Golgi complex is a membrane structure inherent in any eukaryotic cell.

Golgi apparatus presented flattened tanks(or bags) collected in a stack. Each tank is slightly curved and has convex and concave surfaces. The average diameter of the tanks is about 1 micron. In the center of the tank, its membranes are brought closer together, and at the periphery they often form extensions, or ampoules, from which they are detached bubbles. Packages of flat tanks with an average number of about 5-10 are formed dictyosome. In addition to cisternae, the Golgi complex contains transport and secretory vesicles. In the dictyosome, in accordance with the direction of curvature of the curved surfaces of the tanks, two surfaces are distinguished. A convex surface is called immature, or cis-surface. It faces the nucleus or the tubules of the granular endoplasmic reticulum and is connected to the latter by vesicles that detach from the granular reticulum and bring protein molecules to the dictyosome for maturation and formation into the membrane. The opposite transsurface of the dictyosome is concave. It faces the plasmalemma and is called mature because secretory vesicles containing secretion products ready for removal from the cell emerge from its membranes.

The Golgi complex is involved in:

• in the accumulation of products synthesized in the endoplasmic reticulum,
• in their chemical restructuring and maturation.

IN cisternae of the Golgi complex polysaccharides are synthesized and combined with protein molecules.

One of main functions Golgi complex - formation of finished secretory products, which are removed outside the cell by exocytosis. The most important functions of the Golgi complex for the cell are also renewal of cell membranes, including areas of the plasmalemma, as well as replacement of plasmalemma defects in the process of secretory activity of the cell.

The Golgi complex is considered source of formation of primary lysosomes, although their enzymes are also synthesized in the granular network. Lysosomes are intracellularly formed secretory vacuoles filled with hydrolytic enzymes necessary for the processes of phago- and autophagocytosis. At the light-optical level, lysosomes can be identified and the degree of their development in the cell can be judged by the activity of the histochemical reaction to acid phosphatase, a key lysosomal enzyme. By electron microscopy, lysosomes are defined as vesicles bounded by a membrane from the hyaloplasm. Conventionally, there are 4 main types of lysosomes:

• primary,
• secondary lysosomes,
• autophagosomes,
• residual bodies.

Primary lysosomes- these are small membrane vesicles (their average diameter is about 100 nm), filled with homogeneous finely dispersed content, which is a set of hydrolytic enzymes. About 40 enzymes have been identified in lysosomes (proteases, nucleases, glycosidases, phosphorylases, sulfatases), the optimal mode of action of which is designed for an acidic environment (pH 5). Lysosomal membranes contain special carrier proteins for the transport of hydrolytic cleavage products - amino acids, sugars and nucleotides - from the lysosome to the hyaloplasm. The lysosome membrane is resistant to hydrolytic enzymes.

Secondary lysosomes are formed by the fusion of primary lysosomes with endocytic or pinocytotic vacuoles. In other words, secondary lysosomes are intracellular digestive vacuoles, the enzymes of which are supplied by primary lysosomes, and the material for digestion is supplied by the endocytic (pinocytotic) vacuole. The structure of secondary lysosomes is very diverse and changes during the hydrolytic breakdown of the contents. Lysosome enzymes break down biological substances that have entered the cell, resulting in the formation of monomers that are transported through the lysosome membrane into the hyaloplasm, where they are utilized or included in a variety of synthetic and metabolic reactions.

If the cell’s own structures (aging organelles, inclusions, etc.) are subjected to interaction with primary lysosomes and hydrolytic cleavage by their enzymes, autophagosome. Autophagocytosis is a natural process in the life of a cell and plays a large role in the renewal of its structures during intracellular regeneration.

Residual bodies this is one of the final stages of the existence of phago- and autolysosomes and is detected during incomplete phago- or autophagocytosis and is subsequently released from the cell by exocytosis. They have compacted contents, and secondary structuring of undigested compounds is often observed (for example, lipids form complex layered formations).

Organoids- permanent, necessarily present, components of the cell that perform specific functions.

Endoplasmic reticulum

Endoplasmic reticulum (ER), or endoplasmic reticulum (ER), is a single-membrane organelle. It is a system of membranes that form “cisterns” and channels, connected to each other and delimiting a single internal space - the EPS cavities. The membranes are connected on one side to the cytoplasmic membrane and on the other to the outer nuclear membrane. There are two types of EPS: 1) rough (granular), containing ribosomes on its surface, and 2) smooth (agranular), the membranes of which do not carry ribosomes.

Functions: 1) transport of substances from one part of the cell to another, 2) division of the cell cytoplasm into compartments (“compartments”), 3) synthesis of carbohydrates and lipids (smooth ER), 4) protein synthesis (rough ER), 5) place of formation of the Golgi apparatus .

