There are about 70,000 species of unicellular animals in the world fauna.
Almost all simple ones are microscopic in size (from 2 microns to 0.2 mm), among them there are also colonial forms (volvox). Single-celled organisms live in freshwater (common amoeba, green euglena, infusoria-shoe, volvox) and marine water bodies (foraminifera, promenyaks), in the soil (some types of amoebae, flagellates, ciliates).
The simplest are representatives of the animal world, located at the cellular level of organization. Morphologically, they constitute one cell, and functionally they constitute an integral organism. Therefore, the cell of the simplest is built much more complicated than the cell of a multicellular organism.
This is due to the fact that the cells of multicellular organisms perform only certain functions, while one cell of the simplest performs all the vital functions inherent in the whole organism: nutrition, movement, excretion, respiration, reproduction, etc.
Features of the structure and vital activity of unicellular organisms (protozoa)
The protozoan cell, like any eukaryotic cell, has general cellular organelles. In the cytoplasm of protozoa, two layers are distinguished: the outer - ectoplasm and the inner - endoplasm. In addition, the protozoa have organelles characteristic only of them: movements (pseudopodia, flagella, cilia), digestion (digestive vacuoles, ciliates have a cellular mouth, pharynx), excretion and osmoregulation (contractile vacuoles).
A cell of unicellular animals contains one (ameba, euglena) or several (ciliates) nuclei. The vast majority of unicellular organisms have the ability to move. With the help of temporary bulges of the cytoplasm - false legs (pseudo-legs), simple ones, devoid of a dense cell membrane (ameba), move. Flagella (euglena green) and cilia (ciliates-shoe) contribute to the rapid movement of unicellular organisms.
The ways of feeding protozoa are varied. Most of them feed heterotrophically. In amoeba, food enters the cytoplasm with the help of pseudopodia, which capture it. In ciliates, vibrations of the cilia cause food to enter the cellular mouth and pharynx.
Digestion of food occurs in digestive vacuoles. Undigested food residues are removed from the cell in any place to which the digestive vacuole (amoeba) approaches or through special openings (powder in ciliates-shoes).
Among unicellular animals, there are species that feed like green plants (volvox). In their cytoplasm there are chromatophores - organelles with photosynthetic pigments. For some flagellates with chromatophores (euglena green), a characteristic mixed (mixotrophic) type of nutrition. In the light, they are capable of photosynthesis, and in the dark they feed on ready-made organic substances.
Respiration is carried out by the supply of oxygen through the entire surface of the cell. It oxidizes complex organic substances to CO 2 , H 2 O and other compounds. At the same time, energy is released, which is used for the life processes of animals.
For protozoa, asexual and sexual methods of reproduction are characteristic. Asexual reproduction is carried out by division and budding. More often single-celled ones reproduce by dividing the mother's organism into two daughter cells.
For ciliates-shoes, in addition to the section, there is a characteristic sexual process, during which two ciliates are temporarily connected to each other and exchange small nuclei. Thus ciliates exchange genetic (hereditary) information contained in their nuclei.
Unicellular is characterized by irritability - the response-reaction of the body to external influences. Unicellular organisms endure unfavorable environmental conditions in the state of a cyst - the cell is rounded, compressed, draws in the organelles of movement and is covered with a thick membrane.
Soil formation processes are also carried out with the help of protozoa. Flagellar unicellular serve for the biological assessment of the degree of purity of water bodies (biodiagnostics). Foraminifers and promenacs play a significant role in the formation of chalk and limestone deposits, which are valuable building materials.
The most primitive representatives of the animal kingdom are unicellular organisms. 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 description of protozoa
Flagella
Sarcode
Sarcodidae are another group that includes 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 since school with such a representative of the Sarcode as a free-living one (pictured below). The amoeba is a single-celled animal belonging to a large phylum of protozoans that thrive wherever there is adequate moisture.
Rays, sunflowers and sporozoans
Infusoria shoe
Paramecium (shoe ciliate) 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 through the movements of the cilia, and then enter the vacuoles. 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
During 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.
Nutrition of 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) only certain types of ciliates are used for food, 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 current of the endoplasm in one direction and its reversible transformation on the periphery into gelatinous ectoplasm. Thanks to the sharp blows of the flagellum, the flagellates move. Ciliates move with the help of many tiny oscillating cilia.
Bacteria and viruses
The general characteristics and variety of protozoa should be supplemented by a brief account of 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 unfavorable 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 do 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 found in the ocean even at a depth of 9 km. With the deterioration of environmental conditions, bacteria form a stable resting stage - endospore. It is the most stable of known living organisms: 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, still others produce masking substances that help them hide their presence in the 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); and by stimulating allergic reactions in victims.
