Nickel is dangerous. The toxic effect of nickel on the body. Deficiency and excess: causes and symptoms
The structure of the Golgi apparatus
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 cisterns 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 - vesicles that bud off from the granular endoplasmic reticulum (ER) are continuously attached to these tanks, 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, protein transport is carried out using the same vesicular transport mechanisms as the transport route from the ER, and the resident proteins are not included in the budding vesicle. According to the second, there is a continuous movement (maturation) of the tanks 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.
Functions of the Golgi apparatusThe 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.
The Golgi apparatus consists of cisterns (disc-shaped membranous sacs), which are slightly expanded closer to the edges. The structure of the Golgi complex can be divided into 3 departments:
1. Cis-cistern or cis-compartment. Located closer to the nucleus and endoplasmic reticulum;
2. Connecting tanks. Middle section of the Golgi apparatus;
3. Trans cisterns or trans compartment. The section furthest from the nucleus and, accordingly, the closest to the cell membrane.
You can also see what the Golgi complex looks like in a cell using the example of the structure of an animal cell or the structure of a plant cell.
Functions of the Golgi complex (apparatus)
The main functions of the Golgi apparatus include:
1. Removal of substances synthesized in the endoplasmic reticulum;
2. Modification of newly synthesized protein molecules;
3. Separates proteins into 3 streams;
4. Formation of mucous secretions;
5. In the plant cell is responsible for the synthesis of polysaccharides, which then go to the formation of the plant cell wall;
6. Partial proteolysis of proteins;
7. Produces the formation of lysosomes, cell membrane;
8. Sulfation of carbohydrate and protein components of glycoproteins and glycolipids;
9. Formation of carbohydrate components of the glycocalyx - mainly glycolipids.
The Golgi complex, or apparatus, is named after the scientist who discovered it. This cell organelle has the appearance of a complex of cavities bounded by single membranes. In plant cells and in protozoa, it is represented by several separate smaller stacks (dictyosomes).
The structure of the Golgi apparatus
The Golgi complex in appearance, visible in an electron microscope, resembles a stack of disk-shaped sacs superimposed on each other, near which there are many bubbles. Inside each "bag" there is a narrow channel, expanding at the ends into the so-called tanks (sometimes the entire bag is called a tank). Bubbles come out of them. A system of interconnected tubes is formed around the central stack.
With the outer, somewhat convex side of the stack, new cisterns are formed by the merging of bubbles budding from the smooth one. On the inside of the tank, they complete their maturation and break up again into bubbles. Thus, the cisterns (stack pouches) of the Golgi move from the outside to the inside.
The part of the complex located closer to the nucleus is called "cis". The one closer to the membrane is "trance".
Micrograph of the Golgi complex
Functions of the Golgi complex
The functions of the Golgi apparatus are diverse, in total they come down to modification, redistribution of substances synthesized in the cell, as well as their removal outside the cell, the formation of lysosomes and the construction of the cytoplasmic membrane.
The activity of the Golgi complex is high in secretory cells. Proteins coming from the ER are concentrated in the Golgi apparatus, then transferred to the membrane in the Golgi vesicles. Enzymes are secreted from the cell by reverse pinocytosis.
Oligosaccharide chains are attached to the proteins entering the Golgi. In the apparatus, they are modified and serve as markers, with the help of which proteins are sorted and directed along their path.
In plants, during the formation of the cell wall, the Golgi secretes carbohydrates that serve as a matrix for it (cellulose is not synthesized here). The budding Golgi vesicles are transported by microtubules. Their membranes fuse with the cytoplasmic membrane, and the contents are incorporated into the cell wall.
The Golgi complex of goblet cells (located in the thickness of the epithelium of the intestinal mucosa and respiratory tract) secretes the glycoprotein mucin, which forms mucus in solutions. Similar substances are synthesized by the cells of the tip of the root, leaves, etc.
In the cells of the small intestine, the Golgi apparatus performs the function of lipid transport. Fatty acids and glycerol enter the cells. In smooth ER, the synthesis of its lipids occurs. Most of them are covered with proteins and transported through the Golgi to the cell membrane. After passing through it, lipids are in the lymph.
