The term “protist” refers to a wide group of unicellular and multicellular creatures. Protists have membrane-bound organelles and are classified as eukaryotes, meaning they have a nucleus. Protists are polyphyletic, deriving from multiple common evolutionary ancestors. They are organisms that resemble other species such as plants, animals, or fungi but lack the qualities necessary to be classed as actual plants, animals, or fungi. As a result, protists are classified as plant-like, animal-like, or fungus-like. They form their own kingdom known as Protista.
Algae, amoebas, euglena and slime moulds are all examples of common protists.
Protists that resemble animals are frequently referred to as protozoa (singular, protozoan). The majority of protozoa are composed of a single cell. They resemble animals because they are heterotrophs and have the ability to move. While protozoa are not animals, they are believed to be their ancestors.
The majority of protozoa feed by engulfing and digesting other species. They have a variety of roles in food chains and webs as consumers. Certain individuals are predators. They are predators of other single-celled creatures, particularly bacteria. Indeed, protozoa predators maintain control over a large number of bacterial populations. Additionally, some protozoa are herbivores. They subsist on algae. Others act as decomposers. They feed on decomposing organic materials. Additionally, parasitic protozoa exist that live within or on living hosts. For instance, malaria is caused by a protozoan that dwells inside a human host. Additionally, protozoa provide food for a variety of larger species, including insects and worms.
Mainly there are four types of protozoans, classified according to their movement and where they live:
The majority of amoebas, ciliates, and flagellates are free-living organisms that contribute to the ecosystem by inhibiting specific bacteria and providing food for larger organisms.
Amoebas, which live in freshwater or as parasites, and foraminifera, which live in the sea and create shells, are the two principal animal-like protozoa in this group. They are all distinguished by pseudopodia (“false feet”) — cytoplasmic lobes or fingerlike bulges that enable them to move. They consume bacteria and smaller protozoans by engulfing them in their pseudopodia and enclosing them in vacuoles, where enzymes break them down.
Excess waste and water are expelled through pores in the cell membrane. Amoebas reproduce asexually via binary fission, in which the nucleus divides into two and a new cell form around each of the two halves. In alternating generations, foraminifera reproduce differently: asexually via fission, then sexually via nucleic material exchange. Several amoebas are parasitic; for example, entamoeba is the causative agent of amoebic dysentery.
Ciliates, such as paramecium, have cilia, which are little hairlike projections that develop from their surfaces. They are propelled through water by their cilia, which waft food into a mouthlike groove in the surface membrane. They consume algae and bacteria and are then consumed by larger protozoans such as amoeba.
Ciliates have two nuclei: a large one that controls daily functions and a smaller one that controls reproductive functions. Certain ciliates reproduce sexually and asexually; they join together to exchange reproductive nuclei and then split the resulting double nuclei to form new cells.
Flagellates are protozoa that resemble animals and use a whip-like or tail-like structure to propel themselves through water. Only a handful, the phytoflagellates, are capable of photosynthesis, just like plants. Others sequester food particles in vacuoles or absorb nutritional molecules via their surface membrane.
While the majority of flagellates reproduce via fission, others reproduce sexually by fusing together before to dividing. Certain flagellates are parasitic; for example, trypanosoma and giardia are responsible for sleeping sickness and giardiasis, respectively (diarrhoea and vomiting).
Sporozoans are parasitic; they dwell on or within a host body, wreaking havoc. Due to their lack of cilia, flagella, and pseudopodia, sporozoa rely on their host organism for nutrition and vectors such as mosquitoes to transport them there. They spread by spores from host to host, or vector to host.
Sporozoa are also known as apicomplexa due to the presence of a “apical complex,” an enzyme-producing structure that helps the protist to wedge itself into the host cell. Sexual and asexual stages of reproduction exist.
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