Topic 2.1 · Core
Bacterial Cell Structure
A bacterium is a single cell, and its structure is genuinely simpler than a plant or animal cell’s. One fact does almost all of the work in this section: a bacterial cell has no nucleus, and everything else you need follows from that single difference.
The required structures
Cambridge limits what you need here to six named structures: a cell wall, cell membrane, cytoplasm and ribosomes, plus circular DNA and, often, plasmids. The main genetic material is present as circular DNA lying free in the cytoplasm rather than enclosed in a nucleus, and a bacterial cell may also contain plasmids — small additional circles of DNA carrying extra genes.
| Structure | Function |
|---|---|
| Cell wall | Supports and protects the cell and helps it keep its shape |
| Cell membrane | Controls the movement of substances into and out of the cell |
| Cytoplasm | Where many of the cell’s chemical reactions occur |
| Ribosomes | Site of protein synthesis |
| Circular DNA | The main genetic material; carries the genes that control the cell’s activities |
| Plasmids | Small additional circles of DNA carrying extra genes |
Notice what is missing from that list. A bacterial cell has no mitochondria and no chloroplasts, and its DNA is not enclosed inside a nucleus at all — it sits directly in the cytoplasm, alongside any plasmids that happen to be present.
Why “no nucleus” is the headline difference
A nucleus is a compartment that encloses genetic material. A bacterium has no such compartment, so a bacterial cell is described as prokaryotic, while a cell with a nucleus — like a plant or animal cell — is eukaryotic. Everything else about a bacterium’s simpler structure follows the same pattern: no nucleus, and no mitochondria or chloroplasts either. The cell still does every essential job a living cell has to do; it just does them using only the membrane, cytoplasm and ribosomes it actually has.
Comparing a bacterial cell with plant and animal cells
| Feature | Bacterial cell | Plant cell | Animal cell |
|---|---|---|---|
| Cell membrane | Present | Present | Present |
| Cytoplasm | Present | Present | Present |
| Ribosomes | Present | Present | Present |
| Cell wall | Present | Present | Absent |
| Nucleus | Absent | Present in a typical cell | Present in a typical cell |
| Genetic material | Circular DNA and possibly plasmids, free in the cytoplasm | Inside a nucleus | Inside a nucleus |
| Mitochondria | Absent | Present | Present |
| Chloroplasts | Absent | Present in photosynthetic cells | Absent |
A cell wall on its own does not identify a plant cell, because bacteria have cell walls too — and a bacterial cell wall is chemically different from a plant cell wall in any case, since it is not made of cellulose. The clearest single piece of evidence for identifying a bacterial cell in a labelled diagram is circular DNA that is not enclosed inside a nucleus, ideally alongside a visible plasmid.
Applying this to an unfamiliar image
The identification logic here always runs the same way: observed feature, then the relevant criterion, then a conclusion. If a diagram shows circular DNA, a possible plasmid, and no nucleus, that combination strongly supports identifying the cell as bacterial. If a diagram instead shows DNA enclosed inside a clearly bounded nucleus, the cell cannot be bacterial, whatever else is present. Work through the evidence in that order rather than guessing from the overall shape of the cell.
Where marks are actually lost
“No nucleus” does not mean “no DNA” — a bacterium still carries circular DNA, it is simply not enclosed. Do not label the DNA region on a bacterial diagram “nucleus”; it is a molecule of DNA, not a membrane-bound structure, and the two are not interchangeable labels. Circular DNA and a plasmid are also not the same thing as each other: the circular DNA is the cell’s main genetic material, while a plasmid is a small extra circle of DNA that may be present alongside it. And a bacterial cell has no mitochondria at all, so adding one to a bacterial diagram or answer is a straightforward error, not a minor imprecision.
Plasmids come up again much later, in genetic modification — but nothing about that later use is required here. In this chapter you only need to name a plasmid and recognise it on a diagram.