An important function is the formation.
Organelles- permanent, necessarily present, components of the cell that perform specific functions.
Endoplasmic reticulum
Endoplasmic reticulum (ER), or endoplasmic reticulum (EPR), is a single-membrane organelle. It is a system of membranes that form "tanks" and channels, connected to each other and limiting a single internal space - EPS cavities. On the one hand, the membranes are connected to the cytoplasmic membrane, on the other hand, 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 cytoplasm of the cell 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 is a stack of flattened "tanks" with widened edges. A system of small single-membrane vesicles (Golgi vesicles) is associated with them. Each stack usually consists of 4-6 "tanks", 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 the 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, move to the Golgi apparatus, where they are modified and packaged into membrane vesicles, which, after separation from the Golgi apparatus, become lysosomes proper. A lysosome can contain 20 to 60 different types of hydrolytic enzymes. The breakdown of substances by enzymes is called lysis.
Distinguish: 1) primary lysosomes, 2) secondary lysosomes. Primary lysosomes are called lysosomes, detached from the Golgi apparatus. Primary lysosomes are a factor that ensures the exocytosis of enzymes from the cell.
Secondary lysosomes are called lysosomes, formed as a result of the fusion of primary lysosomes with endocytic vacuoles. In this case, they digest substances that have entered the cell by phagocytosis or pinocytosis, so they can be called digestive vacuoles.
Autophagy- the process of destruction of structures unnecessary to the cell. First, the structure to be destroyed is surrounded by a single membrane, then the resulting membrane capsule merges with the primary lysosome, as a result, a secondary lysosome (autophagic vacuole) is also formed, in which this structure is digested. Digestion products are absorbed by the cytoplasm of the cell, but some of the material remains undigested. The secondary lysosome containing this undigested material is called the residual body. By exocytosis, undigested particles are removed from the cell.
Autolysis- self-destruction of the cell, resulting from the release of the contents of lysosomes. Normally, autolysis takes place during metamorphoses (disappearance of the tail of the frog tadpole), involution of the uterus after childbirth, in foci 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 organoids, are "tanks" filled with aqueous solutions of organic and inorganic substances. The ER and the 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, while the nucleus and organelles are pushed back to the cell membrane. The membrane that surrounds the plant vacuole is called the tonoplast. The fluid that fills the plant vacuole is called cell sap. The composition of cell sap includes water-soluble organic and inorganic salts, monosaccharides, disaccharides, amino acids, end or toxic metabolic products (glycosides, alkaloids), some pigments (anthocyanins).
Animal cells contain small digestive and autophagic vacuoles that 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.
Vacuole functions: 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) cm. lysosome functions.
Endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles form single vacuolar network of the cell, whose individual elements can transform into each other.
Mitochondria
1 - outer membrane;
2 - inner membrane; 3 - matrix; 4 - crista; 5 - multienzyme system; 6 - circular DNA.
The shape, size, and number of mitochondria are extremely variable. The shape of the mitochondria can be rod-shaped, round, spiral, cup-shaped, branched. The length of mitochondria ranges from 1.5 to 10 µm, the diameter is 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.
Mitochondria are bounded by two membranes. The outer membrane of mitochondria (1) is smooth, the inner (2) forms numerous folds - cristae(four). Cristae increase the surface area of the inner membrane, which hosts multienzyme systems (5) involved in the synthesis of ATP molecules. The inner space of mitochondria is filled with matrix (3). The matrix contains circular DNA (6), specific mRNA, prokaryotic-type ribosomes (70S-type), Krebs cycle enzymes.
Mitochondrial DNA is not associated with proteins ("naked"), is attached to the inner membrane of the mitochondria and carries information about the structure of about 30 proteins. Many more proteins are required to build a mitochondrion, so information about most mitochondrial proteins is contained in nuclear DNA, and these proteins are synthesized in the cytoplasm of the cell. Mitochondria are able to reproduce autonomously by dividing in two. Between the outer and inner membranes is proton reservoir, where the accumulation of H + occurs.
