What are protists?
Protists are a group of all eukaryotes that are not fungi, plants, and animals. As a result, it is a very diverse group of organisms and exhibits a wide range of diversity in shape and size. The boundary of this kingdom is not very well defined as they contain both unicellular and multicellular organisms. Scientists speculate that kingdom protista form a link between plants, animals, and fungi as these kingdoms diverge from a common protist-like ancestor. Therefore organisms in the protist kingdom are thought to be the first eukaryotic life form and common ancestors of plants, animals, and fungi.
Characteristics of protists
- All protists are eukaryotes means they have well-defined nuclei and membrane-bound organelles.
- Protists vary greatly in shape and size. some are single-celled some are multicellular while some protists can be found as colonies of cells.
- Protists vary greatly in habitat, they live in both fresh and marine water, damp terrestrial environments, or even as parasites.
- They can reproduce both sexually and asexually.
- Protists can be autotrophic, heterotrophic, or saprotrophic.
- Protists can exhibit a symbiotic relationship with other organisms.
Classification of protists
Chrysophytes
This group of protists includes diatoms and golden algae(desmids). They are found in both fresh and marine waters. Most of them are photosynthetic. Diatoms have a cell wall made up of two thin overlapping shells that fit together their cell wall is embedded with silica.
Dinoflagellates
Dinoflagellates are a group of protists that are mostly marine and photosynthetic. They can be of color red, yellow, blue, or green depending on the color of pigment present in the side of their cell. Organisms of this group have stiff cellulose plates on the outer surface. Most of them have two flagella, one lies longitudinally and the other transversally.
Euglenoids
These are mostly freshwater organisms found in stagnant(non-flowing) water. They do not have a cell wall, instead have a protein-rich layer of celled pellicle which makes their body flexible. They are photosynthetic in the presence of sunlight and behave like heterotrophs in the absence of sunlight.
Slime Moulds
Slime molds are saprotrophic protists that feed on dead and decaying under suitable conditions they form an aggregation called plasmodium which may grow over several feet, under unfavourable conditions plasmodium differentiates and forms fruiting bodies bearing spores.
Protozoans
All protozoans are heterotrophs and live as predators or parasites. There are four major groups of protozoa
- Amoeboid Protozoans:- These organisms live in freshwater, seawater, and moist soil. They have pseudopodia (false feet) to catch their prey. eg. Amoeba and Entamoeba
- Flagellated Protozoans:- Members of this group possess flagella. They are either free-living or parasitic. Trypanosoma species can cause diseases like sleeping sickness.
- Ciliated Protozoans:- These are aquatic and actively moving due to the presence of cilia on their body. They exhibit nuclear dimorphism. eg. Plasmodium
- Sporozonas:- This includes diverse organisms that have an infections spore-like stage on their life cycle. eg. Plasmodium(malaria causing protozoa).