Protozoans are heterotrophic protists, predators or parasites, and they are thought to be the ancestors of animals. There are four groups. Amoeboid protozoans move by pseudopodia, for example Amoeba. Flagellated protozoans have flagella, for example Trypanosoma, which causes sleeping sickness. Ciliated protozoans have cilia and a gullet, for example Paramecium. Sporozoans have an infectious spore-like stage, for example Plasmodium, which causes malaria.
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The animal-like protists
The protozoans are protists that live like small animals. All of them are heterotrophs: predators that hunt other small organisms, or parasites that live in another organism. Biologists think that they are the ancestors of animals.
There are four major groups.
The four groups of protozoans. Amoeboid, for example Amoeba: an irregular cell that moves by pseudopodia, which are false feet. Flagellated, for example Trypanosoma: a long thin cell with a flagellum. Ciliated, for example Paramecium: a slipper-shaped cell covered with cilia, with a gullet. Sporozoan, for example Plasmodium: a cell inside a red blood cell, and spore-like cells outside, which are the infectious stage.
Amoeboid protozoans
They live in fresh water, in sea water or in moist soil.
They move and catch prey with pseudopodia (false feet): the cell pushes out a part of its body.
Marine forms have shells of silica on their surface.
Some are parasites, for example Entamoeba.
The freshwater Amoeba has a contractile vacuole, which pumps out the extra water.
Flagellated protozoans
They are free-living or parasitic, and they have flagella.
Parasitic forms cause diseases, for example sleeping sickness, caused by Trypanosoma.
Ciliated protozoans
They are aquatic and move actively, with thousands of cilia.
They have a cavity, the gullet, that opens to the outside of the cell.
Rows of cilia beat together and steer water with food into the gullet.
The example is Paramecium.
Sporozoans
All have an infectious spore-like stage in their life cycle.
The best-known is Plasmodium, the malarial parasite, which has caused great suffering.
Group
How it is known
Example
Amoeboid
pseudopodia
Amoeba
Flagellated
flagella
Trypanosoma
Ciliated
cilia and gullet
Paramecium
Sporozoan
infectious spore-like stage
Plasmodium
How fast do they move?
You should find that Paramecium crosses 2mm in 2 seconds, while Amoeba takes about 17 minutes. Thousands of cilia beat far faster than a cell can flow.
Try it in Python
This program keeps the four groups in a dictionary, and finds the groups that move with a thread-like organ.
Practise with new numbers
Common mistakes
Calling protozoans autotrophs. All of them are heterotrophs.
Mixing up the two parasites.Plasmodium: malaria. Trypanosoma: sleeping sickness.
Giving Amoeba cilia. It moves by pseudopodia.
Thinking the gullet is a mouth with teeth. It is a cavity into which cilia sweep food.
Remember these
Protozoans are heterotrophic protists: predators or parasites, thought to be the ancestors of animals.
Amoeboid: pseudopodia. Flagellated: flagella. Ciliated: cilia and a gullet. Sporozoan: a spore-like infectious stage.