Mitochondrial functions: 1) ATP synthesis, 2) oxygen breakdown of organic substances.
According to one of the hypotheses (the theory of symbiogenesis), mitochondria originated from ancient free-living aerobic prokaryotic organisms, which, having accidentally entered the host cell, then formed a mutually beneficial symbiotic complex with it. The following data support this hypothesis. First, mitochondrial DNA has the same structural features as the DNA of modern bacteria (closed in a ring, not associated with proteins). Second, mitochondrial ribosomes and bacterial ribosomes belong to the same type, the 70S type. Thirdly, the mechanism of mitochondrial division is similar to that of bacteria. Fourth, the synthesis of mitochondrial and bacterial proteins is inhibited by the same antibiotics.
plastids
1 - outer membrane; 2 - inner membrane; 3 - stroma; 4 - thylakoid; 5 - grana; 6 - lamellae; 7 - grains of starch; 8 - lipid drops.
Plastids are found only in plant cells. Distinguish three main types of plastids: leucoplasts are colorless plastids in the cells of unstained 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 microns, the diameter is from 2 to 4 microns. 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(four). A group of thylakoids stacked like a stack of coins is called faceted(5). The chloroplast contains an average of 40-60 grains arranged in a checkerboard pattern. The granules 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 inner space of 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, H + accumulates. Chloroplasts, like mitochondria, are capable of autonomous reproduction by dividing in two. They are contained in the cells of the green parts of higher plants, especially many chloroplasts in leaves and green fruits. The 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 ways (circular, "naked" DNA, 70S-type ribosomes, mode of reproduction).
Leukoplasts. The shape varies (spherical, rounded, cupped, etc.). Leucoplasts are bounded by two membranes. The outer membrane is smooth, the inner one forms small thylakoids. The stroma contains circular "naked" DNA, 70S-type ribosomes, enzymes for the synthesis and hydrolysis of reserve nutrients. There are no pigments. Especially many leukoplasts have cells of the underground organs of the plant (roots, tubers, rhizomes, etc.). Function of leukoplasts: synthesis, accumulation and storage of reserve nutrients. Amyloplasts- leukoplasts that synthesize and accumulate starch, elaioplasts- oils, proteinoplasts- squirrels. Different substances can accumulate in the same leukoplast.
Chromoplasts. Limited by two membranes. The outer membrane is smooth, the inner or also smooth, or forms single thylakoids. The stroma contains circular DNA and pigments - carotenoids, which give the chromoplasts a yellow, red or orange color. The form of accumulation of pigments is different: in the form of crystals, dissolved in lipid drops (8), etc. They are contained in the cells of mature fruits, petals, autumn leaves, rarely - root crops. Chromoplasts are considered the final stage of plastid development.
Function of chromoplasts: coloring of 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 are possible between them. 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 leukoplasts or chloroplasts is considered impossible.
Ribosomes
1 - large subunit; 2 - small subunit.
Ribosomes- non-membrane organelles, about 20 nm in diameter. 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 of the whole ribosome - 80S, small subunit - 40S, large - 60S) and 2) prokaryotic (70S, 30S, 50S, respectively).
Eukaryotic type ribosomes contain 4 rRNA molecules and about 100 protein molecules, while prokaryotic type ribosomes contain 3 rRNA molecules and about 55 protein molecules. During protein biosynthesis, ribosomes can “work” singly or combine into complexes - polyribosomes (polysomes). In such complexes, they are linked to each other by a single mRNA molecule. Prokaryotic cells have only 70S-type ribosomes. Eukaryotic cells have both 80S-type ribosomes (rough ER membranes, cytoplasm) and 70S-type ribosomes (mitochondria, chloroplasts).
Eukaryotic ribosome subunits are formed in the nucleolus. The association of subunits into a whole ribosome occurs in the cytoplasm, as a rule, during protein biosynthesis.
