Movements of protozoa. Features and Functions

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 covered with a cell membrane about 100 A thick. Most protozoa have a denser but elastic membrane, 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 sheath, which is a continuation 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 a tourniquet 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.

Ingested 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 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.

A more complex form is movement carried out with the help of flagella and cilia. The flagellated form of movement is characteristic of the class of flagellates.

Flagella are the thinnest outgrowths of the body. Number of them different types varies - from one to many tens and even hundreds.

Each flagellum originates from a small basal granule, called a blepharoplast, located in the cytoplasm. Thus, the part of the flagellum immediately bordering the basal granule passes inside the cytoplasm (it is called the root filament), and then passes through the pellicle to the outside. The mechanism of flagellar movement differs in different species. In most cases it comes down to a rotational movement. The flagellum describes the shape of a cone, with its apex facing the place of its attachment. The greatest mechanical effect is achieved when the angle formed by the top of the cone is 40-46°. The speed of movement varies; it varies in different species between 10 and 40 revolutions per second. The simplest is, as it were, “screwed” into the liquid environment surrounding it.

Often rotational movement The flagellum is combined with its wave-like movement. Usually, during translational movement, the body of the protozoan itself rotates around the longitudinal axis.

The presented scheme is valid for most single-flagellate forms.

In polyflagellates, the movement of the flagella may be of a different nature; in particular, the flagella may be in the same plane without forming a cone of rotation.

Rice. Cross section of the flagella of Pseudotrichonympha (the figure was made from an electron microscopic photograph). Nine peripheral double fibrils and a central pair of fibrils are visible in each flagellum.

Electron microscopy studies recent years showed that the internal ultramicroscopic structure of flagella is very complex. Outside, the flagellum is surrounded by a thin membrane, which is a direct continuation of the most superficial layer of ectoplasm - the pellicle. The internal cavity of the flagellum is filled with cytoplasmic contents. Eleven very thin filaments (fibrils), which are often double, run along the longitudinal axis of the flagellum (Fig.). These fibrils are always arranged in a regular manner. Nine of them (simple or double) lie along the periphery, collectively forming a kind of cylinder. Two fibrils occupy a central position. To get an idea of ​​the size of all these formations, it is enough to say that the diameter of the peripheral fibrils is about 350A (angstroms). An angstrom is a unit of length equal to 0.0001 microns, and a micron is equal to 0.001 mm. These are the tiny structures that have become available for study thanks to the introduction of the electron microscope into microscopic technology.

The functional significance of flagellar fibrils cannot be considered definitively clarified. Apparently, some of them (probably peripheral) play an active role in motor function flagellum and contains special protein molecules, capable of contracting, while others are supporting elastic structures that have a supporting value.

Cilia serve as organelles for the movement of ciliates. Usually their number in each individual is very large and measured in several hundreds, thousands and even tens of thousands. The mechanism of movement of cilia is somewhat different from that of flagella.

Each eyelash makes a rowing motion. It bends quickly and forcefully to one side, and then slowly straightens.

Joint action large number cilia, the beating of which is coordinated, causes rapid forward movement of the protozoan.

Each ciliate cilium, as the latest research has shown, is complex education, corresponding in structure to the flagellum. At the base of each cilium there is always a so-called basal granule (otherwise known as kinetosome), an important part of the ciliary apparatus.

In many ciliates, individual cilia connect with each other, forming structures of a more complex structure (membranella, cirri, etc.) and more effective mechanical action.

Some highly organized protozoa (ciliates, radiolarians) are characterized by another form of movement - contraction. The body of such protozoa is capable of quickly changing its shape and then returning to its original state.

The ability to rapidly contract is due to the presence in the body of the protozoan of special fibers - myonemes - formations similar to the muscles of multicellular animals.

Some protozoa also have other forms of movement.

Electron micrographs show that cilia and flagella have the same internal structure. Cilia are simply a shortened version of flagella, and unlike flagella, they are most often found in groups rather than singly. Cross sections show that other organelles consist of two central fibrils surrounded by nine peripheral ones (Fig. 17.31) - the so-called structure "9 + 2". This bundle of fibrils - the axoneme - is surrounded by a membrane, which is a continuation of the plasma membrane.

