The description of the structure of the Golgi apparatus is closely related to the description of its main biochemical functions, since the division of this cellular compartment into sections is carried out mainly on the basis of the localization of enzymes located in one or another section.
Most often, there are four main divisions in the Golgi apparatus: cis-Golgi, medial-Golgi, trans-Golgi and trans-Golgi network (TGN)
In addition, the so-called intermediate compartment, which is an accumulation of membrane vesicles between the endoplasmic reticulum and cis-Golgi, is sometimes referred to as the Golgi apparatus. The Golgi apparatus is a highly polymorphic organelle; in cells of different types and even at different stages of development of the same cell, it can look different. Its main characteristics are:
1) the presence of a stack of several (usually 3-8) flattened tanks, more or less tightly adjacent to each other. Such a stack is always surrounded by a certain (sometimes very significant) number of membrane vesicles. In animal cells, one stack is more common, while in plant cells there are usually several; each of them is then called a dictyosome. Individual dictyosomes can be interconnected by a system of vacuoles, forming a three-dimensional network;
2) compositional heterogeneity, expressed in the fact that resident enzymes are not uniformly distributed throughout the organelle;
3) polarity, that is, the presence of a cis-side facing the endoplasmic reticulum and nucleus, and a trans-side facing the cell surface (this is especially true for secreting cells);
4) association with microtubules and the centriole region. Destruction of microtubules by depolymerizing agents leads to fragmentation of the Golgi apparatus, but its functions are not significantly affected. A similar fragmentation is observed in natural conditions, during mitosis. After the restoration of the microtubule system, the elements of the Golgi apparatus scattered throughout the cell are collected (along the microtubules) in the region of the centriole, and the normal Golgi complex is reconstructed.
The Golgi apparatus (Golgi complex) is a membrane structure of a eukaryotic cell, mainly designed to remove substances synthesized in the endoplasmic reticulum. The Golgi complex was named after the Italian scientist Camillo Golgi, who first discovered it in 1898.
The Golgi complex is a stack of disk-shaped membranous sacs (cistern), somewhat expanded closer to the edges, and the system of Golgi vesicles associated with them. In plant cells, a number of separate stacks (dictyosomes) are found, in animal cells there is often one large or several stacks connected by tubes.
Proteins intended for secretion, transmembrane proteins of the plasma membrane, proteins of lysosomes, etc. mature in the tanks of the Golgi Apparatus. Maturing proteins sequentially move through the tanks of the organelle, in which their final folding takes place, as well as modifications - glycosylation and phosphorylation.
The Golgi apparatus is asymmetric - the tanks located closer to the cell nucleus (cis-Golgi) contain the least mature proteins, membrane vesicles continuously join these tanks - vesicles that bud off from the granular endoplasmic reticulum (ER), on the membranes of which proteins are synthesized by ribosomes.
Different tanks of the Golgi Apparatus contain different resident catalytic enzymes and, consequently, different processes sequentially occur with maturing proteins in them. It is clear that such a stepwise process must be somehow controlled. Indeed, maturing proteins are “marked” with special polysaccharide residues (mainly mannose), apparently playing the role of a kind of “quality mark”.
It is not entirely clear how maturing proteins move through the cisternae of the Golgi apparatus while resident proteins remain more or less associated with one cisterna. There are two mutually non-exclusive hypotheses explaining this mechanism. According to the first (1), protein transport is carried out using the same vesicular transport mechanisms as the transport route from the ER, and resident proteins are not included in the budding vesicle. According to the second (2), there is a continuous movement (maturation) of the cisterns themselves, their assembly from vesicles at one end and disassembly at the other end of the organelle, and resident proteins move retrograde (in the opposite direction) using vesicular transport.
Eventually, vesicles containing fully mature proteins bud off from the opposite end of the organelle (trans-Golgi).
In the Golgi complex,
1. O-glycosylation, complex sugars are attached to proteins through an oxygen atom.
2. Phosphorylation (attachment of orthophosphoric acid residue to proteins).
3. Formation of lysosomes.
4. Formation of a cell wall (in plants).
5. Participation in vesicular transport (formation of a three-protein stream):
6. maturation and transport of plasma membrane proteins;
7. maturation and transport of secrets;
8. maturation and transport of lysosome enzymes.
