IGCSE Biology / The variety of living organisms
Plants, animals, fungi, protoctists and bacteria
Edexcel International GCSE Biology (4BI1), chapter 2: The variety of living organisms, specification point 1.2, 1.3. Free lesson.
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Living organisms are grouped into five kingdoms by their cell features and how they feed. Recognising these features helps you identify an organism and explain why viruses are not classified in a kingdom.
How the five kingdoms are divided
The five kingdoms are plants, animals, fungi, protoctists and bacteria. The first four have cells with a nucleus, so they are eukaryotic. Bacteria have no nucleus, so they are prokaryotic. Each named organism belongs to one kingdom. A virus is not in any kingdom because it is not made of cells. Cell features therefore cannot be used to classify it.
What identifies plants and animals
Plants and animals are both multicellular, but their cells and feeding methods differ. Plants make their own food by photosynthesis, while animals take food in from outside and digest it inside their bodies. Plants have cellulose cell walls and chloroplasts. Examples include maize and peas. Animals have no cell walls or chloroplasts. They take in and digest food, and usually move about. Examples include insects and mammals.
What identifies fungi and protoctists
Fungi and protoctists can be separated by their cell features and how they feed. Fungi have chitin walls and absorb digested food. Protoctists are single cells with a nucleus, but they do not all feed in the same way. Fungi have cell walls made of chitin and form a body of thread-like hyphae. They release enzymes onto food, then absorb the dissolved products. This is saprotrophic nutrition. Mucor and yeast are examples. Protoctists are single-celled and have a nucleus. Some are plant-like and have chloroplasts, such as Chlorella. Others are animal-like, such as Amoeba.
What identifies bacteria
Bacteria are single-celled organisms with no nucleus. This absence of a nucleus is the feature that separates them from the other four kingdoms. Bacteria have a circular chromosome and often have plasmids. Different bacteria feed in different ways. Some photosynthesise. Lactobacillus is a named example.
How to identify an organism from a description
Work through the cell features first, then use the feeding method to confirm your choice. A nucleus, cell wall, chloroplasts and number of cells narrow down the possibilities.
Worked example: feeding on dead wood
An organism is multicellular, has cell walls, has no chloroplasts and feeds by releasing enzymes onto dead wood. It is a fungus. Its cell walls rule out animals, and the lack of chloroplasts rules out plants. Its feeding is saprotrophic nutrition: enzymes are released onto the food, then the digested products are absorbed.