What special organelles do protozoa have? Characteristics and significance of the main cell organelles

The body of the protozoan consists of cytoplasm and one or more nuclei. The nucleus is surrounded by a double membrane and contains chromatin, which includes deoxyribonucleic acid (DNA), which determines the genetic information of the cell. Most protozoa have a vesicular nucleus with a small content of chromatin, collected along the periphery of the nucleus or in the intranuclear body, the karyosome. Micronuclei of ciliates are massive nuclei with a large amount of chromatin. Common components of the cell of most protozoa include mitochondria and the Golgi apparatus.

The body surface of amoeboid forms (sarcodidae, as well as some life stages of other groups) is dressed cell membrane about 100 A thick. Most protozoa have a denser but elastic shell, the pellicle. The body of many flagellates is covered with a periplast, formed by a series of longitudinal fibrils fused with the pellicle. Many protozoa have special supporting fibrils, such as the supporting fibril of the undulating membrane in trypanosomes and trichomonads.

Dense and hard shells have resting forms of protozoa, cysts. Testate amoebas, foraminifera and some other protozoa are enclosed in houses or shells.

Unlike the cell of a multicellular organism, the cell of a protozoan is a complete organism. To perform the diverse functions of the body, structural formations and organelles can be specialized in the body of a protozoan. According to their purpose, the organelles of protozoa are divided into organelles of movement, nutrition, excretion, etc.

The organelles of protozoan movement are very diverse. Amoeboid forms move through the formation of cytoplasmic protrusions, pseudopodia. This type of movement is called amoeboid and is found in many groups of protozoa (sarcodes, asexual forms of sporozoans, etc.). Special organelles of movement are flagella and cilia. Flagella are characteristic of the class of flagellates, as well as gametes of representatives of other classes. In most forms they are few in number (from 1 to 8). The number of cilia, which are the organelles of ciliate movement, can reach several thousand in one individual. Electron microscopic studies have shown that flagella and cilia in Protozoa, Metazoa and plant cells are built according to a single type. Their basis is a bundle of fibrils, consisting of two central and nine paired, peripheral ones.

The tourniquet is surrounded by a membrane, which is a continuation of the cell membrane. The central fibrils are present only in the free part of the cord, and the peripheral fibrils extend deep into the cytoplasm, forming the basal grain - the blepharoplast. The tourniquet can be connected to the cytoplasm for a considerable distance by a thin membrane - the undulating membrane. The ciliary apparatus of ciliates can reach significant complexity and differentiate into zones that perform independent functions. The cilia often fuse in groups to form spines and membranellae. Each cilium begins from a basal grain, a kinetosome, located in the surface layer of the cytoplasm. The totality of kinetosomes forms an infraciliation. Knnetosomes reproduce only by dividing into two and cannot arise again. With partial or complete reduction of the flagellar apparatus, the infracilia remains and subsequently gives rise to new cilia.

The movement of protozoa occurs with the help of temporary or permanent movement organelles. The first include pseudopodia, or pseudopods - temporarily formed outgrowths of ectoplasm, for example, in an amoeba, into which the endoplasm seems to “flow”, due to which the simplest itself seems to “flow” from place to place. The permanent organelles of movement are whips, or flagella, and cilia.

All these organelles are outgrowths of the protoplasm of the protozoan. The tourniquet has a denser elastic thread along its axis, dressed, as it were, in a case of more liquid plasma. In the body of the protozoa, the base of the cord is connected to the basal granule, which is considered a homologue of the centrosome. The free end of the tourniquet hits the surrounding liquid, describing circular movements.

The cilia, in contrast to the lashes, are very short and extremely numerous. The cilia quickly bend to one side and then slowly straighten; their movement occurs sequentially, due to which the eye of the observer receives the impression of a flickering flame, and the movement itself is called flickering.
Some protozoa may simultaneously have pseudopodia and cilia or pseudopodia and cilia. Other protozoa may exhibit different modes of movement at different stages of their life cycle.
In some protozoa, contractile fibers, or myonemes, differentiate in the protoplasm, thanks to the work of which the body of the protozoa can quickly change shape.