Or Golgi complex, is a single-membrane organelle. It consists of stacks of flattened “cisterns” with widened edges. Associated with them is a system of small single-membrane vesicles (Golgi vesicles). Each stack usually consists of 4-6 “cisterns”, is a structural and functional unit of the Golgi apparatus and is called a dictyosome. The number of dictyosomes in a cell ranges from one to several hundred. In plant cells, dictyosomes are isolated.

The Golgi apparatus is usually located near the cell nucleus (in animal cells, often near the cell center).

Functions of the Golgi apparatus: 1) accumulation of proteins, lipids, carbohydrates, 2) modification of incoming organic substances, 3) “packaging” of proteins, lipids, carbohydrates into membrane vesicles, 4) secretion of proteins, lipids, carbohydrates, 5) synthesis of carbohydrates and lipids, 6) place of formation lysosomes The secretory function is the most important, therefore the Golgi apparatus is well developed in secretory cells.

Lysosomes

Lysosomes- single-membrane organelles. They are small bubbles (diameter from 0.2 to 0.8 microns) containing a set of hydrolytic enzymes. Enzymes are synthesized on the rough ER and move to the Golgi apparatus, where they are modified and packaged into membrane vesicles, which, after separation from the Golgi apparatus, become lysosomes themselves. A lysosome can contain from 20 to 60 different types of hydrolytic enzymes. The breakdown of substances using enzymes is called lysis.

There are: 1) primary lysosomes, 2) secondary lysosomes. Primary are called lysosomes that are detached from the Golgi apparatus. Primary lysosomes are a factor ensuring the exocytosis of enzymes from the cell.

Secondary are called lysosomes formed as a result of the fusion of primary lysosomes with endocytic vacuoles. In this case, they digest substances that enter the cell by phagocytosis or pinocytosis, so they can be called digestive vacuoles.

Autophagy- the process of destroying structures unnecessary for the cell. First, the structure to be destroyed is surrounded by a single membrane, then the resulting membrane capsule merges with the primary lysosome, resulting in the formation of a secondary lysosome (autophagic vacuole), in which this structure is digested. The products of digestion are absorbed by the cell cytoplasm, but some of the material remains undigested. The secondary lysosome containing this undigested material is called a residual body. By exocytosis, undigested particles are removed from the cell.

Autolysis- cell self-destruction, which occurs due to the release of lysosome contents. Normally, autolysis occurs during metamorphosis (disappearance of the tail in a tadpole of frogs), involution of the uterus after childbirth, and in areas of tissue necrosis.

Functions of lysosomes: 1) intracellular digestion of organic substances, 2) destruction of unnecessary cellular and non-cellular structures, 3) participation in the processes of cell reorganization.

Vacuoles

Vacuoles- single-membrane organelles are “containers” filled with aqueous solutions of organic and inorganic substances. The ER and Golgi apparatus take part in the formation of vacuoles. Young plant cells contain many small vacuoles, which then, as the cells grow and differentiate, merge with each other and form one large central vacuole. The central vacuole can occupy up to 95% of the volume of a mature cell; the nucleus and organelles are pushed towards the cell membrane. The membrane bounding the plant vacuole is called the tonoplast. The fluid that fills a plant vacuole is called cell sap. The composition of cell sap includes water-soluble organic and inorganic salts, monosaccharides, disaccharides, amino acids, final or toxic metabolic products (glycosides, alkaloids), and some pigments (anthocyanins).

Animal cells contain small digestive and autophagy vacuoles, which belong to the group of secondary lysosomes and contain hydrolytic enzymes. Unicellular animals also have contractile vacuoles that perform the function of osmoregulation and excretion.

Functions of the vacuole: 1) accumulation and storage of water, 2) regulation of water-salt metabolism, 3) maintenance of turgor pressure, 4) accumulation of water-soluble metabolites, reserve nutrients, 5) coloring of flowers and fruits and thereby attracting pollinators and seed dispersers, 6) see. functions of lysosomes.

The endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles form single vacuolar network of the cell, the individual elements of which can transform into each other.

Mitochondria

1 - outer membrane;
2 - internal membrane; 3 - matrix; 4 - crista; 5 - multienzyme system; 6 - circular DNA.

The shape, size and number of mitochondria vary enormously. Mitochondria can be rod-shaped, round, spiral, cup-shaped, or branched in shape. The length of mitochondria ranges from 1.5 to 10 µm, diameter - from 0.25 to 1.00 µm. The number of mitochondria in a cell can reach several thousand and depends on the metabolic activity of the cell.

The mitochondrion is bounded by two membranes. The outer membrane of mitochondria (1) is smooth, the inner (2) forms numerous folds - cristas(4). Cristae increase the surface area of ​​the inner membrane, on which multienzyme systems (5) involved in the synthesis of ATP molecules are located. The internal space of mitochondria is filled with matrix (3). The matrix contains circular DNA (6), specific mRNA, prokaryotic type ribosomes (70S type), and Krebs cycle enzymes.

