Animal Like Protists Are Also Called Phytoflagellates

Hall, Richard P. University of California, Los Angeles, California.

An informal grouping of single-celled eukaryotic microorganisms. In previous taxonomic schemes, certain unicellular eukaryotic microorganisms were assigned to an fauna kingdom known as Protozoa. Still, modern-day classifications consider this term to be obsolete and invalid, and the word protozoa (or protozoans) is used commonly equally a descriptive term ( Fig. 1 ). Moreover, although members of the informal protozoan grouping often display animal-like characteristics, they are non classified as animals, and they are not considered to be plants or fungi either. Instead, protozoa or protozoans are animal-similar protists (eukaryotes that are non animals, plants, or fungi; other protists include algae, water molds, and slime molds). Come across too: Fauna ; Biological nomenclature ; Eukaryota ; Protista ; Taxonomy

An ameboid protozoan against a blue background

Protozoans are most equally widely distributed as bacteria, and they can be free-living or parasitic. Free-living types occur in soil, moisture sand, and fresh, brackish, and salt waters. Protozoans of the soil and sand live in films of moisture on the particles. Habitats of endoparasites vary. Some are intracellular, such as malarial parasites in vertebrates. Other parasites, such as Entamoeba histolytica ( Fig. 2 ), invade tissues, but not private cells. Virtually trypanosomes alive in the blood plasma of vertebrate hosts. Many other parasites live in the lumen of the digestive tract or sometimes in coelomic cavities of invertebrates, as practice certain gregarines. Run into also: Coccidia ; Gregarinia ; Trypanosomatidae

A blue-colored trophozoite with various internal organlles seen in its cytoplasm

Many protozoans are uninucleate (possessing a single nucleus), whereas others are binucleate or multinucleate. In addition, the number of nuclei may vary at different stages in a life bike. Size ranges from less than three micrometers (µm) to a number of centimeters. Colonies are known in flagellates, ciliates, and sarcodine ameboids. Although marked differentiation of reproductive and somatic zooids (contained individuals of colonial animals) characterizes certain colonies, protozoa accept non developed tissues and organs. See also: Ameba ; Cell nucleus

Morphology

A protozoan may be a plastic organism (ameboid type), just changes in class are often restricted by the pellicle (a thin protective membrane). In Trypanosoma lewisi, parallel microtubules below the pellicle may support this layer and may also, as probably contractile structures, cause the typical undulations of these flagellates. Although protozoan form varies considerably, there is a tendency toward universal symmetry in floating species and radial symmetry in sessile types. Bilateral symmetry is rare, and nearly swimmers show appreciable spiral torsion.

A protective layer is frequently secreted outside the pellicle, although the pellicle itself may be strengthened by incorporation of minerals. Secreted coverings may fit closely—for example, cellulose-containing thecae, which are somewhat analogous to cell walls in higher plants. Thecae also may be equanimous of plates arranged in a specific pattern. In other protozoans, tests may be composed by and large of inorganic material, although organic (chitinous) tests occur in certain species. Siliceous skeletons, oftentimes elaborate, characterize some protozoans. A vase-shaped lorica (rigid case), from which the anterior part of the organism or its appendages may be extended, occurs in certain flagellates and ciliates. Sure marine ciliates are actively pond loricate forms.

Colonies and aggregates

An important feature of colonies is the material that holds the component zooids together. In spheroid colonies, a matrix is secreted during development of the colony. Spheroid colonies of a few radiolarians (for example, Collozoum), unusually large examples, may measure 4–6 cm (one.half-dozen–2.4 in.) in width. In arboroid colonies, a branching blueprint is produced past a branching stem or by attachment of loricae to ane some other. Dispersal of such colonial species sometimes involves migratory stages. In sure spheroid colonies, reproductive and somatic zooids are differentiated. More than often, members of a colony are similar. In add-on to colonial types, certain protozoa may form aggregates by not separating promptly after fission. Palmella stages, analogous to spheroid colonies, are aggregates of nonflagellated organisms. See likewise: Radiolaria

