We explain what the protista kingdom is, its characteristics and how it is classified. Also, what is its nutrition, reproduction and examples.
What is the protist kingdom?
The kingdom protista, also called protoctista, is one of the groups in which biology classifies eukaryotic living beings that cannot be classified as animals, plants or fungi.
The kingdom protista is a paraphyletic group (it does not contain all the descendants of a common ancestor) and includes a very diverse set of organisms. usually unicellular or simple multicellular that do not form tissues, both autotrophs and heterotrophs. Their enormous variety makes it difficult to characterize them, except in the common features of every eukaryotic being, that is, possessing cells with a defined cell nucleus.
The existence of a protist kingdom It was proposed in 1969 in the theory of the five kingdoms of life but it is currently considered a disused term and its members tend to be classified within the other branches of eukaryotic life.
The word protista comes from Greek and means “primordial” or “first of the first.” Protoctista, similarly, translates as “first creatures.”
See also: Kingdom Monera
Characteristics of protists
The kingdom protista is not a monophyletic group, that is, all organisms included in it did not evolve from a common ancestor. In the biological classification that attributes them the status of kingdom (that of Robert Whittaker in 1969), the characteristics that group protists are that they are simple unicellular or multicellular eukaryotic organisms that do not form any type of tissue.
Except for their relative evolutionary simplicity, there is not much in common between the different protists, which have different models of nutrition, reproduction, locomotion and cellular structures.
Classification of protists
The kingdom of protists is traditionally divided into very different supergroups:
- Archeplastids. They have plastids surrounded by an outer and inner membrane. This group includes the most primitive green and red algae, precursors of plant life, especially terrestrial life.
- Chromalveolated. They are a very diverse group that could have originated as a result of secondary endosymbiosis, in which an ancestral cell engulfed a red alga (the process that occurs when a eukaryotic cell envelops and absorbs another eukaryotic cell is called secondary endosymbiosis). This group includes alveolates, which have cortical alveoli, that is, flattened vesicles that form a flexible film that supports the plasma membrane. Within the alveolates, dinoflagellates, apicomplexans and ciliates are grouped; and stramenopiles, which possess motile cells with two flagella, one of which has small hair-like projections extending from the axis. This group includes oomycetes, diatoms, golden algae and brown algae.
- Rizarios. They are various organisms of the amoeboid or flagellate type that frequently have testae (shells). They include foraminifera, which have hard shells through which cytoplasmic projections (pseudopods) extend; and to actinopods, which have endoskeletons (internal shells) through which axopods (filamentous pseudopods) extend.
- Excavated. They are characterized by having atypical, highly modified mitochondria. They are organisms formerly classified as flagellates, which have a central feeding groove (they are heterotrophic), although many can present chlorophyll as a result of endosymbiosis with green algae. They include diplomonadids, parabasalids, euglenoids and trypanosomes.
- Unicontos. They have cells that have a single flagellum or are amoebas without flagella. Within this group are the amoebozoans, characterized by forming pseudopodia (“fingers”) with extensions of their cytoplasm; and opisthokonts, which are devoid of flagella or have a single posterior flagellum in cells with motility. It is believed that this group would have given rise to the kingdoms animalia and fungi.
Nutrition of protists
The protists can have autotrophic or heterotrophic metabolisms depending on whether they have chloroplasts (chlorophyll) to carry out photosynthesis or if, on the other hand, they lack them and must feed on the surrounding organic matter (through osmosis or ingestion or phagocytosis).
Many of them have both nutrition mechanisms simultaneously, and some lead a parasitic existence: They enter multicellular organisms and feed on them, causing diseases.
However, protists They are originally aerobic (they use oxygen for their metabolic processes), with the exception of those that evolved to live in environments where oxygen is scarce.
Reproduction of protists
The reproduction of protists can be both sexual and asexual and sometimes the same species can alternate between one model and another, according to environmental conditions.
Sexual reproduction occurs through the generation of gametes and cell fusion, while asexual reproduction occurs through cell fission and mitosis.
Importance of protists
Protists are a diverse and difficult group to classify, but fundamental to the emergence of life as we know it. They were the first eukaryotic organisms to emerge in the primitive seas of the Earth and, from them, life took various evolutionary paths, and gave rise to the rest of the kingdoms of eukaryotes: plants, animals and fungi.
The protists constitute a previous and relatively common step to all these forms of eukaryotic life including the human one, and its study is also the study of eukaryogenesis, that is, the appearance of the cell nucleus in the evolutionary history of primitive prokaryotes.
Examples of protists
Some known protists are the following:
- Paramecia. They are unicellular, ciliated, free-living organisms, abundant in stagnant waters and puddles.
- amoebas. Called amoebas or amoebae, they are unicellular protists that move and feed by generating pseudopodia or “fingers” with their cytoplasm, which gives them a changing and dispersed shape. They can be free-living or parasitic-living.
- Plasmodium. They are a genus of parasitic protists of which up to 175 species are known, which are transmitted to vertebrate hosts through the bite of an infected mosquito. They cause the disease known as malaria or malaria.
- Glaucophytes. They are unicellular freshwater algae, of which about 13 species are known, and which are sometimes included among plants. They are generally composed of individual cells, but they can also share the cell wall of their parent, since their reproduction is always asexual.
- Choanozoans. It is a group linked to animals and fungi, and constitutes a kind of intermediate step between these two groups of eukaryotes. It can be divided into holomycota (similar to mushrooms) and holozoa (similar to animals).
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References
- “Protista” on Wikipedia.
- “Kingdom protista” in BioEncyclopedia.
- “Protistas” (video) in Educatina.
- “What are protists?” on LiveScience.
- “Kingdom protista” (video) at 7activestudio.
- “Protist” in The Encyclopaedia Britannica.
- Biology: Solomon E., Berg L., Martin D. (2013) 9th Edition. Cengage Learning Publishing.