Mitochondrial DNA is not associated with proteins (“naked”), is attached to the inner membrane of the mitochondrion and carries information about the structure of about 30 proteins. To build a mitochondrion, many more proteins are required, so information about most mitochondrial proteins is contained in nuclear DNA, and these proteins are synthesized in the cytoplasm of the cell. Mitochondria are capable of autonomous reproduction by fission in two. Between the outer and inner membranes there is proton reservoir, where H + accumulation occurs.

Functions of mitochondria: 1) ATP synthesis, 2) oxygen breakdown of organic substances.

According to one hypothesis (the theory of symbiogenesis), mitochondria originated from ancient free-living aerobic prokaryotic organisms, which, having accidentally penetrated the host cell, then formed a mutually beneficial symbiotic complex with it. The following data support this hypothesis. Firstly, mitochondrial DNA has the same structural features as the DNA of modern bacteria (closed in a ring, not associated with proteins). Secondly, mitochondrial ribosomes and bacterial ribosomes belong to the same type - the 70S type. Thirdly, the mechanism of mitochondrial fission is similar to that of bacteria. Fourth, the synthesis of mitochondrial and bacterial proteins is suppressed by the same antibiotics.

Plastids

1 - outer membrane; 2 - internal membrane; 3 - stroma; 4 - thylakoid; 5 - grana; 6 - lamellae; 7 - starch grains; 8 - lipid drops.

Plastids are characteristic only of plant cells. Distinguish three main types of plastids: leucoplasts are colorless plastids in the cells of uncolored parts of plants, chromoplasts are colored plastids usually yellow, red and orange, chloroplasts are green plastids.

Chloroplasts. In the cells of higher plants, chloroplasts have the shape of a biconvex lens. The length of chloroplasts ranges from 5 to 10 µm, diameter - from 2 to 4 µm. Chloroplasts are bounded by two membranes. The outer membrane (1) is smooth, the inner (2) has a complex folded structure. The smallest fold is called thylakoid(4). A group of thylakoids arranged like a stack of coins is called facet(5). The chloroplast contains on average 40-60 grains, arranged in a checkerboard pattern. The granae are connected to each other by flattened channels - lamellae(6). The thylakoid membranes contain photosynthetic pigments and enzymes that provide ATP synthesis. The main photosynthetic pigment is chlorophyll, which determines the green color of chloroplasts.

The interior space of the chloroplasts is filled stroma(3). The stroma contains circular “naked” DNA, 70S-type ribosomes, Calvin cycle enzymes, and starch grains (7). Inside each thylakoid there is a proton reservoir, and H + accumulates. Chloroplasts, like mitochondria, are capable of autonomous reproduction by dividing into two. They are found in the cells of the green parts of higher plants, especially many chloroplasts in leaves and green fruits. Chloroplasts of lower plants are called chromatophores.

Function of chloroplasts: photosynthesis. It is believed that chloroplasts originated from ancient endosymbiotic cyanobacteria (symbiogenesis theory). The basis for this assumption is the similarity of chloroplasts and modern bacteria in a number of characteristics (circular, “naked” DNA, 70S-type ribosomes, method of reproduction).

Leukoplasts. The shape varies (spherical, round, cupped, etc.). Leukoplasts are bounded by two membranes. The outer membrane is smooth, the inner one forms few thylakoids. The stroma contains circular “naked” DNA, 70S-type ribosomes, enzymes for the synthesis and hydrolysis of reserve nutrients. There are no pigments. The cells of the underground organs of the plant (roots, tubers, rhizomes, etc.) have especially many leucoplasts. Function of leucoplasts: synthesis, accumulation and storage of reserve nutrients. Amyloplasts- leukoplasts that synthesize and accumulate starch, elaioplasts- oils, proteinoplasts- proteins. Different substances can accumulate in the same leukoplast.

Chromoplasts. Bounded by two membranes. The outer membrane is smooth, the inner membrane is either smooth or forms single thylakoids. The stroma contains circular DNA and pigments - carotenoids, which give chromoplasts a yellow, red or orange color. The form of accumulation of pigments is different: in the form of crystals, dissolved in lipid droplets (8), etc. Contained in the cells of mature fruits, petals, autumn leaves, and rarely - root vegetables. Chromoplasts are considered the final stage of plastid development.

Function of chromoplasts: coloring flowers and fruits and thereby attracting pollinators and seed dispersers.

All types of plastids can be formed from proplastids. Proplastids- small organelles contained in meristematic tissues. Since plastids have a common origin, interconversions between them are possible. Leukoplasts can turn into chloroplasts (greening of potato tubers in the light), chloroplasts - into chromoplasts (yellowing of leaves and reddening of fruits). The transformation of chromoplasts into leucoplasts or chloroplasts is considered impossible.

Ribosomes

1 - large subunit; 2 - small subunit.

