Bacterial cell
Before you start
Bacteria are among the oldest forms of life on Earth, and they are still everywhere: in soil, in the oceans and in our own gut, where most of them do us no harm. On the animal cell page, the DNA sits inside a nucleus surrounded by a double membrane. A bacterium is far smaller and has no nucleus at all, yet it stores instructions, makes proteins and reproduces as successfully as any cell. Seeing what a bacterium does without tells us which cell parts are truly essential. The representative bacterium shown here divides more simply than our cells do: it copies its circular DNA and splits in two. That is not mitosis, the careful division our own cells use, which the cell division page shows. You do not need that page first.
“No nucleus” can easily sound like “no genetic information”, but these are different claims. DNA is still inside the cell; it is simply not enclosed in its own membrane-bound compartment. Having found it, look for the ribosomes that make proteins and the cell membrane around the whole cell. The useful question is how these jobs are carried out, rather than how many labels are missing from another cell diagram. Some additional structures are shown too: this is a representative bacterium, not a claim that every bacterium has every part in the model.
The setupThis is an enlarged rod-shaped bacterium, cut open so that its layers can be seen. Select a structure in the model or from the list to read its name, its job and how to recognise it; Look closer moves in to its detail. The cutaway switch closes the cell to show its outer surface with its hair-like pili and its flagellum.
What to do and noticeStart at the boundary: this bacterium has an outer membrane, a thin mesh-like wall, and an inner plasma membrane, so select each layer in turn. Then look inside for the coiled DNA, the nucleoid, and notice that no membrane surrounds it. Find the ribosomes scattered around it and the small DNA rings called plasmids. Having no nucleus does not mean having no DNA.
Predict firstA bacterium has no nucleus. Does it have one boundary layer or several?
Plasma membrane
A cell must take in materials without letting everything enter or leave freely, so it needs a selective boundary. The membrane mainly contains phospholipids and proteins. A phospholipid has a water-friendly end and a water-avoiding part. In water, the water-avoiding parts turn inward and pair up, so the water-friendly ends face the water on both sides. The two layers belong to one membrane, not two separate membranes. Proteins in the membrane can provide passages or help particular substances cross.
The thin membrane surrounding the cell contents. In cells that have a wall, the wall lies outside this membrane. Animal cells have no cell wall.
Follow one set of protein instructions
The cell model shows where structures are. This separate construction shows what “DNA information is used to make protein” means. DNA stores the instructions, and the working copy is made of messenger RNA. Advance one step at a time; nothing moves on by itself.
Each numbered square stands for the information specifying one amino-acid unit, and each circle stands for the unit itself. An amino acid is one small bead in the chain of a protein. Matching colours let us follow the order from the information to the chain. These are instruction markers, not pictures of DNA or RNA molecules. Distances and the number of units are chosen to make each step visible.
Locate the boundary, follow one set of instructions for a protein, then examine the structures that distinguish this cell.
Prerequisites: A cell is a basic unit that carries out life processes. It must take in materials, build its parts and keep information for making them. We will start with these needs and introduce the structures one at a time.Start with three layers
Predict firstA bacterium has no nucleus. Does it have one boundary layer or several?
This example bacterium has three layers from outside in: an outer membrane, a thin cell wall, and the plasma membrane innermost. The outer membrane is an extra boundary; the wall mainly supports the cell; the plasma membrane controls what passes. Selecting the plasma membrane shows the innermost boundary. The wall and outer membrane lie outside it; select each to compare them. Not every bacterium has an outer membrane.
How to observe
- Start with the boundaries, then explore inside; distinguish a wall from a membrane.
- Select a structure, read what it does, then use Focus to inspect its detail.
- Find the nucleoid and ribosomes: no nucleus does not mean no DNA.
What this model simplifies
Sizes, counts and colours are adjusted for teaching; this is not a raw microscope image. Cellular organization is dynamic. Each model represents one example, not every member of its group; bacterial accessory structures, plant chloroplasts and fungal forms particularly vary.
