Cilia and flagella: a concise description, structure and role in cells. Movements of protozoa

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 also consist of two central fibrils surrounded by nine peripheral fibrils (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 the 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 the 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, at certain intervals, "spokes" extend, which connect the doublets to the "axial sheath" surrounding the central tubes. C. An enlarged microtubule doublet

All peripheral fibrils are built from 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 by means of 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 centriole derivative. It differs from the centriole only by the presence at its base of a complex structure called "spoked wheel". The basal body is believed to serve as a matrix for microtubule assembly during the formation of cilia and flagella. 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 differ markedly.

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, which leads to the rotation of the organism around its longitudinal axis while moving forward along a spiral path (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. Flagella of this type usually have the smallest lateral protrusions - mastigonemes, which increase 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 spermatozoon) and pushes it forward. On fig. 17.34 summarizes information about the types of movement carried out with the help of the flagellum.

The beating of the cilia is asymmetrical (Fig. 17.32); after a quick and vigorous blow of a straight cilia, it bends and slowly returns to its original position. With the accumulation of a large number of cilia, some mechanism is needed to coordinate their activity. Infusoria paramecium these functions are usually attributed neurofans- filaments connecting the basal bodies. Usually, the beating of 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 a lot of controversy regarding the mechanism of movement of the cilia or flagella themselves. Based on the latest data, this mechanism is basically very close to the interaction of actin and myosin during muscle contraction. Flagellum curvature is believed to be 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, the five peripheral fibrils on one side initiate the initial bending, while the remaining four fibrils on the other side come into action later, which leads to the return movement of the flagellum (Fig. 17.35). Radial crossbars, preventing slipping, reduce it to local bending of the flagellum. It is possible that the central fibrils conduct a signal about the beginning of sliding from the basal body along the entire length of the cilium or flagellum. It has been shown that cilia can only work in the presence of Mg 2+ ions and that the direction of beating is determined by Ca 2+ concentration inside the cell. Interestingly, the avoidance reaction is controlled in paramecium as follows: when the ciliate encounters an obstacle, the direction of the beating of the cilia is reversed, and then it resumes its forward movement. This change is stimulated by a sudden influx of Ca 2+ ions into the cell as a result of increased permeability for these ions.

In small eukaryotic organisms, cilia or flagella are widely used for movement in water, pushing the body through the surrounding viscous fluid. This method of locomotion can be effective only for very small body sizes, when the surface to volume ratio is much larger 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 series of important functions. They can drive fluid through the ducts, as happens in annelids in metanephridia when removing waste products of metabolism. With the help of cilia, eggs move in the oviducts of mammals and various materials along the inner surface of organs, for example, mucus in respiratory tract, where the work of the cilia allows you to remove dust particles and other "garbage". Cilia can also create an external fluid flow from which some organisms, including paramecium, filter food particles (often using other types of cilia).

The body of the protozoan consists of the 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 amount of chromatin collected along the periphery of the nucleus or in an intranuclear body, the karyosome. The micronuclei of ciliates belong to the massive type nuclei with big amount chromatin. Common cell components of most protozoa include mitochondria and the Golgi apparatus.

The surface of the body of amoeboid forms (sarcodes, as well as some stages of the life cycle of other groups) is covered with a 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 Trichomonas.

Dense and rigid shells have resting forms of protozoa, cysts. Shell amoeba, foraminifera and some other protozoa are enclosed in houses or shells.

