Single-celled organisms are considered the most primitive representatives of the animal kingdom. They form an extensive type of protozoa, the variety of which we will consider today. The Latin name for this type is Protozoa. Since unicellular organisms are difficult to divide into animals (Protozoa) and plants (Protophyta), they are often grouped together as Protista. The variety of protozoa is amazing. They number over 30,000 species, and most of them are invisible to the naked eye because they are no larger than the tip of a needle. Let's try to briefly characterize the whole variety of protozoa.
Brief characteristics of protozoa
All these organisms are divided into 4 classes depending on the mode of movement. Sarcodidae (Sarcodina) move thanks to pseudopodia; with them, these mostly free-living organisms capture prey. Flagella (Mastigophora) move with the help of one or more flagella. Sporozoa, including Plasmodium sp., are parasitic forms lacking special locomotor adaptations. Ciliates (Ciliata) combine mainly free-living forms that move with the help of cilia. The Stenior ciliate, which looks like a miniature tube, feeds on the surface of aquatic plants.
Like all other animals, protozoa are generally mobile, feeding on a variety of chemically complex foods. In their body, it breaks down, supplying it with energy. All the variety of protozoa needs an aquatic environment. Although most of the species live either in sea or fresh water, many of them lead a parasitic lifestyle in the organisms of higher animals, including humans, where they are often the cause of various diseases. The variety of pathogenic protozoa is great.
Flagellates
Flagellates include two genera - Euglena and Chlamydomonas, whose representatives contain chlorophyll and, therefore, are capable of photosynthesis. This also includes the armored flagellates (Dinoflagellata), dressed in a cellulose capsule and included in the plankton. A parasitic form of Trypanosoma (pictured below) lives in humans, causing sleeping sickness.
Sarcode
Sarcodes are another group with a large number of species. All this variety of protozoa is difficult to characterize, so let's say a few words about the most famous. We are all well acquainted with such a representative of Sarcodidae as a free-living Amoeba proteus since school (pictured below). Amoebais a single-celled animal belonging to an extensive phylum of protozoans that thrive wherever there is adequate moisture.
They range in size from microscopic blood parasites Babesia to large foraminifera, whose shells are 5 cm long.
Rayflowers, sunflowers and sporozoans
Rays (Radiolaria) and sunflowers (Heliozoa) have a silica skeleton. Therefore, they are sometimes collected and used as an abrasive material. Falling out of the general rule of variability, sporozoans demonstrate a high degree of homogeneity, which is not the case for the entire variety of protozoa. Parasitic protozoa - all their types. Therefore, sporozoans are deprived of the organelles necessary for movement and nutrition: they do not need to move, and they assimilate food that has already been digested. Their life cycle combines stages that reproduce both asexually and sexually, resulting in the formation of spores that can produce many hundreds of individuals.
Infusoria shoe
Paramecium (Infusoria shoe) is a specialized unicellular animal. It is certainly worth talking about it, characterizing the diversity of aquatic protozoa. The outer layer of the contents of the cell - ectoplasm - is limited by a dense shell that carries many tiny cilia. Their rhythmic coordinated beats allow the animal to move. The peristome leads to a blind outgrowth - the pharynx, surrounded by granular endoplasm. Food particles enter the pharynx due to the movements of the cilia, and thenenter the vacuole. The contents of the digestive vacuoles moving in the endoplasm are digested by enzymes. Undigested residues are thrown out through the powder. The water balance is maintained thanks to the activity of two pulsating vacuoles. Of the two nuclei, the larger (macronucleus) is associated with the metabolism in the cell, and the smaller (micronucleus) is involved in the sexual process.
Plasmodium vivax
Let's consider another well-known species, characterizing the variety of protozoa. Parasitic protozoa are numerous, but this causative agent of malaria is especially troublesome for humans. Plasmodium vivax, getting into the bloodstream of a person after being bitten by a female Anopheles mosquito, penetrates into the liver cells, where it multiplies. When the affected cells rupture, the plasmodium comes out and infects new ones. Then it is repeatedly introduced into erythrocytes, multiplying in them and destroying them. Finally, male and female sex cells (gametocytes) appear. Getting into the next mosquito with blood, male gametocytes divide in his stomach, forming gametes. From the product of their fusion - the zygotes - new plasmodia appear, penetrating into the salivary glands of the mosquito. And the cycle repeats.
Reproduction of protozoa
In asexual reproduction, the protozoa divide in half, forming two individuals. This division of fully formed cells captures both the protoplasm and the nucleus. As a result, two identical daughter cells are formed. Under adverse conditions, some flagellates and sarcodes secrete a dense, impenetrable protective sheath (cyst),within which the cell can divide. When exposed to favorable conditions, the cyst is destroyed, and individuals appear that reproduce asexually.
