Topic 1.3 · Supplement
The Five Kingdoms
The five-kingdom system used in this syllabus places every organism into one of five kingdoms: animal, plant, fungus, prokaryote or protoctist. The kingdoms differ in major features including how their cells are organised and how they obtain nutrients.
Animal kingdom
Animals are multicellular, made of cells containing nuclei, without cell walls, without chloroplasts, and dependent on other organisms for organic nutrients. Many are also capable of coordinated movement from place to place.
Plant kingdom
Plants are multicellular, made of cells containing nuclei, surrounded by cellulose cell walls, and able to photosynthesise in cells that contain chloroplasts. Not every plant cell has chloroplasts — a root cell may lack them entirely — but the organism as a whole is still classified as a plant because classification depends on its overall characteristics, not on any single cell.
Fungus kingdom
Fungi include mushrooms, moulds and yeasts. Their cells contain nuclei, have cell walls, and lack chloroplasts — fungi do not photosynthesise. Many fungi are built from microscopic threads called hyphae, and a network of hyphae forms a mycelium. Fungi obtain organic nutrients by absorption: they release digestive enzymes onto material outside the organism, breaking large molecules down into smaller soluble ones that can then be absorbed. When those nutrients come from dead or decaying organic material, the process is called saprotrophic nutrition. Some fungi, including yeast, are unicellular.
Prokaryote kingdom
Prokaryotes are unicellular organisms whose cells lack a membrane-bound nucleus, so their genetic material isn’t enclosed inside one. Bacteria are prokaryotes. That absence of a membrane-bound nucleus is one of the clearest features separating this kingdom from every other kingdom here.
Protoctist kingdom
Protoctists are eukaryotic organisms— their cells do contain nuclei — and many are unicellular. The group is genuinely diverse: some protoctists contain chloroplasts and photosynthesise, others don’t and obtain organic nutrients from other sources instead. In effect, protoctists are the organisms that don’t fit neatly into the animal, plant or fungus kingdoms of this system.
Comparing the five kingdoms
| Kingdom | Organisation | Cell wall | Chloroplasts | Nutrition |
|---|---|---|---|---|
| Animal | Multicellular | Absent | Absent | Organic nutrients obtained from other organisms |
| Plant | Multicellular | Cellulose wall present | Present in photosynthetic cells | Photosynthesis |
| Fungus | Usually multicellular; some unicellular | Present | Absent | Organic nutrients absorbed |
| Prokaryote | Unicellular | Usually present | No chloroplasts | Varies |
| Protoctist | Many are unicellular | Varies | Present in some | Varies |
The five kingdoms are recognised from combinations of characteristics, never from one feature used in isolation — exactly the same principle running through animals and plants, vertebrate groups and arthropod groups elsewhere in this chapter.