Bacteria Cell Diagram: Every Part Labelled and Explained
Here is a labelled bacterial cell. It is drawn in the page itself rather than being a picture file, so it stays sharp at any size and prints cleanly.
Scroll sideways to see the whole diagram
Many bacteria diagrams still show a mesosome, an infolded loop of membrane. It is not real. It is damage caused by the chemicals used to prepare cells for the electron microscope, and the evidence against it has been settled since the 1980s. There is a full explanation further down, because it is the single most common error in diagrams of this cell.
Every part, and what it does
Bacteria are prokaryotes, which means their DNA is not wrapped in a nucleus. The whole cell is built around that one fact. Work from the outside in.
| Part | What it is | What it does | Always there? |
|---|---|---|---|
| Capsule | A slimy outer layer of polysaccharide | Sticks the cell to surfaces and helps it hide from the host’s immune system | No |
| Cell wall | A mesh of peptidoglycan (also called murein) | Holds the shape and stops the cell bursting when water moves in | Yes, in nearly all |
| Plasma membrane | A phospholipid bilayer, same basic design as ours | Controls what enters and leaves. Also does the respiration work our mitochondria do | Yes |
| Cytoplasm | The watery interior | Where the chemistry happens. Contains no membrane-bound organelles at all | Yes |
| Nucleoid | The region holding one circular chromosome | Carries the genes the cell needs to live | Yes |
| Plasmid | A small separate ring of DNA | Carries extra genes, very often antibiotic resistance. Can be passed to other bacteria | No |
| Ribosomes | 70S — smaller than the 80S ones in our cells | Build proteins. The size difference is what lets antibiotics work | Yes |
| Flagellum | A stiff helical filament of the protein flagellin | Spins like a propeller to move the cell. It rotates — it does not whip | No |
| Pili and fimbriae | Short protein hairs on the surface | Fimbriae grip surfaces. Pili join two cells to transfer DNA, called conjugation | No |
Note the last column. Capsule, plasmid, flagellum and pili are optional. A question asking for structures “found in all bacteria” wants the other five, and including the optional ones costs you the mark.
A bacterium has no nucleus, no mitochondria, no chloroplasts, no endoplasmic reticulum, no Golgi and no lysosomes.
That list of absences is worth as many marks as the list of parts. “Prokaryotic” literally means “before the nut” — before the kernel, the nucleus.
The nucleoid is simply the area of cytoplasm where the DNA happens to sit. It has no membrane around it. A nucleus is a membrane-bound organelle; a nucleoid is a region. Drawing a line around the nucleoid in a diagram turns your prokaryote into something that does not exist.
The mesosome problem
Search for a bacteria cell diagram and a good number of the results will show a mesosome: a folded loop of plasma membrane pushing into the cytoplasm, usually labelled as the site of respiration or involved in cell division.
Mesosomes do not exist in living bacteria. They are an artefact — damage created by the process of preparing the cell, not a structure that was ever there.
How the mistake happened
- 1953. Chapman and Hillier see odd membrane folds in electron micrographs and call them “peripheral bodies”.
- 1960. Fitz-James names them mesosomes. Through the 1960s they are given jobs: respiration, cell wall building, chromosome replication. Textbooks pick them up.
- 1971. Nanninga shows the mesosome in Bacillus subtilis depends on which fixation method you use. That is a bad sign for a real structure.
- 1976. Higgins, Tsien and Daneo-Moore compare fixed and unfixed cells. Mesosomes appear in the fixed ones and not in the unfixed ones.
- 1983. Dubochet and colleagues look at frozen-hydrated bacteria, which skips chemical fixation entirely. No mesosomes.
- Mid-to-late 1980s. With cryofixation established, the consensus settles: mesosomes are artefacts of preparation.
The honest footnote: a small number of researchers, including Stolz (1991) and Murata and colleagues (2008), have argued the case is not completely closed. But the mainstream position is that they are artefacts, and that is what you should write.
Why it is a good story
It is a clean example of method creating the result. The chemicals that made the cell visible also damaged it, and for thirty years people described the damage as anatomy.
Why it still turns up
Diagrams get copied. A drawing made in 1975 gets redrawn, reused and uploaded, and nobody goes back to check whether the thing in it was ever real.
If your textbook shows one, it is not necessarily wrong to know what it is — but do not put it in a diagram you are drawing, and do not write that respiration happens there. In a bacterium, respiration happens on the plasma membrane.
Bacterial against animal and plant cells
This comparison appears on every paper that covers cells. The useful way to hold it is that a bacterium is smaller, simpler and has nothing wrapped in its own membrane.
| Feature | Bacterial (prokaryotic) | Animal and plant (eukaryotic) |
|---|---|---|
| Size | About 0.5–5 µm | About 10–100 µm, so roughly ten times wider |
| Nucleus | None. DNA sits free in a nucleoid | A true nucleus with a double membrane |
| DNA shape | One circular chromosome, no histones | Several linear chromosomes, wound round histones |
| Plasmids | Common | Essentially absent |
| Ribosomes | 70S | 80S |
| Membrane-bound organelles | None at all | Mitochondria, ER, Golgi, lysosomes, and chloroplasts in plants |
| Cell wall | Peptidoglycan | None in animals; cellulose in plants; chitin in fungi |
| Division | Binary fission | Mitosis or meiosis |
Bacteria: peptidoglycan. Plants: cellulose. Fungi: chitin. Writing that a bacterium has a cellulose wall is one of the most common errors in cell biology, and it is an easy mark to lose.
