Prokaryotic cells

The prokaryotes

The prokaryotic cells are the bacteria, the blue-green algae (cyanobacteria), the mycoplasma and the PPLO, pleuro-pneumonia like organisms. They are generally smaller than eukaryotic cells, and they multiply much faster.

Bacteria come in four basic shapes:

• coccus, round;
• bacillus, rod-like;
• vibrio, shaped like a comma;
• spirillum, spiral.

Their shapes and jobs differ widely, but their build is much the same. Almost all have a cell wall around the plasma membrane; the mycoplasma are the exception. The fluid inside is the cytoplasm.

No nucleus, but plenty of DNA

There is no well-defined nucleus. The genetic material is naked: no membrane encloses it. This region of DNA is the nucleoid.

Many bacteria also carry plasmids. These are small circles of DNA, outside the main genomic DNA. Plasmids give bacteria special traits. One is resistance to antibiotics. Scientists also use plasmids to carry foreign DNA into bacteria, in genetic engineering.

Apart from ribosomes, prokaryotes have no organelles like those of eukaryotes.

The cell envelope

Most prokaryotes, and bacteria in particular, have a tough cell envelope of three layers. Each layer does its own job, but they act together as one protective unit.

1. The glycocalyx is the outermost layer. In some bacteria it is a loose sheath, the slime layer. In others it is thick and tough, a capsule.
2. The cell wall lies beneath it. It gives the cell its shape and strong support, so the cell does not burst or collapse.
3. The plasma membrane is innermost. It is semi-permeable and much like the membrane of a eukaryote.

Bacteria are sorted into two groups by Gram staining. Bacteria that take up the stain are Gram positive. Those that do not are Gram negative. The difference lies in their cell envelopes.

Membranes folded inside

The plasma membrane folds into the cell to form a mesosome. It may be a vesicle, a set of tubules or a stack of lamellae. Mesosomes help in:

• forming the cell wall;
• copying the DNA and sharing it between daughter cells;
• respiration and secretion;
• adding surface area to the plasma membrane, and adding enzymes.

In blue-green algae and some other bacteria, membranes stretch into the cytoplasm as chromatophores. These hold pigments for photosynthesis.

Moving and sticking

• Flagella let motile bacteria swim. A bacterial flagellum has three parts: the long filament, the hook and the basal body, which sits in the envelope.
• Pili are long tubes made of a special protein. They do not help the cell move.
• Fimbriae are small bristle-like fibres. They help some bacteria stick to rocks in streams, or to the tissues of a host.

Ribosomes and inclusion bodies

The ribosomes of a prokaryote sit close to the plasma membrane. Each is about $15$ to $20\ \text{nm}$ across. It has a large 50S and a small 30S subunit, which join into a 70S ribosome. The S units measure how fast a particle settles; they do not simply add up.

Several ribosomes may attach to one mRNA, forming a chain called a polysome or polyribosome. They all translate the mRNA into proteins at once.

Reserve material is stored in inclusion bodies. These are not bound by any membrane. They include phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles are found in blue-green, purple and green photosynthetic bacteria.

You should find that a $900\ \text{nm}$ mRNA with a ribosome every $100\ \text{nm}$ carries ten of them. A shorter gap packs more ribosomes onto the same mRNA.

Try it in Python

The program places a ribosome at the start of an mRNA and then one every gap, and counts them.

Practise with new numbers

Common mistakes

• Giving bacteria a nucleus. Their DNA is naked, in a nucleoid.
• Putting the cell wall outermost. The glycocalyx lies outside the wall.
• Adding the S units. A 70S ribosome has 50S and 30S subunits; S values do not add.
• Thinking pili help bacteria swim. Flagella do that; fimbriae help them stick.
• Giving mycoplasma a cell wall. They are the prokaryotes without one.

Remember these

• Prokaryotes: bacteria, blue-green algae, mycoplasma, PPLO.
• Cell envelope: glycocalyx (slime layer or capsule), cell wall, plasma membrane.
• Nucleoid of naked DNA; plasmids carry extra traits, such as antibiotic resistance.
• Mesosomes: wall formation, DNA copying, respiration, secretion, more surface.
• Flagellum: filament, hook, basal body. Pili and fimbriae do not move the cell.
• 70S ribosomes (50S and 30S); polysomes; inclusion bodies without membranes.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the nucleoid?

2. Name the three layers of a bacterial cell envelope, from the outside in.

3. Give two jobs of a mesosome.

4. What are the three parts of a bacterial flagellum?

5. What are the two subunits of a 70S ribosome?