Methods of sexual reproduction of protozoa are very diverse. Paramecium, for example, reproduces by conjugation: two individuals belonging to different lines merge sideways, and then, after nuclear fission and exchange of nuclear material, diverge. Later, both partners can share, producing up to eight (four from each) daughter individuals with nuclei with mixed heredity. Amoeba, reproducing asexually, divides into two daughter cells. They are of the same size. At the beginning of division in the nucleus, which becomes shorter and thicker, chromosomes appear; each consists of two chromatids. The pulsating vacuole divides and its halves separate. At the same time, the chromatids separate, and the cytoplasm begins to lace up in half. With the end of chromosome division, the cytoplasm is also divided. The resulting daughter cells are identical.
Nutrition for protozoa
Like other animals, protozoa get energy by eating complex organic compounds. Amoeba sp. captures food particles with pseudopodia, and they are digested in digestive vacuoles with the participation of enzymes. Paramecium sp. lives mainly due to bacteria, driving them into the cirrus by movements of cilia. Trichonypha sp. lives in the intestines of termites and feeds there on those substances that are not absorbed by the host. Acineta sp. (pictured below) use only certain types of ciliates, which are sometimes larger than themselves.
Movement
Protozoa move in three main ways. Sarcodes "crawl" by forming outgrowths of protoplasm. The movement is created due to the direction of the endoplasmic current in one direction and its reversible transformation on the periphery into gelatinous ectoplasm. Thanks to the sharp blows of the flagellum, the flagellates move. Infusoria move with many tiny oscillating cilia.
Bacteria and viruses
The general characteristics and variety of protozoa should be supplemented by a brief account of bacteria and viruses, which are often confused with them. They cause a lot of trouble to man, but they play a special role in nature. Bacteria and viruses are the smallest organisms on the planet. Although they are relatively simple organized beings, they cannot be called primitive. They are able to survive in very adverse conditions, and their great ability to adapt to changing conditions puts them on a par with the most advanced and successful forms. Viruses are not cells, so they cannot be classified as unicellular, but bacteria can be considered as such. However, they are not the simplest, since they do not have a nucleus. Let's talk about them in more detail.
Where bacteria live
Unlike viruses, bacteria are cells. However, they are much simpler than the cells of highly organized creatures, and vary greatly in size and shape. Bacteria are found everywhere. They can live even under conditions that preclude the existence of more complex organisms. They are met in the oceaneven at a depth of 9 km. With the deterioration of environmental conditions, bacteria form a stable resting stage - endospore. It is the most stable living organism known: some endospores do not die even when boiled.
Of all possible habitats, the most risky is another organism. Bacteria enter it usually through wounds. But, having penetrated inside, they must resist the defenses of their victim, especially against phagocytes (cells that can capture and digest them) and antibodies that can neutralize their harmful effects. Therefore, some bacteria are surrounded on the outside by a mucous membrane that is invulnerable to phagocytes; others, after being captured by phagocytes, can live in them; finally, others produce masking substances that help them hide their presence in affected cells, and the latter do not produce antibodies.
Harmful and beneficial bacteria
Bacteria can cause harm in three ways: for example, by blocking various vital channels in the body due to their abundance; the release of toxic substances (the toxin of the soil bacterium Clostridium tetani (pictured below), which causes tetanus, is one of the most powerful poisons known to science); as well as stimulating allergic reactions in victims.
Antibiotics were effective against microbial infections for a while, but many bacteria have developed resistance to a number of drugs. They multiply rapidly, dividing under favorable conditions every 10 minutes. At the same time, of course, the chances of the appearance of mutants resistant to thoseor other antibiotics are increasing. But not all bacteria living in other organisms are harmful. So, in the gastrointestinal tract of a cow, sheep or goat there is a special section - the scar, which is home to many bacteria that help animals digest plant fiber.
Mycoplasmas
Mycoplasmas - the smallest of all cellular organisms and possibly a transitional stage between viruses and bacteria - occur naturally in wastewater, but can also infect animals, causing them diseases such as, for example, some forms arthritis in pigs.
Meaning of bacteria
These organisms decompose corpses and return their organic matter to the soil. Without this constant cycle of organic building blocks, life could not exist. Man widely uses the vital activity of bacteria to turn organic waste and raw materials into useful products when composting, making cheese, butter, vinegar.
In conclusion
As you can see, the variety and importance of the simplest is great. Despite the fact that their size is very small, they play an important role in maintaining life on our planet. Of course, we have only briefly described the diversity of the simplest animals. We hope you have the desire to get to know them better. The systematics and diversity of protozoa is an interesting and extensive topic.