Golgi apparatus. The Golgi apparatus (Golgi complex) is a specialized part of the endoplasmic reticulum, consisting of stacked flat membrane sacs. It is involved in the secretion of proteins by the cell (the packing of secreted proteins into granules occurs in it) and therefore is especially developed in cells that perform a secretory function. The important functions of the Golgi apparatus also include the attachment of carbohydrate groups to proteins and the use of these proteins to build the cell membrane and lysosome membrane. In some algae, cellulose fibers are synthesized in the Golgi apparatus.
Golgi apparatus: functions
The function of the Golgi apparatus is the transport and chemical modification of the substances entering it. The initial substrate for enzymes are proteins that enter the Golgi apparatus from the endoplasmic reticulum. Once modified and concentrated, the enzymes in the Golgi vesicles are transported to their "destination", such as where a new kidney is formed. This transfer is most actively carried out with the participation of cytoplasmic microtubules.
The functions of the Golgi apparatus are very diverse. These include:
1) sorting, accumulation and excretion of secretory products;
2) completion of post-translational modification of proteins (glycosylation, sulfation, etc.);
3) accumulation of lipid molecules and formation of lipoproteins;
4) formation of lysosomes;
5) synthesis of polysaccharides for the formation of glycoproteins, waxes, gums, mucus, substances of the matrix of plant cell walls
(hemicellulose, pectins), etc.
6) formation of a cell plate after nuclear fission in plant cells;
7) participation in the formation of the acrosome;
8) the formation of contractile vacuoles of protozoa.
This list is no doubt incomplete, and further research will not only allow a better understanding of the already known functions of the Golgi apparatus, but will also lead to the discovery of new ones. So far, the most studied from a biochemical point of view are the functions associated with the transport and modification of newly synthesized proteins.
A- Granular cytoplasmic reticulum.
B- Microbubbles.
B- Microfilaments.
Mr. Cistern.
D- Vacuoles.
Answer: B, D, D.
16. Indicate what functions the Golgi complex performs:
A - Protein synthesis.
B- Formation of complex chemical compounds (glycoproteins, lipoproteins).
B- Formation of primary lysosomes.
G- Participation in the excretion of the secretory product from the cell.
D- Formation of hyaloplasm.
Answer: B, C, D.
What structural elements of the cell are most actively involved in exocytosis?
A Cytolemma.
B- Cytoskeleton.
B- Mitochondria.
G- Ribosomes.
Answer: A, B.
18 . What determines the specificity of the synthesized protein?
A- Messenger RNA.
B- Ribosomal RNA.
D- Membranes of the cytoplasmic reticulum.
Answer: A, B
19 . What structural elements are actively involved in the implementation
phagocytic function?
A Karyolemma.
B- Endoplasmic reticulum.
B - Cytolemma.
G- Lysosomes.
D- Microfilaments.
Answer: B, D, D.
20 .What structural components of the cell determine the basophilia of the cytoplasm?
A- ribosomes.
B. Agranular endoplasmic reticulum.
B- Lysosomes.
G- Peroxisomes.
D- Golgi complex.
E- Granular endoplasmic reticulum.
Answer: A, E.
21 . Which of the following organelles have a membrane structure?
A - Cell center.
B- Mitochondria.
B- Golgi complex.
G- Ribosomes.
D - Cytoskeleton.
Answer: B, C.
22 .What do mitochondria and peroxisomes have in common?
A- They belong to the organelles of the membrane structure.
B- They have a double membrane.
D- These are organelles of general importance.
Answer: A, B, D.
What are the functions of lysosomes in the cell?
A- Protein biosynthesis
B- Participation in phagocytosis
B- Oxidative phosphorylation
D- Intracellular digestion
Answer: B.G.
What is the structural organization of lysosomes?
A- Surrounded by a membrane.
B- Filled with hydrolytic enzymes.
G- Formed in the Golgi complex.
Answer: A, B, D.
25. Glycocalyx:
A- It is located in the smooth endoplasmic reticulum.
B- It is located on the outer surface of the cytolemma.
B- Formed by carbohydrates.
G- Participates in cell adhesion and cell recognition.
D- It is located on the inner surface of the cytolemma.
Answer: B, C, D.
26. Marker enzymes of lysosomes:
A- Acid phosphatase.
B- ATP-ase.
B- hydrolases.
G- Catalase and oxidases.
Answer: A, B.
What is the importance of the nucleus in the life of the cell?
A- Storage of hereditary information.
B- Energy storage center.
B- Control center of intracellular metabolism.