Ribosome function: assembly of the polypeptide chain (protein synthesis).
cytoskeleton
cytoskeleton made up of 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 - threads with a diameter of 5-7 nm, consist of actin protein. Microtubules and microfilaments form complex tangles in the cytoplasm. Functions of the cytoskeleton: 1) determination of the shape of the cell, 2) support for organelles, 3) formation of a division spindle, 4) participation in cell movements, 5) organization of the flow of the cytoplasm.
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 paired, 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 spindle of division, which contributes to the uniform 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-reproducing organelles of the cytoplasm, they arise as a result of duplication of already existing centrioles. Functions: 1) ensuring the divergence of chromosomes to the poles of the cell during mitosis or meiosis, 2) the center of organization of the cytoskeleton.
Organelles of movement
They are not present in all cells. The organelles of movement include cilia (ciliates, respiratory tract epithelium), flagella (flagellates, spermatozoa), pseudopods (rhizomes, leukocytes), myofibrils (muscle cells), etc.
Flagella and cilia- organelles of a filamentous form, represent an axoneme bounded by a membrane. Axoneme - 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 µm, the cilia are several times shorter.
myofibrils consist of actin and myosin myofilaments, which provide contraction of muscle cells.
Go to lectures number 6"Eukaryotic cell: cytoplasm, cell wall, structure and functions of cell membranes"
The Golgi complex is located near the nucleus after the ER and often near the centriole, formed by a stack of 3-10 flattened and slightly curved cisterns with widened ends. Place of maturation and sorting of proteins.
In many animal cells, such as nerve cells, it takes the form of a complex network located around the nucleus. In the cells of plants and protozoa, the Golgi complex is represented by separate sickle-shaped or rod-shaped bodies. The structure of this organoid is similar in the cells of plant and animal organisms, despite the variety of its shape.
The composition of the Golgi complex includes: cavities limited by membranes and located in groups (5-10 each); large and small bubbles located at the ends of the cavities. All these elements form a single complex.
Tanks to. G. form three main compartments: cis-side, trans-side, intermediate compartment. With k.g.
18. Golgi complex, its structure and functions. Lysosomes. Their structure and functions. types of lysosomes.
closely connected and always seen together is the trans-Golgi network.
The cis-side (forming) includes cisterns facing the expanded elements of the granular endoplasmic reticulum, as well as small transport vesicles.
The trans side (mature) is formed by cisterns facing vacuoles and secretory granules. At a short distance from the marginal cistern lies the trans-network of G.
The intermediate compartment includes a small number of cisterns between the cis and trans sides.
Functions of the Golgi complex
1. Modification of the secretory product: enzymes c.G. glycosylate proteins and lipids, the glycoproteins, proteoglycans, glycolipids and sulfated glycosaminoglycans formed here are intended for subsequent secretion.
2. Concentration of secretory products occurs in condensing vacuoles located on the trans side.
3. Packaging of the secretory product, formation of secretory granules involved in exocytosis.
4. Sorting and packaging of secretory product, formation of secretory granules.
The Golgi complex performs many important functions. Through the channels of the endoplasmic reticulum, the products of the synthetic activity of the cell - proteins, carbohydrates and fats - are transported to it. All these substances first accumulate, and then enter the cytoplasm in the form of large and small bubbles and are either used in the cell itself during its life activity, or removed from it and used in the body. For example, in the cells of the pancreas of mammals, digestive enzymes are synthesized, which accumulate in the cavities of the organoid. Then vesicles filled with enzymes form. They are excreted from the cells into the pancreatic duct, from where they flow into the intestinal cavity. Another important function of this organoid is that fats and carbohydrates (polysaccharides) are synthesized on its membranes, which are used in the cell and which are part of the membranes. Thanks to the activity of the Golgi complex, the renewal and growth of the plasma membrane occurs.
The Golgi complex is involved in the accumulation of products synthesized in the endoplasmic reticulum, in their chemical rearrangement and maturation. In the tanks of the Golgi complex, polysaccharides are synthesized and complexed with protein molecules. One of the main functions of the Golgi complex is the formation of finished secretory products that are excreted outside the cell by exocytosis. The most important functions of the Golgi complex for the cell are also the 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 the source of the formation of primary lysosomes, although their enzymes are also synthesized in the granular network.