Rice. 17.31. The structure of a cilium or flagellum. A. Layout of tubes and related components (side view). B. Cross section. Each doublet consists of microtubules A and B. A - a microtubule has a pair of processes (“handles”). Please note that in cross section A, the tube appears as a hollow cylinder, and part of its wall becomes common to both tubes of the doublet. Along the entire length of the tubes, “spokes” extend at certain intervals, which connect the doublets to the “axial case” surrounding the central tubes. B. Microtubule doublet enlarged

All peripheral fibrils are made of protein tubulin and consist of microtubules A and B. Each A-microtubule carries a pair of “handles” formed by another protein - dynein, which has the ability to hydrolyze ATP, i.e. acts as an ATPase. The central fibrils are connected to the A-microtubules of the peripheral fibrils using radial crossbars.

At the point of attachment to the cell, the cilium or flagellum ends basal body, which is almost identical in structure to the axoneme (has a 9 + 0 structure) and is a derivative of the centriole. It differs from the centriole only in the presence at its base of a complex structure called "spoked wheel". It is believed that the basal body during the formation of cilia and flagella serves as a matrix for the assembly of microtubules. Often, fibers extend from the basal body into the cytoplasm, fixing it in a certain position. But although cilia and flagella have the same structural plan, the way they work is noticeably different.

The flagellum makes symmetrical movements, with each this moment Several successive waves pass through it (Fig. 17.32). The beating of the flagellum can occur in one plane, but (less often) it can also be spiral-shaped, which leads to rotation of the organism around its longitudinal axis while simultaneously moving forward along a spiral trajectory (Fig. 17.33). In some flagellates, the flagellum is located at the anterior end of the body and, as it were, pulls the animal along with it. Flagella of this type usually have tiny lateral projections - mastigonemas, increasing the efficiency of this method of locomotion. More often, the flagellum is located at the posterior end of the cell (as, for example, in a sperm) and pushes it forward. In Fig. 17.34 summarizes information about the types of movement carried out with the help of a flagellum.

The beating of the cilia is asymmetrical (Fig. 17.32); after a quick and energetic strike of a straight cilium, it bends and slowly returns to its original position. When a large number of cilia accumulate, some mechanism is needed to coordinate their activity. In ciliates Paramecium these functions are usually attributed neurofans- filaments connecting the basal bodies. Usually the beating of the cilia is synchronized, so that waves of their activity run along the body in one specific direction. It is called metachronous rhythm.

There has been much debate regarding the mechanism of movement of the cilia or flagella themselves. Judging by the latest data, this mechanism is fundamentally very close to the interaction of actin and myosin during muscle contraction. It is believed that the bending of the flagellum is associated with the attachment of two dynein processes of A-microtubules to adjacent B-microtubules in peripheral fibrils. In this case, ATP hydrolysis occurs, and microtubules A and B slide over each other, setting the flagellum in motion. Apparently, five peripheral fibrils on one side initiate the initial bending, and the remaining four fibrils on the other side are involved later, resulting in the return movement of the flagellum (Fig. 17.35). Radial crossbars, preventing sliding, reduce it to local bending of the flagellum. It is possible that the central fibrils transmit the signal to initiate sliding from the basal body along the entire length of the cilium or flagellum. It was shown that cilia can only work in the presence of Mg 2+ ions and that the direction of beating is determined by the concentration of Ca 2+ inside the cell. Interestingly, the avoidance response in Paramecia is controlled in the following way: when the ciliate encounters an obstacle, the direction of its cilia beat is reversed, and then it resumes moving forward. This change is stimulated by a sudden influx of Ca 2+ ions into the cell as a result of increased permeability to these ions.

