Major Groups of Protista
Protozoa: Animal –like Protists
All protozoans are unicellular. Most ingest their food by endocytosis. A summary or protozoan diversity is given in Table 7.1
Table 7.1 some groups of protozoa
|Amoebae||Unicellular, no definite shape||pseudopods||Amoeba, entamoeba,|
|Zooflagellates||Unicellular,some colonial||One or more flagella||Trypanosome, Eugleria,|
|Ciliates||Unicellular||Cilia||Paramecium, vorticella, stentor.|
This group includes all free living freshwater, marine and soil amoebae as well as those that are parasites of animals. Amoebae lack flagella and move by forming specialized cytoplamic projections called pseudopodia (false feet). (fig. 7.2).
These protest are mostly unicellular (a few are colonial) organisms with spherical or elongated bodies with a single central nucleus. They possess from one to many long, whip-like flagella that enable them to move. Flagellates move rapidly, pulling themselves forward by lashing flexible flagella. That are usually located at the anterior end.
Flagellates obtain their food either by ingesting living or dead organisms or by absorbing nutrients from dead or decomposing organic matter. They may be free-living, symbionts or parasites. Trichonymphas are complex, specialized flagellates with many flagella which live as symbionts in the guts of termites and help in the digestion of dry wood (fig. 7.3a)
Fig. 7.3Zooflagellates (a) Trichonympha has hundreds of flagella (b) Trypanosome causes sleeping sickness.
Parasitic flagellates cause diseases. For example Trypanosoma is a human parasite causing African sleeping sickness. It is transmitted by the bits of infected tsetse fly (fig. 7.3b)
Choanoflagellates are sessile marine or freshwater flagellates which are attached by a stalk and their single flagellum is surrounded by a delicate collar. They are of special interest because of their striking resemblance to collar cells in sponges (fig. 7.4).
Fig. 7.4A colonial choanoflagellste(c) Ciliates
ciliates are unicellular organisms with a flexible outer covering called a pellicle that gives them a definite but changeable shape. In paramecium, the surface of the cell is covered with several thousand fine, short, their-like structures called cilia. The cilia beat in such a precisely coordinated fashion that the organism can go forward. Can also go back and turn around.
Some ciliates are sessile and remain attached to a rock or other surface. Their cilia set up water currents that draw food towards them. Most ciliates ingest bacteria or other tiny protists.
Fig. 7.5(a) paramecium, conjugating individuals (b) stentor, a sessile ciliate.
Water regulation in freshwater ciliates is controlled by special organelles called contractile vacuoles. Ciliates differ from other protozans in having two kinds of nuclei. One or more small diploid micronuclei that function in sexual process, and a large, polyploidy macronucleus that controls cell metabolism and growth. Most ciliates are capable of a sexual process called conjugation. During conjugation two individuals come together and exchange genetic material (Fig. 7.5)
(d) Foraminiferans and Actinopods
These marine protozoans produce shells (ortests). Tests of foraminifera are made fo calcium whereas those of actinopods are made of silica. The shells or tests contain pores through which yctoplasmic projections can be extended. These cytoplasmic projections form a sticky, interconnected net that entangles prey. Dead foraminiferans sink to the bottom of the ocean where their shells form a grey mud that is gradulally transformed into chalk. Formaminiferans of the past have created vast limestone deposits.
Fig. 7.6(a) foraminiferan test have (a) beautiful geometric patterns and (b) pores through which cytoplasmic projections are extended (c) radiolarians are actinopods with glassy shells.