In the first case, the ingestion of food is carried out by the work of pseudopodia, the so-called phagocytic nutrition, for example, the ingestion of protozoan cysts and bacteria by intestinal amoeba or by cilia that drive particles into the cell mouth (cytostome, for example, the ciliates Balantidium coll and starch grains). Endosmotic nutrition is characteristic of protozoa that do not have nutritional organelles, for example, trypanosomas, leishmania, gregarines, some ciliates, and many others. etc. Nutrition in such cases occurs due to the absorption of organic dissolved substances from environment; This form of nutrition is also called saprophytic.

Swallowed nutrients enter the endoplasm where they are digested. Unused residues are thrown out or anywhere on the surface of the protozoan’s body or in a certain area of ​​it (analogous to the process of defecation).

In the endoplasm of the protozoan, reserve nutrients are deposited in the form of glycogen, paraglycogen (insoluble in cold water and in alcohol), fat and other substances.
The endoplasm also contains the excretory apparatus, if it is morphologically expressed at all in a given species of protozoan. The organelles of excretion, as well as osmoregulation, and partly of respiration, are pulsating vacuoles, which, rhythmically contracting, empty out their liquid contents, which are again collected into the vacuole from the adjacent parts of the endoplasm. The endoplasm contains the nucleus of the protozoan. Many protozoa have two or more nuclei, which have a varied structure in different Protozoa.
The core is essential integral part the simplest, because all life processes can take place only in its presence; Nuclear-free sections of the protoplasm of a protozoan can only survive for a while under experimental conditions.

Protozoa also have specificity for vectors. Some species adapt only to one specific vector, while for others the carriers can be several species, often belonging to any one class.

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 has great importance to understand the evolution of the animal world. The type under consideration includes up to 40,000 species. Protozoa are widespread on our planet and live in various environments - in seas and oceans, fresh waters, and some species - in soil. Many protozoa have adapted to living in the body of other organisms - plants, animals, humans. All of them perform different functions: they actively participate in the cycle of substances, purify water from bacteria and rotting organic matter, influence soil-forming processes, and serve as food for larger invertebrates. Many marine unicellular organisms have hard mineral skeletons. Over tens of millions of years, the microscopic skeletons of dead animals sank to the bottom, forming powerful deposits of limestone, chalk, and green sandstone. The skeletons of some protozoa are used in geological exploration to identify oil-bearing layers.

Protozoa 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 rhizomes, living in the polar seas off the coast of Russia, and fossil nummulites reach 2-3 cm in diameter.

The body of a protozoan 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 single-celled animals (Monocytozoa). However, physiologically, they cannot be equated to individual cells of multicellular animals (Metazoa), since their body performs all the functions characteristic of multicellular animals. A single cell, which is an organism of protozoa, moves, captures food, reproduces, defends itself from enemies, i.e., 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 multinuclear forms. Ciliates always have two nuclei: a large vegetative one - the macronucleus and a small generative one - the micronucleus. The nucleus regulates vital processes and plays an important role in reproduction and the transmission of hereditary properties to offspring.

Most of the body of the protozoan is protoplasm. Under a microscope, one can distinguish the outer dense, transparent, homogeneous (uniform) layer - ectoplasm and the usually granular endoplasm of a more liquid consistency located inside. Protoplasm serves as the main substrate of life.

The surface of the ectoplasm in most forms is represented by a thin elastic membrane - pellicle (Latin pellicula - skin), consisting of proteins and fat-like substances. Having the property of semi-permeability, the shell regulates the flow of substances from external environment(water, salts, oxygen, etc.). The pellicle is part of 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 addition to the nucleus, the endoplasm contains organelles general purpose- mitochondria, endoplasmic reticulum, reticular apparatus, etc. In addition, in accordance with the functions inherent in the whole organism, protozoa have organelles special purpose, performing the functions of movement, nutrition, excretion, protection, etc.

Special Purpose Organoids

In connection with nutrition, excretion, movement and other functions in the body of protozoa, separate sections of protoplasm are isolated that perform certain vital functions of single-celled organisms as independent organisms. These areas are collectively called organelles, or organelles. In protozoa, organelles are allocated for special purposes in accordance with functions, unlike any other cells that have organelles general meaning(mitochondria, centrosomes, ribosomes, etc.)

Nutrition organoids have different structures. Depending on the type of assimilation and method of feeding, protozoa are divided into several groups (Fig. 1).