Ribosomes- non-membrane organelles, diameter approximately 20 nm. Ribosomes consist of two subunits - large and small, into which they can dissociate. The chemical composition of ribosomes is proteins and rRNA. rRNA molecules make up 50-63% of the mass of the ribosome and form its structural framework. There are two types of ribosomes: 1) eukaryotic (with sedimentation constants for the whole ribosome - 80S, small subunit - 40S, large - 60S) and 2) prokaryotic (70S, 30S, 50S, respectively).

Ribosomes of the eukaryotic type contain 4 rRNA molecules and about 100 protein molecules, while the prokaryotic type contains 3 rRNA molecules and about 55 protein molecules. During protein biosynthesis, ribosomes can “work” individually or combine into complexes - polyribosomes (polysomes). In such complexes they are linked to each other by one mRNA molecule. Prokaryotic cells have only 70S-type ribosomes. Eukaryotic cells have both 80S-type ribosomes (rough EPS membranes, cytoplasm) and 70S-type (mitochondria, chloroplasts).

Eukaryotic ribosomal subunits are formed in the nucleolus. The combination of subunits into a whole ribosome occurs in the cytoplasm, usually during protein biosynthesis.

Function of ribosomes: assembly of a polypeptide chain (protein synthesis).

Cytoskeleton

Cytoskeleton formed by microtubules and microfilaments. Microtubules are cylindrical, unbranched structures. The length of microtubules ranges from 100 µm to 1 mm, the diameter is approximately 24 nm, and the wall thickness is 5 nm. The main chemical component is the protein tubulin. Microtubules are destroyed by colchicine. Microfilaments are filaments with a diameter of 5-7 nm and consist of the protein actin. Microtubules and microfilaments form complex weaves in the cytoplasm. Functions of the cytoskeleton: 1) determination of the shape of the cell, 2) support for organelles, 3) formation of the spindle, 4) participation in cell movements, 5) organization of cytoplasmic flow.

Includes two centrioles and a centrosphere. Centriole is a cylinder, the wall of which is formed by nine groups of three fused microtubules (9 triplets), interconnected at certain intervals by cross-links. Centrioles are united in pairs where they are located at right angles to each other. Before cell division, centrioles diverge to opposite poles, and a daughter centriole appears near each of them. They form a division spindle, which contributes to the even distribution of genetic material between daughter cells. In the cells of higher plants (gymnosperms, angiosperms), the cell center does not have centrioles. Centrioles are self-replicating organelles of the cytoplasm; they arise as a result of duplication of existing centrioles. Functions: 1) ensuring the divergence of chromosomes to the cell poles during mitosis or meiosis, 2) the center of organization of the cytoskeleton.

Organoids of movement

Not present in all cells. Organelles of movement include cilia (ciliates, epithelium of the respiratory tract), flagella (flagellates, sperm), pseudopods (rhizopods, leukocytes), myofibrils (muscle cells), etc.

Flagella and cilia- filament-shaped organelles, representing an axoneme bounded by a membrane. Axoneme is a cylindrical structure; the wall of the cylinder is formed by nine pairs of microtubules; in its center there are two single microtubules. At the base of the axoneme there are basal bodies, represented by two mutually perpendicular centrioles (each basal body consists of nine triplets of microtubules; there are no microtubules in its center). The length of the flagellum reaches 150 microns, the cilia are several times shorter.

Myofibrils consist of actin and myosin myofilaments that provide contraction of muscle cells.

Go to lectures No. 6“Eukaryotic cell: cytoplasm, cell membrane, structure and functions of cell membranes”

Structure of the Golgi complex

Golgi complex (KG), or internal mesh apparatus , is a special part of the metabolic system of the cytoplasm, participating in the process of isolation and formation of membrane structures of the cell.

CG is visible in an optical microscope as a mesh or curved rod-shaped bodies lying around the nucleus.

Under an electron microscope, it was revealed that this organelle is represented by three types of formations:

All components of the Golgi apparatus are formed by smooth membranes.

Note 1

Occasionally, AG has a granular-mesh structure and is located near the nucleus in the form of a cap.

AG is found in all cells of plants and animals.

Note 2

The Golgi apparatus is significantly developed in secretory cells. It is especially visible in nerve cells.

The internal intermembrane space is filled with a matrix that contains specific enzymes.

The Golgi apparatus has two zones:

• formation zone, where, with the help of vesicles, the material that is synthesized in the endoplasmic reticulum enters;
• ripening zone, where the secretion and secretory sacs are formed. This secretion accumulates at the terminal sites of the AG, from where secretory vesicles bud. As a rule, such vesicles carry secretions outside the cell.

• Localization of CG

In apolar cells (for example, in nerve cells), the CG is located around the nucleus; in secretory cells, it occupies a place between the nucleus and the apical pole.

The Golgi sac complex has two surfaces:

formative(immature or regenerative) cis-surface (from the Latin Cis - on this side); functional(mature) – trans-surface (from Latin Trans – through, behind).

The Golgi column with its convex formative surface faces the nucleus, is adjacent to the granular endoplasmic reticulum and contains small round vesicles called intermediate. The mature concave surface of the sac column faces the apex (apical pole) of the cell and ends in large vesicles.

