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.
Animal-Like Protists: Protozoa
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.
Ecology of Protozoa
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.
Characteristics of Protozoa
- Protozoa are predominantly single-celled, motile protists that feed via phagocytosis, a type of endocytosis.
- However, there are numerous exceptions. The majority of protozoa are too tiny to be observed without a microscope.
- The majority of protozoa are between 10 and 50 metres in length, although some are greater.
- They are found wherever there is water, including damp soil and leaf litter, as well as within and on the bodies of multicellular organisms.
- The term protozoa is not a taxonomic categorization term, and taxonomists do not use it. Rather than that, it is a helpful phrase for protists that resemble animals. Historically, protozoa were classified according to their mode of motility.
- Protozoa play a variety of roles in the food chain as consumers. Certain protozoa are predators, preying on unicellular algae, bacteria, and fungi; others are herbivores or decomposers.
- Additionally, they serve a critical function in bacterial population control.
- Protozoa are a significant source of food for a wide variety of larger creatures.
- Additionally, protozoa such as the malaria parasite (plasmodium), trypanosomes, and leishmania are significant as parasites and symbionts of multicellular creatures.
- Certain protozoa have the ability to create a cyst in order to protect themselves from harsh environments, allowing them to survive exposure to extreme temperatures and toxic chemicals, as well as periods of time without food, water, or oxygen.
- The cyst also enables parasitic organisms to persist outside of their host, allowing them to be transmitted from one host to another.
Types of Protozoans
Mainly there are four types of protozoans, classified according to their movement and where they live:
- Rhizopoda (animal-like protists having “false feet” called pseudopodia)
- Ciliates (protists covered in the tiny hairlike cilia)
- Flagellates (protists having whiplike “tails”)
- Sporozoa (parasitic protists)
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.
Rhizopoda
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
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
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).
Sporozoan
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.
Conclusion
- The majority of protozoa are single-celled, motile protists that feed via phagocytosis.
- They frequently exhibit animal-like features such as movement and heterotrophy.
- The majority of protozoa are too tiny to be observed without a microscope.
- They are found wherever there is water, including damp soil and leaf litter, as well as inside and on the bodies of multicellular organisms.
- Historically, protozoa were classified according to their mode of motility.
- Certain protozoa are capable of forming a cyst in order to protect themselves from harsh environments.