The first group consists of autotrophic protozoa. They feed like green plants, absorbing from the environment carbon dioxide, water and mineral salts (holophytic nutrition). Their assimilation organelles are chromatophores containing chlorophyll. On sunlight with their participation carbohydrates are synthesized. Autotrophic protozoa do not require 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 substances for food. Most of them feed on bacteria, algae, and protozoa. This method of feeding is called holozoic (animal). In this case, food is digested in special organelles - digestive vacuoles, which have the appearance of 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 protozoan. Digestion of food occurs with the participation of digestive juices coming from protoplasm. Many protozoa have organelles that serve to enter their body with food particles and throw undigested food debris out. These include the cellular mouth - cytostome, cellular pharynx - cytopharynx and anal pore.

Organoids of excretion. Most freshwater species have special pulsating vacuoles. They have the appearance of vesicles, to which a system of tubules approaches from the protoplasm. The pulsating vacuoles gradually fill with liquid, after which, quickly contracting, they throw the liquid out. In this way, the protozoa are freed from excess water, which, when living in a freshwater body of water, according to the law of osmosis [show] enters their body all the time. If the water is not removed, the protozoan will swell and die.

The phenomenon of osmosis is as follows: if two solutions with different concentrations are separated by a semi-permeable membrane, then the solvent (water) moves from a solution with a lower concentration to a solution with a higher concentration.

Organoids of movement in protozoa (Fig. 2) they serve:

• pseudopodia or pseudopods (Greek pseudos - false, podos - leg), which are temporary protoplasmic projections; occur in amoeba anywhere on its body. The movement is carried out due to the current of protoplasm, which gradually flows into one of the pseudopodia; at the same time, the opposite end of the body is shortened.
• flagella (or whips) are permanent organelles that look like long protoplasmic filaments, usually starting at the anterior end; They produce helical movements.
• cilia are permanent organelles, representing numerous short protoplasmic filaments. Their movements consist of quick swings in one direction and a slow subsequent straightening.

Movement is closely related to irritability and often serves as its external manifestation. Irritability is the body’s ability to respond to the influence of the external and internal environment with certain active reactions.

Protozoa 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). There are taxis 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 cell deformation by the formation of protoplasmic protrusions in the form of pseudopodia. The formation of pseudopodia manifests the ability of protoplasm to pass from the state of gel to sol and back. The flickering movements are carried out by flagella and cilia.

Some species have special organelles for the perception of stimuli. These include light-sensitive eyes, tactile bristles, etc.

Skeletal formations are found in the body of protozoa. The exoskeleton is often represented by calcareous or flint shells. Among the internal skeletal formations, a special axial rod should be mentioned - the axostyle (Gech. acson - axis, stylos - stick).

Defense organoids. Some protozoa have protective devices - trichocysts - short rods located in the ectoplasm under the pellicle. When irritated, the trichocysts shoot out, turning into a long elastic thread that strikes an 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, nuclear structures are evenly distributed between two newly formed nuclei (mitosis). Following the nucleus, the protoplasm divides, after which two newly emerged daughter individuals begin independent life.

In most protozoa it occurs in the form of copulation, in ciliates - in the form of conjugation (Fig. 4).

During copulation (Latin copulare - to connect), two individuals approach each other, their protoplasm and nuclei merge, forming one individual - a zygote, which then reproduces asexually.

Conjugation (lat. conjagatio - pairing, copulation) is a form of sexual reproduction characteristic of ciliates. During conjugation, two ciliates touch each other with their bodies. Their nuclei undergo complex restructuring. The macronuclei of both partners are destroyed and disappear. Micronuclei, after double division and destruction of 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 the partner, where it merges with its stationary core. Then the partners separate, 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.

encystment

If an encysted individual reenters 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 protozoal diseases.

Life cycle

In the life cycle of some protozoa, there is an alternation of morphological various forms. 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. Encysted forms (cysts) are characterized by resistance to unfavorable environmental conditions.

Classification

The division of the type of protozoa into classes is based mainly on the structure of the organelles of movement and the characteristics of reproduction. The generally accepted classification is according to which all protozoa are divided into 4 classes.

An organelle is a permanent formation in a cell that performs specific functions. They are also called organelles. An organelle is what allows a cell to live. Just as animals and humans are made up of organs, so every cell is made up of organelles. They are diverse and perform all the functions that ensure the life of the cell: metabolism, storage, and division.

What types of organelles are there?

An organelle is a complex structure. Some of them may even have their own DNA and RNA. All cells contain mitochondria, ribosomes, lysosomes, a cell center, the Golgi apparatus (complex), and the endoplasmic reticulum (reticulum). Plants also have specific cellular organelles: vacuoles and plastids. Some also classify microtubules and microfilaments as organelles.