Flagella

Flagella occur in the active or flagellated stages of various protozoans. A flagellum consists of a sheath enclosing a matrix in which an axoneme extends from the cytoplasm to the flagellar tip. In certain groups, the sheath shows lateral fibrils (mastigonemes), which increase surface area and likewise may modify direction of the thrust effective locomotion. The base (blepharoplast) is considered a self-replicating structure from which new flagella arise in fission. Flagellar number ranges from 1 or two (for well-nigh gratuitous-living species) to many (every bit seen on certain symbionts of termites and wood roaches). Most flagella extend forward in swimming; others extend posteriorly, sometimes driving the body forwards, sometimes serving as a ski in gliding, or forming the margin of an undulating membrane. Certain organelles, as the kinetoplast, axostyle, parabasal body, or sometimes other structures, may be attached to or adjoin the blepharoplasts. In full general, flagellar movements include planar sinusoidal waves, helical waves, and lateroposterior strokes. See too: Cilia and flagella

Cilia

Although typically shorter than flagella, cilia are similar in structure. Multiplicity of cilia entails circuitous fibrillar systems. Information technology has been suggested that several sets of fibrils help support the cortex and ciliary organisation. A more frail fibril, or tubule, running forth each row of basal granules (as seen in Tetrahymena) seemingly offers fiddling support and may function in coordination. Other ciliates, such equally hypotrichs, with compound organelles, may take quite complex fibrillar systems. Compound ciliary organelles represent fused bundles or rows of cilia, with the basal granules detectable in basal plates of the organelles. Cirri, typically ventral, are fused tufts of cilia and are important in locomotion. Meet also: Ciliophora

Pseudopodia

Two major types have been described—the contraction-hydraulic type and the two-mode menstruation type. Wrinkle-hydraulic pseudopodia are lobopodia with rounded tips and ectoplasm denser than endoplasm. The larger ones commonly incorporate granular endoplasm and clear ectoplasm. Co-ordinate to the contraction-hydraulic theory, pseudopodia arise as a result of pressure from the contracting ectoplasm on the endoplasm, forcing the latter forward, abroad from the posterior contraction. Local weakening of the ectoplasm results in a bulge, or developing pseudopodium. Endoplasm reaching the tip of the growing pseudopodium is diverted to the ectoplasm and converted into gel, thus becoming part of the ectoplasm. In the concurrently, ectoplasm at the other end is existence converted into endoplasm, which flows forward once more under pressure from the ectoplasm.

Two-way menstruum pseudopodia include reticulopodia, filoreticulopodia, and axopodia. In all these types, cytoplasmic menstruation in opposite directions occurs on reverse sides of a pseudopodium.

Trichocysts

At maturity, trichocysts (minute structures that release filamentous or fibrillar threads when discharged) prevarication in the cortex of certain ciliates and flagellates. These structures probably serve in secretion of cyst walls and similar materials. Fibrillar trichocysts of Paramecium ( Fig. iii ) may exist extruded partially to anchor a feeding ciliate. In improver, toxicysts of diverse protozoans may induce paralysis of prey. Run across besides: Paramecium

An oval paramecium (with many internal organelles in its cytoplasm) on a dark background

Inclusions

Nuclei, food vacuoles (gastrioles), chromatophores (which contain chlorophylls), stigmas (which possibly function in orientation with relation to a light source), and water-elimination vesicles are found in the cytoplasm of protozoans. In add-on, the cytoplasm may contain mitochondria, Golgi material, pinocytotic vacuoles, stored nutrient materials, an endoplasmic reticulum, and sometimes pigments of various kinds. Minor vacuoles (0.5–1.0 µm) in Tetrahymena pyriformis ( Fig. 4 ) are considered lysosomes containing acid hydrolases. See also: Cell organization ; Chromatophore ; Endoplasmic reticulum ; Golgi apparatus ; Lysosome ; Mitochondria ; Vacuole