Golgi complex is a stack of membrane sacs (cistern) and a system of bubbles associated with it.
On the outer, concave side of the pile of vesicles, budding from smooth. EPS, new cisterns are constantly formed, and on the inside of the cisterns turn back into bubbles.
The main function of the Golgi complex is the transport of substances into the cytoplasm and extracellular environment, as well as the synthesis of fats and carbohydrates. The Golgi complex is involved in the growth and renewal of the plasma membrane and in the formation of lysosomes.
The Golgi complex was discovered in 1898 by K. Golgi. With extremely primitive equipment and a limited set of reagents, he made a discovery, thanks to which, together with Ramon y Cajal, he received the Nobel Prize. He treated the nerve cells with a dichromate solution, after which he added silver and osmium nitrates. With the help of precipitation of osmium or silver salts with cellular structures, Golgi discovered a dark-colored network in neurons, which he called the internal reticulum apparatus. When stained by general methods, the lamellar complex does not accumulate dyes; therefore, the zone of its concentration is visible as a light area. For example, near the nucleus of the plasma cell, a light zone is visible, corresponding to the area where the organelle is located.
Most often, the Golgi complex is adjacent to the nucleus. Under light microscopy, it can be distributed in the form of complex networks or separate diffusely located areas (dictyosomes). The shape and position of the organelle are of no fundamental importance and may change depending on the functional state of the cell.
The Golgi complex is a place of condensation and accumulation of secretion products produced in other parts of the cell, mainly in the EPS. During protein synthesis, radioisotope-labeled amino acids accumulate in gr. EPS, and then they are found in the Golgi complex, secretory inclusions or lysosomes. This phenomenon makes it possible to determine the significance of the Golgi complex in synthetic processes in the cell.
Electron microscopy shows that the Golgi complex consists of clusters of flat cisterns called dictyosomes. The tanks are closely adjacent to each other at a distance of 20 ... 25 nm. The lumen of the tanks in the central part is about 25 nm, and extensions are formed on the periphery - ampoules, the width of which is not constant. There are about 5…10 tanks in each stack. In addition to densely spaced flat cisterns, a large number of small vesicles (vesicles) are located in the zone of the Golgi complex, especially along the edges of the organelle. Sometimes they are laced from the ampoules.
On the side adjacent to the ER and to the nucleus, the Golgi complex has a zone containing a significant number of small vesicles and small cisterns.
The Golgi complex is polarized, that is, qualitatively heterogeneous from different angles.
golgi apparatus
It has an immature cis surface lying closer to the nucleus and a mature trans surface facing the cell surface. Accordingly, the organelle consists of several interconnected compartments that perform specific functions.
The cis compartment usually faces the cell center. Its outer surface has a convex shape. Microvesicles (transport pinocytic vesicles), heading from the EPS, merge with the cisterns. The membranes are constantly renewed by vesicles and, in turn, replenish the contents of the membrane formations of other compartments. The post-translational processing of proteins begins in the compartment and continues in the following parts of the complex.
The intermediate compartment carries out glycosylation, phosphorylation, carboxylation, sulfation of biopolymer protein complexes. The so-called post-translational modification of polypeptide chains occurs. There is a synthesis of glycolipids and lipoproteins. In the intermediate compartment, as in the cis compartment, tertiary and quaternary protein complexes are formed.
Some proteins undergo partial proteolysis (destruction), which is accompanied by their transformation necessary for maturation. Thus, the cis- and intermediate compartments are required for the maturation of proteins and other complex biopolymer compounds.
The trans compartment is located closer to the cell periphery. Its outer surface is usually concave. Partially, the trans-compartment passes into the trans-network - a system of vesicles, vacuoles and tubules.
In cells, individual dictyosomes can be connected to each other by a system of vesicles and cisterns adjacent to the distal end of a cluster of flat sacs, so that a loose three-dimensional network, the trans network, is formed.
