Description
Most protists live in water, though some live in moist soil or even the human body. All protists are eukaryotic, meaning they all have a nuclues. Any organism that is not a plant, animal, of fungus is included in Kingdom Protista. It is basically a group of the ones who don't belong to a group.
- Animal-like Protists - also called protozoa (means "first animal") - heterotrophs
- Plant-like Protists - also called algae - autotrophs
- Fungus-like Protists - heterotrophs, decomposers, external digestion
Protists represent an important step in early evolution, evolving from prokaryotes and eventually giving rise to the entire line of eukaryotes. The first protists probably evolved 1.7 billion years ago, 2.3 billion years after the origin of life, from simple communities of prokaryotic cells. Membrane infolding was one of the defining processes in this evolution: in some prokaryotic cells, parts of the plama membrane folded into the cell to create the nuclear envelope and the other organelles of the endomembrane system. The second major step in the evolution of protists from bacteria was the process of endosymbiosis, which introduced the mitochondrion and chloroplast as organelles of eukaryotic cells. Small prokaryotic cells capable of cellular respiration or photosynthesis entered eukaryotic cells, either as parasites or indigestible food, and these prokaryotes evolved into mitochondria and chloroplasts as they developed a symbiotic relationship with the host cell. (Because mitochondria are present in all eukaryoptic cells, this process probably happened to mitochondria first.) As a result of these two processes, protists evolved as sucessful organisms. Eventually, colonial protists evolved into plants, fungi, and animals, eukaryotic kingdoms that came to dominate the earth.
Body Plan
Some are unicellular, like any protist in Division Rhizopoda, and some are multicellular, like sea lettuce.
Some are unicellular, like any protist in Division Rhizopoda, and some are multicellular, like sea lettuce.
Metabolism
Digestion
Some animal-like ones possess extracellular digestive systems, and these animal-like protists have complete digestive systems, as they dissolve complex organic compounds. Most protists possess intracellular digestion though.
- autotrophic - photosynthesis
- ex. Giant kelp
- heterotrophic - ingest food
- ex. Paramecium
- mixotrophic - combine photosynthesis and heterotrophic nutrition
- ex. Phytophlankton
Digestion
Some animal-like ones possess extracellular digestive systems, and these animal-like protists have complete digestive systems, as they dissolve complex organic compounds. Most protists possess intracellular digestion though.
Nervous: no specialized system.
Circulatory: no specialized system.
Respiratory
All are aerobic and have mitochondria to do cellular respiration.
All are aerobic and have mitochondria to do cellular respiration.
Reproduction
Some protists reproduce asexually, using binary fission, such as amoebas. Others reproduce sexually, using conjugation, such as Plasmodium falciparum.
Some protists reproduce asexually, using binary fission, such as amoebas. Others reproduce sexually, using conjugation, such as Plasmodium falciparum.
Examples
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| Diatoms |
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| Amobea |
6 Phyla
-Ciliophora
There are roughly 8,000 species of Ciliophora. Ji et. al. (2005) recently discovered two new speices of Ciliophora: Pseudovorticella clampi and Zoothamnium pararbuscula. There are numerous types of Ciliophora. Some of the major ones include Didinium, Paramecium, Stentor, Suctoria, and Vorticella. Ciliophora get their name based on their method of locomotion: they swim with cilia. Cilia are short, hairlike projections of cytoplasm composed of pairs of microtubules surrounded by cell membrane. They line the cell membrane. Cilia can also be used for obtaining food. In some species, they are fused into sheets, making them efficient at sweeping up food. Ciliophora are mainly freshwater organisms. Ciliophora often form relationships with bacteria. Some of these may be harmful to the Ciliophora, but others are not destructive. These relationships can help increase the environmental resiliency of the bacteria. In addition Ciliophora may benefit from these relationships. The research of Fujishima et. al. (2005) illustrates that the relationship Paramecium caudatum forms with the bacteriumHolospora obtusa helps the ciliate develop heat-shock resistance. Some species of Ciliophora are parasitic towards humans and other animals. Some Ciliophora species, such as Stentors, form symbiotic relationships with algae, giving them a green tint.
Vorticella
Paramecium
-Zoomastigina
These are holozoic or saprozoic flagellates. This division contains some organisms which are free-living, others which are symbionts, and yet others which are parasites. An example of a symbiotic member of this division is the protozoans which live in the gut of termites and digest cellulose in the wood the termites eat. An example of a parasitic form would be Trypanosoma gambiense, which causes African sleeping sickness and is spread by the bite of the tsetse fly.
Trypanosoma gambiense
-Euglenophyta
Euglenoids (or euglena) are one of the best-known groups of flagellates, commonly found in freshwater especially when it is rich in organic materials, with a few marine and endosymbiotic members. Most euglenids are unicellular. Many euglenids have chloroplasts and produce energy through photosynthesis, but others feed by phagocytosis or strictly by diffusion. They belong to the phylum Euglenophyta, and their cell structure is typical of that group.
Euglenids are believed to descend from an ancestor that took up green algae by secondary endosymbiosis.
Euglenids are distinguished mainly by the presence of a pellicle, which is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral microtubules. This varies from rigid to flexible, and gives the cell its shape, often giving it distinctive striations. In many euglenids the strips can slide past one another, causing an inching motion called metaboly. Otherwise they move using the flagella.
Euglena
-Chlorophyta
Chlorophyta is a division of green algae, informally called chlorophytes. The name is used in two very different senses so that care is needed to determine the use by a particular author. In older classification systems, it refers to a highly paraphyletic group of all the green algae within the green plants (Viridiplantae), and thus includes about 7,000 species of mostly aquatic photosynthetic eukaryotic organisms. Like the land plants (bryophytes and tracheophytes), green algae contain chlorophylls a and b, and store food as starch in their plastids.
In newer classifications, it refers to one of the two clades making up the Viridiplantae, which are the chlorophytes and the streptophytes or charophytes. In this sense it includes only about 4,300 species.
Chlorophyta
-Phaeophyta
The largest of the chromists are the Phaeophyta, the brown algae -- the largest brown algae may reach over 30 meters in length. The rockweed shown at left, Fucus distichous, visible at low tide at the Berkeley Marina in California, is somewhat smaller. Almost all phaeophytes are marine.
Phaeophytes, like most photosynthetic protists, have traditionally been classified as plants. However, phaeophytes are not closely related to land plants; their cells contain different pigments, such as chlorophyll c and fucoxanthin. They also lack the plasmodesmata and starch production of land plants and their relatives.
Like plants and many protists, brown algae undergo a complex life cycle involving alternation of generations. In this picture, you can see a diploid kelp with flat photosynthetic structures, the blades, branching from the stipe, or stalk. The "puffy" regions attached to the blades are receptacles, structures in which the gametes are produced.
Fucus distichous
-Rhodophyta
The red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species of mostly multicellular, marine algae, including many notable seaweeds. Other references indicate as many as 10,000 species; more detailed counts indicate ~4000 in ~600 genera.
The red algae form a distinct group characterized by the following attributes: eukaryotic cells without flagella and centrioles, using floridean starch as food reserve, with phycobiliproteins as accessory pigments (giving them their red color), and with chloroplasts lacking external endoplasmic reticulum and containing unstacked thylakoids. Most red algae are also multicellular, macroscopic, marine, and have sexual reproduction.
Many of the coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong here. Red algae such as dulse (Palmaria palmata) and laver are a traditional part of European and Asian cuisine and are used to make other products like agar, carrageenans and other food additives.
Palmaria palmata
