The big idea: Eukaryotes (organisms whose cells have a nucleus) are sorted into four kingdoms:
- Plants — make their own food. - Animals — eat other organisms. - Fungi — feed on dead or living matter by digesting it outside the body. - Protists — a mixed 'leftover' group, mostly single-celled.
We decide which kingdom an organism belongs to using its cell features and its way of feeding.
- Eukaryote
- An organism whose cells have a nucleus and membrane-bound organelles.
- Kingdom
- One of the large top-level groups used to classify living things.
- Cell wall
- A tough outer layer outside the cell membrane that supports and protects the cell.
- Chloroplast
- The organelle where photosynthesis happens (it traps light to make food).
Plant wall vs fungal wall: Plants and fungi both have a cell wall, but they are made of different materials: plant walls are cellulose, fungal walls are chitin.
So 'has a cell wall' alone does not tell you the kingdom — you must also check for chloroplasts and how the organism feeds.
The four eukaryote kingdoms at a glance — each tagged with its cell-wall material, whether it has chloroplasts, and how it feeds.
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Each kingdom has a typical set of cell features. In the exam you are often given a table of features (cell wall, chloroplasts, single- or many-celled) and asked which kingdom each organism belongs to.
Read the table below from left to right, like a checklist.
| Kingdom | Cell wall? | Chloroplasts? | Body plan | How it feeds |
|---|---|---|---|---|
| Plants | yes (cellulose) | yes | multicellular | autotroph (makes food) |
| Animals | no | no | multicellular | heterotroph (eats food) |
| Fungi | yes (chitin) | no | mostly multicellular | heterotroph (saprotroph) |
| Protists | some do, some don't | some do, some don't | many are unicellular | varies (auto, hetero or mixo) |
Plants vs Animals
- Plants: cell wall (cellulose) and chloroplasts → make their own food
- Animals: no cell wall, no chloroplasts → must eat food
- Both are multicellular (made of many cells)
Fungi vs Protists
- Fungi: cell wall of chitin, no chloroplasts → feed on dead/living matter
- Protists: the odd-one-out group — features vary
- Most protists are unicellular (a single cell does everything)
Protists are the 'everything else' kingdom: Protists are grouped together mainly because they do not fit the plant, animal or fungus kingdoms.
Some have chloroplasts (like algae), some hunt other cells (like amoeba), and some do both — so you cannot describe 'a typical protist'.
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Three ways to get food: An organism's mode of nutrition is how it obtains its food (carbon compounds). There are three modes:
- Autotroph — makes its own food (e.g. by photosynthesis). - Heterotroph — takes in ready-made food from other organisms. - Mixotroph — can do both, depending on conditions.
- Autotroph
- An organism that makes its own organic food from simple inorganic molecules (e.g. plants photosynthesising).
- Heterotroph
- An organism that obtains food by taking in organic molecules made by other organisms.
- Mixotroph
- An organism that can feed as an autotroph OR a heterotroph, depending on conditions.
- Holozoic nutrition
- Heterotrophic feeding where food is taken INTO the body and digested internally (as animals do).
- Saprotrophic nutrition
- Heterotrophic feeding where enzymes are released ONTO dead matter and digested OUTSIDE the body, then the products are absorbed (as fungi do).
| Mode of nutrition | What it does | Typical example |
|---|---|---|
| Autotroph | makes its own food | a plant photosynthesising |
| Heterotroph (holozoic) | eats food and digests it inside | an animal eating a meal |
| Heterotroph (saprotrophic) | digests dead matter outside, then absorbs it | a fungus on rotting wood |
| Mixotroph | makes food AND eats food | some protists (e.g. Euglena) |
Holozoic vs saprotrophic — where digestion happens: Both are heterotrophic, but the digestion site differs:
- Holozoic (animals): food is taken inside the body and digested internally. - Saprotrophic (fungi): enzymes are released outside onto the food, which is digested externally, then the small products are absorbed.
How this is tested: Paper 1A often gives a table of cell features and asks you to assign each organism to a kingdom, or to identify a mode of nutrition (autotroph, heterotroph, mixotroph; holozoic vs saprotrophic).
Paper 1B / Paper 2 can ask you to outline why an organism is a mixotroph, or to describe holozoic or saprotrophic nutrition in a sentence or two.
IB-style question — assign an organism to its kingdom
A biologist examines an unknown organism. Its cells have a cell wall made of chitin but no chloroplasts, and it feeds by releasing enzymes onto a dead log and absorbing the digested products. State the kingdom this organism belongs to and the term for its mode of nutrition. [2]
How to score both marks
- Use the cell features. A chitin cell wall with no chloroplasts points to the fungi kingdom (plants would have cellulose + chloroplasts; animals have no wall).
- Use the feeding method. Releasing enzymes onto dead matter and absorbing the products = digestion outside the body = saprotrophic nutrition.
- Answer the command term. Kingdom: fungi. Mode of nutrition: saprotrophic (a form of heterotrophic nutrition).
Final answer
Kingdom: fungi. Mode of nutrition: saprotrophic (heterotrophic, external digestion of dead matter).
Why each feature was decisive: Chitin wall rules out animals (no wall) and protists. No chloroplasts rules out plants. External digestion of dead matter is the signature of fungi — and the exact word for that feeding is saprotrophic.