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How the Golgi apparatus works and works in a living cell

The cell is an integral system

A living cell is a unique, perfect, smallest unit of the body; it is designed to use oxygen and nutrients as efficiently as possible while performing its functions. Vital organelles for the cell are the nucleus, ribosomes, mitochondria, endoplasmic reticulum, and Golgi apparatus. Let's talk about the latter in more detail.

What is it

This membrane organelle is a complex of structures that remove substances synthesized in it from the cell. Most often it is located close to the outer cell membrane.

Golgi apparatus: structure

It consists of membrane-shaped “sacs” called cisterns. The latter have an elongated shape, slightly flattened in the middle and widened at the edges. The complex also contains round Golgi vesicles - small membrane structures. The cisternae are “folded” into stacks called dictyosomes. The Golgi apparatus contains various types of “sacs”; the entire complex is divided into certain parts according to the degree of distance from the nucleus. There are three of them: the cis section (closer to the nucleus), the middle section, and the trans section - the farthest from the nucleus. They are characterized by a different composition of enzymes, and therefore the work performed. There is one feature in the structure of dictyosomes: they are polar, that is, the section closest to the nucleus only receives vesicles coming from the endoplasmic reticulum. The part of the “stack” facing the cell membrane only forms and releases them.

Golgi apparatus: functions

The main tasks performed are the sorting of proteins, lipids, mucous secretions and their removal. Non-protein substances secreted by the cell and carbohydrate components of the outer membrane also pass through it. At the same time, the Golgi apparatus is not at all an indifferent mediator that simply “transmits” substances; processes of activation and modification (“maturation”) take place in it:

  1. Sorting of substances, transport of proteins. The distribution of protein substances occurs into three streams: for the membrane of the cell itself, export, and lysosomal enzymes. In addition to proteins, the first stream also includes fats. Interesting fact that any export substances are carried within the vesicles. But proteins intended for the cell membrane are embedded in the membrane of the transport vesicle and move in this way.
  2. The release of all products produced in the cell. The Golgi apparatus “packs” all products, both protein and other nature, into secretory vesicles. All substances are released through the complex interaction of the latter with the cell membrane.
  3. Synthesis of polysaccharides (glycosaminoglycans and components of the cell wall glycocalyx).
  4. Sulfation, glycosylation of fats and proteins, partial proteolysis of the latter (necessary to convert them from an inactive form to an active one) - these are all processes of “maturation” of proteins necessary for their future full-fledged work.

In conclusion

Having examined how the Golgi complex is structured and works, we are convinced that it is the most important and integral part of any cell (especially secretory ones). A cell that does not produce substances for export also cannot do without this organelle, since the “completion” of the cell membrane and other important factors depend on it. internal processes life activity.

The Golgi apparatus, also called the Golgi complex, is found in both animals and animals, and usually consists of a collection of cup-shaped, membrane-bound sections called cisternae that look like a stack of deflated balloons.

However, some unicellular flagellates have 60 cisternae that form the Golgi apparatus. Similarly, the number of Golgi complex stacks varies depending on its functions. , as a rule, contain from 10 to 20 stacks per cell, united into one complex by tubular connections between the tanks. The Golgi apparatus is usually located close to.

History of discovery

Due to relatively large sizes The Golgi complex was one of the first organelles observed in cells. In 1897, an Italian doctor named Camillo Golgi, studying nervous system, used new technology coloring, which he himself developed (and which is relevant today). Thanks to the new method, the scientist was able to discern the cellular structure and called it the internal reticular apparatus.

Soon 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 the time did not believe that Golgi was observing a real cell organelle, and attributed his discovery to 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 pouches consisting of two main sections: a cis section and a trans section. The cis compartment is a complex of flattened membranous discs known as cisternae, derived from vesicular clusters that rush from the endoplasmic reticulum.

