Chapter 3

The bacterial cell

One cell, drawn in cutaway, with everything the chapter asks you to name. Five structures are in every bacterium; the other eight are what make one organism different from another.

Wording follows the Chapter 3 lecture slides. Where the slides and the textbook differ in emphasis, the slides win — they are what the exam is written from.

The specimen

Hover or tap any structure — in the drawing or in the list — to lift it out. The list is grouped from the outside in, which is the order the exam tends to ask them in.

Fig. 1A rod-shaped bacterium in cutaway. Not to scale: the cell is about 1 µm long, and the ribosomes are drawn far larger than they are.

Nothing selected

Thirteen structures, and only five of them are in every bacterium.

The five that are always there: cytoplasmic membrane, cytoplasm, nucleoid, ribosomes, and — for almost everything you will meet — a cell wall. Everything else is optional, and the optional ones are usually the ones that make an organism dangerous.

Outside the wall

Cell wall

Membrane

Inside the cell

Every structure at a glance

The table a revision sheet would have. Presence matters: a question that says “all bacteria have…” is usually testing whether you know which five are universal.

Structure	Where	Present in	Made of	Does
Capsule	Outside the wall	Some	Polysaccharides, polypeptides, or both, in organised repeating units	A firmly attached glycocalyx. Protects the cell and helps it evade the host.
Slime layer	Outside the wall	Some	Polysaccharides, water soluble	A loosely attached, sticky glycocalyx that lets the cell attach to surfaces.
Flagellum	Outside the wall	Some	Filament, hook and basal body; the basal body anchors through rings of integral protein	Movement. Rotation propels the cell, and it can turn either way.
Fimbriae	Outside the wall	Some	Protein	Sticky, bristle-like projections used to stick to surfaces, to hosts, and to each other.
Pilus (conjugation pilus)	Outside the wall	Some	Protein	Transfers DNA from one cell to another — conjugation.
Cell wall	Cell wall	Almost all	Peptidoglycan — alternating NAG and NAM sugars, cross-linked by peptide bridges	Gives the cell its shape, holds it together against osmotic pressure, and helps some cells attach.
Cytoplasmic membrane	Membrane	All	Phospholipid bilayer with embedded proteins	Controls what enters and leaves. Also where energy production happens, since bacteria have no mitochondria.
Cytoplasm	Inside the cell	All	Mostly water, with dissolved ions, sugars, amino acids and enzymes	The medium every reaction in the cell takes place in.
Nucleoid	Inside the cell	All	A single circular chromosome of DNA	Holds the genome.
Plasmid	Inside the cell	Some	Small circular DNA, separate from the chromosome	Carries extra genes the cell does not strictly need to survive.
Ribosome	Inside the cell	All	70S in bacteria — a 50S and a 30S subunit	Builds proteins.
Inclusions	Inside the cell	Some	Stored nutrients — lipid, glycogen, phosphate, sulfur	A reserve the cell draws on when supplies run short.
Endospore	Inside the cell	Some	A dehydrated core wrapped in tough protein coats, with dipicolinic acid and calcium	A dormant survival structure, not a reproductive one — one cell makes one spore.

The pairs that get confused

Capsule vs slime layer

Both are glycocalyx. A capsule is firmly attached and organised into repeating units; a slime layer is loose and water soluble. Capsules hide cells from the immune system, slime layers stick cells to teeth.

Fimbriae vs pili

Fimbriae are short, numerous, and for sticking. A pilus is longer, there are one or two, and it moves DNA. Neither moves the cell — that is the flagellum.

Endospore vs reproduction

One cell makes one spore, so the count never rises. It is survival, not reproduction. That distinction is worth a mark on its own.

The wall difference — thick versus thin peptidoglycan — has its own page: the Gram stain. How this cell differs from one of ours is on prokaryote vs eukaryote.