In the structures of the trans-compartment and trans-network, sorting of proteins and other substances, the formation of secretory granules, precursors of primary lysosomes, and spontaneous secretion vesicles occur. Secretory vesicles and prelysosomes are surrounded by proteins - clathrins.
Clathrins are deposited on the membrane of the emerging vesicle, gradually splitting it off from the distal cistern of the complex. The bordered vesicles depart from the trans network, their movement is hormone-dependent and is controlled by the functional state of the cell. The process of transport of bordered vesicles is influenced by microtubules. Protein (clathrin) complexes around the vesicles disintegrate after the vesicle is cleaved from the trans network and re-form at the moment of secretion. At the moment of secretion, the vesicle protein complexes interact with microtubule proteins, and the vesicle is transported to the outer membrane. The vesicles of spontaneous secretion are not surrounded by clathrins, their formation occurs continuously and they, heading towards the cell membrane, merge with it, ensuring the restoration of the cytolemma.
In general, the Golgi complex is involved in segregation - this is separation, separation of certain parts from the main mass, and the accumulation of products synthesized in EPS, in their chemical rearrangements, maturation. In tanks, polysaccharides are synthesized, they are combined with proteins, which leads to the formation of complex complexes of peptidoglycans (glycoproteins). With the help of the elements of the Golgi complex, ready-made secrets are removed outside the secretory cell.
Small transport bubbles are split off from gr. EPS in zones free from ribosomes. Bubbles restore the membranes of the Golgi complex and deliver to it the polymer complexes synthesized in EPS. The vesicles are transported to the cis compartment where they fuse with its membranes. Consequently, the Golgi complex receives new portions of membranes and products synthesized in gr. EPS.
In the tanks of the Golgi complex, secondary changes occur in proteins synthesized in gr. EPS. These changes are associated with rearrangement of oligosaccharide chains of glycoproteins. Inside the cavities of the Golgi complex, lysosomal proteins and secretion proteins are modified with the help of transglucosidases: there is a sequential replacement and growth of oligosaccharide chains. Modifying proteins move from the cis-compartment cisterna to the trans-compartment cisternae by transport in protein-containing vesicles.
In the trans-compartment, proteins are sorted: protein receptors are located on the inner surfaces of the membranes of the tanks, which recognize secretory proteins, membrane proteins and lysosomes (hydrolases). As a result, three types of small vacuoles split off from the distal trans-sites of dictyosomes: prelysosomes containing hydrolases; with secretory inclusions, vacuoles replenishing the cell membrane.
The secretory function of the Golgi complex is that the exported protein synthesized on the ribosomes, which is separated and accumulated inside the EPS tanks, is transported to the vacuoles of the lamellar apparatus. Then the accumulated protein can condense, forming secretory protein granules (in the pancreas, mammary and other glands), or remain in a dissolved form (immunoglobulins in plasma cells). Vesicles containing these proteins are split off from the ampullar extensions of the cisterns of the Golgi complex. Such vesicles can merge with each other, increase in size, forming secretory granules.
After that, the secretory granules begin to move towards the cell surface, come into contact with the plasma membrane, with which their own membranes merge, and the contents of the granules are outside the cell. Morphologically, this process is called extrusion, or excretion (ejection, exocytosis) and resembles endocytosis, only with the reverse sequence of stages.
The Golgi complex can dramatically increase in size in cells that actively carry out the secretory function, which is usually accompanied by the development of EPS, and in the case of protein synthesis, the nucleolus.
During cell division, the Golgi complex disintegrates into individual cisterns (dictyosomes) and/or vesicles, which are distributed between two dividing cells and, at the end of telophase, restore the structural integrity of the organelle. Outside of division, there is a continuous renewal of the membrane apparatus due to vesicles migrating from the EPS and distal cisterns of the dictyosome due to the proximal compartments.
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Golgi complex: description
How does the Golgi apparatus work?
Golgi apparatus (Golgi complex) - AG
The structure known today as complex or golgi apparatus (AG) first discovered in 1898 by the Italian scientist Camillo Golgi
It was possible to study in detail the structure of the Golgi complex much later using an electron microscope.