In small eukaryotic organisms, cilia or flagella are widely used for locomotion in water, propelling the body through the surrounding viscous fluid. This method of locomotion can only be effective at very small body sizes, when the surface-to-volume ratio is much greater than in large animals. For the latter, the power generated by the cilia would be insufficient. However, cilia are often found inside bodies of multicellular organisms, where they perform a number of important functions. They can push fluid through ducts, as happens in the metanephridia of annelids when removing metabolic waste. With the help of cilia, eggs move in the oviducts of mammals and various materials along the internal surface of organs, for example, mucus in the respiratory tract, where the work of cilia allows the removal of dust particles and other “garbage”. Cilia can also create a flow of external fluid from which some organisms, including Paramecium, filter food particles (often using other types of cilia).

MOVEMENT OF PROTOZOTS

There are five main forms of protozoan movement: flagellar, ciliary, amoeboid, metabolizing and gliding.
Flagellar/ciliary apparatus. The mobility of many protozoa is ensured by the presence and functioning of flagella or cilia. Flagella and cilia are identical in ultrastructure, work according to the same functional scheme, but differ in the way they beat. Usually the flagellum makes helical movements, and the flagellate is “screwed” into the water. If there are many flagella/cilia, then the movement of each individual flagella/cilium resembles the work of an oar, and in general, the entire set of flagella/cilia is characterized by coordinated movement like metachronal waves. In the case of multiflagellar forms, such formal differences between the flagellum and the cilium as number (the cell has few flagella, but many cilia) and length (the flagella are long, and the cilia are short) disappear.
The fact that for some animals the term “cilium” is used, and for others “flagellum” is associated with an established tradition.
The flagellum/cilium includes a free part or undulipodium containing an axoneme, transition zone, kinetosome and roots.
The undulipodium is covered on the outside with a membrane, which is a continuation of the cell membrane. The surface of the undulipodium can be smooth; in other cases, the undulipodium on its surface bears ultramicroscopic outgrowths of different structures (scales or rod-shaped outgrowths - mastigonemes). An axoneme is located inside the undulipodium. In some species, a paraxial cord runs parallel to the axoneme along the entire length of the flagellum (euglenaceae, kinetoplastids). The paraxial cord consists of microfilaments and probably increases the elasticity of the flagellum.
The axoneme is a cylinder, the wall of which is built of 9 pairs of microtubules (peripheral microtubules). In the center of this cylinder there are two central microtubules (9+2 structure). The peripheral microtubules that make up the pair (doublet) are not identical; they are called A and B. Outgrowths (dynein arms) extend from microtubules A, directed to the neighboring doublet, as well as radial spokes to the central pair of microtubules.
The transition zone is located at the level of the flagella exiting the cell, in which the structure of the axoneme changes (doublets are replaced by triplets) and additional fibrillar formations appear. There are often a variety of support structures present in this area.
The kinetosome lies under the base of the flagellum/cilium; it is a hollow cylinder consisting of 9 triplets of microtubules. Most flagellates have 2 flagella and, accordingly, 2 kinetosomes. Uniflagellate species also have 2 kinetosomes, but one of them does not have a direct connection with the flagellum. The relative position of the two kinetosomes differs in different groups.
Kinetosomes are associated with roots, which can be fibrillar or microtubular and are characterized by different sizes and locations. In general, the roots perform a cytoskeletal function.
The driving force behind the flagellum/cilium is the peripheral microtubules and their handles, which have ATPase activity. Due to the released energy, the dynein arms of one doublet are attached to the neighboring doublet and bend. As a result, this doublet is shifted relative to the other. Consecutive repetition of the cycle ensures alternate displacement of the doublets, which causes bending of the flagellum. The central fibrils play a supporting role.
Not all flagella/cilia of one cell are always arranged identically. There are flagellates in which the longer flagellum bears mastigonemes and is directed forward, and the shorter one is devoid of mastigonemes, but has a characteristic thickening and is directed backward. In many ciliates, the cilia covering the body are short, and the cilia of the oral apparatus are noticeably longer. In all such cases, different flagella/cilia have different functions. In ciliates, cilia can be united into bristle-like bundles (cirrhi) or into plates - membranellae. Cirri serve as organelles of movement, and membranelles are involved in driving food to the cytostome.
Amoeboid movement. This method of movement externally looks like a flow of cytoplasm: on the surface of the body of the protozoan, a protrusion (pseudopodia) appears in the direction of movement, into which a flow of cytoplasm rushes, and accordingly the whole body moves in the same direction. The growth of pseudopodia is accompanied by a change in the consistency of the cytoplasm. Ectoplasm is a dense gel, and endoplasm is a liquid sol. Pseudopodia is formed where the ectoplasm, which was previously in a gel state, liquefies. Thanks to this, the liquid cytoplasm is able to flow out. At different times, different hypotheses were put forward to explain amoeboid movement. Currently, the most widely accepted hypothesis is generalized cortical contraction. Amoeboid organisms have a submembrane layer of microfilaments represented by actin and myosin. These proteins are arranged in a three-dimensional network in the peripheral regions of the cell. With a coordinated contraction of this network, a cytoplasmic current occurs, causing cell movement. Simultaneously with the movement, a restructuring of the actin-myosin network occurs, as well as changes in the viscous properties of the cytoplasm. This hypothesis explains part of the data on amoeboid movement, but the mechanism of amoeboid movement cannot be considered fully disclosed.
Metabolic movement. This type of movement is characteristic of euglenoids and is carried out by active sliding of the protein plates of the integument relative to each other. Externally, this movement looks like a wave-like bending of the cell body.
Sliding movement. Gliding is a unidirectional movement without changing the shape of the cell, this type of movement is characteristic of gregarines. On the surface of the body of gregarines there are longitudinal folds (epicytic ridges), at the apexes of which microfilaments pass. The undulating (undulating) movements of these epicytic ridges ensure the gliding movement of the cell. In turn, the movements of the epicytic ridges are probably due to the interactions of microfilaments with the internal membrane complex of the pellicle, which leads to a shift of this membrane complex relative to the plasmalemma and temporary deformation of the ridge.

