Ray was a member of Science Olympiad, participates in science and health writing competitions, and studied at a sci-tech school.
What Are Protists?
The organisms that belong to Kingdom Protoctista are ordinarily called protists. Protists live in a variety of environments. Some are said to be free-living. Those living inside or on other organisms' bodies are said to be parasitic. Protists occupy specific habitats; some float in water, while some live at the bottom of a body of water. Some live only in freshwater, others in saltwater. Some live on moist surfaces, such as fallen leaves and branches of trees.
Some leaves are parasites of plants, some animals, and some other protists. Parasitic protists also have specific habitats. Some live only on the surface of the host's body and are called ectoparasites. On the other hand, those that live inside the host's body are called endoparasites. Endoparasites themselves have their habitats; most of them live inside the host's food tube and some in the blood.
Protists abound in nature. The number of species of protists, as well as the number of individuals for each species, is tremendous. It is estimated that a pond of ordinary size contains billions of free-living protists, and a human body hosts a large number of parasitic protists. Kingdom Protoctista includes three big groups of organisms: protozoans, eukaryotic algae, and fungus-like slime molds.
What Is a Protozoa?
Protozoans consist primarily of eukaryotic and single-celled organisms. They are represented by four major groups: Flagellates, Ciliates, Sarcodina, and Sporozoans. In some systems of biological classification, protozoan is a high-level taxonomic group.
When first introduced in 1818, protozoa was erected as a taxonomic class, but in later classification schemes, it was elevated to a variety of higher ranks. These higher ranks pertain to phylum, subkingdom, and kingdom.
Characteristics of Protozoa
- All protozoans are heterotrophic. Heterotrophic organisms derive nutrients from other organisms by ingesting them whole or consuming their organic remains and waste products.
- Unlike plants, fungi, and most types of algae, protozoans do not typically have a rigid cell wall but are usually enveloped by elastic structures of membranes that permit movement of the cell.
- All protozoans reproduce asexually by binary fission or multiple fission.
- Protozoans are important food sources for microinvertebrates. Thus, the ecological role of protozoa in transferring bacterial and algal production to successive trophic levels is important.
Flagellates are a unicellular type of protozoans and are about 2,000 species. Some flagellates are free-living, such as Chlamydomonas. Some are parasitic, such as Trichomonas. Some live singly, such as Euglena. Some live in colonies, such as Synura. Euglena belongs to a group of protists that move about with the help of one or more thread-like flagella, commonly referred to as flagellates.
How do flagellates affect man? Certain flagellates, such as Synura uvella, give an unpleasant odor and taste to our drinking water when present in large numbers. Other flagellates cause diseases in man. One example is Trypanosoma gambiense, which causes Gambian sleeping sickness. This disease is characterized by increasing physical weakness and mental depression and leads to death unless treated.
Several flagellates live in a person's body without making him sick unless present in great numbers. They are mostly found in the food tube, mouth, and intestines. An example of these is Trichomonas.
Examples of Flagellates
- Termites.The interesting group of flagellates is those that live in the intestines of termites. One of them is Trichonympha campanula. Termites, by themselves, cannot make use of hardwood they ear, because they cannot digest the substance cellulose of wood. The flagellates in a termite's intestine change the cellulose into a form that can be digested by the termite. The relationship between the termite and its intestinal flagellate is advantageous to the flagellates since they also get their food from the intestine of their host.
- Volvox. Another flagellate, Volvox, is of great interest in the study of biology. Comparing Volvox with Synura, both of them are unicellular flagellates that live in groups. In Synura, there is no obvious advantage of living together. Each cell performs all the functions necessary to keep it alive and reproduce. On the other hand, the cells in a Volvox colony are of two types - those that perform reproduction only, and those that perform all activities necessary to keep the cells alive but cannot reproduce. Hence, there is a division of labor among the cells of a Volvox colony. Strands of living matter connect neighboring cells of a Volvox colony, such is not true in a Synura colony. This phenomenon in Volvox can be considered a significant event toward the appearance of multicellular forms.
Paramecium belongs to a group of protists that move about through numerous hairlike cilia, commonly referred to as ciliates. "Ciliophora" comes from the Latin term cilium, which means eyelid, and the Greek word phoros, which means bear. The name pertains to the cilia which resembles eyelashes, and which are the main structures for the movement of members of this group. There are about 5000 species of ciliates.