Gram positive and Gram negative
Bacteria split into two groups based on their wall, and you can tell them apart with a stain invented in 1884. The difference matters because it decides which antibiotics work.
Gram positive — stains purple
A thick peptidoglycan wall and no outer membrane. The thick mesh traps the purple crystal violet dye, so the cell stays purple through the whole process.
Easier to kill with penicillin, which attacks peptidoglycan.
Gram negative — stains pink
A thin peptidoglycan layer with an extra outer membrane over it. The dye washes out, so a pink counterstain is needed to see the cell at all.
Harder to treat — the outer membrane keeps many antibiotics out.
The naming confuses people, so be clear about it: “positive” and “negative” have nothing to do with electrical charge or with being harmful. They simply mean the cell did or did not hold the stain, and the test is named after Hans Christian Gram.
The shapes bacteria come in
The diagram above is a rod, because rods are the shape most often drawn. There are three main forms and the names turn up in questions.
| Shape | Name | Looks like | Example |
|---|---|---|---|
| Sphere | Coccus plural: cocci | A ball. Often found in chains or clusters | Staphylococcus |
| Rod | Bacillus plural: bacilli | A capsule or sausage, as drawn above | E. coli |
| Spiral | Spirillum plural: spirilla | A corkscrew or a comma | Vibrio cholerae (comma-shaped) |
The shape does not change the internal structure. A coccus has the same parts as the rod drawn above, just packed into a sphere.
Why 70S ribosomes matter more than you think
“70S” looks like a detail to memorise. It is actually the reason antibiotics are possible, and a question asking why antibiotics do not poison us is really asking about this.
Bacterial ribosomes are 70S. Ours are 80S. They are built differently as well as being different sizes. So a drug that jams a 70S ribosome stops the bacterium making proteins and leaves your own cells working. That selectivity is the whole basis of antibiotic treatment.
The same logic runs through the rest of the cell. Penicillin attacks peptidoglycan, and human cells have no cell wall at all, so there is nothing for it to damage. Antibiotics work by aiming at the things on this diagram that we do not have.
The S is a Svedberg unit, which measures how fast something sinks in a centrifuge, not its mass. That is why the numbers do not add up the way you expect: a 70S ribosome is made of a 50S and a 30S subunit, and 50 + 30 does not make 80. Svedberg units depend on shape as well as size, so they simply do not add.
Drawing it in an exam
Biological drawing is marked on rules that have nothing to do with artistic skill. Follow these and a plain drawing scores full marks.
- Sharp pencil, clean single lines. No sketchy or doubled lines, and no shading or colouring in — shading loses marks in biological drawing.
- Draw it large. At least half the space you are given. Small drawings lose detail marks.
- Label lines must be straight, drawn with a ruler, and must not cross. Each one should touch the structure it names.
- Write labels horizontally, outside the drawing, and keep them on one side where you can.
- Draw only what is asked. If the question says a typical bacterium, include the five structures all bacteria have. Add a capsule, flagellum or plasmid only if the question mentions them.
- No mesosome, and no membrane around the nucleoid.
If the question asks for a magnification or a scale, remember that magnification = image size ÷ actual size, and that both must be in the same units. A bacterium around 2 µm drawn at 10 cm has been magnified 50,000 times.
Where the marks get lost
- Drawing a membrane around the nucleoid. It is a region, not an organelle. Give it a membrane and you have drawn a eukaryote.
- Including a mesosome. It is an artefact of fixation.
- Saying the cell wall is cellulose. Peptidoglycan for bacteria, cellulose for plants, chitin for fungi.
- Adding mitochondria. Bacteria respire on the plasma membrane. No bacterium has one.
- Writing 80S instead of 70S. 80S is ours.
- Including optional structures in a list of what all bacteria have. Capsule, flagellum, pili and plasmids are all optional.
- Confusing pili with fimbriae. Fimbriae grip surfaces; pili transfer DNA between cells.
- Saying the flagellum whips. A bacterial flagellum rotates, driven by a motor in the membrane. It is structurally unrelated to the eukaryotic flagellum.
- Mixing up Gram positive and negative. Thick wall, purple, positive. Thin wall plus outer membrane, pink, negative.
Frequently asked questions
What are the parts of a bacterial cell?
Does a bacterial cell have a nucleus?
What is the difference between a nucleoid and a nucleus?
What is a mesosome, and why is it not in this diagram?
What is the bacterial cell wall made of?
Why are bacterial ribosomes 70S?
What is a plasmid?
What is the difference between pili and fimbriae?
How big is a bacterial cell?
What is the difference between Gram positive and Gram negative bacteria?
Do bacteria have mitochondria?
How do you draw a bacterial cell for an exam?
Cell biology is where the easy marks are, if the detail is right
Most of the marks on this topic go to precision rather than difficulty — peptidoglycan not cellulose, 70S not 80S, nucleoid not nucleus. Our tutors teach IB and A-level biology and will work through your own past papers rather than a generic worksheet.
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