What Characteristics Of Protists Prevent Them From Being Classified As Plants, Animals, Or Fungi

Learning Objectives

By the end of this section, you volition exist able to do the following:

Describe the cell structure characteristics of protists
Describe the metabolic diversity of protists
Describe the life cycle diversity of protists

There are over 100,000 described living species of protists, and it is unclear how many undescribed species may exist. Since many protists live as commensals or parasites in other organisms and these relationships are often species-specific, at that place is a huge potential for protist diversity that matches the multifariousness of their hosts. Because the name "protist" serves as a catchall term for eukaryotic organisms that are non animal, plant, or fungi, information technology is non surprising that very few characteristics are mutual to all protists. On the other manus, familiar characteristics of plants and animals are foreshadowed in various protists.

Cell Structure

The cells of protists are amidst the nigh elaborate of all cells. Multicellular plants, animals, and fungi are embedded among the protists in eukaryotic phylogeny. In most plants and animals and some fungi, complication arises out of multicellularity, tissue specialization, and subsequent interaction because of these features. Although a rudimentary course of multicellularity exists among some of the organisms labelled as "protists," those that have remained unicellular show how complexity tin can evolve in the absenteeism of true multicellularity, with the differentiation of cellular morphology and part. A few protists live as colonies that deport in some means as a grouping of complimentary-living cells and in other means as a multicellular organism. Some protists are composed of enormous, multinucleate, single cells that wait similar amorphous blobs of slime, or in other cases, similar ferns. In some species of protists, the nuclei are different sizes and have singled-out roles in protist cell part.

Unmarried protist cells range in size from less than a micrometer to three meters in length to hectares! Protist cells may be enveloped past animal-similar cell membranes or constitute-like cell walls. Others are encased in glassy silica-based shells or wound with pellicles of interlocking poly peptide strips. The pellicle functions like a flexible glaze of armor, preventing the protist from being torn or pierced without compromising its range of move.

Metabolism

Protists exhibit many forms of diet and may be aerobic or anaerobic. Those that store free energy by photosynthesis belong to a grouping of photoautotrophs and are characterized by the presence of chloroplasts. Other protists are heterotrophic and consume organic materials (such equally other organisms) to obtain nutrition. Amoebas and some other heterotrophic protist species ingest particles by a procedure chosen phagocytosis, in which the cell membrane engulfs a food particle and brings it inwards, pinching off an intracellular membranous sac, or vesicle, chosen a nutrient vacuole (Figure 23.7). In some protists, food vacuoles can exist formed anywhere on the trunk surface, whereas in others, they may be restricted to the base of a specialized feeding structure. The vesicle containing the ingested particle, the phagosome, and so fuses with a lysosome containing hydrolytic enzymes to produce a phagolysosome, and the food particle is broken down into minor molecules that tin diffuse into the cytoplasm and be used in cellular metabolism. Undigested remains ultimately are expelled from the cell via exocytosis.

In this illustration, a eukaryotic cell is shown consuming a food particle. As the food particle is consumed, it is encapsulated in a vacuole. The vacuole fuses with a lysosome, and proteins inside the lysosome digest the food particle. Indigestible waste material is ejected from the cell when an exocytic vesicle fuses with the plasma membrane.

Figure 23.seven Phagocytosis. The stages of phagocytosis include the engulfment of a food particle, the digestion of the particle using hydrolytic enzymes contained inside a lysosome, and the expulsion of undigested materials from the prison cell.

Subtypes of heterotrophs, called saprobes, absorb nutrients from dead organisms or their organic wastes. Some protists can function as mixotrophs, obtaining nutrition by photoautotrophic or heterotrophic routes, depending on whether sunlight or organic nutrients are available.

Motility

The majority of protists are motile, but different types of protists have evolved varied modes of movement (Effigy 23.8). 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 a coordinated manner to swim. Still others form cytoplasmic extensions called pseudopodia anywhere on the cell, ballast the pseudopodia to a substrate, and pull themselves forward. Some protists can move toward or abroad from a stimulus, a move referred to as taxis. For example, motility toward lite, termed phototaxis, is achieved by coupling their locomotion strategy with a lite-sensing organ.

Part a shows a shoe-shaped Paramecium, which is covered with fine, hair-like cilia. Part b shows an Amoeba, which is irregular in shape with long extensions of cytoplasm jutting out from the main body. The extensions are called pseudopods. Part c shows an oval Euglena, which has a narrow front end. A long, whip-like flagellum protrudes from the back end.

Figure 23.8 Locomotor organelles in protists. Protists use various methods for transportation. (a) Paramecium waves hair-like appendages called cilia to propel itself. (b) Amoeba uses lobe-like pseudopodia to anchor itself to a solid surface and pull itself forward. (c) Euglena uses a whip-like tail called a flagellum to propel itself.

Life Cycles

Protists reproduce by a variety of mechanisms. Most undergo some class of asexual reproduction, such every bit binary fission, to produce 2 daughter cells. In protists, binary fission tin 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 showroom multiple fission and simultaneously split up into many daughter cells. Others produce tiny buds that keep to separate 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 exist 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 specially resistant to temperature extremes, desiccation, or low pH. This strategy allows certain protists to "await out" stressors until their environment becomes more favorable for survival or until they are carried (such as past wind, water, or transport on a larger organism) to a different environment, because cysts exhibit well-nigh no cellular metabolism.

Protist life cycles range from simple to extremely elaborate. Certain parasitic protists have complicated life cycles and must infect dissimilar host species at different developmental stages to consummate 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.

Habitats

Nearly all protists exist in some type of aquatic surroundings, including freshwater and marine environments, damp soil, and even snowfall. Several protist species are parasites that infect animals or plants. A few protist species live on expressionless organisms or their wastes, and contribute to their disuse.

Source: https://openstax.org/books/biology-2e/pages/23-2-characteristics-of-protists

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