Antibiotics have been effective against microbial infections for some time, 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, naturally, the chances of the emergence of mutants resistant to certain antibiotics increase. 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 - a 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 diseases such as some forms of arthritis in pigs.
Importance 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 convert organic waste and raw materials into useful products in composting, making cheese, butter, and vinegar.
Finally
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.
The type of protozoa includes animals, the ancient forms of which were the progenitors of the entire diverse animal world. In this regard, the study of protozoa is of great importance for understanding the evolution of the animal world. The type under consideration includes up to 40,000 species. The simplest are widespread on our planet and live in various environments - in the seas and oceans, fresh waters, and some species - in the soil. Many protozoa have adapted to living in the body of other organisms - plants, animals, humans. All of them perform various functions: they actively participate in the cycle of substances, purify water from bacteria and decaying organic matter, affect soil-forming processes, and serve as food for larger invertebrates. Many marine unicellular organisms have hard mineral skeletons. For tens of millions of years, the microscopic skeletons of dead animals sank to the bottom, forming powerful deposits of limestone, chalk, and green sandstone there. The skeletons of some protozoa are used in the practice of geological exploration to determine oil-bearing layers.
The simplest are microscopically small animals of various shapes, the sizes of which range from 2-3 to 50-150 microns and even up to 1-3 mm. The largest representatives of this type, for example, shell rhizopods living in the polar seas off the coast of Russia, and fossil nummulites reach a diameter of 2-3 cm.
The body of protozoa consists of the same components as a multicellular cell - the outer membrane, cytoplasm, nucleus and organelles, and at the same time morphologically corresponds to one cell. Because of this, protozoa are often called unicellular animals (Monocytozoa). However, in physiological terms, they cannot be equated with individual cells of multicellular (Metazoa), since their body performs all the functions characteristic of multicellular animals. The only cell, which is the organism of the simplest, moves, captures food, multiplies, defends itself from enemies, that is, it has all the properties of the whole organism and physiologically corresponds to it. Therefore, protozoa are now called organisms at the cellular level or "non-cellular" organisms.
The nucleus is an essential part of the body of protozoa. Usually there is one core. However, there are also multi-core forms. Ciliates always have two nuclei: a large vegetative - macronucleus and a small generative - micronucleus. The nucleus regulates life processes and plays an important role in reproduction and the transfer of hereditary properties to offspring.
Most of the body of the protozoan is made up of protoplasm. Under a microscope, one can distinguish in it an outer dense, transparent, homogeneous (homogeneous) layer - ectoplasm and a usually granular endoplasm located inside of a more liquid consistency. Protoplasm serves as the main substrate of life activity.
The surface of ectoplasm in most forms is represented by a thin elastic shell - pellicle (Latin pellicula - skin), consisting of proteins and fat-like substances. Possessing the semi-permeable property, the shell regulates the flow of substances from the external environment (water, salts, oxygen, etc.). The pellicle is part of the living protoplasm. In some species, a thick shell develops on the surface of the body (pellicles) - the cuticle (Latin cuticula - skin), which plays a protective and supporting role. The cuticle does not have the properties of living protoplasm.
In the endoplasm, in addition to the nucleus, there are general-purpose organelles - mitochondria, the endoplasmic reticulum, the reticulum apparatus, etc. In addition, in accordance with the functions inherent in the whole organism, protozoa have special-purpose organelles that perform the functions of movement, nutrition, excretion, protection, etc.
Special purpose organelles
In connection with nutrition, excretion, movement and other functions in the body of protozoa, separate sections of protoplasm are isolated, which perform certain vital functions of unicellular organisms as independent organisms. These areas are collectively known as organelles, or organelles. In protozoa, special-purpose organelles are isolated in accordance with their functions, unlike any other cells that have organelles of general importance (mitochondria, centrosomes, ribosomes, etc.)
Food organelles have a different structure. Depending on the type of assimilation and the mode of nutrition, protozoa are divided into several groups (Fig. 1).
The first group consists of autotrophic protozoa. They feed like green plants, absorbing carbon dioxide, water and mineral salts from the external environment (holophytic nutrition). Assimilation organelles in them are chromatophores containing chlorophyll. In sunlight, with their participation, carbohydrates are synthesized. Autotrophic protozoa do not need ready-made organic substances. They synthesize carbohydrates, fats and proteins from inorganic substances.
The second group consists of heterotrophic protozoa that do not have chlorophyll. They can only use ready-made organic matter for food. Most of them feed on bacteria, algae, protozoa. This way of eating is called holozoic (animal). At the same time, food is digested in special organelles - digestive vacuoles that look like a bubble. Vacuoles form in the protoplasm around an ingested food particle. If there is a lot of food, several vacuoles appear simultaneously in the body of the simplest. Digestion of food occurs with the participation of digestive juices coming from the protoplasm. Many protozoa have organelles that serve to enter food particles into their body and throw undigested food debris out. These include the cell mouth - the cytostome, the cell pharynx - the cytopharynx, and the anal pore.
