APBio: Protist Word Link
Protists are very diverse. The can be photoautotrophs, heterotrophs, or mixotrophs (a
combination of photosynthesis and heterotrophic nutrition). Protists can either use a flagella, which is a
long whip-like structure, or cilia, which is a short hair-like structure, for motility. The Protista Kingdom
classification of protists crumbled because scientists realized that some protists are more similar to
organisms in the Animal Kingdom, Plant Kingdom, Monera Kingdom, or the Fungi Kingdom than to other
protists. Hence, unlike bacteria and archaea bacteria, which are prokaryotes, protists are essentially
eukaryotes that can be divided into three groups: photosynthetic plant like protists (algae), ingestive
animal like protists (protozoans), and absorptive fungi like protists.
The diversity of protists can be explained by the Endosymbiotic Hypothesis. This says that
certain unicellular organisms engulfed other cells, which became endosymbionts and ultimately
organelles in the host cell. This process of endosymbiosis is very different from phagocytosis. Biologists
relate the origin of plastids with the endosymbiosis of a particular endosymbiont called cyanobacteria.
This plastid-bearing lineage eventually gave rise to green algae and red algae, which then again became
endosymbionts in the process of secondary endosymbiosis.
Red algae can be classified under Rhodophyta. Green algae is divided into two main groups:
chlorophytes and charophyceans. Chlorophyta includes protists such as the Ulva (multicellular), Volvox
(colonial), Spyrogyra (conjugating algae), and Chlamydomonas (unicellular). The charophyceans are
ancestors of land plants.
Under the major clade of Diplomonadida and Parabasala, protists have a modified mitochondria
and are thus anaerobic.
Under the major clade of Alveolata, the protists called called alveolates are characterized by
membraneous sacs under the plasma membrane. There are a few important protists in this major clade.
Paramecium and Stentor are protists that are ciliates, which are protists that use cilia to move and feed,
that are classified under Ciliaphora. A distinctive feature of ciliates is having both a macronucleus and a
micronucleus. Plasmodium, a parasite causing Malaria through mosquito populations of Anopheles, is a
protist that is an apicomplexan and is classified under Apicomplexa. Parasites such as Plasmodium
spread through their host as tiny infectious cells called sporozoites. Apicomplexans are all parasitic.
Pfesteria and Ceratium are dinoflagellates that are classified under Dinoflagellata. Dinoflagellate blooms
cause red tides. Furthermore, dinoflagellates are spectacularly bioluminescent.
Under the major clade of Euglenozoa, euglenozoans have a unique characteristic of having a
spiral or crystalline rod inside their flagella. Euglenozoa has two subgroups of Kinetoplastida
(kinetoplastids) and Euglenophyta (euglenids). Trypanosoma is a kinetoplastid that causes African
sleeping sickness that is spread to humans by a tetse fly. Euglena is a euglenid that has paramylon as a
storage molecule.
In the major clade of Stramenophila, stramenophiles have hairy and smooth flagella.
Stramenophila has four subgroups: Oomycota (oomycetes), Bascillariophyta (diatoms), Chrysophyta
(golden algae), and Phaeophyta (brown algae). Oomycetes include water molds, white rusts, and downy
mildews. An oomycetes called Phytophthora caused potato blight. Diatoms are glass-like structures that
have cell walls made from silica (silica shells). These silica shells form an earth-like substance when they
are fossilized. Diatoms are also a major component of phytoplankton both in the ocean and in lakes.
Golden algae are usually biflagellated, with both flagella attached near one end of the cell. Brown algae
include many of the species most commonly called seaweed. Seaweed is the largest, most complex
protist. Seaweed is used as a thickening agent in ice cream and other foods. Brown algae can grow at
great depths. Kelp, a brown algae, can be as long as 60 m in some waters.
In the major clade of Cercozoa (cercozoans), protists called amoebas with thread-like
pseudopodia to move and feed are present. The two subgroups are Foraminfera (forarms) and
Radiolaria (radiolarians). Forarms have a porous shell while radiolarians have pseudopodia radiating
from a central body.
In the major clade of Amoebozoa (amoebozoans), amoebas with lobe shaped pseudopodia exist.
The four subgroups are Gymnamoeba (gymnamoebas), Entamoeba (entamoebas), Myxogastrida
(plasmodial slime molds), and Dictyostelida (cellular slime molds).