Illustration of an oval protozoan; various structures, including many vacuoles, are labeled

Nutrition

In protozoan feeding, either phagotrophic (holozoic) or saprozoic (osmotrophic) methods predominate in item species. In addition, chlorophyll-bearing flagellates profit from photosynthesis; in fact, certain species have not been grown in darkness and may be obligate phototrophs. Encounter also: Nutrition ; Photosynthesis

Phagotrophic feeding

Ingestion of food, followed by digestion in vacuoles, is characteristic of ameboids, ciliates, and many flagellates. Digestion follows synthesis of advisable enzymes and their transportation to the food vacuole. Ingestion of nonnutritive latex particles also leads to acid-phosphatase activity of the food vacuoles. Primary lysosomes, which tin fuse with the food vacuole, may transport hydrolases from the crude endoplasmic reticulum, as suggested for Tetrahymena ( Fig. four ). Some parasitic alveolates, including sporozoans (organisms whose adult stage is not motile), are known to ingest host proteins or sometimes tissue cells or portions of them. Details of ingestion vary. Formation of food cups, or gulletlike invaginations, to enclose casualty is common in ameboid organisms, many flagellates, and at least a few sporozoans. Entrapment in a sticky reticulopodial internet occurs in ameboids. Phagotrophic ciliates and a few flagellates have a persistent cytostome and gullet. Many ciliates ( Fig. 4 ) have buccal organelles (membranes, membranelles, and closely set rows of cilia) arranged to drive particles to the cytostome. Particles pass through the cytostome into the cytopharynx (gullet), at the base of operations of which food vacuoles (gastrioles) are formed. Digestion occurs in such vacuoles. An boosted characteristic, reported in Paramecium and Tetrahymena, is entanglement of particles in mucus secreted past the peristomial cortex or buccal cavity. The result is a clumping of particles into larger aggregates. Come across also: Enzyme

Saprozoic feeding

By definition, saprozoic feeding involves passage of dissolved foods through the cortex. It is uncertain to what extent improvidence is responsible, but enzymatic activities presumably are involved in uptake of various unproblematic sugars, acetate, and butyrate, as well as potassium and phosphate ions. In addition, external factors (for example, the pH of the medium) may strongly influence uptake of fatty acids and phosphate.

Pinocytosis

Pinocytosis resembles phagocytosis in miniature, but with ingestion apparently limited to materials in solution. First recognized in Amoeba proteus, the procedure has since been described in other amebas, certain ciliates, and some sporozoans. Pinocytosis is stimulated by "inducers," such as proteins, sure salts, and toluidine blue (a bones dye). In the process, pocket-sized vacuoles are formed at the base of a tubular invagination from a surface, either cortex or the cytopharynx wall. The vacuoles migrate into the endoplasm and mingle with other inclusions. In certain ciliates, a similar process yields secondary vacuoles from a food vacuole. In certain malarial parasites, merely not in others, such pinocytosis precedes completion of digestion. Come across also: Endocytosis ; Phagocytosis

Nutritional requirements

In nature, nutritional requirements are met past the available nutrition, which includes other microorganisms and materials dissolved in the external medium. Certain mineral materials, including calcium, silicon, and strontium salts, are needed for tests and other skeletal structures, and metal ions are needed in traces for enzyme systems. Nitrogen requirements of many phytoflagellates are met by an ammonium table salt or past either a nitrate or an ammonium salt. The simply carbon source needed by sure chlorophyll-bearing flagellates is carbon dioxide (CO₂). Organic substances, such as vitamins, are needed in traces. Some few phytoflagellates are known to synthesize all required vitamins. Other protozoa need ane or more vitamins and sometimes other growth factors. See also: Carbon dioxide ; Vitamin

Reproduction

Reproduction occurs after a menses of growth that ranges, in unlike species, from less than half a day to several months. General reproductive methods include binary fission, budding, plasmotomy, and schizogony. Fission, involving nuclear division and replication of organelles, yields two organisms similar in size. Budding (either internal or external) produces ii organisms—i smaller than the other. In plasmotomy, a multinucleate organism divides into several girl cells, with each containing a number of nuclei. Schizogony, which is characteristic of sporozoans, follows repeated nuclear division, yielding many uninucleate buds. In addition, reproduction includes germination of new organelles, either by partition of old structures or past neoformation that is sometimes preceded by resorption of an onetime organelle. Blepharoplasts and basal granules, in detail, seem to exist self-replicating. Formation of a new mouth (stomatogenesis) also occurs in ciliates.