An organelle is a ribosome, a vacuole, a cell center, and many others. Let's take a closer look at the structure and functions of organelles.

Mitochondria

These organelles provide the cell with energy - they are responsible for They are found in plants, animals, and fungi. These cellular organelles have two membranes: external and internal, between which there is an intermembrane space. What is inside the shells is called the matrix. It contains a variety of enzymes - substances necessary to accelerate chemical reactions. The inner membrane has folds called cristae. It is on them that the process of cellular respiration occurs. In addition, the mitochondrial matrix contains mitochondrial DNA(mDNA) and mRNA, as well as ribosomes, almost similar to those possessed

Ribosome

This organelle is responsible for the translation process, in which protein is synthesized from individual amino acids. The structure of the ribosome organelle is simpler than mitochondria - it does not have membranes. This organelle consists of two parts (subunits) - small and large. When the ribosome is inactive, they are separated, and when it begins to synthesize protein, they unite. Several ribosomes can also come together if the polypeptide chain synthesized by them is very long. This structure is called a "polyribosome".

Lysosomes

The functions of organelles of this type are limited to cellular digestion. Lysosomes have one membrane, inside which there are enzymes that catalyze chemical reactions. Sometimes these organelles not only break down but also digest entire organelles. This can happen during prolonged starvation of the cell and allows it to live for some time. Although if nutrients still do not begin to flow, the cell dies.

and functions

This organelle consists of two parts - centrioles. These are cylinder-shaped formations consisting of microtubules. The cell center is a very important organelle. It is involved in the process of spindle formation. In addition, it is the center of microtubule organization.

Golgi apparatus

It is a complex of disc-shaped membrane sacs called cisternae. The functions of this organelle include sorting, storing and converting certain substances. Carbohydrates, which are part of the glycocalyx, are synthesized here mainly.

Structure and functions of the endoplasmic reticulum

It is a network of tubes and pockets surrounded by a single membrane. There are two types of endoplasmic reticulum: smooth and rough. Ribosomes are located on the surface of the latter. Smooth and rough reticulum perform different functions. The first is responsible for the synthesis of hormones, storage and conversion of carbohydrates. In addition, the rudiments of vacuoles, organelles characteristic of plant cells, are formed in it. The rough endoplasmic reticulum contains ribosomes on its surface, which produce a polypeptide chain of amino acids. Then it enters the endoplasmic reticulum, and here a certain secondary, tertiary and quaternary protein structure (chain) is formed in the right way twists).

Vacuoles

These are organelles. They have one membrane. Cell sap accumulates in them. The vacuole is necessary to maintain turgor. It also participates in the process of osmosis. In addition, there are They are found mainly in single-celled organisms living in bodies of water, and serve as pumps that pump out excess fluid from the cell.

Plastids: varieties, structure and functions

These are also organelles. They come in three types: leucoplasts, chromoplasts and chloroplasts. The first ones are used to store spare parts. nutrients, mostly starch. Chromoplasts contain various pigments. Thanks to them, the petals of plants are multi-colored. The body needs this primarily in order to attract pollinating insects.

Chloroplasts are the most important plastids. The largest amount of them is found in the leaves and stems of plants. They are responsible for photosynthesis - a chain of chemical reactions during which the body produces organic substances from inorganic substances. These organelles have two membranes. The matrix of chloroplasts is called "stroma". It contains plastid DNA, RNA, enzymes, and starch inclusions. Chloroplasts contain thylakoids, coin-shaped membrane formations. Photosynthesis occurs inside them. It also contains chlorophyll, which serves as a catalyst for chemical reactions. The thylakoids of chloroplasts are combined into stacks - grana. The organelles also contain lamellae, which connect individual thylakoids and provide communication between them.

Organelles of movement

They are characteristic mainly of unicellular organisms. These include flagella and cilia. The former are present in euglena, trypanosomes, and chlamydomonas. Flagella are also present in animal sperm. Ciliates and other single-celled organisms have cilia.

Microtubules

They ensure the transport of substances, as well as the constant shape of the cell. Some scientists do not classify microtubules as organelles.

