Protozoa: animal-like protists

Biology · Kingdom Protista. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Kingdom Protista holds the eukaryotes that are not plants, animals or fungi. Most are single-celled and most live in water. John Hogg proposed the kingdom Protoctista in about 1860 and Ernst Haeckel suggested the name Protista in 1866. The kingdom has animal-like, plant-like and fungus-like members.

The animal-like protists are the protozoa. They are heterotrophs that take in food and digest it in food vacuoles. They are grouped mainly by how they move: amoebas by pseudopodia, zooflagellates by flagella, ciliates by cilia. Apicomplexans have no organelles for movement.

Several protozoa cause serious human diseases, so learn each parasite with its disease and its carrier.

Rules to remember

Amoebas: move and feed with pseudopodia. Entamoeba histolytica causes amoebic dysentery.
Zooflagellates: Trypanosoma causes African sleeping sickness, spread by the tsetse fly. Trichonympha lives in the gut of termites and digests cellulose.
Ciliates (Paramecium, Vorticella, Stentor): cilia, a macronucleus for everyday metabolism and growth, a micronucleus for conjugation.
Contractile vacuoles of freshwater protozoa pump out the water that enters by osmosis.
Foraminiferans: shells (tests) of calcium carbonate with pores; their remains form chalk and limestone.
Actinopods (e.g. radiolarians): glassy silica shells and thin axopodia.
Apicomplexans: all parasites, form spores, no locomotory organelles in adults. Plasmodium causes malaria and is spread by the female Anopheles mosquito.

Common mistake

Students swap the shells: foraminiferans have calcium carbonate, actinopods and diatoms have silica. Also, malaria is caused by Plasmodium; the mosquito only carries it.

Worked example

A single-celled organism from a pond is covered with rows of short hair-like structures, has one large and one small nucleus and takes in bacteria through an oral groove. To which group does it belong, and what does the small nucleus do?

Short hair-like structures all over the cell are cilia, so it moves by cilia.
Two kinds of nuclei (macronucleus and micronucleus) are found only in ciliates.
The macronucleus runs daily metabolism; the micronucleus is used in conjugation.

Answer: It is a ciliate such as Paramecium; the small micronucleus takes part in sexual reproduction (conjugation).

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