Features of organisms
The main groups of living things, and why viruses sit outside them.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 1.3.1List the main features used to place organisms into kingdoms and groups.
- 1.3.2Describe the main groups of vertebrates, arthropods and plants.
6 minute read
Features of organisms
Classification groups organisms by shared features, and each group has a small set of defining characteristics worth learning precisely.
The five kingdoms
- Animals — multicellular, no cell walls, no chloroplasts, feed on organic material.
- Plants — multicellular, cellulose cell walls, chloroplasts, photosynthesise.
- Fungi — cell walls not made of cellulose, no chloroplasts, feed by saprophytic or parasitic nutrition, often multinucleate.
- Protoctists — mostly single-celled with a nucleus; some resemble plants, some animals.
- Prokaryotes (bacteria) — no nucleus, DNA loose in the cytoplasm, plus plasmids; cell wall not made of cellulose.
The dividing line that matters most: prokaryotes have no nucleus; everything else does.
Vertebrate groups
| Group | Key features |
|---|---|
| Fish | wet scales, gills, external fertilisation |
| Amphibians | moist permeable skin, larval stage in water |
| Reptiles | dry scaly skin, soft-shelled eggs on land |
| Birds | feathers, beak, hard-shelled eggs |
| Mammals | fur/hair, mammary glands, external ear |
Arthropods
All have a hard exoskeleton and jointed legs. The four groups are told apart by leg count and body sections: insects (6 legs, 3 body parts, usually wings), arachnids (8 legs, 2 body parts), crustaceans (many legs, chalky exoskeleton), myriapods (many segments, one or two pairs of legs each).
Viruses
Viruses are not placed in any kingdom and are not usually considered living. They have no cell structure — just genetic material in a protein coat — and can only reproduce inside a host cell. Applying MRS GREN shows why: they carry out none of the seven processes independently.
Think of it like this
Classification groups are nested boxes with a label on each: "has a backbone", "has fur". Viruses are the object that fits in none of the boxes, which is precisely why biologists set them aside rather than forcing them in.
Worked examples
Method, step by step
An organism is multicellular, has cell walls that are not cellulose, and feeds on dead organic matter. Which kingdom is it in, and why?
- 1Cell walls are present, so it is not an animal.
- 2The walls are not cellulose and there are no chloroplasts, so it is not a plant.
- 3Feeding on dead organic material is saprophytic nutrition, characteristic of fungi.
The fungi kingdom — it has non-cellulose cell walls, no chloroplasts, and feeds saprophytically.
Common misconceptions
- Placing viruses in a kingdom. They are excluded because they have no cell structure and cannot carry out life processes alone.
- Thinking fungi are plants. Fungi have no chloroplasts and do not photosynthesise — they feed on organic material.
- Confusing arachnids with insects. Spiders have 8 legs and 2 body sections, not 6 and 3.
In the exam
- For "which kingdom" questions, quote the decisive feature — presence of a nucleus, cell wall composition, or chloroplasts.
- When asked why viruses are not classed as living, refer to MRS GREN: they cannot respire, grow or reproduce independently.