SUB-KINGDOM PROTOZOTS,
OR SINGLE CELL (PROTOZOA)

The subkingdom of unicellular organisms includes animals whose body consists of a single cell. Morphologically, they are similar to the cells of multicellular animals, but physiologically they differ in that, in addition to the usual functions of the cell (metabolism, protein synthesis, etc.), they perform the functions of an entire organism (nutrition, movement, reproduction, protection from unfavorable environmental conditions). Certain functions in multicellular organisms are performed by special organs, tissues or cells, while in unicellular organisms the functions of the body are performed by structural elements one cell - organelles. Cell division in multicellular animals leads to the growth of the body, and in protozoa it leads to reproduction.

Thus, protozoa are organisms at the unicellular level of organization. The integrity of the organism of protozoa is maintained by the functions of a single cell, and in multicellular organisms it is maintained through the interaction of cells, tissues and organs.

The life cycle of protozoa consists of developmental phases with a unicellular organization, and in multicellular organisms, unicellular developmental phases alternate with multicellular ones.

Currently, more than 39 thousand species of protozoa are known, but tens and hundreds of new species are discovered every year, which is an indicator of insufficient knowledge of this group of animals.

Protozoa were first discovered by the Dutch scientist A. van Leeuwenhoek, the first inventor of the microscope (1675). His microscopes were highly magnifying loupes that provided magnification of 100 and even 200 times. The first microscopists discovered especially many protozoa in herbal infusions (infusum - means “tincture”), so at first these animals were called “tincture” or ciliates. Now this name is retained only by one group of protozoa. In the first system of animals by C. Linnaeus (1759), the protozoa were assigned to one genus - Chaos - of the class of worms. Only

in the 19th century Kölliker and Siebold identified them as an independent type (1845). At the International Congress of Protozoologists in 1977 it was adopted new system the simplest, reflecting the latest achievements of science. According to new principles published in 1980 (Levine et al.), protozoa are grouped into the subkingdom Unicellular and divided into seven phyla.

The body shape of protozoa is extremely diverse. Among them there are species with a variable body shape, like amoebas. There are various types of symmetry in protozoa. Forms with radial symmetry are widespread: radiolarians, sunfishes. These are mainly floating planktonic protozoans. Bilateral symmetry is observed in some

flagellates, foraminifera, radiolarians. Translational-rotational symmetry is characteristic of foraminifera with a spirally twisted shell. In some species, metamerism is observed - repeatability of structures along the longitudinal axis. The life forms of protozoa, or morphoadaptive types, are diverse. The most common forms are: amoeboid, which lead a crawling lifestyle on various substrates in water or in a liquid medium in the host’s body; conch shell- sedentary benthic forms; actively floating flagellates and ciliates floating in plankton radial or radiant, forms; sedentary - stalked, narrow-bodied or flat-bodied substrate boreholes - interstitials, as well as round, stationary, resting forms (cysts, spores).

The structure of a protozoan cell is characterized by all the main features cellular structure eukaryotes. The ultrastructure of the structure of protozoa has been studied by biologists using electron microscopy technology. The resolution capabilities of a modern electron microscope make it possible to obtain a magnification of 200-300 thousand times.

The protozoan cell is typical of eukaryotic organisms and consists of cytoplasm and one or more nuclei. The cytoplasm is bounded externally by a three-layer membrane. The total thickness of the membrane is about 7.5 nanomicrons (1 nm = 10 - 6 mm). In the cytoplasm of protozoa there is an outer, more transparent and dense layer - ectoplasm and an inner, granular layer - endoplasm. All the main organelles of the cell are concentrated in the endoplasm: the nucleus, mitochondria, ribosomes, lysosomes, endoplasmic reticulum, Golgi apparatus, etc. In addition, protozoa have special organelles: supporting, contractile fibrils, digestive and contractile vacuoles, etc. The nucleus is covered with a double-layer membrane with at times. Inside the nucleus there is karyoplasm, in which chromatin and nucleoli are distributed. Chromatin is despiralized chromosomes consisting of DNA and proteins such as histones. Nucleoli are similar to ribosomes and are made of RNA and proteins. The nuclei of protozoa are varied in composition, shape, and size.

In protozoa, special functional complexes of organelles can be distinguished, which correspond to the systems of organs and tissues of multicellular organisms.

