Still others form cytoplasmic extensions called pseudopodia anywhere on the cell, anchor the pseudopodia to a substrate, and pull themselves forward. Some protists can move toward or away from a stimulus, a movement referred to as taxis. For example, movement toward light, termed phototaxis, is accomplished by coupling their locomotion strategy with a light-sensing organ.
Life Cycles 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, some of which may be better suited to surviving changes in a new or changing environment.
However, sexual reproduction is often associated with resistant cysts that are a protective, resting stage. Depending on habitat of the species, the cysts may be particularly resistant to temperature extremes, desiccation, or low pH. Protist life cycles range from simple to extremely elaborate. Certain parasitic protists have complicated life cycles and must infect different host species at different developmental stages to complete their life cycle.
Some protists are unicellular in the haploid form and multicellular in the diploid form, a strategy employed by animals. Other protists have multicellular stages in both haploid and diploid forms, a strategy called alternation of generations, analogous to that used by plants. 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 few protist species live on dead organisms or their wastes, and contribute to their decay.
Protists are extremely diverse in terms of their biological and ecological characteristics, partly because 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 amoeba E. The ability to perform sexual reproduction allows protists to recombine their genes and produce new variations of progeny that may be better suited to the new environment.
In contrast, asexual reproduction generates progeny that are clones of the parent. Giardia lamblia is a cyst-forming protist parasite that causes diarrhea if ingested. 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. 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. Certain strides have been made in reducing the rates of incidence occurrence of new cases and mortality rates in part by supplying insecticide treated mosquito nets, spraying for mosquitoes and improving diagnostics.
Between and the rate of incidence fell by 37 percent globally and mortality rates fell by 60 percent globally. The WHO has a goal of eliminating malaria in at least 35 countries by 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. They may ingest, absorb, or make their own organic molecules. Sexual or asexual reproduction for protists? Protist Habitats Most protists are aquatic organisms. Motility of Protists Most protists have motility. Protists use cilia, pseudopods, or flagella to move. Protist Reproduction Protists have complex life cycles.
Protist Nutrition Protists get food in one of three ways. Ingestive protists ingest, or engulf, bacteria and other small particles. They extend their cell wall and cell membrane around the food item, forming a food vacuole. Then enzymesdigest the food in the vacuole. Absorptive protists absorb food molecules across their cell membranes.
This occurs by diffusion. These protists are important decomposers. Photosynthetic protists use light energy to make food. They are major producers in aquatic ecosystems.
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