Ciliates are all unicellular, but they exhibit a great deal of variation in shape and the arrangement of cilia. Paramecium, for instance, is slipper-shaped. Its cilia are generally of equal length and distributed over the entire surface of the cell.
Examples of Ciliates
- Vorticella. Vorticella is shaped like a bell. The adult is attached to a surface through a contractile stalk. The swarm stage is free-swimming. In Vorticella the cilia are limited to the mouth area.
- Balantidium coli. Most ciliates are free-living in freshwater, and some in the sea. Several ciliates are parasites of animals. Balantidium coli lives in the intestine of man, monkeys, and pigs. In man, it may cause dysentery and ulcers on the walls of the large intestine. Except for this, ciliates as a group have little effect on man.
Sarcodina, a type of protozoa, is a unicellular organism without definite shape. Its cell membrane is so flexible that it changes its shape constantly. It moves about by extending fingerlike projections called pseudopodia, at the side of the cell towards the direction where it is going. Pseudopodia mean false feet. The pseudopodia of an amoeba are feet in the sense that they are not permanent. They are formed when needed and disappear after use.
Sarcodina comes from the Greek term sarx, which means flesh, and eidos, which means form. There are about 8000 species in this group. Like the ciliates, most Sarcodina is free-living in freshwater and the sea. But there are also species which are parasites of animals. Six species of amoeba have long been known to infect people, one in the mouth and five in the large intestine.
Examples of Sarcodina
- Amoeba Dubia. Amoeba Dubia, an amoeba, is a type of Sarcodina. An amoeba uses pseudopodia for another purpose besides movement. This protist has no mouth. An example is an amoeba capturing a flagellate called Chilomonas. It extends a pseudopodium over the food particle and simply takes it at any point on the surface of the cell. This manner of food-getting is termed "engulfing." Amoeba and other protists that move by the pseudopodia are known as Sarcodina.
- Entamoeba gingivalis. Entamoeba gingivalis lives in the tartar at the base of the teeth. It feeds on bacteria in the mouth, and it is believed to be harmless when only a few are present. It may be transmitted to others by kissing. Entamoeba histolytica lives in man's large intestine. Man gets infected with it through contaminated food or water. This amoeba can produce ulcers on the walls of the intestine and cause amoebic dysentery. Organisms that belong to this group are referred to as foraminiferans. Most of them live in great numbers in the mud at the bottom of the ocean. Foraminiferans are useful in prospecting for oil deposits.
Sporozoans, a type of protist, are species of parasitic protists that produce spores at a certain stage of their life cycle. There about 2,000 known species of sporozoans. The word spore is probably no longer new to you. You have heard of spores of mushroom, fern, and mold. These plants multiply through spores. A plant spore is a single cell with a relatively thick outer wall. In the presence of water, the wall breaks and a new individual develops from the mature spore. In short, a spore is specialized for reproduction.
There is another kind of cell that is specialized for reproduction and is called gamete or sex cell. A gamete is either a male sperm or a female egg. A spore differs from a gamete in that it has no sex, that is, it is neither male nor female.
All sporozoans are parasites of animals. They usually pass from one host to another in the spore stage. Because the spores are light and tiny, they are easily scattered by the wind, water, and other agents from one host to another. Although the sporozoans are among the most widely distributed parasites, only a few of them are very harmful to their hosts.
Examples of Sporozoans
- Plasmodium vivax. Plasmodium spends part of its life cycle in the body of a mosquito of the genus Anopheles. Thus malaria is transmitted through the bite of an Anopheles mosquito. In the Philippines, it is spread by female Anopheles minimus. Two effective ways of controlling the spread of malaria are the following:
- The more immediate measure is to use mosquito nets or screens to prevent being bitten by mosquitoes that have previously bitten a malarial patient.
- Another measure is to destroy the insect host. The adult mosquitoes can be destroyed by directly spraying homes with insecticides. The young larvae can be destroyed right at their breeding places. The breeding places of Anopheles minimus are the shady banks of streams with clear and gentle-flowing water.
- P. malariae
- P. Falcifarum
- P. Ovale
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2020 Ray
Godwin on June 22, 2020:
Ray (author) from Philippines on March 07, 2020:
Thank you Sir Eric! Glad your 10 year old loves reading my science articles. More to come!
Eric Dierker from Spring Valley, CA. U.S.A. on March 07, 2020:
Of course he did not read the whole thing but my 10 year old learned a whole lot. Now we want to know about the bacteria, normal in our body.