Phytophthora infestans is an oomycete responsible for potato late blight, which causes potato stalks and stems to decay into black slime [Figure 6] b. Widespread potato blight caused by P. Late blight continues to plague potato crops in certain parts of the United States and Russia, wiping out as much as 70 percent of crops when no pesticides are applied. They are equally important on the other end of food webs as decomposers.
Protists are essential sources of nutrition for many other organisms. In some cases, as in plankton, protists are consumed directly. Alternatively, photosynthetic protists serve as producers of nutrition for other organisms by carbon fixation. For instance, photosynthetic dinoflagellates called zooxanthellae pass on most of their energy to the coral polyps that house them [Figure 7].
In this mutually beneficial relationship, the polyps provide a protective environment and nutrients for the zooxanthellae. The polyps secrete the calcium carbonate that builds coral reefs. Without dinoflagellate symbionts, corals lose algal pigments in a process called coral bleaching, and they eventually die. This explains why reef-building corals do not reside in waters deeper than 20 meters: Not enough light reaches those depths for dinoflagellates to photosynthesize.
Protists themselves and their products of photosynthesis are essential—directly or indirectly—to the survival of organisms ranging from bacteria to mammals. On land, terrestrial plants serve as primary producers. Protists do not create food sources only for sea-dwelling organisms. For instance, certain anaerobic species exist in the digestive tracts of termites and wood-eating cockroaches, where they contribute to digesting cellulose ingested by these insects as they bore through wood.
The actual enzyme used to digest the cellulose is actually produced by bacteria living within the protist cells. The termite provides the food source to the protist and its bacteria, and the protist and bacteria provide nutrients to the termite by breaking down the cellulose. Many fungus-like protists are saprobes , organisms that feed on dead organisms or the waste matter produced by organisms saprophyte is an equivalent term , and are specialized to absorb nutrients from nonliving organic matter.
For instance, many types of oomycetes grow on dead animals or algae. Saprobic protists have the essential function of returning inorganic nutrients to the soil and water. This process allows for new plant growth, which in turn generates sustenance for other organisms along the food chain.
Protists are extremely diverse in terms of biological and ecological characteristics due in large part to the fact that they are an artificial assemblage of phylogenetically unrelated groups. Protists display highly varied cell structures, several types of reproductive strategies, virtually every possible type of nutrition, and varied habitats. Most single-celled protists are motile, but these organisms use diverse structures for transportation.
The process of classifying protists into meaningful groups is ongoing, but genetic data in the past 20 years have clarified many relationships that were previously unclear or mistaken. The majority view at present is to order all eukaryotes into six supergroups. The goal of this classification scheme is to create clusters of species that all are derived from a common ancestor.
Which parasitic protist evades the host immune system by altering its surface proteins with each generation? 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. The eukaryotes that make up this kingdom, Kingdom Protista , do not have much in common besides a relatively simple organization. Protists can look very different from each other. Some are tiny and unicellular, like an amoeba , and some are large and multicellular, like seaweed.
However, multicellular protists do not have highly specialized tissues or organs. This simple cellular-level organization distinguishes protists from other eukaryotes, such as fungi, animals, and plants.
The vesicle containing the ingested particle, the phagosome, then fuses with a lysosome containing hydrolytic enzymes to produce a phagolysosome , 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 via exocytosis.
Figure 2. The stages of phagocytosis include the engulfment of a food particle, the digestion of the particle using hydrolytic enzymes contained within a lysosome, and the expulsion of undigested materials from the cell. Subtypes of heterotrophs, called saprobes, absorb nutrients from dead organisms or their organic wastes. Some protists, like, euglena, can function as mixotrophs , obtaining nutrition by photoautotrophic or heterotrophic routes, depending on whether sunlight or organic nutrients are available.
Protists reproduce by a variety of mechanisms. Most undergo some form of asexual reproduction, such as binary fission, to produce two daughter cells. In protists, binary fission can be divided into transverse or longitudinal, depending on the axis of orientation; sometimes Paramecium exhibits this method. Some protists such as the true slime molds exhibit multiple fission and simultaneously divide into many daughter cells.
Others produce tiny buds that go on to divide and grow to the size of the parental protist. Sexual reproduction, involving meiosis and fertilization, is common among protists, and many protist species can switch from asexual to sexual reproduction when necessary.
Sexual reproduction is often associated with periods when nutrients are depleted or environmental changes occur. Sexual reproduction may allow the protist to recombine genes and produce new variations of progeny that may be better suited to surviving in the new environment.
However, sexual reproduction is often associated with resistant cysts that are a protective, resting stage. Depending on their habitat, the cysts may be particularly resistant to temperature extremes, desiccation, or low pH. Whether an organism contains one or multiple cells, the nucleus — the first and largest organelle discovered — is essential to cell life. Protected in a double envelope, cytoplasm crosses through pores in the outer membrane; this is unlike prokaryotes, where cytoplasm directly crosses the cell wall.
Appearing like a darkened area inside the nucleus under a light microscope, the nucleolus is surrounded by chromatin, which contains DNA and RNA — necessary for cell division. Ciliates Paramecium from BiologyCorner. In addition, parasitic protists such as sporozoans also fall under animal-like, but are categorized by the way they survive and diseases brought about in hosts. Animal-like protozoan are also heterotrophic and contain organelles such as a cell membrane and food vacuole.
Considered the foundation of many aquatic food chains, responsible for over forty percent of photosynthesis that occurs in salt and fresh bodies of water, as well as essential to the production of oxygen in the atmosphere, plant-like protista are classified into three phylum:.
See Euglena under the microscope. Although each contains chlorophyll , organisms such as Dinoflagelles have properties that are both animal and plant like. This overlap is exclusive to Eukaryotes in the Protista Kingdom.
Fungus-like protists have cells walls similar to plants, which contain chitin, but possess the animal-like function of heterotrophy. They release spores into the air to reproduce and have the ability to move, although this might only happen once during a lifespan.
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