G- Place of formation of lysosomes.
D- Reproduction and transmission of genetic information to daughter cells.
Answer: A, B, D.
28. What does not apply to the structural components of the nucleus:
A Karyolemma.
B- Nucleoli.
B- karyoplasm.
G- Ribosomes.
D- Chromatin, chromosomes.
E- Peroxisomes.
Answer: G, E.
What is transported from the nucleus through the nuclear pores to the cytoplasm?
A - DNA fragments.
B- Ribosome subunits.
B- Messenger RNA.
D- Fragments of the endoplasmic reticulum.
Answer: B, C.
What is the nuclear-cytoplasmic ratio and how does it change with an increase in the functional activity of the cell?
A- The position of the nucleus in the cytoplasm.
B- The shape of the nucleus.
B- The ratio of the size of the nucleus to the size of the cytoplasm.
G- Decreased with increased functional activity of the cell.
Answer: V, G.
What is true for nucleoli?
A- Well visible during mitosis.
B- Consist of granular and fibrillar components.
B- Granules of the nucleolus are subunits of ribosomes.
G- Threads of the nucleolus - ribonucleoproteins
Answer: B, C, D.
Which of the following are signs of necrosis?
A- This is genetically programmed cell death.
B- At the beginning of apoptosis, RNA and protein synthesis increases.
B- membranes are destroyed
G-enzymes of lysosomes are released into the cytoplasm
D- Fragmentation of the cytoplasm with the formation of apoptotic bodies
Answer: V, G.
Everything is true, except
1. The function of the Golgi complex (everything is true except):
A - sorting of proteins by transport vesicles
B- protein glycosylation
B- reutilization of membranes of secretory granules after exocytosis
G- packaging of the secretory product
D- synthesis of steroid hormones
2. Microtubules provide (all are true except):
A - organization of the internal space of the cell
B- maintaining the shape of the cell
B- cell polarization during division
G- form the contractile apparatus
D- organization of the cytoskeleton
E- organelle transport
3. The specialized structures built on the basis of the cytoskeleton include (all are true except):
A- cilia, flagella
B- basal striation
B- microvilli
4. Localization of cilia (all are true except):
A - epithelium of the mucous membrane of the airways
B- epithelium of the proximal nephron
B- epithelium of the mucosa of the reproductive tract of women
G- epithelium of the mucosa of the vas deferens
5. Localization of microvilli (all are true except):
A - epithelium of the mucous membrane of the small intestine
B- epithelium of the mucous membrane of the trachea
B - epithelium of the proximal nephron
6. Basal striation (all true except):
A- provides transport of substances against the concentration gradient
B - a part of the cell where highly energy-intensive processes take place
B - area of the cell where simple diffusion of ions occurs
G- where the reabsorption of elements of primary urine occurs in the proximal tubule of the nephron
D- participates in the concentration of salivary secretion
7. Brush border (all are true except):
A - located on the apical surface of the cells
B- increases the area of the suction surface
B- consists of cilia
G- consists of microvilli
D- increases the transport surface in the proximal tubules of the nephron
8. General purpose organelles (all are true except):
A- mitochondria
B-Golgi complex
G- cilia
D-lysosomes
E- peroxisomes
F- centrioles
H-elements of the cytoskeleton
9.Function of peroxisomes (all are true except):
A- oxidation of an organic substrate with the formation of hydrogen peroxide
B- synthesis of the enzyme - catalase
B- utilization of hydrogen peroxide
10. Ribosomes (all true except):
A - with light microscopy, their presence is judged by pronounced basophilia of the cytoplasm
B- consist of small and large subunits
B- are formed in the granular endoplasmic reticulum
G- consist of rRNA and proteins
D - non-membrane structure
11. What organelles are well developed in steroid-producing cells (all are correct except):
A- granular endoplasmic reticulum
B- agranular endoplasmic reticulum
B- mitochondria with tubular cristae
12. Trophic inclusions (everything is true except):
A- carbohydrate
B- mucous
B-protein
G- lipid
13.Nuclear envelope (all are true except):
A- consists of a single membrane
B- consists of two membranes
B - ribosomes are located on the outside
G- the nuclear plate is connected with it from the inside
D- riddled with pores
14. Structural components of the kernel (all are true except):
A - nucleoplasm
B- nucleolemma
B- microtubules
G- chromatin
D - nucleoli
15. The structure of the nuclear pore (all are true except):
A - membrane component
B- chromosomal component
B-fibrillar component
G- granular component
16. Nucleolus (all are true except):
A - surrounded by a membrane
B - not surrounded by a membrane
B- five pairs of chromosomes are involved in its organization
G- contains a granular and fibrillar component
