea explains why bacteria can manage without the internal compartments of eukaryotic cells. A bacterium is so small that substances diffuse right across it very quickly and reach everywhere they are needed, so it has no need for organelles such as mitochondria to organise its chemistry. A large eukaryotic cell, on the other hand, gains a real advantage from dividing its work between separate compartments — respiration in the mitochondria, photosynthesis in the chloroplasts, DNA in the nucleus — so that different reactions can be controlled independently.
It is easy to finish a first look at cells thinking they are all much alike, but in truth they are astonishingly varied. Among the eukaryotes, an oak tree, a mushroom, a human and a single-celled Amoeba are all built from eukaryotic cells, yet those cells differ enormously 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 contain chloroplasts; and some protist cells are among the largest, most elaborate single cells known. Among the prokaryotes, bacteria come as spheres, rods and spirals, and live almost everywhere on Earth, from hot springs to the inside of your gut. Yet for all this variety, every one of these cells obeys the same basic rule: it is either eukaryotic (with a nucleus) or prokaryotic (without one). That single organising idea is exactly why the eukaryote/prokaryote split is so useful, and why it opens the whole 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 picks up marks for the nucleus difference, the relative size and the DNA loop, and sensibly notes the shared features too. It is held back by vague wording ("bigger") and by leaving out the organelle differences. To move up a band, put a number on 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: A clear, well-ordered comparison that uses the key terms, puts a figure on the size difference and contrasts both an organelle (mitochondria) and the DNA arrangement. To reach the top band, make the comparison point-by-point and add that the bacterial ribosomes are smaller.
Top-band response: "A typical animal cell is eukaryotic: its genetic material is stored 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 with plasmids as well, and it has no membrane-bound organelles such as mitochondria. The two also differ greatly in size — an animal cell is around 20 μm across whereas a bacterium is around 2 μm, roughly one order of magnitude smaller. The 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 built as clear point-by-point contrasts, uses precise terms, puts an order-of-magnitude figure on the size difference, separates chromosomal DNA from plasmids, and ends with the shared features — exactly the balance of differences and similarities examiners reward.
This content is aligned with OCR Gateway Combined Science A (J250), Topic B1 Cell-level systems (eukaryotic and prokaryotic cells). Refer to the official OCR specification for exact wording.