Formation of the Golgi complex

KG membranes are synthesized by the granular endoplasmic reticulum, which is adjacent to the complex. The areas of the EPS adjacent to it lose ribosomes, and small, so-called, ribosomes bud from them. transport or intermediate vesicles. They move to the formative surface of the Golgi column and merge with its first sac. On the opposite (mature) surface of the Golgi complex there is an irregularly shaped sac. Its expansion - prosecretory granules (condensing vacuoles) - continuously buds and turns into vesicles filled with secretion - secretory granules. Thus, to the extent that the membranes of the mature surface of the complex are used for secretory vesicles, the sacs of the formative surface are replenished at the expense of the endoplasmic reticulum.

Functions of the Golgi complex

The main function of the Golgi apparatus is the removal of substances synthesized by the cell. These substances are transported through the cells of the endoplasmic reticulum and accumulate in the vesicles of the reticular apparatus. Then they are either released into the external environment or the cell uses them in the process of life.

The complex also concentrates some substances (for example, dyes) that enter the cell from the outside and must be removed from it.

In plant cells, the complex contains enzymes for the synthesis of polysaccharides and the polysaccharide material itself, which is used to build the cellulose membrane of the cell.

In addition, CG synthesizes those chemicals that form the cell membrane.

In general, the Golgi apparatus performs the following functions:

1. accumulation and modification of macromolecules that were synthesized in the endoplasmic reticulum;
2. formation of complex secretions and secretory vesicles by condensation of the secretory product;
3. synthesis and modification of carbohydrates and glycoproteins (formation of glycocalyx, mucus);
4. modification of proteins - adding various chemical formations to the polypeptide (phosphate - phosphorylation, carboxyl - carboxylation), the formation of complex proteins (lipoproteins, glycoproteins, mucoproteins) and the breakdown of polypeptides;
5. is important for the formation and renewal of the cytoplasmic membrane and other membrane formations due to the formation of membrane vesicles, which subsequently merge with the cell membrane;
6. formation of lysosomes and specific granularity in leukocytes;
7. formation of peroxisomes.

The protein and, partially, carbohydrate contents of CG come from the granular endoplasmic reticulum, where it is synthesized. The main part of the carbohydrate component is formed in the sacs of the complex with the participation of glycosyltransferase enzymes, which are located in the membranes of the sacs.

In the Golgi complex, cellular secretions containing glycoproteins and glycosaminoglycans are finally formed. In the CG, secretory granules mature, which turn into vesicles, and the movement of these vesicles towards the plasma membrane. The final stage of secretion is the pushing of the formed (mature) vesicles outside the cell. The removal of secretory inclusions from the cell is carried out by installing the membranes of the vesicle into the plasmalemma and releasing secretory products outside the cell. In the process of moving secretory vesicles to the apical pole of the cell membrane, their membranes thicken from the initial 5-7 nm, reaching a plasmalemma thickness of 7-10 nm.

Note 4

There is an interdependence between cell activity and the size of the Golgi complex - secretory cells have large columns of CG, while non-secretory cells contain a small number of complex sacs.

In 1898, the Italian scientist C. Golgi discovered mesh formations in nerve cells, which he called the “internal mesh apparatus” (Fig. 174). Reticulate structures (Golgi apparatus) are found in all cells of any eukaryotic organisms. Typically, the Golgi apparatus is located near the nucleus, near the cell center (centriole).

Fine structure of the Golgi apparatus. The Golgi apparatus consists of membrane structures assembled together in a small zone (Fig. 176, 177). A separate zone of accumulation of these membranes is called dictyosome(Fig. 178). In the dictyosome, flat membrane sacs, or cisterns, are located close to each other (at a distance of 20-25 nm) in the form of a stack, between which thin layers of hyaloplasm are located. Each individual tank has a diameter of about 1 μm and variable thickness; in the center its membranes can be close together (25 nm), and at the periphery they can have expansions, ampoules, the width of which is not constant. The number of such bags in a stack usually does not exceed 5-10. In some single-celled organisms their number can reach 20. In addition to densely located flat cisterns, many vacuoles are observed in the AG zone. Small vacuoles are found mainly in the peripheral areas of the AG zone; sometimes you can see how they are laced from the ampullary extensions at the edges of the flat cisterns. It is customary to distinguish in the dictyosome zone the proximal or developing, cis-section, and the distal or mature, trans-section (Fig. 178). Between them is the middle or intermediate section of the AG.

During cell division, the reticulate forms of AG disintegrate into dictyosomes, which are passively and randomly distributed among daughter cells. As cells grow, the total number of dictyosomes increases.

In secreting cells, the AG is usually polarized: its proximal part faces the cytoplasm and nucleus, and the distal part faces the cell surface. In the proximal area, the stacks of closely spaced cisterns are adjacent to a network-like or sponge-like system of membrane cavities. It is believed that this system represents the zone of transition of ER elements into the zone of the Golgi apparatus (Fig. 179).

