Part 1: Core structure of prokaryotic cells

Prokaryotic cells are much smaller and simpler than eukaryotic cells. Bacteria are the standard example. A typical prokaryotic cell is around 0.1-10 μm long.

The key A-level idea is that a prokaryotic cell is still a cell, but it does not have the internal membrane-bound compartments seen in eukaryotes.

All prokaryotic cells have:

• cytoplasm with no membrane-bound organelles
• 70S ribosomes, which are smaller than eukaryotic 80S ribosomes
• no nucleus
• one circular DNA molecule lying free in the cytoplasm
• DNA that is not associated with proteins
• a cell wall containing murein (peptidoglycan)

Just inside the cell wall is the cell-surface membrane, which controls movement into and out of the cell. The wall gives protection and helps prevent osmotic lysis.

For this topic, you do not need fine detail about the chemistry of murein or the mechanics of bacterial movement. You do need to remember the structural differences clearly.

Part 2: Plasmids, capsules and flagella

Many, but not all, prokaryotic cells also have extra structures.

Plasmids are small circular DNA molecules separate from the main circular chromosome. They can replicate independently and may carry useful genes, such as genes for antibiotic resistance.

Capsules are slimy outer layers around some bacterial cells. They protect the bacterium and can help bacteria stick to surfaces or to each other.

Flagella are long structures that rotate and allow movement. Some bacteria have one flagellum; others have more than one; some have none.

A quick way to compare prokaryotes with eukaryotes is:

Part 3: Viruses

Viruses are not cells. They are acellular and are treated as non-living. They are also much smaller than bacteria, typically about 20-300 nm across.

The AQA specification only needs three core structural features:

• genetic material - either DNA or RNA
• capsid - a protein coat around the genetic material
• attachment proteins - proteins on the outside that allow the virus to bind to a host cell

The attachment proteins matter because a virus can only attach to cells with complementary receptors. That is one reason viruses are often specific to particular hosts or particular cell types.

Some viruses also have a lipid envelope, but that is extra detail. The must-know structure is still: genetic material, capsid, attachment proteins.

Viruses are described as non-living because they do not have cytoplasm, ribosomes or independent metabolism. They can only reproduce inside living host cells.

Part 4: Prokaryotes versus viruses

Students often blur prokaryotic cells and viruses together because both are small. The safer comparison is this:

• a prokaryote is a living cell
• a virus is a non-living particle
• a prokaryote has cytoplasm, ribosomes, a cell-surface membrane and DNA
• a virus has genetic material inside a capsid and attachment proteins, but no cytoplasm or ribosomes

If an exam question asks for cell features, do not talk about viruses as though they are cells. If it asks for virus structure, keep your answer tight and structural: genetic material, capsid, attachment proteins.