Look at a drop of pond water under a microscope and you will likely see protists: mostly single cells, but each with a nucleus. They are in the domain Eukarya, with the fungi, plants and animals, but they belong to none of those kingdoms.
Key idea
Protists are eukaryotes that are not plants, animals or fungi. They are often grouped by how they get food: animal-like (amoebas, paramecia hunt and move), plant-like (algae, diatoms make food by photosynthesis) and fungus-like (water molds and slime molds absorb food).
Why it works
Protists are a "leftover" group: everything with a nucleus that is not a plant, animal or fungus. That is why they vary so much. Some are as big as a forest (kelp), most are single cells, and many are much larger than bacteria, so you can often see them at 100×.
Worked examples
Worked example: Bacterium or protist?
A single cell from pond water has a nucleus. Is it a bacterium?
- Bacteria never have a nucleus.
Answer: No. It is most likely a protist.
Worked example: How does it move?
How do an amoeba, a paramecium and a euglena move?
- Amoeba: pseudopods ("false feet") that flow forward.
- Paramecium: thousands of tiny hairs called cilia.
- Euglena: one whip-like flagellum.
Answer: False feet, cilia, flagellum.
Common mistake
Thinking every microscopic organism is a bacterium. Protists have a nucleus and are often much larger. Check: does it have a nucleus? Then it is not a bacterium.