The structure of the smooth endoplasmic reticulum. Endoplasmic reticulum: structure, types and functions

Endoplasmic reticulum(endoplasmic reticulum) was discovered by C. R. Porter in 1945.

This structure is a system of interconnected vacuoles, flat membrane sacs or tubular formations that create a three-dimensional membrane network within the cytoplasm. The endoplasmic reticulum (ER) is found in almost all eukaryotes. It binds organelles together and transports nutrients. There are two independent organelles: granular (granular) and smooth non-granular (agranular) endoplasmic reticulum.

Granular (rough, or granular) endoplasmic reticulum. It is a system of flat, sometimes expanded tanks, tubules, transport bubbles. The size of the cisterns depends on the functional activity of the cells, and the width of the lumen can range from 20 nm to several microns. If the cistern expands sharply, it becomes visible under light microscopy and is identified as a vacuole.

The cisterns are formed by a two-layer membrane, on the surface of which there are specific receptor complexes that provide attachment to the membrane of ribosomes, translating polypeptide chains of secretory and lysosomal proteins, cytolemmal proteins, etc., that is, proteins that do not merge with the contents of the karyoplasm and hyaloplasm.

The space between the membranes is filled with a homogeneous matrix of low electron density. Outside, the membranes are covered with ribosomes. Ribosomes are visible under electron microscopy as small (about 20 nm in diameter), dark, almost rounded particles. If there are many of them, then this gives a granular appearance to the outer surface of the membrane, which served as the basis for the name of the organelle.

On the membranes, ribosomes are located in the form of clusters - polysomes, which form rosettes, clusters or spirals of various shapes. This feature of the distribution of ribosomes is explained by the fact that they are associated with one of the mRNAs, from which they read information, synthesize polypeptide chains. Such ribosomes are attached to the ER membrane using one of the regions of the large subunit.

In some cells, the granular endoplasmic reticulum (GR. EPS) consists of rare scattered cisterns, but can form large local (focal) accumulations. Weakly developed gr. EPS in poorly differentiated cells or in cells with low protein secretion. Accumulations gr. EPS are found in cells that actively synthesize secretory proteins. With an increase in the functional activity of the cisterna, the organelles become multiple and often expand.

Gr. EPS is well developed in the secretory cells of the pancreas, the main cells of the stomach, in neurons, etc. Depending on the type of cells gr. EPS can be diffusely distributed or localized in one of the poles of the cell, while numerous ribosomes stain this zone basophilically. For example, in plasma cells (plasmocytes) a well-developed gr. EPS causes a bright basophilic color of the cytoplasm and corresponds to areas of concentration of ribonucleic acids. In neurons, the organelle is located in the form of compactly lying parallel tanks, which, under light microscopy, appears as basophilic granularity in the cytoplasm (the chromatophilic substance of the cytoplasm, or tigroid).

In most cases, gr. ER synthesizes proteins that are not used by the cell itself, but are secreted into external environment: proteins of the exocrine glands of the body, hormones, mediators (protein substances of the endocrine glands and neurons), proteins of the intercellular substance (proteins of collagen and elastic fibers, the main component of the intercellular substance). Proteins formed by gr. EPS are also part of the lysosomal hydrolytic enzyme complexes located on the outer surface of the cell membrane. The synthesized polypeptide not only accumulates in the EPS cavity, but also moves, is transported through channels and vacuoles from the site of synthesis to other parts of the cell. First of all, such transport is carried out in the direction of the Golgi complex. Under electron microscopy, the good development of EPS is accompanied by a parallel increase (hypertrophy) of the Golgi complex. In parallel with it, the development of nucleoli increases, the number of nuclear pores increases. Often in such cells there are numerous secretory inclusions (granules) containing secretory proteins, the number of mitochondria increases.

Proteins accumulating in the EPS cavities, bypassing the hyaloplasm, are most often transported to the Golgi complex, where they are modified and are part of either lysosomes or secretory granules, the contents of which remain isolated from the hyaloplasm by the membrane. Inside tubules or vacuoles gr. EPS is the modification of proteins, their binding to sugars (primary glycosylation); condensation of synthesized proteins with the formation of large aggregates - secretory granules.