17. Nucleolus (all are true except):
A - the amount depends on the metabolic activity of the cell
B- participates in the formation of ribosome subunits
B- chromosomes 13,14, 15, 21 and 22 participate in the organization
G- chromosomes 7, 8, 10, 11 and 23 participate in the organization
D - consists of three components
18. Cell center (all are true except):
A- localized near the nucleus
B- is the center of the organization of the division spindle
B- consists of two centrioles
G-centrioles are formed by 9 doublets of microtubules
D-centrioles are duplicated in the S period of interphase
19. Mitochondria (all true except):
A - the presence of cristae
B- the ability to share
20. Functions of actin filaments (all are true except):
A - cell movement
B- change in the shape of the cell
B- participation in exo- and endocytosis
G- provide movement of cilia
D- are part of the microvilli
21. Everything is true for the nucleolus, except:
A- Formed in the region of nucleolar organizers (secondary chromosome constrictions)
B- Granules of the nucleoli enter the cytoplasm
B- Nucleolar proteins are synthesized in the cytoplasm
D- nucleolar RNA is produced in the cytoplasm
For compliance
1. Compare the periods of interphase with the processes taking place in them:
1. Presynthetic A - DNA doubling, increased RNA synthesis
2. Synthetic B- synthesis of rRNA, mRNA, tubulins
3. Postsynthetic B-cell growth, preparing them for DNA synthesis
Answer: 1-B; 2-A; 3-B.
2 .Compare the phases of mitosis with the processes occurring in them:
1. Prophase A - the formation of an equatorial plate from chromosomes
2. Metaphase B - formation of a nucleolemma, despiralization of chromosomes,
nucleolus formation, cytotomy
3. Anaphase B-spiralization of chromosomes, disappearance of the nucleolus,
fragmentation of the nucleolemma
4. Telophase G - divergence of chromatids to opposite poles
Answer: 1-B; 2-A; 3-G; 4-B.
3. Changing the structure of the kernel is called (match):
1. karyolysis A - reduction in size and compaction of chromatin
2. karyorrhexis B - fragmentation
3. karyopyknosis B- dissolution of its components
Answer: 1-B, 2-B, 3-A.
4. Characteristics of the drug components:
1. chromophobic A - stained with sudan dye
2. chromophilic B - does not stain with dye
3. sudanophilic B - stained with dye
Golgi complex is a membrane structure inherent in any eukaryotic cell.
The Golgi apparatus is represented flattened tanks(or bags) collected in a pile. 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 together, and on the periphery they often form extensions, or ampoules, from which they lace up. bubbles. Packages of flat tanks with an average of about 5-10 form dictyosome. In addition to cisterns, the Golgi complex contains transport and secretory vesicles. In the dictyosome, two surfaces are distinguished in accordance with the direction of curvature of the curved surfaces of the cisterns. The convex surface is called immature, or cis-surface. It faces the nucleus or tubules of the granular endoplasmic reticulum and is connected with the latter by vesicles that detach from the granular reticulum and bring protein molecules into the dictyosome for maturation and formation into the membrane. The opposite transsurface of the dictyosome is concave. It faces the plasmolemma and is called mature because secretory vesicles are laced from its membranes, containing secretion products ready for removal from the cell.
The Golgi complex is involved in:
- in the accumulation of products synthesized in the endoplasmic reticulum,
- in their chemical restructuring and maturation.
AT cisterns of the Golgi complex there is a synthesis of polysaccharides, their complexation with protein molecules.
One of main functions Golgi complex - formation of finished secretory products, which are removed from the cell by exocytosis. The most important functions of the Golgi complex for the cell are also renewal of cell membranes, including sections of the plasmolemma, as well as the replacement of defects in the plasmolemma during the 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 judged on the degree of their development in the cell by the activity of the histochemical reaction to acid phosphatase, the key lysosomal enzyme. Under electron microscopy, lysosomes are defined as vesicles, limited from the hyaloplasm by a membrane. 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 a homogeneous finely dispersed content, which is a set of hydrolytic enzymes. About 40 enzymes (proteases, nucleases, glycosidases, phosphorylases, sulfatases) have been identified in lysosomes, the optimal mode of action of which is designed for an acidic environment (pH 5). Lysosomal membranes contain special carrier proteins for transport from the lysosome to the hyaloplasm of hydrolytic cleavage products - amino acids, sugars and nucleotides. The lysosome membrane is resistant to hydrolytic enzymes.
