Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Setting the Scene
The Kingdom Protista
Protozoa

The kingdom Protists (Protista) generally includes both Unicellular Eukaryotes and Multicellular Organisms in which all Cells perform similar Functions (i.e., tissue differentiation is absent or poorly expressed). Fungi may also be included here, although they are sometimes classified as a separate kingdom.

Among the best-known animal-like Protozoans is the amoeba [subphylum Sarcodina, or rhizopods (Rhizopoda)]. The most remarkable feature of the amoeba (Fig. 1-7) is its method of locomotion, which involves the transition of Cytoplasm from a liquid state to a semisolid gel. As the amoeba moves, the cytoplasm at the rear of The Cell liquefies and flows forward into the advancing pseudopodia, where it subsequently solidifies at the edges. This Organism poses several fundamental questions for biochemists. What is the Chemical Nature of the reversible transition of cytoplasm from a liquid to a gel? What chemical processes cause contractile vacuoles [33] to periodically expel excess fluid—acting essentially as a rudimentary excretory system within a single cell? Finally, how do rapid rupture and repair of cell membranes occur when an amoeba engulfs food particles?

Organisms related to the amoeba include radiolarians (Radiolaria)—marine protozoans that are exceptionally symmetrically organized creatures with a complex internal Skeleton composed of the carbohydrate polymer Chitin and silicon dioxide (SiО₂) or strontium sulfate. Foraminifera (Foraminifera) are small amoebas bearing external shells made of calcium carbonate or silicon dioxide. More than 20,000 species of foraminifers are currently known, and to this day, just as in the distant past, their tiny skeletons drift to the ocean floor and form limestone deposits.

The smallest protozoans, belonging to the subphylum Sporozoa, parasitize animals of all types. Several genera of pathogenic coccidia (Coccidia) parasitize rabbits and poultry, causing enormous economic losses.

Humans frequently fall victim to protozoans of the genus Plasmodium (Fig. 1-7), which invade the Blood and other Tissues to cause malaria, one of the most widespread human diseases globally. Throughout human history, malaria has likely claimed more lives than any other disease.

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FIG. 1-7. Well-known representatives of protists.

Another protozoan subphylum, the flagellates (Mastigophora), which move using a small number of flagella, serves as an evolutionary link between animals and Algae. Euglena viridis (euglena), a tiny freshwater organism with a flexible, cone-shaped body and a long flagellum at the anterior end, contains green METABOLISM/14.html">Chloroplasts and a light-sensitive eyespot that enables it to accumulate in sunlit areas (Fig. 1-7). In the absence of light, euglena adopts a lifestyle characteristic of animals. Treatment with streptomycin (Supplement 12-A) causes euglena to lose its chloroplasts, irreversibly converting it into an animal-like form. Marine plankton abound in armored flagellates (dinoflagellates, Dinoflagellata) (Fig. 1-7), which include both colorless and green forms.

Haemoflagellates (Haemoflagellata) are the causative agents of some of the most devastating human diseases. Trypanosomes (genus Trypanosoma) invade cells of The Nervous system, causing sleeping sickness. Certain flagellates maintain symbiotic relationships with other organisms. Some of the most complex flagellates known today (Fig. 1-7) inhabit the digestive tracts of termites and roaches. In turn, the cells of these protozoans harbor symbiotic Bacteria that provide termites with the Enzymes necessary to digest wood Cellulose.

Members of the subphylum Ciliophora (Ciliates) are the most structurally complex protozoans. They bear numerous cilia, whose synchronized beating serves as their means of locomotion. This immediately raises a question: How is the coordination among the cilia achieved? Biochemists most frequently utilize two representatives of this subphylum in their research—Tetrahymena [34] (Fig. 1-7), the simplest ciliate, and Paramecium, which is more complexly organized.

Myxomycetes (slime Molds) are probably more closely related to protozoans than to fungi. Members of the family Acrisieae, among which Dictyostelium discoideum is the most extensively studied, begin their life cycle as small amoebas. After a time, when food supplies are depleted, individual amoebas begin releasing small bursts of a chemical attractant—cyclic AMP (Ch. 6, Sec. E.5). Nearby amoebas respond to this signal after approximately 15 s by also releasing a pulse of cyclic AMP, and then begin migrating toward the original source of the attractant [35]. Eventually, the amoebas cluster around several centers, aggregate, and form mushroom-like fruiting bodies. Within these bodies, spores are produced vegetatively, and the entire cycle begins anew. Other slime molds grow as a multinucleate (diploid) plasmodium containing thousands of nuclei not separated by cell membranes.