On the ribosomes ERs are synthesized membrane integral proteins that are embedded in the thickness of the membrane. Here, from the side of the hyaloplasm, lipid synthesis and their incorporation into the membrane take place. As a result of these two processes, the EPS membranes themselves and other components of the vacuolar system grow.

The main function of gr. EPS is the synthesis of exported proteins on the ribosomes, isolation from the contents of the hyaloplasm inside the membrane cavities and the transport of these proteins to other parts of the cell, chemical modification or local condensation, as well as the synthesis structural components cell membranes.

During translation, ribosomes attach to the membrane gr. EPS in the form of a chain (polysomes). The ability to bind to the membrane is provided by signaling regions that attach to special ER receptors - the mooring protein. After that, the ribosome binds to a protein that fixes it to the membrane, and the resulting polypeptide chain is transported through the pores of the membranes, which open with the help of receptors. As a result, protein subunits are in the intermembrane space gr. EPS. An oligosaccharide (glycosylation) can join the resulting polypeptides, which is cleaved off from dolichol phosphate attached to the inner surface of the membrane. Subsequently, the contents of the lumen of the tubules and cisterns gr. EPS is transported by transport vesicles to the cis-compartment of the Golgi complex, where it undergoes further transformation.

Smooth (agranular) EPS. It may be related to Mr. EPS is a transition zone, but, nevertheless, is an independent organelle with own system receptor and enzymatic complexes. It consists of a complex network of tubules, flat and expanded cisterns and transport bubbles, but if in gr. EPS is dominated by tanks, then in a smooth endoplasmic reticulum(smooth EPS) more tubules with a diameter of about 50 ... 100 nm.

To the membranes smooth. ERs do not attach to ribosomes, which is due to the absence of receptors for these organelles. Thus, smooth. EPS, although it is a morphological continuation of the granular, is not just an endoplasmic reticulum, on which in this moment no ribosomes, but is an independent organelle to which ribosomes cannot attach.

Glad. EPS is involved in the synthesis of fats, the metabolism of glycogen, polysaccharides, steroid hormones and some drugs (in particular, barbiturates). In smooth EPS go through the final steps in the synthesis of all lipids in cell membranes. On the membranes smooth. EPS are lipid-transforming enzymes - flippases, moving fat molecules and maintaining the asymmetry of the lipid layers.

Glad. EPS is well developed in muscle tissues, especially striated ones. In skeletal and cardiac muscles, it forms a large specialized structure - the sarcoplasmic reticulum, or L-system.

The sarcoplasmic reticulum consists of mutually passing networks of L-tubules and marginal cisterns. They braid special contractile organelles of muscles - myofibrils. In striated muscle tissues, the organelle contains a protein - calsequestrin, which binds up to 50 Ca 2+ ions. In smooth muscle cells and non-muscle cells in the intermembrane space there is a protein called calreticulin, which also binds Ca 2+ .

Thus, smooth. EPS is a reservoir of Ca 2+ ions. At the moment of excitation of the cell during the depolarization of its membrane, calcium ions are removed from the EPS into the hyaloplasm, the leading mechanism that triggers muscle contraction. This is accompanied by contraction of cells and muscle fibers due to the interaction of actomyosin or actominimyosin complexes of myofibrils. At rest, Ca 2+ is reabsorbed into the lumen of the tubules smooth. EPS, which leads to a decrease in the calcium content in the cytoplasmic matrix and is accompanied by relaxation of myofibrils. Calcium pump proteins regulate transmembrane ion transport.

An increase in the concentration of Ca 2+ ions in the cytoplasmic matrix also accelerates the secretory activity of non-muscle cells, stimulates the movement of cilia and flagella.

Glad. EPS deactivates various substances harmful to the body due to their oxidation with the help of a number of special enzymes, especially in liver cells. So, with some poisonings, acidophilic zones (not containing RNA) appear in the liver cells, completely filled with a smooth endoplasmic reticulum.