AG is a stack of flattened "tanks" with widened edges. A system of small single-membrane vesicles (Golgi vesicles) is associated with them. Each stack usually consists of 4-6 "tanks", 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.
The Golgi apparatus is usually located near the cell nucleus, near the EPS (in animal cells often near the cell center).
Golgi complex
On the left - in the cell, among other organelles.
On the right is the Golgi complex with membrane vesicles separating from it.
All substances synthesized on EPS membranes transferred to golgi complex in membrane vesicles, which bud off from the ER and then merge with the Golgi complex. Arrived organic substances from EPS undergo further biochemical transformations, accumulate, are packed into membranous vesicles and delivered to those places in the cell where they are needed. They are involved in building cell membrane or stand out ( are secreted) from the cell.
Functions of the Golgi apparatus:
1 Participation in the accumulation of products synthesized in the endoplasmic reticulum, in their chemical rearrangement and maturation. In the tanks of the Golgi complex, polysaccharides are synthesized and complexed with protein molecules.
2) Secretory - the formation of ready-made secretory products that are excreted outside the cell by exocytosis.
3) Renewal of cell membranes, including sections of the plasmolemma, as well as replacement of defects in the plasmolemma during the secretory activity of the cell.
4) Place of formation of lysosomes.
5) Transport of substances
Lysosomes
The lysosome was discovered in 1949 by K. de Duve (Nobel Prize for 1974).
Lysosomes- single-membrane organelles. They are small bubbles (diameter from 0.2 to 0.8 microns) containing a set of hydrolytic enzymes - hydrolases. A lysosome may contain from 20 to 60 different types of hydrolytic enzymes (proteinases, nucleases, glucosidases, phosphatases, lipases, etc.) that degrade various biopolymers. The breakdown of substances by enzymes is called lysis (lysis-decay).
Lysosome enzymes are synthesized on the rough ER, move to the Golgi apparatus, where they are modified and packaged into membrane vesicles, which, after separation from the Golgi apparatus, become lysosomes proper. (Lysosomes are sometimes called the "stomachs" of the cell)
Lysosome - Membrane vesicle containing hydrolytic enzymes
Functions of lysosomes:
1. Cleavage of substances absorbed as a result of phagocytosis and pinocytosis. Biopolymers are broken down into monomers that enter the cell and are used for its needs. For example, they can be used to synthesize new organic substances, or they can be further broken down for energy.
2. Destroy old, damaged, excess organelles. Destruction of organelles can also occur during starvation of the cell.
3. Carry out autolysis (self-destruction) of the cell (liquefaction of tissues in the area of inflammation, destruction of cartilage cells in the process of bone tissue formation, etc.).
Autolysis - this is self-destruction cells resulting from the release of contents lysosomes inside the cell. Because of this, lysosomes are jokingly called "suicide tools" Autolysis is a normal phenomenon of ontogeny; it can spread both to individual cells and to the entire tissue or organ, as occurs during resorption of the tadpole's tail during metamorphosis, i.e., during the transformation of a tadpole into a frog
Endoplasmic reticulum, Golgi apparatus and lysosomesform single vacuolar system of the cell, individual elements of which can pass into each other during the rearrangement and change in the function of membranes.
Mitochondria
The structure of the mitochondria:
1 - outer membrane;
2 - inner membrane; 3 - matrix; 4 - crista; 5 - multienzyme system; 6 - circular DNA.
The shape of the mitochondria can be rod-shaped, round, spiral, cup-shaped, branched. The length of mitochondria ranges from 1.5 to 10 microns, the diameter is from 0.25 to 1.00 microns. The number of mitochondria in a cell can reach several thousand and depends on the metabolic activity of the cell.