Integumentary and supporting organelles. Some unicellular species do not have integumentary or supporting structures. The cell of such protozoa is limited only by a soft cytoplasmic membrane. Such species do not have a constant body shape (amoeba). Other species have a dense elastic shell - a pellicle, formed due to the compaction of the peripheral layer of ectoplasm and the presence of various

supporting fibrils. In this case, the protozoa have a certain body shape (ciliates, euglena) and at the same time they retain flexibility and can bend when moving and partially contract. Other unicellular organisms secrete a shell of scales on the outside, which prevents changes in body shape (diatoms). The body shape can additionally be supported by other supporting structures - fibrils, which form, for example, the cortex in some ciliates.

The supporting structures also include the skeleton. The skeleton of protozoa can be external (shell) or internal (skeletal capsules, needles). The shell is secreted by the ectoplasm of the cell, and at the same time an extracellular formation is formed, which has protective function. The internal skeleton is formed in the endoplasm of the cell. The formation of skeletal capsules and needles occurs by biocrystallization. Skeletal formations consist of organic and mineral substances. Most often, protozoan skeletons include calcium carbonate (CaCO 3) or silicon oxide (SiO 2), less often strontium sulfate (SrSO 4).

Motor organelles. The most primitive method of movement in protozoa can be considered amoeboid movement with the help of false legs, or pseudopodia. In this case, special protrusions of the cell are formed into which the cytoplasm flows. Such organelles of movement are characteristic of single-celled organisms with a variable body shape.

More complex movement is characteristic of protozoa, which have flagella or cilia as movement organelles. The structure of the flagellum and cilia is similar (Fig. 16). Each flagellum is externally covered with a three-layer cytoplasmic membrane. Inside the flagellum there are fibrils: two central and nine double peripheral. The flagellum is attached to the cytoplasm using a basal body - a kinetosome. Typically, flagella produce a rotating motion, and cilia produce a rowing motion. Flagella are characteristic of flagellates, and cilia are characteristic of ciliates.

Rice. 16. Diagram of the structure of the flagellum (according to Noirot-Timote): A - longitudinal section of the flagellum, B, C, D, E - transverse sections of the flagellum on different levels; 1 - central fibrils, 2 - peripheral fibrils. 3 - outer membrane of the flagellum, 4 - axial granule, 5 - kinetosome

Some protozoa are capable of rapid contraction of the body due to special contractile fibrils - myonemes. For example, sessile ciliates - suvoiki - are capable of sharply shortening their long stalk and folding it into a spiral. Radiolarians are capable of either stretching the cell body on radial spines or contracting it using contractile fibers. This provides them with the regulation of free swimming in the water column. Under unfavorable conditions, many protozoa become encysted, i.e. secrete a dense membrane around themselves and turn into a cyst.

Types of nutrition and trophic organelles. Protozoa are varied in their type of nutrition. Among them there are autotrophs capable of photosynthesis. This unicellular algae from flagellates. They have chlorophyll grains, or chromatophores, in their cytoplasm.

Most protozoa are heterotrophs, feeding like animals on ready-made organic substances. Some of them have a holozoic method of feeding, swallowing solid lumps of food. Others feed saprophytically, absorbing dissolved organic matter. Food particles are swallowed by amoebas and ciliates. Digestive vacuoles are formed in their cytoplasm, where food is digested. This ingestion of solid food by the cell is called phagocytosis. With the saprophytic method of nutrition, digestive vacuoles are not formed. However, it is known that many protozoa can swallow liquid through a temporary invagination of the membrane - a special funnel. This absorption of liquid is called pinocytosis.

Some species have mixed type nutrition (mixotrophs). They are capable of photosynthesis, like plants, and of feeding on ready-made organic matter, like animals. They have chlorophyll grains in the cytoplasm, but digestive vacuoles can also form. Such protozoa with a mixed type of nutrition include, for example, euglena, which feed in the light like plants and in the dark like animals.

Nuclear apparatus consists of one or more nuclei. The nuclei regulate the metabolic processes of protozoan cells and ensure reproduction. The nuclei of protozoa vary in shape, number, ploidy, and functions. In some multinucleated protozoa, two types of nuclei are distinguished: generative and vegetative. This phenomenon is called nuclear dualism. Vegetative nuclei regulate all life processes in the cell, and generative ones participate in the sexual process. Nuclear dualism is characteristic of ciliates and some foraminifera. Protozoan nuclei may be haploid at some stage of the life cycle, or diploid, or polyploid. Most protozoa are mononuclear (monoenergetic). Species that have many nuclei are called polyenergetic.

