Rotate the bacterium and click any structure — or use the buttons and colored legend on the right — to see its job. Notice there is no nucleus and no membrane-bound organelles: that is what makes this cell prokaryotic.
Bacteria were the first life on Earth, and they are still everywhere — in soil, in the ocean, and trillions of them living in and on you. A bacterium is a prokaryotic cell, from Greek for "before the nut (kernel)," meaning before the nucleus. It is tiny: a typical rod is only about $1$–$5\,\mu\mathrm{m}$ long, roughly ten times smaller across than an animal cell. Everything it needs to live is packed into one simple compartment.
The defining feature is what a bacterium lacks: there is no nucleus and no membrane-bound organelles — no mitochondria, no ER, no Golgi. Instead the main DNA, a single circular chromosome, sits loose in a region called the nucleoid. Floating nearby are small DNA rings called plasmids and huge numbers of ribosomes making proteins. Around the cell are a plasma membrane, a rigid cell wall of peptidoglycan, and often a slimy capsule. Many bacteria also have a flagellum for swimming and pili for sticking and sharing DNA.
"Simple" does not mean weak. Because there is no nuclear membrane separating the DNA, a bacterium can transcribe a gene into mRNA and translate it into protein at the same time, so it responds to its environment almost instantly. Its small size gives it a large surface-area-to-volume ratio, $\frac{A}{V}$, so nutrients reach every part fast. It can divide as often as every $20$ minutes, and share plasmid genes (including antibiotic resistance) with its neighbours. The cell wall is so important — it stops the cell bursting under osmotic pressure — that antibiotics like penicillin kill bacteria precisely by attacking it.
Rotate the cell and click the nucleoid — notice the DNA is loose, not boxed in a nucleus. Click a plasmid (the small ring) and compare it to the big chromosome. Turn off "Cell wall" and "Capsule" to peel back the layers, then toggle "Swim" to watch the flagellum rotate like a propeller. Use the dropdown to find the thin pili on the surface.
| Part | Main job | In eukaryotes too? |
|---|---|---|
| Capsule | Sticky outer slime layer; protection and sticking to surfaces | No (some have a coat) |
| Cell wall | Rigid peptidoglycan layer; shape and stops bursting | Plants/fungi have walls (different material) |
| Plasma membrane | Selective barrier; also does jobs the membrane organelles do in eukaryotes | Yes |
| Cytoplasm | Watery interior where reactions happen | Yes |
| Nucleoid | Region holding the single circular chromosome (main DNA) | No (eukaryotes use a nucleus) |
| Plasmid | Small extra DNA ring with bonus genes (e.g. resistance) | Rare |
| Ribosomes | Build proteins; smaller (70S) than eukaryotic (80S) | Yes (larger) |
| Flagellum | Whip-like tail spun by a motor for swimming | Different kind exists |
| Pili / fimbriae | Hair-like surface fibres for attaching and transferring DNA | No |
Since a bacterium has no mitochondria or chloroplasts, the reactions those organelles perform in eukaryotes happen on the plasma membrane instead. The proteins of the electron transport chain that make ATP sit right in the bacterial membrane, and in photosynthetic bacteria the light reactions happen on infolded membrane patches. One simple boundary does many of the jobs that eukaryotes spread across separate compartments — an elegant consequence of being small.
Bacterial ribosomes (70S) are built slightly differently from our own (80S). Several antibiotics — such as tetracyclines and erythromycin — exploit this difference, jamming the bacterial ribosome while leaving human ribosomes untouched. This idea, called selective toxicity, is why we can kill bacteria inside our bodies without poisoning ourselves, and it is a direct, practical payoff of understanding prokaryotic cell structure.
A prokaryotic cell is a cell with no membrane-bound nucleus and no membrane-bound organelles. Bacteria and archaea are prokaryotes. Their DNA floats freely in a region of the cytoplasm called the nucleoid rather than being enclosed in a nucleus. Prokaryotic cells are also much smaller and simpler than eukaryotic cells.
Key takeaway: a prokaryote keeps its DNA loose in the cytoplasm and has no membrane-bound nucleus or organelles.A bacterium's main DNA is a single, usually circular chromosome that sits in an irregular region of the cytoplasm called the nucleoid. It is not wrapped in a membrane, so it is in direct contact with the rest of the cell. Many bacteria also carry small extra rings of DNA called plasmids.
Key takeaway: bacterial DNA is a circular chromosome in the nucleoid, with no surrounding nuclear membrane.A plasmid is a small, circular piece of DNA separate from the main chromosome. It carries a few extra genes — often ones for antibiotic resistance or for breaking down unusual nutrients — that are useful but not essential. Bacteria can copy plasmids and pass them to other bacteria, which is a major way antibiotic resistance spreads. Plasmids are also the workhorses of genetic engineering.
Key takeaway: a plasmid is a small extra DNA ring carrying bonus genes that bacteria can share, including resistance.The bacterial cell wall is a rigid layer outside the plasma membrane, made mainly of peptidoglycan. It gives the cell its shape and, crucially, stops it from bursting when water flows in by osmosis. Because peptidoglycan is unique to bacteria, many antibiotics such as penicillin work by attacking the cell wall without harming our own cells.
Key takeaway: the peptidoglycan cell wall gives shape and prevents bursting, and is a key antibiotic target.Many bacteria swim using a flagellum, a long whip-like tail driven by a tiny rotary motor embedded in the membrane and wall. The motor spins the flagellum like a propeller, pushing the cell through liquid. Bacteria steer toward food or away from danger by changing how the motor turns, a behaviour called chemotaxis.
Key takeaway: bacteria swim by spinning a flagellum with a rotary motor, steering by chemotaxis.Prokaryotic cells (bacteria, archaea) have no nucleus and no membrane-bound organelles, their DNA is a single circular chromosome in the nucleoid, and they are small (about $1$–$5\,\mu\mathrm{m}$) with small (70S) ribosomes. Eukaryotic cells have a true nucleus, many membrane-bound organelles, usually linear chromosomes, and are larger with larger (80S) ribosomes. Both have a plasma membrane, cytoplasm, ribosomes, and DNA.
Key takeaway: prokaryotes are small and lack a nucleus and organelles; eukaryotes are larger and compartmentalised — but both share membranes, cytoplasm, ribosomes, and DNA.Being small and simple is an advantage. Bacteria reproduce extremely fast (some divide every $20$ minutes), so they evolve quickly and adapt to new environments. They can share genes directly through plasmids, exploit almost every habitat on Earth, and form protective communities called biofilms. Their lack of a nucleus actually lets transcription and translation happen at the same time, speeding up protein production.
Key takeaway: fast reproduction, gene sharing, and metabolic flexibility make "simple" prokaryotes one of the most successful forms of life on Earth.