Life cycles

Elementary life cycles include a cyst and an active (trophic) stage undergoing growth and reproduction. In sure free-living and parasitic species, no cyst is developed. Dimorphic cycles show two active stages [for example, adult and larva; flagellate and ameba; flagellate and palmella (nonflagellated); and ameba and plasmodium], whereas polymorphic cycles show several active stages.

Encystment

Encystment involves resorption of locomotor and feeding organelles, loss of water, often rounding upwardly of the body, and secretion of one or more cyst membranes. The wall unremarkably is composed of secreted materials (for example, proteins, carbohydrates, and keratin), although foreign particles may be cemented into a wall. Precystic activities may include storage of reserves. Protective cysts, with fairly thick walls, frequently resist desiccation. In fact, stale cysts of certain amebas and flagellates take remained viable for well-nigh 50 years in dry soil samples. Within reproductive cysts, different reproductive methods occur, including fission, budding or schizogony, gametogenesis, and syngamy. The usually thin-walled reproductive cysts afford relatively little protection.

Excystment involves absorption of water leading to rupture of cyst membranes and, when necessary, development of organelles for feeding and locomotion.

Sexual activities

Sexual activities involve production of haploid (gametic) nuclei and their fusion into a synkaryon (zygotic nucleus). Meiosis occurs at different stages in life cycles. In haploid organisms (for example, certain flagellates, coccidian parasites, malarial parasites, and gregarines), the zygote is the merely diploid phase. Meiosis occurs equally the zygotic nucleus divides. Diploid cycles, in which meiosis occurs during gametogenesis, are found in ciliates. See likewise: Meiosis

Syngamy, autogamy, and conjugation are common sexual activities. Syngamy involves fusion of two gametes, which may be morphologically similar (isogamy) or different (anisogamy). Autogamy, which is observed in certain ciliates and a few parasitic flagellates, occurs within a single organism; haploid nuclei are formed and afterwards fuse into a synkaryon. Conjugation involves pairing of ciliates, apparently following appearance of specific poly peptide-containing substances at the surface, including cilia.

Parasitic protozoans

Parasites occur in all major groups of protozoans. Sporozoans are exclusively parasitic, as are some flagellates (for instance, trichomonads and hypermastigids) and several ciliates (for case, apostomes). Diverse other protozoan groups contain both parasitic and free-living types. Protozoans too serve every bit hosts of other protozoans, certain bacteria, fungi, and algae. Many protozoan symbionts, chosen commensals, are more or less harmless. Cellulose-digesting flagellates of termites and ciliates of ruminants are symbiotic in the sense that they benefit their hosts. In addition to helpful digestive activities, rumen ciliates are potential food for the herbivores. Encounter also: Parasitology

Relatively few parasites are distinctly pathogenic, causing amebiasis, visceral leishmaniasis (kala azar), African sleeping sickness (African trypanosomiasis), Chagas disease (American trypanosomiasis), malaria, tick fever of cattle, dourine of horses, and other diseases. Harmful furnishings may be produced in different means. Tissue cells are invaded and destroyed in Chagas affliction, leishmaniasis, coccidiosis, and malaria. Tissues may exist destroyed, as in abscesses and ulcers involving Entamoeba histolytica. Transfer may involve direct migration of parasites living in aquatic hosts; direct contact (osculatory or venereal); ingestion with contaminated food or potable; and vector transfer, as in malaria or Chagas illness. See also: Malaria ; Medical parasitology ; Trypanosomiasis