As a collective group, protists display an astounding diversity of morphologies, physiologies, and ecologies. There are over , described living species of protists, and it is unclear how many undescribed species may exist. Since many protists live in symbiotic relationships with other organisms and these relationships are often species specific, there is a huge potential for undescribed protist diversity that matches the diversity of the hosts.
As the catchall term for eukaryotic organisms that are not animals, plants, fungi, or any single phylogenetically related group, it is not surprising that few characteristics are common to all protists. Nearly all protists exist in some type of aquatic environment, including freshwater and marine environments, damp soil, and even snow. Several protist species are parasites that infect animals or plants. A parasite is an organism that lives on or in another organism and feeds on it, often without killing it.
A few protist species live on dead organisms or their wastes, and contribute to their decay. The cells of protists are among the most elaborate of all cells. Most protists are microscopic and unicellular, but some true multicellular forms exist. A few protists live as colonies that behave in some ways as a group of free-living cells and in other ways as a multicellular organism.
Still other protists are composed of enormous, multinucleate, single cells that look like amorphous blobs of slime or, in other cases, like ferns. In fact, many protist cells are multinucleated; in some species, the nuclei are different sizes and have distinct roles in protist cell function. Single protist cells range in size from less than a micrometer to the 3-meter lengths of the multinucleate cells of the seaweed Caulerpa. Protist cells may be enveloped by animal-like cell membranes or plant-like cell walls.
Others are encased in glassy silica-based shells or wound with pellicles of interlocking protein strips. The pellicle functions like a flexible coat of armor, preventing the protist from being torn or pierced without compromising its range of motion. The majority of protists are motile, but different types of protists have evolved varied modes of movement. Some protists have one or more flagella, which they rotate or whip. Others are covered in rows or tufts of tiny cilia that they beat in coordination to swim.
Still others send out lobe-like pseudopodia from anywhere on the cell, anchor the pseudopodium to a substrate, and pull the rest of the cell toward the anchor point. Some protists can move toward light by coupling their locomotion strategy with a light-sensing organ. Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Photosynthetic protists photoautotrophs are characterized by the presence of chloroplasts. Other protists are heterotrophs and consume organic materials such as other organisms to obtain nutrition.
Amoebas and some other heterotrophic protist species ingest particles by a process called phagocytosis, in which the cell membrane engulfs a food particle and brings it inward, pinching off an intracellular membranous sac, or vesicle, called a food vacuole [Figure 2]. This vesicle then fuses with a lysosome, and the food particle is broken down into small molecules that can diffuse into the cytoplasm and be used in cellular metabolism. Undigested remains ultimately are expelled from the cell through exocytosis.
Some heterotrophs absorb nutrients from dead organisms or their organic wastes, and others are able to use photosynthesis or feed on organic matter, depending on conditions. 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. They have been redistributed amongst different branches of the family tree. According to Simpson, we now know most of the evolutionary relationships amongst protists, and these are often counterintuitive. He cited the example of dinoflagellate algae, which are more closely related to the malaria parasite than they are to diatoms another group of algae or even to land plants.
Still, there are pressing questions that remain. This point is called the "root" of the eukaryotic tree of life. Pinpointing the root will cement the understanding of eukaryotic origins and their subsequent evolution. As author Tom Williams said in a article published in the journal Current Biology, "For the eukaryotic tree, the root position is critical for identifying the genes and traits that may have been present in the ancestral eukaryote, for tracing the evolution of these traits throughout the eukaryotic radiation, and for establishing the deep relationships among the major eukaryotic groups.
Protists are responsible for a variety of human diseases including malaria, sleeping sickness , amoebic dysentery and trichomoniasis.
Malaria in humans is a devastating disease. It is caused by five species of the parasite Plasmodium , which are transmitted to humans by female Anopheles mosquitoes, according to the Centers for Disease Control and Prevention CDC.
The species Plasmodium falciparum infects red blood cells, multiplies rapidly and destroys them. Infection can also cause red blood cells to stick to the walls of small blood vessels. This creates a potentially fatal complication called cerebral malaria according to the CDC.
According to their recent malaria fact sheet , in there were an estimated , deaths due to malaria in the world, the majority of which 90 percent occurred in Africa. As the name of the class suggests, these protists have a characteristic structure- the apical complex. This apical complex is used by the protist for intrusion into the host cell. This apical complex has a secretory organelle known as rhoptries.
The rhoptries release enzymes while intruding into the cell membrane of the host. This organelle is made up of a four-layer membrane and encases short circular DNA. Although the exact function of this organelle is not yet known, it is being explored for developing drugs for targeting Plasmodium.
Phytophthora infestans is a pathogenic protist that causes diseases in plants. It caused late blight potato , which was the cause of severe Irish famine. Plasmopara viticola is the parasitic protist that causes a disease known as downy mildew in grapes. This protist was the cause of the almost collapse of the French wine industry in the 19th century.
In parasitic kinetoplastids and the free-living euglenids and dinoflagellates, a complex structure is known as paraflagellar rod PFR is found. Its function seems to be a photoreceptor. However, new findings indicate its utility in the attachment to the host cell during infection. Foraminiferans or forams resemble tiny snails and exhibit porous shells, called tests.
The shell is hardened by calcium. Try to answer the quiz below to check what you have learned so far about protists. New Zealand is known for its unique biodiversity, caused by its remarkable geography and geologic history.
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Learn about the general structure of a eukaryotic gene, the transcription factors, and post-transcriptional regulation Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum.
Table of Contents. Protist biology definition : Any of a group of eukaryotic organisms belonging to the Kingdom Protista. Protists include: 1 protozoa , the animal-like protists, 2 algae , the plant-like protists, and 3 slime molds and water molds, the fungus-like protists. Protoctista definition : It is the fourth kingdom of nature as proposed by John Hogg.
It consists of unicellular or primitive multicellular organisms that are not plants, animals, or fungi. They do not have much in common apart from a relatively simple organization. They may occur as unicellular, multicellular, coenocytic, or colonial organisms.
Endosymbiotic theory is well supported by several pieces of evidence: Mitochondrial and chloroplasts DNA is different from the nuclear DNA of the cell. The plasma membranes around Mitochondria and chloroplasts are similar to bacterial membranes. Mitochondria and chloroplasts both divide by the process of binary fission, similar to bacteria. Chloroplasts have structural and biochemical similarities to cyanobacteria. Agar-agar is a cell wall component of the red algae, especially Gelidium and Gracilaria.
Agar is the primary growth media used in microbiology. It is also extensively used in the food industry as a thickener for eg. Agarose, purified from agar, is an essential component to carry out gel-electrophoresis in research laboratories. Agar is also used as a bulk-laxative. Animal-like protists Algae. Slime molds.
Protists that have chlorophyll and therefore are photosynthetic Algae. Which of the following is a characteristic of protists? With nucleus, mitochondria, and other organelles. Mostly multicellular. An asexual mode of reproduction in protists Conjugation.
Multiple fission. Protists that feed on organic decaying matter Protozoa. Send Your Results Optional. Your Name. To Email. This organism consists of a single cell with several flagella. Is it a prokaryote, such as a bacterium? Therefore, this organism belongs to the domain Eukarya, the domain that includes humans. This particular eukaryote is one of the smallest, simplest organisms in the domain, called a protist. As a human parasite, it can make us sick. Figure 1: This scanning electron micrograph revealed some of the external ultrastructural details displayed by a flagellated Giardia lamblia protozoan parasite, which is the organism responsible for causing the diarrheal disease "giardiasis".
Protists are a group of all the eukaryotes that are not fungi, animals, or plants. As a result, it is a very diverse group of organisms.
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