This is the ability to move. Protists have three types of appendages for movement. There may be one or more whip-like flagella. Cilia are similar to flagella, except they are shorter and there are more of them. They may completely cover the surface of the protist cell.
Pseudopods are temporary, foot-like extensions of the cytoplasm. Protists have complex life cycles. Many have both asexual and sexual reproduction. An example is a protist called Spirogyra , a type of algae, shown Figure below.
It usually exists as haploid cells that reproduce by binary fission. In a stressful environment, such as one that is very dry, Spirogyra may produce tough spores that can withstand harsh conditions. Spores are reproductive cells produced by protists and various other organisms.
If two protist spores are close together, they can fuse to form a diploid zygote. This is a type of sexual reproduction. The zygote then undergoes meiosis, producing haploid cells that repeat the cycle. Spirogyra is a genus of algae with a complex life cycle. Each organism consists of rectangular cells connected end-to-end in long filaments. Protists get food in one of three ways.
Often complex, the long history of protist classification introduced two terms, still used today, into the scientific lexicon: protozoa and protists. However, the meaning of these terms has also evolved over time. The observable living world was once neatly divided between plants and animals.
But the discovery of various microscopic organisms including what we now know as protists and bacteria brought forth the need to understand what they were, and where they fit taxonomically. The first instinct of scientists was to relate these organisms to plants and animals by relying on morphological characteristics. The term protozoan plural: protozoa or protozoans , meaning "early animals," was introduced in by naturalist Georg A.
Goldfuss, according to a article published in the journal International Microbiology. This term was used to describe a collection of organisms including ciliates and corals. By , Protozoa was established as a phylum or subset of the animal kingdom by German scientist Carl Theodor von Seibold.
This phylum included certain ciliates and amoebas, which were described by von Seibold as single-celled animals. In , the concept of protozoans was further refined and they were elevated to the level of a taxonomic kingdom by paleontologist Richard Owen. The members of this Kingdom Protozoa, in Owen's view, had characteristics common to both plants and animals. Though the scientific rationale behind each of these classifications implied that protozoans were rudimentary versions of plants and animals, there was no scientific evidence of the evolutionary relationships between these organisms International Microbiology, According to Simpson, nowadays "protozoa" is a term of convenience used in reference to a subset of protists, and is not a taxonomic group.
The term protista, meaning "the first of all or primordial" was introduced in by German scientist Ernst Haeckel. He suggested Protista as a third taxonomic kingdom, in addition to Plantae and Animalia, consisting of all "primitive forms" of organisms, including bacteria International Microbiology, Since then, the kingdom Protista has been refined and redefined many times.
Different organisms moved in and out notably, bacteria moved into a taxonomic kingdom of their own. American scientist John Corliss proposed one of the modern iterations of Protista in the s. His version included the multicellular red and brown algae, which are considered to be protists even today. Scientists, often concurrently, have debated kingdom names and which organisms were eligible for example, versions of yet another kingdom, Protoctista had been proposed over the years.
However, it is important to note the lack of correlation between taxonomy and evolutionary relationships in these groupings. According to Simpson, these groupings were not monophyletic, meaning that they did not represent a single, whole branch of the tree of life; that is, an ancestor and all of its descendants.
Today's classification has shifted away from a system built on morphology to one based on genetic similarities and differences. The result is a family tree of sorts, mapping out evolutionary relationships between various organisms. In this system there are three main branches or "domains" of life: Bacteria, Archaea both prokaryotic and Eukarya the eukaryotes. Within the eukaryotic domain, the protists are no longer a single group. Heterotrophic protists can be categorized based on their type of movement or lack of locomotion.
Examples of heterotrophic protists include amoebas, paramecia, sporozoans, water molds, and slime molds. Amoebas are examples of protists that move using pseudopodia. These temporary extensions of the cytoplasm allow the organism to move as well as to capture and engulf organic material through a type of endocytosis known as phagocytosis, or cell eating. Amoebas are amorphous and move by changing their shape.
They reside in aquatic and moist environments, and some species are parasitic. Trypanosomes are examples of heterptrophic protists that move with flagella. These long, whip-like appendages move back an forth enabling movement. Trypanosomes are parasites that can infect animals and humans.
Some species cause African sleeping sickness which is transmitted to humans by biting flies. Paramecia are examples of protists that move with cilia. Cilia are short, thread-like protrusions that extend from the body and move in a sweeping motion. This motion allows the organism to move and also pulls food bacteria, algae. Some paramecia live in mutualistic symbiotic relationships with green algae or with certain bacteria. Slime molds and water molds are examples of protists that exhibit limited motion.
These protists are similar to fungi in that they decompose organic matter and recycle nutrients back into the environment. They live in moist soils among decaying leaves or wood. There are two types of slime molds: plasmodial and cellular slime molds. A plasmodial slime mold exists as an enormous cell formed by the fusion of several individual cells.
This huge blob of cytoplasm with many nuclei resembles slime that moves slowly in an amoeba-like fashion. Under harsh conditions, plasmodial slime molds produce reproductive stalks called sporangia that contain spores.
When released into the environment, these spores may germinate producing more plasmodial slime molds. Cellular slime molds spend most of their life cycle as single-celled organisms. They too are capable of amoeba-like movement. When under stressful conditions, these cells unite forming a large group of individual cells that resemble a slug.
The cells form a reproductive stalk or fruiting body that produces spores. Water molds live in aquatic and moist terrestrial environments. They feed on decaying matter, and some are parasites that live off of plants, animals, algae, and fungi. Species of the Oomycota phylum exhibit filamentous or thread-like growth, similar to fungi.
However, unlike fungi, oomycetes have a cell wall that is composed of cellulose and not chitin. They can also reproduce both sexually and asexually. Sporozoans are examples of protists that do not posses structures that are used for locomotion. These protists are parasites that feed off of their host and reproduce by the formation of spores. Sporozoans exhibit a type of alternation of generations in their life cycle, in which they alternate between sexual and asexual phases.
Sporozoans are transmitted to humans by insect or other animal vectors. Toxoplasmosis is a disease caused by the sporozoan Toxoplasma gondii that can be transmitted to humans by animals or can be contracted by ingesting contaminated food or water. In severe toxoplasmosis, T. Toxoplasmosis does not typically develop in people with healthy immune systems. Another sporozoan, known as plasmodium , causes malaria in humans.
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