Kingdom Prokaryotae — the bacteria
The bacteria are single prokaryotic cells: they have no nucleus and no membrane-bound organelles, their DNA lying free as a single circular loop, sometimes with small extra rings called plasmids. A tough cell wall gives them shape, and many have a flagellum to move. They reproduce very fast by simple binary fission.
Bacteria are everywhere and enormously important — decomposers that recycle nutrients, nitrogen-fixers that feed plants, and the microbes behind yoghurt and antibiotics — though some cause disease.
Kingdom Protista
The protists are a mixed kingdom of mostly single-celled eukaryotes — they do have a nucleus and organelles. Some are animal-like (the Amoeba and Paramecium, which hunt and engulf food), and some are plant-like (algae, which photosynthesise). A few cause serious disease, such as the Plasmodium that causes malaria.
| Group | Cell type | Example |
|---|---|---|
| Prokaryotae | prokaryotic (no nucleus) | bacteria |
| Protista (animal-like) | eukaryotic | Amoeba |
| Protista (plant-like) | eukaryotic | algae |
Viruses — on the edge of life
Viruses are not placed in any kingdom, because they are not truly alive by the usual tests. A virus is just genetic material (DNA or RNA) inside a protein coat. It cannot respire, grow or reproduce on its own — it must invade a living host cell and hijack the cell's machinery to make copies of itself, usually destroying the host cell. Viruses cause diseases such as flu, measles and COVID-19, and antibiotics do not affect them.
Why viruses are not "alive"
A virus shows none of the characteristics of life on its own — no respiration, no growth, no independent reproduction. It only replicates by taking over a host cell, which is why it sits outside the five kingdoms.
A worked example
Give two structural differences between a bacterial cell and a human (animal) cell, and explain why a virus is not classed as a living organism.
- A bacterial cell has no nucleus (its DNA is a free circular loop); a human cell has a nucleus.Bacteria are prokaryotic, human cells eukaryotic.
- A bacterial cell has no membrane-bound organelles (e.g. mitochondria); a human cell does.Another prokaryote/eukaryote difference.
- A virus cannot respire, grow or reproduce by itself.It lacks the machinery of a cell.
- It can only replicate inside a host cell, so it does not meet the criteria for life.Independent life processes are needed to be called living.
Bacteria lack a nucleus and membrane-bound organelles (unlike human cells); a virus is non-living because it cannot carry out life processes without hijacking a host cell.
How bacteria cause — and fight — disease
Some bacteria are pathogens: they invade the body, multiply rapidly and often release poisons (toxins) that damage tissues, causing diseases like cholera, tuberculosis and tetanus. The body defends itself with white blood cells, and doctors treat bacterial infection with antibiotics, which kill bacteria or stop them dividing. Overusing antibiotics, however, breeds resistant strains — so they must be used carefully. Antibiotics have no effect on viral diseases.
Bacteria that help us
Most bacteria are harmless or useful. As decomposers they recycle nutrients from dead matter; nitrogen-fixing bacteria in the soil and in root nodules turn nitrogen gas into compounds plants can use; and in industry bacteria make yoghurt and cheese, antibiotics, and — through genetic engineering — medicines such as insulin. Their fast growth and simple needs make them ideal microscopic factories.
Most microbes are friends
Only a minority of bacteria are pathogens. Far more are essential — recycling nutrients, fixing nitrogen for plants, and making our food and medicines.