Mammalian cells typically contain between 40 and 100 stacks. As a rule, each stack includes from 4 to 8 tanks. However, some have about 60 cisterns. This set of cisterns is divided into cis, medial and trans divisions. The trans compartment is the terminal 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 the chemical distribution and delivery department of the cell. It modifies proteins and lipids (fats) that are produced in the cell 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 sent in a vesicle to their destination inside or outside the cell. The exported products are secretions of proteins or glycoproteins that are part of the function of cells in the body. Other substances return to the endoplasmic reticulum or may mature to become.

Molecular modifications that occur in the Golgi complex occur in an orderly manner. Each cistern has two main compartments: the cis compartment 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 are sent.

Chemical composition each section, as well as the enzymes contained in the lumens (internal open spaces of the tanks) between the sections are distinctive. Proteins, carbohydrates, phospholipids and other molecules produced in the endoplasmic reticulum are transported to the Golgi apparatus to undergo biochemical modification during the transition from the cis to trans compartments of the complex. Enzymes present in the Golgi lumen modify the carbohydrate portion 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 compartment ultimately fuse with plasma membrane cells. Among the most important functions of the complex is sorting large quantity macromolecules produced by the cell and their transportation to the necessary destinations. Specialized molecular identification marks or tags, such as phosphate groups, are added by Golgi enzymes to aid in this sorting process.

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The Golgi complex was discovered in 1898. This membrane structure is designed to remove compounds that are synthesized in the endoplasmic reticulum. Next, we will take a closer look at this system.

Golgi complex: structure

The device is a stack of membrane disk-shaped tanks. These bags are somewhat expanded towards the edges. The Golgi vesicle system is associated with the cisternae. In animal cells there is one large or several stacks, which are connected by tubes; in plant cells, dictyosomes (several separate stacks) are found. The Golgi complex includes three sections. They are surrounded by membrane vesicles:

  • cis-compartment closest to the nucleus;
  • medial;
  • trans department (furthest from the nucleus).

These systems differ in their enzyme set. In the cis department, the first pouch is called the “rescue tank.” With its help, receptors that come from the endoplasmic intermediate reticulum move back. The enzyme in the cis section is called phosphoglycosidase. It adds phosphate to mannose (a carbohydrate). The medial compartment contains two enzymes. These are, in particular, mennadiase and N-acetylglucosamine transferase. The latter adds glycosamines. Enzymes of the trans department: peptidase (it carries out proteolysis) and transferase (with its help the transfer of chemical groups occurs).

Golgi complex: functions

This structure ensures the separation of proteins into the following three streams:

  1. Lysosomal. Through it, glycosylated proteins penetrate into the cis-compartment of the Golgi apparatus. Some of them are phospholytized. As a result, mannose-6-phosphate is formed - marketlysosomal enzymes. In the future, these phospholated proteins will enter the lysosomes and will not be modified.
  2. Constitutive exocytosis (secretion). This flow includes proteins and lipids, which have become components of the surface cellular apparatus, including the glycocalyx. Also present here are compounds that are part of the extracellular matrix.
  3. Inducible secretion. Proteins that function outside the cell, the surface apparatus, and in the internal environment of the body penetrate into this flow. Inducible secretion is characteristic of secretory cells.

The Golgi complex takes part in the formation of mucous secretion - mucopolysaccharides (glycosaminoglycans). The apparatus also forms the carbohydrate components of the glycocalyx. They are mainly represented by glycolipids. The system also ensures sulfation of protein and carbohydrate elements. The Golgi complex is involved in partial proteolysis of proteins. In some cases, due to this, the compound changes from inactive to active form (for example, proinsulin is transformed into insulin).

