Cambridge O Level Biology · Syllabus 5090 · Classification
Kingdom
What is Kingdom?
A kingdom is the largest group used in this classification system. Cambridge O Level Biology recognises five kingdoms — animal, plant, fungus, prokaryote and protoctist — separated by cell features such as the presence and composition of a cell wall, the presence of a nucleus and of chloroplasts, together with the way the organism obtains its food and how its body is organised.
This definition is part of the Classification chapter in Cambridge O Level Biology.
Kingdom in context
Classification is the placing of living organisms into groups on the basis of the features they share, so that organisms with many features in common sit in the same group. Cambridge O Level Biology (5090) Chapter 2 covers four things: the definition of a species and the binomial naming system, dichotomous keys, the five kingdoms (animal, plant, fungus, prokaryote and protoctist), and the observable features that separate the vertebrate groups, the arthropod groups, ferns from flowering plants and monocotyledons from dicotyledons. Viruses sit apart from all five kingdoms because they have no cellular structure.
Common mistakes with Kingdom
- 6. Putting viruses in a kingdom Fix Viruses are not placed in any of the five kingdoms. State their features — genetic material in a protein coat — and that they replicate only inside living cells.
- 7. "All plant cells contain chloroplasts" Fix Chloroplasts are found in the photosynthetic cells of a plant. Root cells have none. The kingdom feature is "chloroplasts in photosynthetic cells", not "every cell green".
- "A fungus is a plant that has lost its chlorophyll." Repair A fungus is its own kingdom. Its cell walls are not made of cellulose, it has no chloroplasts, and it feeds saprotrophically by releasing enzymes onto its food and absorbing the products. Nothing about it is a modified plant.
- "Viruses are in the prokaryote kingdom because they are tiny and have no nucleus." Repair Viruses are not placed in any kingdom. A prokaryote is a complete cell with cytoplasm and a cell wall; a virus is genetic material in a protein coat with no cell structure at all, and it can replicate only inside a living cell.
- "Every plant cell has chloroplasts." Repair Chloroplasts are present in the photosynthetic cells of a plant. Root cells have none. Write the kingdom feature the way the syllabus writes it.
Questions students ask about Kingdom
What are the five kingdoms and how are they separated?
The five kingdoms are animal, plant, fungus, prokaryote and protoctist. They are separated by cell features: whether there is a nucleus, whether there is a cell wall and what it is made of, whether chloroplasts are present in the photosynthetic cells, how the organism obtains its food and how its body is organised. Two questions carry most of the marks: "Is there a nucleus?" separates prokaryotes from everything else, and "Is there a cell wall, and what is it made of?" separates animals, plants and fungi.
Why are viruses not placed in any of the five kingdoms?
A virus is genetic material inside a protein coat, with no cellular structure at all — no cytoplasm, no cell wall and no nucleus. Because it is not a cell it cannot belong to any kingdom, and it cannot replicate on its own: it can make new virus particles only inside a living host cell, using that cell's machinery. That is also why a virus will not grow on nutrient agar, which contains nutrients but no living cells.