Organelles of excretion. Most freshwater species have special pulsating vacuoles. They look like bubbles, to which a system of tubules approaches from the protoplasm. Pulsating vacuoles gradually fill with liquid, after which, rapidly contracting, they throw the liquid out. In this way, the protozoa are freed from excess water, which, when living in a freshwater reservoir, according to the law of osmosis [show] all the time enters their body. If water is not removed, swelling and death of the protozoan will occur.
The phenomenon of osmosis is as follows: if two solutions with different concentrations are separated by a semi-permeable membrane, then the solvent (water) passes from a solution with a lower concentration to a solution with a higher concentration.
Organelles of movement in protozoa (Fig. 2) they serve:
- pseudopodia or pseudopodia (Greek pseudos - false, podos - leg), which are temporary protoplasmic protrusions; occur in an amoeba anywhere on its body. The movement is carried out due to the current of protoplasm, which gradually overflows into one of the pseudopodia; while the opposite end of the body is shortened.
- flagella (or scourges) - permanent organoids that look like long protoplasmic filaments, usually starting at the anterior end; They produce helical movements.
- cilia are permanent organelles, which are numerous short protoplasmic filaments. Their movements consist of quick swings in one direction and a slow subsequent straightening.
Movement is closely connected with irritability and often serves as its external manifestation. Irritability is the ability of the organism to respond to the influence of the external and internal environment with certain active reactions.
The simplest are irritable. They respond to the action of various mechanical, light, chemical or other environmental stimuli with a directed movement, called taxis (Greek taxis - arrangement in order). Taxis are distinguished, directed either towards the stimulus or away from it, and depending on the stimuli, thermo-, photo-, hydro, chemo-, galvanotaxis, etc. are distinguished. One of the forms of movement characteristic of taxis is amoeboid movements associated with the deformation of the cell by the formation of protoplasmic protrusions in the form of pseudopodia. In the formation of pseudopodia, the ability of protoplasm to pass from the state of gel to sol and vice versa is manifested. Flickering movements are carried out by flagella and cilia.
Some species have special organelles for the perception of stimuli. These include photosensitive eyes, tactile bristles, etc.
Skeletal formations are found in the body of protozoa. The outer skeleton is often represented by calcareous or flint shells. Of the internal skeletal formations, we should mention a special axial rod - axostyle (Gech. acson - axis, stylos - stick).
Protection organelles. Some protozoa have protective devices - trichocysts - short sticks located in the ectoplasm under the pellicle. When irritated, the trichocysts shoot out, turning into a long elastic thread that strikes the enemy or prey.
reproduction
Protozoa reproduce asexually and sexually. Asexual reproduction occurs both in the form of division into two parts, and in the form of multiple division (Fig. 3).
In the form of division into two parts, it begins with the division of the cell nucleus. In this case, the nuclear structures are evenly distributed between the two newly formed nuclei (mitosis). Following the nucleus, the protoplasm divides, after which the two newly emerged daughter individuals begin an independent life.
In most protozoa, it proceeds in the form of copulation, in ciliates - in the form of conjugation (Fig. 4).
During copulation (lat. copulare - to connect), two individuals approach each other, their protoplasm and nuclei merge, forming one individual - a zygote, which then reproduces asexually.
Conjugation (Latin conjagatio - conjugation, copulation) is a form of sexual reproduction characteristic of ciliates. During conjugation, two ciliates are applied to each other with their bodies. Their nuclei undergo complex restructuring. The macronuclei of both partners are destroyed and disappear. Micronuclei after double fission and destruction of a part of the nuclear material form a stationary and wandering nucleus in each ciliate. The first remains in place, and the second, moving, passes into a partner, where it merges with its stationary core. Then the partners diverge, and their nuclei after division form a micro- and macronucleus. Conjugation is a kind of fertilization and is associated with the combination of hereditary factors (genes) of two individuals.
encysting
If the encysted individual again finds itself in favorable conditions, excystation occurs; the animal leaves the cyst, turns into a vegetative form and resumes active life. Encystation of pathogenic protozoa plays an important role in the spread of protozoan diseases.
Life cycle
In the life cycle of some protozoa, morphologically different forms alternate. There are vegetative, sexual and encysted forms. The former are characterized by active nutrition and growth. They usually reproduce asexually. The latter are represented by micro- and macrogametes. Their appearance precedes the sexual process. The encysted forms (cysts) are characterized by resistance to adverse environmental conditions.
Classification
The division of the type of protozoa into classes is based mainly on the structure of the organoids of movement and the characteristics of reproduction. The classification is generally accepted, according to which all protozoa are divided into 4 classes.