Unlike the cell of a multicellular organism, the cell of the simplest is whole organism. To perform the diverse functions of the body in the body of the simplest, structural formations, organelles, can specialize. 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 protrusions of the cytoplasm, pseudopodia. This type of movement is called amoeboid and is found in many groups of protozoa (sarcode, asexual forms of sporozoans, etc.). Flagella and cilia serve as special organelles for movement. Flagella are characteristic of the class of flagellates, as well as gametes of representatives of other classes. They are few in most forms (from 1 to 8). The number of cilia, which are the organelles of the movement of ciliates, can reach several thousand in one individual. An electron microscope study showed that flagella and cilia in Protozoa, Metazoa and plant cells are built according to 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 of the cell membrane. Central fibrils are present only in the free part of the tourniquet, and peripheral fibrils go deep into the cytoplasm, forming a basal grain - blepharoplast. The tourniquet can be connected to the cytoplasm for a considerable distance by a thin membrane - an undulating membrane. The ciliary apparatus of ciliates can reach considerable complexity and differentiate into zones that perform independent functions. The cilia often coalesce in groups, forming spikes and membranellae. Each cilium starts from a basal grain, a kinetosome, located in the surface layer of the cytoplasm. The collection of kinetosomes forms the infracilia. Knetosomes reproduce only by dividing in two and cannot arise anew. With a 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 organelles of movement. The former include pseudopodia, or pseudopodia, - temporarily formed outgrowths of ectoplasm, for example, in an amoeba, into which the endoplasm, as it were, “overflows”, due to which the simplest itself, as it were, “flows” 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 filament along the axis, dressed as if with a case of more liquid plasma. In the body of the protozoan, the base of the tourniquet is connected to the basal grain, 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 whips, are very short and extremely numerous. Cilia quickly tilt 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 have both pseudopodia and tourniquet, or pseudopodia and cilia. In other protozoa, different ways of locomotion can be observed at different stages of the life cycle.
In some protozoa, contractile fibers, or myonemes, differentiate in the protoplasm, due 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 cysts of protozoa and bacteria by intestinal amoeba or by cilia that drive particles into the cell mouth (cytostomes, for example, ciliates Balantidium coll and starch grains). Endosmotic nutrition is characteristic of protozoa that do not have nutritional organelles, for example, trypanosomes, leishmania, gregarines, some ciliates, and many others. etc. Nutrition in such cases occurs due to the absorption of organic solutes from environment; this form of nutrition is also called saprophytic.

Ingested food substances enter the endoplasm where they are digested. Unused residues are thrown out or anywhere on the surface of the body of the protozoan or in a certain area of ​​\u200b\u200bit (analogy of the defecation process).

In the endoplasm of the protozoan, spare nutrients 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 this species of protozoa. The organelles of excretion, as well as osmoregulation, and partially respiration, are pulsating vacuoles, which, rhythmically contracting, empty their liquid contents outward, which are again recruited into the vacuole from the adjacent parts of the endoplasm. In the endoplasm, the nucleus of the protozoan is laid. Many protozoa have two or more nuclei, which vary in structure in different Protozoa.
The nucleus is a necessary component of the simplest, for all life processes can proceed only if it is present; the nuclear-free sections of the protoplasm of the protozoan under experimental conditions can only survive for a while.

In protozoa, specificity is also noted for carriers. Some species adapt only to one specific carrier, for others, several species may be carriers, often belonging to any one class.

Both prokaryotic and eukaryotic can contain structures known as cilia and flagella. These outgrowths on the surface of the cells help in their.

Features and functions

Cilia and flagella are outgrowths from some cells necessary for cellular locomotion (movement). They also help move substances around the cells and guide them to the right places.

Cilia and flagella are formed from specialized groups of microtubules called basal bodies.

If the outgrowths are short and numerous, they are called cilia. If they are longer and less numerous (usually only one or two), they are called flagella.

Structure

Typically, cilia and flagella have a core consisting of microtubules connected to , arranged in a 9 + 2 pattern. The ring of nine microtubules has two special microtubules in its center that flex the cilia or flagella. This type of organization is found in the structure of most cilia and flagella.

Where do they meet?

Both cilia and flagella are found in many cell types. For example, the sperm of many animals, algae, and even ferns have flagella. Cilia can be found in cells in tissues such as the respiratory tract and the female reproductive tract.

Cilia and flagella have a similar internal structure; they differ only in the nature of the beat. The flagellum, like the tail of the spermatozoon, makes symmetrical wave-like oscillations that propagate along it. Unlike the flagellum, the cilium strikes asymmetrically, quickly, with a jerk in one direction, and then produces a slow reverse movement, as a result of which it returns to its original position (Fig. 11.1) .. When

When the flagellum moves, water is repelled parallel to its longitudinal axis, and when the cilium moves, it moves parallel to the surface bearing the cilia (Fig. 11.2).
The cell usually possesses one or only a few flagella; on a ciliary cell, for example, on a paramecium cell, there may be several thousand cilia, evenly distributed

Rice. 11.1. Beating of cilia in the ctenophore Pleurobrachia. (Sleigh, 1968.)
The scheme was compiled according to the data of filming made at 17 °C. The position of one cilium is shown at 5 ms intervals. The working movement takes 10ms and the return movement takes 50ms. The cilium then remains in its original position for 20 ms until the next beat begins at the 65th millisecond.

nyh on its surface. However, it is difficult to clearly distinguish between flagella and cilia: their internal structure is identical, movements occur in the organelles themselves, and intermediate forms of movement are often found.
The flagella of bacteria are completely different. They are thinner (their diameter is about 0.02 μm, as opposed to 0.25 μm in true flagella and cilia), short and relatively rigid, they are turned by forces acting at the base of the flagellum, where it is attached to the cell (Berg and Anderson, 1973 ).
Many protozoans have cilia and flagella and are of paramount importance for locomotion. Spermatozoa set
Animals swim using flagella. Cilia-covered gills and tentacles, often found in invertebrates, perform two main functions: respiratory gas exchange and filtering water to capture food particles. Cilia are also common in more highly organized animals, in which they serve, for example, to move fluid through tubes.