In the middle part of the dictyosome, the periphery of each cistern is also accompanied by a mass of small vacuoles about 50 nm in diameter.

In the distal or trans-section of dictyosomes, the last membrane flat cistern is adjacent to a section consisting of tubular elements and a mass of small vacuoles, often having fibrillar pubescence along the surface on the cytoplasmic side - these are pubescent or bordered vesicles of the same type as the bordered vesicles during pinocytosis. This is the so-called trans-Golgi network (TGN), where the separation and sorting of secreted products occurs. Even more distal is a group of larger vacuoles - this is the product of the fusion of small vacuoles and the formation of secretory vacuoles.

Using a megavoltage electron microscope, it was established that in cells individual dictyosomes can be connected to each other by a system of vacuoles and cisterns and form a loose three-dimensional network that can be detected in a light microscope. In the case of the diffuse form of AG, each individual section is represented by a dictyosome. In plant cells, the diffuse type of AG organization predominates; usually, on average, there are about 20 dictyosomes per cell. In animal cells, centrioles are often associated with the membrane zone of the Golgi apparatus; between the bundles of microtubules extending radially from them lie groups of stacks of membranes and vacuoles, which concentrically surround the cell center. This connection indicates the participation of microtubules in the movement of vacuoles.

Secretory function of the Golgi apparatus. The main functions of AG are the accumulation of products synthesized in the ER, ensuring their chemical rearrangements and maturation.

In the AG tanks, the synthesis of polysaccharides and their interaction with proteins occurs. and the formation of mucoproteins. But the main function of the Golgi apparatus is to remove ready-made secretions outside the cell. In addition, AG is a source of cellular lysosomes.

The exported protein synthesized on ribosomes is separated and accumulates inside the ER cisterns, through which it is transported to the AG membrane zone. Here, small vacuoles containing the synthesized protein are split off from the smooth areas of the ER and enter the vacuole zone in the proximal part of the dictyosome. At this point, the vacuoles merge with each other and with the flat cis cisternae of the dictyosome. In this way, the protein product is transferred already inside the cavities of the AG tanks.

As proteins in the cisternae of the Golgi apparatus are modified, small vacuoles are used to transport them from cisternae to cisternae into the distal part of the dictyosome until they reach the tubular membrane network in the trans region of the dictyosome. In this area, small bubbles containing an already mature product are separated. The cytoplasmic surface of such vesicles is similar to the surface of bordered vesicles, which are observed during receptor pinocytosis. The separated small vesicles merge with each other and form secretory vacuoles. After this, the secretory vacuoles begin to move towards the cell surface, the plasma membrane and the vacuole membranes fuse, and thus the contents of the vacuoles appear outside the cell. Morphologically, this process of extrusion (throwing out) resembles pinocytosis, only with the reverse sequence of stages. It is called exocytosis.

In the Golgi apparatus, not only the movement of products from one cavity to another occurs, but also the modification of proteins occurs, which ends with the targeting of products either to lysosomes, the plasma membrane or to secretory vacuoles.

Modification of proteins in the Golgi apparatus. Proteins synthesized in the ER enter the cis-zone of the Golgi apparatus after primary glycosylation and reduction of several saccharide residues. After which all proteins receive the same oligosaccharide chains, consisting of two molecules of N-acetylglucosamine and six molecules of mannose (Fig. 182). In cis-cisternae, secondary modification of oligosaccharide chains occurs and their sorting into two classes. The sorting results in one class of phosphorylatable oligosaccharides (mannose-rich) for hydrolytic enzymes destined for lysosomes, and another class of oligosaccharides for proteins destined for secretory granules or the plasma membrane

The transformations of oligosaccharides are carried out with the help of enzymes - glycosyltransferases, which are part of the membranes of the Golgi apparatus cisterns. Since each zone in dictyosomes has its own set of glycosylation enzymes, glycoproteins are transferred, as if in a relay race, from one membrane compartment (“floor” in a stack of dictyosome tanks) to another and in each are subjected to the specific action of enzymes. Thus, in the cis-site, phosphorylation of mannoses in lysosomal enzymes occurs and a special mannose-6 group is formed, characteristic of all hydrolytic enzymes, which then enter the lysosomes.

In the middle part of dictyosomes, secondary glycosylation of secretory proteins occurs: additional removal of mannose and addition of N-acetylglucosamine. In the trans region, galactose and sialic acids are added to the oligosaccharide chain (Fig. 183).

In a number of specialized cells in the Golgi apparatus, the synthesis of polysaccharides themselves occurs.

In the Golgi apparatus of plant cells, polysaccharides of the cell wall matrix (hemicelluloses, pectins) are synthesized. Dictyosomes of plant cells are involved in the synthesis and secretion of mucus and mucins, which also include polysaccharides. The synthesis of the main framework polysaccharide of plant cell walls, cellulose, occurs on the surface of the plasma membrane.