During asexual reproduction of protozoa, the nuclei divide by mitosis. The nuclei of protozoa for which the sexual process is known undergo meiosis, or reduction division. Unlike multicellular organisms, meiosis in unicellular organisms is varied. In the primitive case, meiosis occurs during one cell division, in others, as in higher animals, as a result of two successive divisions. In some cases, reduction division occurs after the formation of the zygote (zygotic reduction), in others, as in multicellular organisms, during the formation of gametes (gametic reduction).

Types of reproduction protozoa are diverse. They are characterized by asexual and sexual reproduction. Asexual reproduction occurs by dividing a cell into two or many cells (agamogamy) during mitotic nuclear division. Sexual reproduction protozoa is characterized by the formation of sex cells - gametes (gamogamy) with their subsequent fusion (copulation), which leads to the formation of a zygote, from which a new daughter organism develops. In some protozoa (ciliates), the sexual process - conjugation occurs not by the fusion of gametes, but by the fusion of generative nuclei from different cells. During the process of copulation, the merging gametes can be the same in size and shape (isogamy) or different (heterogamy). In the case of sharp differences between gametes, when one of the gametes is large, immobile, without flagella (oogamete), and the other is small in size, with flagella, such copulation is called oogamy. In this case, the macrogamete (oogamete) is equated to the egg of multicellular organisms, and the microgamete to the sperm.

Life cycle of protozoa represents a cyclically repeating segment of the development of a species between two phases of the same name (for example, from zygote to zygote). The life cycle of protozoa can be characterized by only asexual reproduction (from division to division), or only sexual reproduction (from zygote to zygote), or

alternating sexual and asexual reproduction(metagenesis). Will be discussed in more detail later Various types life cycles protozoa.

Classification. According to modern concepts, in protozoology, protozoa are divided into seven types:

The division of protozoa into types is based on the principles of the structure of their nuclear apparatus, organelles of movement, a number of microstructures, types of reproduction and life cycles.

Ciliates move with the help of movement organelles - cilia or their derivatives; have nuclear dualism and polyenergy. The sexual process is carried out through conjugation.

Labyrinthulae live on aquatic marine plants and are a labyrinth of cytoplasmic strands along which spindle-shaped cells move. They reproduce by zoospores with flagella.

Comparative characteristics of protozoan types are given in Table 1.

Covers of the body.

Body shape, symmetry.

The body shape of protozoa and its color are extremely diverse and determined by specific living conditions. Functionally, the anterior end of the flagellate is where the flagellum is attached.

All protozoa, regardless of their type of organization, are protected from the effects of the external environment by cell membranes of various structures. The main structural unit of all types of integument in protozoa is the cytoplasmic membrane. WITH inside The plasmalemma usually contains submembrane microfilaments or microtubules.

The appearance of flagella as a locomotor apparatus led to the appearance of relatively another type of integument in flagellates - dense pellicles. The pellicle is formed due to the compaction of the peripheral layer of the cytoplasm and the presence of supporting fibrils in it. It is strengthened by outgrowths of the radicular system.

The next stage in the complication of the integument is the external skeleton, formed by protein, cellulose and even chitinous plates, calcareous, silica structures, as well as glycoprotein gelatinous secretions in some flagellates.

Some protozoa have integuments different types complicated by the appearance of a more or less complex sculpture, that is, a system of more or less regularly located recesses and protrusions that form something like stiffening ribs (Opalinidomorpha), “reinforced” with microtubules. Such covers are called folded or comb tubulemma.

Characteristic of ciliates cortex. The cortex includes: a pellicle (formed by a membrane and a system of alveoli), under the pellicle there is a protein layer - epiplasm and a complex of kinetosomes.

TO general cellular structures include: cytoplasm, nucleus, mitochondria, endoplasmic reticulum, ribosomes, lysosomes, Golgi apparatus, centriole.

One core or several of them. Depending on the number of nuclei, protozoa are divided into monoenergetic and polyenergetic. Ciliates are characterized by nuclear dualism: the functions of the nuclei (micronucleus and macronucleus) differ.

Special organelles cells are: contractile and digestive vacuoles, microfilaments (participate in the processes of contraction and cell division, form fibrils), microtubules (the main function is the formation of the cytoskeleton, take part in cell division, in the formation oral apparatus, hold organelles in a certain position), extrusomes (various in shape, release contents in response to irritation), powder, stigma, flagella and cilia.

Inclusions are: droplets of fat, protein crystals, symbiotic organisms.