Abcte Biology

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Kingdom Protista

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The word protozoa means "first animals" and it refers to the animal-like organisms in various phyla of the Kingdom Protista

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Protozoa

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are unicellular heterotrophs, mainly free-living or parasitic. They reproduce asexually via binary fission, though some have sexual cycles and others undergo conjugation, or nucleus exchange between cells.

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Protozoa are organized into phyla based primarily on their mode of locomotion. What are these 3 modes?

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by flagella, by pseudopodia (false feet) and by cilia

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Phylum Mastigophora

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contains the most primitive protozoa, which generally have two whip-like flagella, though larger members of this phylum may have many. Most Mastigophora reproduce asexually via binary fission, though some reproduce sexually or through syngamy. Most Mastigophora are parasites, though some are free-living; many have no cell wall and are able to form pseudopodia (brief extensions of the cell body) used for locomotion or to engulf prey. Among the parasitic Mastigophora is the flagellate Trypanosoma gambiense, which causes African sleeping sickness, transferred to humans by the nefarious tse-tse fly. Members of the genus Trichonympha enjoy a symbiotic relationship in the gut of termites by helping them digest wood.

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Phylum Sarcodina

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contains the ever-popular amoebas, for whom the formation and use of pseudopodia is a way of life. Locomotion solely with pseudopodia is called streaming. Amoebas create pincer-shaped pseudopodia to catch and engulf prey, a process called phagocytosis. The prey is then drawn into the cell and digested. The cell is sensitive to the chemical stimulus of nearby prey and streams toward its victim when it detects the prey's chemical signals. Although amoebas are nonphotosynthetic, they do have photoreceptors. When a pseudopodium is exposed to bright light, the amoeba withdraws it and streams away. Interestingly, if an entire amoeba is exposed to constant strong light, it will eventually adjust to live with it and carry on its normal behavior.

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phagocytosis

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the process in which a cell engulfs and digests or destroys particulate material

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Sarcodines

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They reproduce both asexually via mitosis (but without breakdown of the nuclear envelope), or sexually via meiosis and the formation of gametes and zygotes. They are usually free-living and found mainly in water, both salt and fresh, though some, like the one that causes amoebic dysentery, are parasites. Some stiffen their pseudopodia with protective spines made from microtubules (e.g. Actinosphaerium).

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foraminifera

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the most common of all protozoans, produce beautiful and delicate shells of calcium carbonate extracted from ocean water. The shells of dead Foraminifera accumulate on the seabed, creating million-year-old foraminiferanooze. Britain’s white cliffs of Dover are made from these skeletons. Another Sarcodine group, the radiolarians, also has skeletons that form a radiolarian ooze on the ocean bottom.

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Phylum Ciliophora

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contains another popular protozoan, the paramecium. As the phylum’s name suggests, its members move by the rapid beating of the cilia on the outside of the cell. In some Ciliophora, the cilia bind together to form clumps, called cirri, or “brushes,” called membranelles, both of which actually enable the cell to “jump” or “walk” along on these stiffened protrusions. Cilia arise from cilia basal bodies, which control and coordinate the even, sequential movement of cilia that propel the cell. Ciliophora have both contractile vacuoles, which rid the cell of liquid, and food vacuoles that fill with enzymes when food particles are swept into it. Ciliophora, oddly, have two types of nuclei, the micronuclei and the macronuclei, which have a very complicated mode of genetic communication. Each cell may have one or more of each type. Reproduction involves conjugation in which genetic information is transferred from cell to cell through direct contact between the cells. During Ciliophora conjugation, micronuclei undergo meiosis, and the two cells exchange haploid micronuclei, which then fuse to form diploid micronuclei. One macronucleus dissolves, and a new one develops from one of the diploid. Like amoebas, paramecia are responsive to external stimuli, including subtle changes in the temperature and chemistry of their environment. A paramecium’s cilia beat constantly, directing water toward an oral groove that is forever sampling the environment. Negative stimuli, such as the presence of a toxic substance, cause a paramecium to stop dead in its tracks, begin beating its cilia in reverse, and beat a hasty retreat. If the negative stimulus is not so dire—a change of temperature, for example—the paramecium will simply turn or move away.

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Phylum Opalinida

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are evenly covered in cilia or flagella and have at least two nuclei (though not the differentiated nuclei of the Ciliophora). Opalinids produce gametes that fuse to form a zygote. Most members of this phylum live their lives in the guts of frogs and toads, or sometimes in the digestive tracts of fish and reptiles.

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Phylum Sporozoa

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are parasites, and some, like the malaria-causing Plasmodium, are quite nasty. All Sporozoa lack cilia and flagella, and thus have no means of locomotion. Sporozoa, as their name suggests, produce spores as part of their complex life cycle. Plasmodia begin life in an amoeboid state, as sporozoites, in the bloodstream of a warm-blooded host. Asexual reproduction occurs in the host’s liver and the resulting merozoites enter the bloodstream and cause malaria by reproducing asexually in red blood cells and then causing the cells to explode. Eventually, the merozoites enter a sexual stage, when they are called gamonts, which produce gametes. A mosquito will take up gamonts when it bites an infected person. Then, fertilization occurs and zygotes develop in the insect’s gut. The oocysts (fertilized eggs) divide asexually to produce sporozoites, which travel to the mosquito’s salivary glands. The next bite transfers the sporozoites to their next victim.

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Which phylum contains protozoa that use pseudopodia almost exclusively as their means of locomotion?

A Mastigophora
B Sarcodina
C Opalinida
D Ciliophora

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The correct answer is B. Sarcodines, which include the amoebas, create pseudopodia and use them to move via streaming. Most Mastigophora use mainly flagella, so A is not correct. Opalinida use cilia or flagella for locomotion, so C is incorrect. Ciliophora use cilia exclusively, so D is not correct.

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The skeletons of which protozoan genus accumulate on the ocean floor to form an ooze?

A Paramecia
B Trypanosomes
C Radiolarians
D Plasmodia

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The correct answer is C. Radiolarians create calcium carbonate shells that accumulate on the ocean floor when the organisms die. Paramecia do not form rigid outer coverings, so A is not correct. Trypanosomes are Mastigophora that have no cell walls and that cause sleeping sickness, so B is incorrect. Plasmodia are parasites that cause malaria, so D is also not correct.

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Which phylum includes the most pathogenic parasites?

A Sarcondina
B Opalinida
C Ciliophora
D Sporozoa

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The correct answer is D. All members of the Phylum Sporozoa are parasitic and many are pathogens. Sarcodina contains some pathogenic parasites, but most members of this phylum are not, so A is incorrect. Opalinida live symbiotically in the guts of amphibians or reptiles, so B is not correct. Ciliophora are ciliates that are generally neither parasitic nor pathogenic, so C is incorrect.

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Phagocytosis is accomplished by which of the following structures?

A Pseudopodium
B Oral groove
C Food vacuole
D Membranelle

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The correct answer is A. A pseudopodium is a temporary extension of the cell body, as in an amoeba, that engulfs prey, which is then ingested. This is the process of phagocytosis. Paramecia have an oral groove that receives food that is swept in by cilia, so B is incorrect. Paramecia also have food vacuoles that receive food from the oral groove, so C is also incorrect. A membranelle is a tuft of cilia used for locomotion in Ciliophora, so D is not correct.