In the Golgi apparatus of animal cells, long unbranched polysaccharide chains of glycosaminoglycans are synthesized. Glucosaminoglycans covalently bind to proteins and form proteoglycans (mucoproteins). Such polysaccharide chains are modified in the Golgi apparatus and bind to proteins, which are secreted by cells in the form of proteoglycans. Sulfation of glycosaminoglycans and some proteins also occurs in the Golgi apparatus.

Sorting of proteins in the Golgi apparatus. Ultimately, three streams of non-cytosolic proteins synthesized by the cell pass through the Golgi apparatus: a stream of hydrolytic enzymes for lysosomes, a stream of secreted proteins that accumulate in secretory vacuoles and are released from the cell only upon receipt of special signals, a stream of constantly secreted secretory proteins. Consequently, in the cell there is a mechanism for the spatial separation of different proteins and their pathways.

In the cis- and middle zones of dictyosomes, all these proteins go together without separation, they are only separately modified depending on their oligosaccharide markers.

The actual separation of proteins, their sorting, occurs in the trans region of the Golgi apparatus. The principle of selection of lysosomal hydrolases occurs as follows (Fig. 184).

Precursor proteins of lysosomal hydrolases have an oligosaccharide, more specifically a mannose group. In cis cisternae, these groups are phosphorylated and, together with other proteins, are transferred to the trans region. The membranes of the trans-network of the Golgi apparatus contain a transmembrane protein receptor (mannose-6-phosphate receptor or M-6-P receptor), which recognizes phosphorylated mannose groups of the oligosaccharide chain of lysosomal enzymes and binds to them. Consequently, M-6-F receptors, being transmembrane proteins, bind to lysosomal hydrolases, separate them, sort them from other proteins (for example, secretory, non-lysosomal) and concentrate them in bordered vesicles. Having become detached from the trans-network, these vesicles quickly lose their borders and merge with endosomes, thus transferring their lysosomal enzymes associated with membrane receptors into this vacuole. Inside endosomes, due to the activity of the proton transporter, acidification of the environment occurs. Starting at pH 6, lysosomal enzymes dissociate from M-6-P receptors, are activated and begin to work in the cavity of the endolysosome. Sections of membranes, together with M-6-F receptors, are returned by recycling membrane vesicles back into the trans-network of the Golgi apparatus.

It is possible that part of the proteins that accumulate in secretory vacuoles and are removed from the cell after receiving a signal (for example, nervous or hormonal) undergoes the same selection and sorting procedure on the receptors of the trans-cisterns of the Golgi apparatus. Secretory proteins also first enter small clathrin-clad vacuoles and then fuse with each other. In secretory vacuoles, proteins accumulate in the form of dense secretory granules, which leads to an increase in protein concentration in these vacuoles by approximately 200 times compared to its concentration in the Golgi apparatus. As proteins accumulate in secretory vacuoles and after the cell receives the appropriate signal, they are released from the cell by exocytosis.

The third stream of vacuoles, associated with constant, constitutive secretion, also emanates from the Golgi apparatus. For example, fibroblasts secrete a large amount of glycoproteins and mucins that are part of the ground substance of connective tissue. Many cells constantly secrete proteins that facilitate their binding to substrates; there is a constant flow of membrane vesicles to the cell surface, carrying elements of the glycocalyx and membrane glycoproteins. This flow of components released by the cell is not subject to sorting in the receptor trans-system of the Golgi apparatus. The primary vacuoles of this flow also split off from the membranes and are related in their structure to bordered vacuoles containing clathrin (Fig. 185).

Concluding the consideration of the structure and operation of such a complex membrane organelle as the Golgi apparatus, it is necessary to emphasize that despite the apparent morphological homogeneity of its components, the vacuole and the cisterna, in fact, it is not just a collection of vesicles, but a slender, dynamic, complexly organized, polarized system.

In the AG, not only the transport of vesicles from the ER to the plasma membrane occurs. There is reverse vesicle transfer. Thus, vacuoles split off from secondary lysosomes and return together with receptor proteins to the trans-AG zone; there is a flow of vacuoles from the trans-zone to the cis-zone of AG, as well as from the cis-zone to the endoplasmic reticulum. In these cases, the vacuoles are coated with proteins of the COP I complex. It is believed that various secondary glycosylation enzymes and receptor proteins in membranes are returned in this way.

Features of the behavior of transport vesicles served as the basis for the hypothesis that there are two types of transport of AG components (Fig. 186).

According to the first type, AG contains stable membrane components to which substances are relayed from the ER by transport vacuoles. According to another type, AG is a derivative of the ER: membrane vacuoles split off from the transition zone of the ER merge with each other into a new cis-tank, which then moves through the entire AG zone and finally breaks up into transport vesicles. According to this model, retrograde COP I vesicles return resident Ag proteins to younger cisternae.