Mitochondria is limited two membranes . The outer membrane of mitochondria is smooth, the inner one forms numerous folds - cristae. The cristae increase the surface area of the inner membrane. The number of cristae in mitochondria can vary depending on the energy needs of the cell. It is on the inner membrane that numerous enzyme complexes involved in the synthesis of adenosine triphosphate (ATP) are concentrated. Here, the energy of chemical bonds is converted into energy-rich (macroergic) bonds of ATP . Besides, in the mitochondria, fatty acids and carbohydrates are broken down with the release of energy, which is accumulated and used for the processes of growth and synthesis.The internal environment of these organelles is called matrix. It contains circular DNA and RNA, small ribosomes. Interestingly, mitochondria are semi-autonomous organelles, since they depend on the functioning of the cell, but at the same time they can maintain a certain independence. So, they are able to synthesize their own proteins and enzymes, as well as reproduce on their own (mitochondria contain their own DNA chain, in which up to 2% of the DNA of the cell itself is concentrated).
Mitochondrial functions:
1. Conversion of the energy of chemical bonds into macroergic bonds of ATP (mitochondria are the "energy stations" of the cell).
2. Participate in the processes of cellular respiration - oxygen breakdown of organic substances.
Ribosomes
The structure of the ribosome:
1 - large subunit; 2 - small subunit.
Ribosomes - non-membrane organelles, about 20 nm in diameter. Ribosomes consist of two fragments - large and small subunits. 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.
During protein biosynthesis, ribosomes can "work" singly or combine into complexes - polyribosomes (polysomes). In such complexes, they are linked to each other by a single mRNA molecule.
Ribosome subunits are formed in the nucleolus. After passing through the pores in the nuclear envelope, the ribosomes enter the membranes of the endoplasmic reticulum (ER).
Ribosome function: assembly of a polypeptide chain (synthesis of protein molecules from amino acids).
cytoskeleton
The cellular cytoskeleton is formed microtubules and microfilaments .
microtubules are cylindrical formations with a diameter of 24 nm. Their length is 100 µm-1 mm. The main component is a protein called tubulin. It is incapable of contraction and can be destroyed by colchicine.
Microtubules are located in the hyaloplasm and perform the following functions:
- create an elastic, but at the same time, a strong frame of the cell, which allows it to maintain its shape;
- take part in the process of distribution of cell chromosomes (form a division spindle);
- provide movement of organelles;
- contained in the cell center, as well as in flagella and cilia.
Microfilaments- filaments that are located under the plasma membrane and consist of the protein actin or myosin. They can contract, resulting in movement of the cytoplasm or protrusion of the cell membrane. In addition, these components are involved in the formation of constriction during cell division.
Cell Center
The cell center is an organoid consisting of 2 small granules - centrioles and a radiant sphere around them - the centrosphere. A centriole is a cylindrical body 0.3–0.5 µm long and about 0.15 µm in diameter. The walls of the cylinder consist of 9 parallel tubes. Centrioles are arranged in pairs at right angles to each other. The active role of the cell center is revealed during cell division. Before cell division, centrioles diverge to opposite poles, and a daughter centriole appears near each of them. They form a spindle of division, which contributes to the uniform distribution of genetic material between daughter cells.
Centrioles are self-reproducing organelles of the cytoplasm, they arise as a result of duplication of already existing centrioles.
Functions:
1. Ensuring uniform divergence of chromosomes to the poles of the cell during mitosis or meiosis.
2. Center for the organization of the cytoskeleton.
Organelles of movement
Not present in all cells
The organelles of movement include cilia, as well as flagella. These are tiny growths in the form of hairs. The flagellum contains 20 microtubules. Its base is located in the cytoplasm and is called the basal body. The length of the flagellum is 100 µm or more. Flagella that are only 10-20 microns are called cilia . When microtubules slide, cilia and flagella are able to oscillate, causing movement of the cell itself. The cytoplasm may contain contractile fibrils called myofibrils. Myofibrils, as a rule, are located in myocytes - cells of muscle tissue, as well as in the cells of the heart. They are made up of smaller fibers (protofibrils).
In animals and humans cilia they cover the airways and help to get rid of small solid particles such as dust. In addition, there are also pseudopods that provide amoeboid movement and are elements of many unicellular and animal cells (for example, leukocytes).