Secondary lysosomes are formed by the fusion of primary lysosomes with endocytic or pinocytic 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 endocytic (pinocytic) vacuoles. The structure of secondary lysosomes is very diverse and changes in the process of hydrolytic cleavage 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 to the hyaloplasm, where they are utilized or included in various synthetic and metabolic reactions.
If the cell's own structures (senescent organelles, inclusions, etc.) undergo interaction with primary lysosomes and hydrolytic cleavage by their enzymes, a autophagosome. Autophagocytosis is a natural process in the life of a cell and plays an important 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 found during incomplete phago- or autophagocytosis and subsequently isolated from the cell by exocytosis. They have a compacted content, often there is a secondary structuring of undigested compounds (for example, lipids form complex layered formations).
The Golgi apparatus, also called the Golgi complex, is found in both humans and animals, and usually consists of a collection of cup-shaped membrane-bound sections called cisterns that look like a stack of deflated balloons.
However, some unicellular flagellates have 60 cisterns that form the Golgi apparatus. Similarly, the number of stacks of the Golgi's complex varies depending on its functions. , as a rule, contain from 10 to 20 stacks per cell, combined into one complex by tubular connections between tanks. The Golgi apparatus is usually located close to.
Discovery history
Due to its relatively large size, the Golgi complex was one of the first organelles observed in cells. In 1897, an Italian physician named Camillo Golgi, who studies the nervous system, used a new staining technique that he himself developed (and is still relevant today). Thanks to the new method, the scientist was able to see the cellular structure and called it the internal reticular apparatus.
Shortly after he publicly announced his discovery in 1898, the structure was named after him, becoming universally known as the Golgi apparatus. However, many scientists of that time did not believe that Golgi observed a real cell organelle, and attributed the scientist's discovery to a visual distortion caused by staining. The invention of the electron microscope in the twentieth century finally confirmed that the Golgi apparatus is a cellular organelle.
Structure
In most eukaryotes, the Golgi apparatus is formed from stacks of sacs, consisting of two main sections: a cis section and a trans section. The cis compartment is a complex of flattened membrane discs known as cisterns, derived from vesicular clusters that rush out from the endoplasmic reticulum.
Mammalian cells typically contain 40 to 100 stacks. As a rule, from to each stack includes from 4 to 8 tanks. However, some have seen around 60 cisterns. This set of cisterns is broken down into cis, medial, and trans divisions. The trans compartment is the final cisternal structure from which proteins are packaged into vesicles destined for lysosomes, secretory vesicles, or the cell surface.
Functions
The Golgi apparatus is often considered to be the cell's chemical distribution and delivery department. It modifies the proteins and lipids (fats) that are produced in and prepares them for export outside the cell or for transport to other locations within the cell. Proteins and lipids built in the smooth and rough endoplasmic reticulum are packed into tiny vesicles that move through until they reach the Golgi complex.
The vesicles fuse with the Golgi membranes and release the molecules contained within into the organelle. Once inside, the compounds are further processed by the Golgi apparatus and then routed in the vesicle to their destination inside or outside the cell. Exported products are the secretions of proteins or glycoproteins, which are part of the cell's function in the body. Other substances return to the endoplasmic reticulum or may mature to become.
Modifications of molecules that are carried out in the Golgi complex occur in an orderly manner. Each cisterna has two main compartments: the cis compartment, which is the end of the organelle, where substances enter from the endoplasmic reticulum for processing, and the trans compartment, where they exit in the form of smaller individual vesicles. Therefore, the cis section is located near the endoplasmic reticulum, where most of the substances come from, and the trans section is located near the cell, where many of the substances modified in the Golgi apparatus go.
The chemical composition of each department, as well as the enzymes contained in the lumens (internal open spaces of cisterns) between departments, are distinctive. Proteins, carbohydrates, phospholipids and other molecules formed in the endoplasmic reticulum are transferred to the Golgi apparatus to undergo biochemical modification when moving from cis to trans divisions of the complex. Enzymes present in the Golgi lumen modify the carbohydrate moiety of glycoproteins by adding or subtracting individual sugar monomers. In addition, the Golgi apparatus itself produces a wide variety of macromolecules, including polysaccharides.