Golgi complex, or Golgi apparatus , - These are single-membrane organelles of eukaryotic cells, the main functions of which are the storage and removal of excess substances from the cells of the body and the formation of lysosomes. These organelles were discovered in 1898 by the Italian physicist C. Golgi.

Structure . Constructed from bags called tanks, tube system And bubbles various sizes. The cisternae of the Golgi complex (CG) are also polar: vesicles with substances that detach from the ER (formation zone) approach one pole, and vesicles with substances separate from the other pole (maturation zone). In cells, the Golgi complex is located mainly near the nucleus. CG is present in all eukaryotic cells, but its structure may differ in different organisms. Thus, in plant cells there are several structural units called dictyosomes. Membranes of the Golgi complex are synthesized granular EPS, adjacent to it. During cell division, the CG breaks down into separate structural units, which are randomly distributed between daughter cells.

Functions . The Golgi complex performs quite diverse and important functions related to the formation and transformation of complex substances. Here are some of them:

1) participation in the construction of biological membranes - for example, in protozoan cells, with the help of its elements, contractile vacuoles, is formed in the sperm acrosomsa;

2 ) formation of lysosomes- hydrolase enzymes synthesized in EPS are packaged into a membrane vesicle, which is separated into the cytoplasm;

3) peroxisome formation- bodies with the catalase enzyme are formed to destroy hydrogen peroxide, which is formed during the oxidation of organic substances and is a toxic composition for cells;

4) synthesis of surface apparatus compounds- lipo-, glyco-, and mucoproteins are formed, which are part of the glycocalyx, cell walls, and mucous capsules;

5) participation in the secretion of substances from the cell- in the CG, the maturation of secretory granules into vesicles occurs, and the movement of these vesicles in the direction of the plasma membrane.

Lysosomes, structure and functions

Lysosomes (from Greek Lysis - dissolution, soma - body) - These are single-membrane organelles of eukaryotic cells that look like round bodies. In unicellular organisms their role is intracellular digestion, in multicellular organisms they perform the function of breaking down substances foreign to the cell. Lysosomes can be located anywhere in the cytoplasm. Lysosomes were discovered by the Belgian cytologist Christian de Duve in 1949.

Structure . Lysosomes have the form of vesicles with a diameter of about 0.5 microns, surrounded by a membrane and filled with hydrolytic enzymes that act in an acidic environment. The enzyme composition of lysosomes is very diverse, it is formed by proteases (enzymes that break down proteins), amylases (enzymes for carbohydrates), lipases (lipid enzymes), nucleases (for the breakdown of nucleic acids), etc. In total, there are up to 40 different enzymes. When the membrane is damaged, enzymes enter the cytoplasm and cause rapid dissolution (lysis) of the cell. Lysosomes are formed by the interaction of CG and granular EPS. Lysosomal enzymes are synthesized in the granular ER and, using vesicles, are transported to the CG located next to the endoplasmic reticulum. Therefore, through the tubular expansion of the CG, enzymes move to its functional surface and are packaged into lysosomes.

Functions . Depending on their functions, different types of lysosomes are distinguished: phagolysosomes, autophagolysosomes, residual bodies, etc. Autophogolysosomes are formed by the fusion of a lysosome with an autophagosome, that is, vesicles containing the cell’s own macromolecular complexes, for example, entire cellular organelles, or their fragments that have lost their functional ability and are subject to destruction. phagolysosomes (phagosomes) are formed by combining lysosomes with phagocytic or pinocytotic vesicles, which contain material captured by the cell for intracellular digestion. The active enzymes in them are in direct contact with biopolymers that are subject to breakdown. Residual bodies- these are undivided particles surrounded by a membrane; they can remain in the cytoplasm for a long time and be utilized here or removed outside the cell by exocytosis. The residual bodies accumulate material, the breakdown of which is difficult (for example, a brown pigment - lipofuscin, which is also called the “aging pigment”). So, the main functions of lysosomes are:

1) autophagy - cleavage of the cell's own components, whole cells or their groups into autophagolysosomes (for example, resorption of the tail of a tadpole, the pectoral gland in adolescents, lysis of liver cells during poisoning)

2) heterophasia- breakdown of foreign substances in phagolysosomes (for example, breakdown of organic particles, viruses, bacteria that have entered the cell in one way or another)

3) digestive function - in unicellular organisms, endosomes fuse with phagocytic vesicles and form a digestive vacuole, which carries out intracellular digestion

4) excretory function- removal of undigested residues from the cell using residual bodies.

BIOLOGY +Storage diseases- hereditary diseases associated with the loss of certain enzymes by lysosomes. The consequence of this loss is the accumulation of undigested substances in the cells, interfering with the normal functioning of the cell. These diseases can be manifested by the development of the skeleton, individual internal organs, the central nervous system, etc. The development of atherosclerosis, obesity, etc. is associated with a deficiency of lysosome enzymes.