Rice. 11.2. A typical beating of a flagellum (left) drives water parallel to its main axis (arrow). The beat of the cilium (right) drives water parallel to the surface to which the cilium is attached. (Sleight, 1974.)

in the reproductive and excretory systems (in the nephridia of annelids, etc.). In mammals, ciliary epithelium assists in the transport of various materials along internal surfaces, such as the movement of mucus in the respiratory tract or eggs in the oviduct.
Cilia are found in animals of all types. Previously, insects were considered an exception, as they do not have functioning cilia. However, modified ciliary structures, identifiable by the characteristic arrangement of internal filaments (scheme 9+2), are found in insects in the eyes, as well as in most other sense organs in animals of almost all types. The most important exceptions are the eyes of some invertebrates and the taste buds of vertebrates. In many cases, when a ciliary structure is found in a sense organ, it is not easy to establish its significance for photo- or chemoreception. These questions are difficult to resolve, and only an examination of them on a broad comparative basis is likely to lead to Acceptable Generalizations (Barber, 1974). .
As shown by electron microscopy, cilia and flagella have the same structure: a pair of filaments in the center is surrounded by nine more thin filaments. Such an arrangement according to the 9 + 2 scheme is typical for all groups of animals from protozoa to vertebrates and is characteristic of the vast majority of spermatozoa.

Photo 11.1. The coordinated beating of cilia in the ciliate Paramecium caudatum creates the appearance of waves running over its surface. The length of the animal is about 150 microns. (Photo courtesy of R. L. Hammersmith, Indiana University.)

matozoids. This 9+2 combination is almost universal, although not mandatory, as exceptions are known: there are spermatozoa with three central filaments, with one or without them at all (Blum and Lubbiner, 1973).
The cilia used for movement can only act in aquatic environment and therefore are found only on the surfaces of cells facing the liquid or covered with a liquid film, such as mucus.
Intuitively, one would expect that an organism with a single flagellum, such as a spermatozoon, would swim in the direction opposite to the direction of the wave traveling along the flagellum. This is true in the case of a smooth undulating thread: it will actually push the organism in the opposite direction. It seems surprising, therefore, that some flagellates move by means of a long cord which strikes anteriorly in the direction of movement, so that the waves move from the body to the anterior end of the flagellum (Jahn et al. 104).
Such a seemingly paradoxical situation, when wave propagation and locomotion are directed in the same direction, is possible if the thread is rough or covered with protrusions. In species that move in this way, the flagella are equipped with tiny appendages in the form of thin lateral outgrowths, which

Photo 11.2. An electron micrograph of the flagella of the protozoan Trichonumoka clearly shows the characteristic distribution of filameites (2 + 91 in each tick. (Photo courtesy of A. V. Grimstone, Cambridge University) D M Y

responsible for this unexpected direction of movement (Figure 11.3).
The mechanism of movement of cilia and flagella has long been the subject of speculative hypotheses. Mechanisms of three types were assumed: 1) the flagellum moves passively, like a whip, under the action of
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viem forces applied at its base; 2) the elements lying along the internal curvature of the propagating wave are reduced, while the elements of the opposite side are not; 3) thin threads inside the cilia are displaced relative to each other due to the forces acting between them, similar to how it happens with myofilaments during muscle contraction.
The notion that the flagellum moves passively under the action of forces applied at its base is inconsistent with the shape of its waves (Machin, 1958). Waves down the tail of the spermatozoon

Figure 11.3. The flagellated Ochromonas swims with a single flagellum pointing forward. The undulating movement of the flagellum consists in the movement of waves from its base to the tip (i.e., in the same direction as the animal moves). This paradoxical situation is explained by the presence of fixed protrusions located at right angles to the flagellum and causing the movement of water in the direction indicated by curved arrows; as a result, the body moves in the opposite direction. (Jahn et al., 1964.)

