Protozoa

Among the best known of the animal-like protista is the ameba (subphylum Sarcodina or Rhizopoda). The most striking feature of the ameba is its method of locomotion, which involves the transformation of cytoplasm from a liquid state to a semisolid gel. As the ameba moves, the cytoplasm at the rear liquifies and flows to the front and into the extending pseudopodia where it solidifies along the edges. The ameba poses several important biochemical questions: What chemistry underlies the reversible change from liquid to solid cytoplasm? How can the cell membranes break and reform so quickly when an ameba engulfs food particles? Relatives of the ameba include the Radiolaria, marine organisms of remarkable symmetry with complex internal skeletons containing the carbohydrate polymer chitin together with silica (SiO2) or strontium sulfate. The Foraminifera deposit external shells of calcium carbonate or silicon dioxide. Over 20,000 species are known and now as in the distant past their minute shells fall to the bottom of the ocean and form limestone deposits. Tiny ameboid parasites of the subphylum Sporozoa attack members of all other animal phyla. Several genera of Coccidia parasitize rabbits and poultry causing enormous damage. Humans are often the victims of species of the genus Plasmodium which invade red blood cells and other tissues to cause malaria, one of our most serious ailments on a worldwide basis. Throughout history malaria has probably killed more persons than any other disease. Toxoplasma gondii is another parasite which, in its haploid phase, is found throughout the world in wild animals and in humans. Although its presence usually elicits no symptoms, it sometimes causes blindness and mental retardation in children and can be fatal to persons with AIDS. Its sexual cycle occurs exclusively in cats. Another subphylum of protozoa, the Mastigophora, are propelled by a small number of flagella and are intermediate between animals and the algae. One of these is Euglena viridis, a small freshwater organism with a long flagellum in front, a flexible tapered body, green chloroplasts, and a light-sensitive “eyespot” which it apparently uses to keep itself in the sunshine. Euglena is also able to live as a typical animal if there is no light. Treatment with streptomycin causes Euglena to lose its chloroplasts and to become an animal permanently. The dinoflagellates, some colorless and some green, occur in great numbers among the plankton of the sea. Giardia lamblia is a troublesome intestinal parasite. The hemoflagellates are responsible for some of our most terrible diseases. Trypanosomes (genus Trypanosoma) invade the cells of the nervous system causing African sleeping sickness. Mutating their surface proteins frequently by genescrambling mechanisms, these and other parasites are able to evade the immune response of the host. For the same reason it is difficult to prepare vaccines against them. Other flagellates live in a symbiotic relationship within the alimentary canals of termites and roaches. Termites depend upon bacteria that live within the cells of these symbiotic protozoans to provide the essential enzymes needed to digest the cellulose in wood. Members of the subphylum Ciliophora, structurally the most complex of the protozoa, are covered with a large number of cilia which beat together in an organized pattern. The following question immediately comes to mind: How are the cilia able to communicate with each other to provide this organized pattern? Two ciliates that are often studied by biochemists are Tetrahymena, one of the simplest, and Paramecium, one of the more complex. TheMyxomycetes or “slime molds” are more closely related to protozoa than to fungi. Members of the family Acrasieae, the best studied member of which is Dictyostelium discoideum, start life as small amebas. After a time, when the food supply runs low, some of the amebas begin to secrete pulses of a chemical attractant cyclic AMP. Neighboring amebas respond to the pulses of cyclic AMP by emitting their own pulses about 15 s later, then moving toward the original source. The ultimate effect is to cause the amebas to stream to centers where they aggregate and form fungus-like fruiting bodies. Asexual spores are formed and the life cycle begins again. Other Myxomycetes grow as a multinucleate (diploid) plasmodium containing millions of nuclei but no individual cell membranes. Physarumpolycephalum, a species whose plasmodium may spread to a diameter of 30 cm, has become popular with biochemists. The 800,000 nuclei per square millimeter all divide synchronously.

A few well-known protists.