The type of protozoa or protists are single-celled animals that are not visible to the naked eye. The type of protozoa was discovered only after the invention of magnifying glasses, magnifiers, microscopes. Anton Leeuwenhoek was the first who began to study unicellular animals. Leeuwenhoek was an intelligent and talented person. He made important scientific discoveries while studying the type of protozoa. Improving his magnifying glasses and microscopes, Anton Leeuwenhoek achieved an increase in the objects under consideration by 300 times. Once he accidentally discovered in a drop of water a whole world of previously unknown, the simplest invertebrate animals of the smallest sizes.
Amoeba is a large amoeboid organism, is the simplest microscopic animal of the protozoan type, which can be found in aquariums, ponds and swamps.
Ciliates are a highly organized type of protozoa. Ciliates live in fresh water and seas, the most common type of ciliates is the shoe.
Euglena green belongs to the kingdom of eukaryotes. This type of protozoa lives in swamps, fresh water bodies and ditches.
All microscopic animals related to unicellular organisms have a number of common features. For example, euglena, amoeba and slipper are animals whose body consists of one cell. Therefore, they are called unicellular. Among other animals, they have a simple structure. This indicates the great antiquity of this type of animal. From the simplest living beings that inhabited the Earth in the distant past, in the process of further development, the first plants and the first animals originated.
Today, more than 30,000 species of unicellular microscopic animals are known.
Types of protozoa
Euglyph- lat. Euglipha, a member of the sarcomastigophora type, belongs to the class Rhizome. This is a very small animal, located in a round-shaped shell.
Nightlight or noctiluca - lat. Noctiluca miliaris, belongs to the phylum Protozoa. Nightlight has the ability to glow in water. A characteristic feature of the night light is the presence of flagella, which are organelles of movement.
The phylum Protozoa includes approximately 25,000 species of unicellular animals living in water, soil, or organisms of other animals and humans. Having a morphological similarity in the structure of cells with multicellular organisms, the protozoa differ significantly from them in functional terms.
If the cells of a multicellular animal perform special functions, then the cell of the simplest is an independent organism capable of metabolism, irritability, movement and reproduction.
The simplest are organisms at the cellular level of organization. Morphologically, the protozoan is equivalent to a cell, but physiologically it is a whole independent organism. The vast majority of them are microscopically small in size (from 2 to 150 microns). However, some of the living protozoa reach 1 cm, and the shells of a number of fossil rhizopods are up to 5-6 cm in diameter. The total number of known species exceeds 25 thousand.
The structure of the protozoa is extremely diverse, but they all have features characteristic of the organization and function of the cell. Common in the structure in the structure of protozoa are the two main components of the body - the cytoplasm and the nucleus.
cytoplasm
The cytoplasm is bounded by an outer membrane that regulates the flow of substances into the cell. In many protozoa, it is complicated by additional structures that increase the thickness and mechanical strength of the outer layer. Thus, formations such as pellicles and shells arise.
The cytoplasm of protozoa usually breaks up into 2 layers - the outer one is lighter and denser - ectoplasm and internal, equipped with numerous inclusions, - endoplasm.
General cellular organelles are localized in the cytoplasm. In addition, a variety of special organelles may be present in the cytoplasm of many protozoa. Various fibrillar formations are especially widespread - supporting and contractile fibers, contractile vacuoles, digestive vacuoles, etc.
Core
The simplest have a typical cell nucleus, one or more. The nucleus of protozoa has a typical two-layer nuclear envelope. Chromatin material and nucleoli are distributed in the nucleus. The nuclei of protozoa are characterized by exceptional morphological diversity in terms of size, number of nucleoli, amount of nuclear juice, etc.
Features of the vital activity of protozoa
Unlike somatic cells, multicellular protozoa are characterized by the presence of a life cycle. It is composed of a series of successive stages, which are repeated in the existence of each species with a certain regularity.
Most often, the cycle begins with the stage of the zygote, which corresponds to the fertilized egg of multicellular organisms. This stage is followed by singly or repeatedly repeated asexual reproduction, carried out by cell division. Then sex cells (gametes) are formed, the pairwise fusion of which again gives a zygote.
An important biological feature of many protozoa is the ability to encystment. At the same time, the animals round out, shed or draw in the organelles of movement, secrete a dense shell on their surface, and fall into a state of rest. In the encysted state, protozoa can tolerate drastic environmental changes while remaining viable. When conditions favorable for life return, the cysts open and the protozoa emerge from them in the form of active, mobile individuals.
According to the structure of the organelles of movement and the characteristics of reproduction, the protozoan type is divided into 6 classes. The main 4 classes are Sarcodaceae, Flagellates, Sporozoans and Ciliates.