propagate without a decrease in amplitude, and in some cases the amplitude even increases (Rickmenspoel, 1965). It follows from this that there must be active elements in the flagellum and that energy is generated locally.
If we assume that the cilia are driven by threads (filaments), then two mechanisms are possible: 1) the contraction of the threads on one side could bend the cilium; 2) the movement could be the result of longitudinal sliding of the threads relative to each other. The first hypothesis is contradicted by the fact that the cilia tubules maintain a constant length throughout the entire bending cycle (Satir, 1968, 1974), and the currently available data are fully consistent with the second hypothesis.
The characteristic fine structure of a flagellum or cilium is shown in Fig. 11.4. Double tubules consist of tubulin protein with a mol. weighing about 55,000. Attached to each A-tubule is a pair of protrusions consisting of the dinvin protein. This protein bears little resemblance to any muscle contractile proteins, except for its ability to catalyze the breakdown of ATP, a property it shares with the important muscle protein myosin (Gibbons and Rowe, 1965; Gibbons, 1977).
The flagellum flexes when the dynein protrusions attach to the adjacent tubule B, causing active gliding motions powered by ATP. This process is similar to the sliding of filaments in a muscle, but what exactly are the molecular movements during the movement of the flagellum has not yet been exactly clarified.
It is possible to create an experimental model of the ciliary apparatus of paramecium and study the regulation of the movement of cilia on this model. The cell is treated with a detergent, which leads to destruction

Rice. 11.4. Scheme internal structure flagellum or cilia (in cross section). The double tubules are made up of the protein tubulin. The protrusions containing the protein dynein are attached to the A-tubules. (Brokaw and Gibbons, 1975.)

the cell membrane, while the function of the ciliary apparatus is preserved. The infusoria can then be reactivated and will float in an ATP solution containing magnesium ions. It can be shown that the direction in which the cilia strike is determined by the concentration of calcium ions (Naitoh and Kaneko, 1972). When a living paramecia encounters an obstacle, it retreats and swims back as the direction of the beat is reversed; this reaction begins with an increase in the permeability of the cell to calcium ions (Eckert, 1972). As we will see later, calcium ions play an important role in controlling many processes, including muscle contraction.
The overall efficiency in converting metabolic energy into mechanical energy in a swimming spermatozoon appears to be at least 19% and probably closer to 25% (Rickmenspoel et al, 1969). This value is strikingly similar to the efficiency value for muscles that perform external work.

Body protozoan is made up of cytoplasm and one or more cores. 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 amount of chromatin collected along the periphery of the nucleus or in an intranuclear body, the karyosome. Micronuclei of ciliates are massive nuclei with a large amount of chromatin. Common cell components of most protozoa include mitochondria and the Golgi apparatus.

Surface bodies of amoeboid forms(sarcodal, as well as some stages of the life cycle of other groups) is covered with a 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 Trichomonas.

Thick and hard shells have resting forms of protozoa, cysts. Shell amoeba, foraminifera and some other protozoa are enclosed in houses or shells.

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

Very diverse protozoan movement organelles. Amoeboid forms move through the formation of protrusions of the cytoplasm, pseudopodia. This type of movement is called amoeboid and is found in many groups of protozoa (sarcode, asexual forms of sporozoans, etc.). Flagella and cilia serve as special organelles for movement. Flagella are characteristic of the class of flagellates, as well as gametes of representatives of other classes. They are few in most forms (from 1 to 8). The number of cilia, which are the organelles of the movement of ciliates, can reach several thousand in one individual. An electron microscope study showed that flagella and cilia in Protozoa, Metazoa and plant cells built in the same way. Their basis is a bundle of fibrils, consisting of two central and nine paired, peripheral ones.

tourniquet surrounded by a shell which is an extension of the cell membrane. Central fibrils are present only in the free part of the tourniquet, and peripheral fibrils go deep into the cytoplasm, forming a basal grain - blepharoplast. The tourniquet can be connected to the cytoplasm for a considerable distance by a thin membrane - an undulating membrane. The ciliary apparatus of ciliates can reach considerable complexity and differentiate into zones that perform independent functions. The cilia often coalesce in groups, forming spikes and membranellae. Each cilium starts from a basal grain, a kinetosome, located in the surface layer of the cytoplasm. The collection of kinetosomes forms the infracilia. Knetosomes reproduce only by dividing in two and cannot arise anew. With a partial or complete reduction of the flagellar apparatus, the infracilia remains and subsequently gives rise to new cilia.