The Golgi complex in plant cells produces pectins and other polysaccharides essential for plant structure and metabolism. Products exported by the Golgi apparatus through the trans region eventually fuse with the plasma membrane of the cell. Among the most important functions of the complex is the sorting of a large number of macromolecules produced by the cell and their transportation to the required destinations. Specialized molecular identification marks or labels such as phosphate groups are added by Golgi enzymes to assist in this sorting process.
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In 1898, the Italian scientist K. Golgi identified net formations in nerve cells, which he called the “internal net apparatus” (Fig. 174). Mesh structures (Golgi apparatus) are found in all cells of any eukaryotic organisms. Usually the Golgi apparatus is located near the nucleus, near the cell center (centrioles).
Fine structure of the Golgi apparatus. The Golgi apparatus consists of membrane structures brought together in a small area (Fig. 176, 177). A separate zone of accumulation of these membranes is called dictyosome(Fig. 178). In the dictyosome, close to each other (at a distance of 20-25 nm), flat membrane sacs, or tanks, are located in the form of a stack, between which there are thin layers of hyaloplasm. Each individual tank is about 1 µm in diameter and variable in thickness; in the center of its membranes can be brought together (25 nm), and on the periphery they can have extensions, ampoules, the width of which is not constant. The number of such bags in a stack usually does not exceed 5-10. In some unicellular organisms, their number can reach 20 pieces. In addition to densely spaced 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 on the edges of flat tanks. It is customary to distinguish between a proximal or emerging, cis-section in the dictyosome zone, and a distal or mature, trans-section (Fig. 178). Between them is the middle or intermediate section of the AG.
During cell division, reticular forms of AG disintegrate into dictyosomes, which are passively and randomly distributed to daughter cells. As cells grow, the total number of dictyosomes increases.
In secreting cells, AG is usually polarized: its proximal part faces the cytoplasm and nucleus, while its distal part faces the cell surface. In the proximal area, a network-like or sponge-like system of membrane cavities adjoins the stacks of contiguous cisterns. It is believed that this system is a 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 cisterna is also accompanied by a mass of small vacuoles about 50 nm in diameter.
In the distal or trans region of dictyosomes, the last membranous squamous cistern is adjacent to a region consisting of tubular elements and a mass of small vacuoles, often with fibrillar pubescence on the surface from the side of the cytoplasm - these are pubescent or bordered vesicles of the same type as bordered vesicles in pinocytosis. This is the so-called trans-Golgi network (TGN), where secreted products are separated and sorted. A group of larger vacuoles is located even more distally - this is already the product of the fusion of small vacuoles and the formation of secretory vacuoles.
Using a megavolt electron microscope, it was found that individual dictyosomes in cells 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 a diffuse form of AH, each of its individual sections 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 that extend radially from them, there are groups of stacks of membranes and vacuoles that concentrically surround the cell center. This relationship 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 tanks of AG, the synthesis of polysaccharides occurs, their relationship with proteins. and the formation of mucoproteins. But the main function of the Golgi apparatus is to remove ready-made secrets outside the cell. In addition, AG is a source of cellular lysosomes.
The exported protein synthesized on ribosomes is separated and accumulated inside the ER cisterns, along which it is transported to the zone of AG membranes. Here, small vacuoles containing the synthesized protein are cleaved 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 fuse with each other and with the flat cis-cistern of the dictyosome. Thus, the transfer of the protein product occurs already inside the cavities of the AG tanks.
As the proteins in the cisternae of the Golgi apparatus are modified, they are transported from cisternae to cisternae to the distal part of the dictyosome with the help of small vacuoles until they reach the tubular membrane network in the trans region of the dictyosome. In this area, small vesicles containing an already mature product are split off. The cytoplasmic surface of such vesicles is similar to the surface of bordered vesicles, which are observed during receptor pinocytosis. Separated small vesicles merge with each other, form secretory vacuoles. After that, the secretory vacuoles begin to move towards the cell surface, the plasma membrane and vacuole membranes merge, and thus the contents of the vacuoles are outside the cell. Morphologically, this process of extrusion (ejection) 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 addressing of products, either to lysosomes, the plasma membrane, or to secretory vacuoles.