The mobility of many protists is ensured by the presence of flagella or cilia. Both are structured the same way. Flagella in both protozoa and flagellar or ciliated cells of multicellular animals and plants are always only part of the locomotor system of the cell, which consists of a kinetosome (or centriole), a flagellum (or undulipodium) and root processes of the kinetosome (or its derivatives). In addition to movement in the water column, flagella and cilia are used for temporary or permanent attachment to the substrate or for creating food flows of water when feeding on suspended particles.

Flagellum- This is a tubular outgrowth of the cell surface, surrounded by a membrane that serves as a continuation of the membrane covering the entire cell. It contains a bundle of protein fibrils, the so-called axoneme. An axoneme or axial filament is a microtubule formation that consists of two central microtubules surrounded by a ring of nine pairs (doublets) of microtubules consisting of subfibrils tightly fused to each other. The fine structure of the flagella of all eukaryotic organisms is surprisingly uniform in its main features.

The most important element of the flagellar system is the basal body or kinetosome. This is a cylinder, the walls of which are formed by nine groups of microtubules, united in groups of three (triplets). Most often, a cell contains two kinetosomes located approximately at right angles to each other. One or two flagella extend from them. The kinetosome does not float in the cytoplasm by itself, since it is secured by a system of roots.

Modern ideas about the Protista system are largely based on the structure of the flagellum and its derivatives. The wide distribution of flagella and cilia among them makes it possible to compare almost all taxa with each other, and also makes it possible to use additional characters of the flagellar apparatus, the number of which is already approaching 100, in systematics and phylogeny. Many structural features of flagellates, including body shape, are determined by the presence of this unique system.

The number of flagella, their relative and absolute length, the place and method of attachment of the flagella, the nature of their movement, their direction are very diverse in different groups, but are constant within individual groups of related organisms.

There are usually 4 morphotypes of flagellates.

Isokonts have from 2 to 8 flagella equal length, directed in one direction, with the same beating methods. These include most of the motile cells of green algae.