Movement of compounds from the endoplasmic reticulum (ER)

The complex is asymmetrical. Cells located closer to the nucleus contain the most immature proteins. Vesicles - membrane vesicles - are continuously attached to these sacs. They bud from the endoplasmic granular reticulum. The process of protein synthesis by ribosomes takes place on its membranes. Transport connections from endoplasmic reticulum into the Golgi complex is carried out indiscriminately. In this case, incorrectly or incompletely folded proteins continue to remain in the EPS. The return movement of compounds into the endoplasmic reticulum requires the presence of a special signal sequence and is made possible by the binding of these substances to membrane receptors in the cis compartment.

Protein modification

In the tanks of the complex, maturation of compounds that are intended for secretion, transmembrane, lysosomal and other substances occurs. These proteins move sequentially through cisterns into organelles. Their modifications begin in them - phospholation and glycosylation. During the first process, an orthophosphoric acid residue is added to the proteins. In O-glycosylation, complex sugars are docked via an oxygen atom. Different tanks contain different catalytic enzymes. Consequently, proteins maturing in them occur sequentially various processes. Undoubtedly, such a step phenomenon must be controlled. Polysaccharide residues (mannose, mainly) act as a kind of “quality mark”. They mark maturing proteins. Further movement of compounds through tanks is not fully understood by science, despite the fact that resistant substances remain in less or less to a greater extent associated with one pouch.

Transport of proteins from the apparatus

Vesicles bud from the trans-compartment of the complex. They contain fully mature protein compounds. The main function of the complex is considered to be the sorting of proteins passing through it. The apparatus carries out the formation of a “three-directional flow of proteins” - maturation and transport:

  1. Plasma membrane connections.
  2. Secrets.
  3. Lysosomal enzymes.

Through vesicular transport, proteins that have passed through the Golgi complex are delivered to certain areas in accordance with the “tags”. This process is also not fully understood by science. It has been established that the transport of proteins from the complex requires the participation of special membrane receptors. They recognize the connection and provide selective docking of the vesicle and a particular organelle.

Lysosome formation

Many hydrolytic enzymes pass through the apparatus. The addition of the tag mentioned above is carried out with the participation of two enzymes. Specific recognition of lysosomal hydrolases by the elements of their tertiary structure and the addition of N-acetylglucosamine phosphate is carried out by N-acetylglucosamine phosphotransferase. Phosphoglycoside, the second enzyme, cleaves N-acetylglucosamine, resulting in the formation of the M6P tag. It, in turn, is recognized by a receptor protein. With its help, hydrolases enter the vesicles and send them into lysosomes. In them, under acidic conditions, phosphate is separated from the mature hydrolase. In the presence of disturbances in the activity of N-acetylglucosamine phosphotransferase due to mutations or due to genetic defects in the M6P receptor, all lysosomal enzymes are delivered by default to the outer membrane. They are then secreted into the extracellular environment. It has also been established that some of the M6F receptors are also transported to the outer membrane. They carry out the return of accidentally introduced lysosomal enzymes from the external environment into the cell during endocytosis.

Transport of substances to the outer membrane

Usually, even at the stage of synthesis, protein compounds of the outer membrane are embedded in the wall of the endoplasmic reticulum with their hydrophobic regions. They are then transported to the Golgi complex. From there they are transported to the cell surface. During the fusion of plasmalemma and vesicles, such compounds are not released into external environment.

Secretion

Almost all produced compounds in the cell (both protein and non-protein nature) pass through the Golgi complex. There they form secretory vesicles. In plants, with the participation of dictyosomes, the production of material occurs

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 in the terminal areas 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, KG synthesizes those chemicals, which 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.

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 various types 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 cell sap contains water-soluble organic and inorganic salts, monosaccharides, disaccharides, amino acids, final or toxic metabolic products (glycosides, alkaloids), some pigments (anthocyanins).

Animal cells contain small digestive and autophagic 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 flowers and fruits and thereby attracting pollinators and seed dispersers, 6) see the 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 - inner 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 - colorless plastids in the cells of uncolored parts of plants, chromoplasts - colored plastids usually yellow, red and orange flowers 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 green 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, 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 common origin, mutual transformations 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 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, the 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”