Eukaryotic cells
Nucleus, endomembranes, usually mitochondria or remnants (mitosomes, hydrogenosomes).
Protozoa is a useful classroom word for animal-like single cells: amoebae that crawl, flagellates that swim, ciliates that row with thousands of cilia. Modern trees split them across several eukaryotic kingdoms.
They are not bacteria, not yeasts, not viruses. A typical cell is 10–50 µm, with a nucleus, organelles and — in parasites — often complex host cycles.
Traits
Motility styles
Pseudopodia, flagella or cilia. The style is a teaching label, not a single family crest.
No typical fungal wall
Many protozoa are naked or have pellicles and tests, not chitin walls like fungi.
Teaching grade
Amoebozoa, excavates, alveolates and others do not form one clade. The word still organises clinics and schools.
Free-living majority
Ciliates in a hay infusion were Leeuwenhoek's delight. Predators of bacteria, not villains.
Parasite minority
Plasmodium, Trypanosoma, Giardia, Entamoeba — famous because they meet people, not because they are typical.
Malaria's Plasmodium made protozoa medically famous. Most species are free-living in soil and water and never infect a person.
Keep exploring
More in Tree of microbes
Bacteria
Single-celled prokaryotes with cell walls ? the oldest widespread life form, and both partners and pathogens.
Familiarity 95Viruses
Packets of DNA or RNA that are not cells — they replicate only inside living hosts.
Familiarity 96Fungi
A eukaryotic kingdom of yeasts, moulds and mushrooms — chitin walls, external digestion, partners of plants.
Familiarity 80Archaea
A prokaryotic domain that is not bacteria — methanogens, extremophiles and no known human pathogens.
Familiarity 35Bacteriophages
Viruses that infect bacteria — Earth's most abundant biological entities, and a concept-level idea in medicine.
Familiarity 42