Protein modification 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 that, all proteins receive the same oligosaccharide chains, consisting of two molecules of N-acetylglucosamine, six molecules of mannose (Fig. 182). In cis-cisterns, the secondary modification of oligosaccharide chains occurs and they are sorted into two classes. Sorting results in one class of phosphorylated oligosaccharides (rich in mannose) for hydrolytic enzymes destined for lysosomes and another class of oligosaccharides for proteins targeted to secretory granules or to the plasma membrane
The transformation of oligosaccharides is carried out with the help of enzymes - glycosyltransferases, which are part of the membranes of the tanks of the Golgi apparatus. Since each zone in dictyosomes has its own set of glycosylation enzymes, glycoproteins, as it were, are transferred from one membrane compartment (“floor” in the stack of dictyosome cisterns) to another, as if by a relay race, and in each they are subjected to a specific effect of enzymes. So in the cis-site, mannoses are phosphorylated in lysosomal enzymes and a special mannose-6-group is formed, which is characteristic of all hydrolytic enzymes, which then enter the lysosomes.
Secondary glycosylation of secretory proteins occurs in the middle part of dictyosomes: additional removal of mannose and addition of N-acetylglucosamine. In the trans region, galactose and sialic acids are attached to the oligosaccharide chain (Fig. 183).
In a number of specialized cells in the Golgi apparatus, the synthesis of polysaccharides proper takes place.
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. Synthesis of the main scaffold 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 that are secreted by cells as proteoglycans. In the Golgi apparatus, sulfation of glycosaminoglycans and some proteins also occurs.
Protein sorting 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-section of the Golgi apparatus. The principle of selection of lysosomal hydrolases occurs as follows (Fig. 184).
Lysosomal hydrolase precursor proteins have an oligosaccharide, more specifically a mannose group. In cis-cisterns, 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 receptor protein (mannose-6-phosphate receptor or M-6-P receptor), which recognizes and binds to phosphorylated mannose groups of the oligosaccharide chain of lysosomal enzymes. Therefore, M-6-P receptors, being transmembrane proteins, bind to lysosomal hydrolases, separate them, sort them out from other proteins (for example, secretory, non-lysosomal) and concentrate them in bordered vesicles. Having broken away from the trans network, these vesicles quickly lose their borders, merge with endosomes, thus transferring their lysosomal enzymes associated with membrane receptors into this vacuole. Inside endosomes, due to the activity of the proton carrier, acidification of the environment occurs. Starting from pH 6, lysosomal enzymes dissociate from M-6-P receptors, are activated, and begin to work in the cavity of the endolysosome. The sections of the membranes, together with the M-6-P receptors, return by recycling the membrane vesicles back to the trans-network of the Golgi apparatus.
It is possible that some of the proteins that accumulate in secretory vacuoles and are excreted from the cell after receiving a signal (for example, a nerve or hormonal one) undergo the same selection and sorting procedure on the trans-cistern receptors 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 the protein concentration in these vacuoles by about 200 times, compared with its concentration in the Golgi apparatus. As proteins accumulate in secretory vacuoles and after the cell receives the appropriate signal, they are ejected from the cell by exocytosis.
The third stream of vacuoles also comes from the Golgi apparatus, associated with constant, constitutive secretion. For example, fibroblasts secrete a large number of glycoproteins and mucins, which are part of the main substance of the connective tissue. Many cells constantly secrete proteins that promote 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 secreted by the cell is not subject to sorting in the trans-receptor system of the Golgi apparatus. The primary vacuoles of this flow also split off from the membranes and are structurally related to the bordered vacuoles containing clathrin (Fig. 185).
Finishing the consideration of the structure and operation of such a complex membrane organelle as the Golgi apparatus, it must be emphasized that despite the apparent morphological homogeneity of its components, vacuoles and cisterns, in fact, this is not just a collection of vesicles, but a slender, dynamic, complexly organized, polarized system.
In AH, not only the transport of vesicles from the ER to the plasma membrane occurs. There is reverse transport of vesicles. So, 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 dressed with COP I-complex proteins. It is believed that various secondary glycosylation enzymes and receptor proteins in the membranes are returned in this way.
Features of the behavior of transport vesicles served as the basis for the hypothesis of the existence of two types of transport of AG components (Fig. 186).
According to the first type, AG has 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 ER transition zone merge with each other into a new cis-cistern, which then moves through the entire AG zone and finally breaks up into transport vesicles. In this model, retrograde COP I vesicles return permanent AG proteins to younger cisterns.