Anisokonts have 2 flagella of unequal length, directed in one direction, differing in the method of beating. Such flagella are characteristic of colorless flagellates.

U heterokontnyh there are 2 flagella of unequal length (one directed forward, the other backward), differing in the way they beat. They are characteristic of motile algae cells, so-called zoosporic fungi, and colorless flagellates.

Stefanokonty have a corolla of flagella at the anterior end of the cell. This is typical for multiflagellate gametes and zoospores of some green algae.

Uniflagellate forms are usually not classified as a separate group. Many of them are considered to be individuals that have lost their flagellum for the second time, since the vast majority have another flagellated kinetosome.

The main function of the flagellum is movement. In the active work of the flagellum, the driving principle is the peripheral microtubules and their handles, which have ATPase activity. The central microtubules have a supporting role. The forms of movement of the flagellum are different, but usually it is a helical movement, allowing the flagellum to “screw in” into the water, making up to 40 revolutions per second. In ciliates and multiflagellate protists, the movement of cilia is organized according to the type of metachronal waves. Flagella and cilia are often used for nutrition. Among flagellates, there are species that spend most of their life cycle in an attached state. During this period, the flagellum loses its usual function of movement and turns into an attachment organelle, stalk or stalk. Another function of the flagellum is that with its movements it cleans the surface of the body from small foreign particles adhering to it.

Endoplasmic organelles

The endoplasm of protists contains one or more nuclei, as well as all organelles and structures characteristic of a eukaryotic cell: ER, ribosomes, Golgi apparatus, mitochondria, peroxisomes, hydrogenosomes, plastids (in autotrophic protists), lysosomes, digestive vacuoles. Some protists also have organelles specific to them.

Extrusomes. These organelles are special vacuoles surrounded by a membrane, which in mature extrusomes is usually in contact with the plasmalemma. In response to various external irritations (mechanical, chemical, electrical, etc.), they throw out their contents. In terms of their structure, they are mucopolysaccharides (complex compounds of carbohydrates and proteins). There are 10 different types of extrusomes known. Some contain toxic substances that can immobilize and kill the victim (protozoa and other small organisms). Others perform a protective function or facilitate movement by secreting mucus.

Plastids. Plastids are present in phototrophic and related protists and are represented by chloroplasts and leucoplasts. The main pigments of chloroplasts are chlorophylls. Different groups of phototrophic protists are characterized by certain sets of chlorophylls. Of the secondary pigments in algae, there are carotenes and xanthophylls, which in high concentrations can mask green chlorophyll and give chloroplasts a variety of colors from yellow-green to reddish-brown.

The Golgi apparatus has been found in almost all protist species studied. Most often, the Golgi apparatus is located adjacent to the nucleus and is represented by one or several stacks of flat cisterns (dictyosomes) surrounded by small vesicles. However, in some protists the Golgi apparatus is formed by single cisternae. The absence of dictyosomes is usually interpreted as a primitive feature. However, the absence of dictyosomes in modern protists cannot unambiguously indicate their primitiveness, since the formation and disassembly of dictyosomes largely depend on external influences on the cell (for example, a decrease in oxygen concentration in the environment) or on physiological changes in the protist itself (the transition to encystment).

Lysosomes and other organelles and inclusions. In the cells of protists, as in the cells of multicellular animals, lysosomes are present. These cytoplasmic bodies in the form of small vesicles (primary lysosomes) are formed in the Golgi apparatus. Digestive hydrolytic enzymes are localized in them. Secondary lysosomes, or digestive vacuoles, are well expressed only in heterotrophic protists that feed by phagocytosis.

In the endoplasm of different protists, reserve nutrients used in metabolic processes are present in greater or lesser quantities. Most often these are various polysaccharides (glycogen, starch, amyloplectin, etc.), often lipids and other fatty inclusions. The amount of reserve substances depends on the physiological state of the protozoan, the nature and amount of food, and the stage of the life cycle and varies widely. However, some large groups of protists store specific substances. For example, euglenoids store paramyl, which is not found in other protists.