In the adrenal cortex, in the endocrine cells of the gonads smooth. ER is involved in the synthesis of steroid hormones, and key enzymes of steroidogenesis are located on its membranes. In such endocrinocytes, glad. EPS has the appearance of abundant tubules, which are visible in cross section as numerous vesicles.

Glad. EPS is formed from gr. EPS. In some areas smooth. EPS are formed new lipoprotein membrane areas, devoid of ribosomes. These areas can grow, split off from granular membranes and function as an independent vacuolar system.

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 matter, 3) “packaging” of proteins, lipids, carbohydrates into membrane vesicles, 4) secretion of proteins, lipids, carbohydrates, 5) synthesis of carbohydrates and lipids, 6) site of formation of 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 from 20 to 60 various kinds 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 formed 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) see. 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(4). 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 unique to plant cells. Distinguish three main types of plastids: leukoplasts - colorless plastids in the cells of unstained parts of plants, chromoplasts - colored plastids are 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 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(4). 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 is responsible for green color 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 found 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 (respectively 70S, 30S, 50S).

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, epithelium of the respiratory tract), 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 structure of the endoplasmic reticulum

Definition 1

Endoplasmic reticulum(EPS, endoplasmic reticulum) is a complex ultramicroscopic, highly branched, interconnected system of membranes that more or less evenly permeates the mass of the cytoplasm of all eukaryotic cells.

EPS is a membrane organelle consisting of flat membrane sacs - cisterns, channels and tubules. Due to this structure, the endoplasmic reticulum significantly increases the area of ​​\u200b\u200bthe inner surface of the cell and divides the cell into sections. It's filled inside matrix(moderately dense loose material (synthesis product)). Content of various chemical substances in sections is not the same, therefore, in the cell, both simultaneously and in a certain sequence, various chemical reactions in a small cell volume. The endoplasmic reticulum opens into perinuclear space(a cavity between two membranes of a karyolem).

The membrane of the endoplasmic reticulum consists of proteins and lipids (mainly phospholipids), as well as enzymes: adenosine triphosphatase and enzymes for the synthesis of membrane lipids.

There are two types of endoplasmic reticulum:

• smooth (agranular, AES), represented by tubules that anastomose with each other and do not have ribosomes on the surface;
• Rough (granular, grES), also consisting of interconnected tanks, but they are covered with ribosomes.

Remark 1

Sometimes they allocate more passing or transient(tES) endoplasmic reticulum, which is located in the area of ​​transition of one type of ES to another.

Granular ES is characteristic of all cells (except spermatozoa), but the degree of its development is different and depends on the specialization of the cell.

GRES of epithelial glandular cells (pancreas producing digestive enzymes, liver synthesizing serum albumins), fibroblasts (connective tissue cells producing collagen protein) is highly developed, plasma cells(production of immunoglobulins).

Agranular ES prevails in the cells of the adrenal glands (synthesis of steroid hormones), in muscle cells (calcium metabolism), in the cells of the fundic glands of the stomach (release of chloride ions).

Another type of EPS membranes are branched membrane tubules containing a large number of specific enzymes inside, and vesicles - small, membrane-surrounded vesicles, mainly located next to the tubules and cisterns. They provide the transfer of those substances that are synthesized.

EPS functions

The endoplasmic reticulum is an apparatus for the synthesis and, in part, the transport of cytoplasmic substances, thanks to which the cell performs complex functions.

Remark 2

The functions of both types of EPS are associated with the synthesis and transport of substances. The endoplasmic reticulum is a universal transport system.

Smooth and rough endoplasmic reticulum with their membranes and contents (matrix) perform common functions:

• dividing (structuring), due to which the cytoplasm is orderly distributed and does not mix, and also prevents random substances from entering the organelle;
• transmembrane transport, due to which the necessary substances are transferred through the membrane wall;
• synthesis of membrane lipids with the participation of enzymes contained in the membrane itself and ensuring the reproduction of the endoplasmic reticulum;
• due to the potential difference that occurs between the two surfaces of the ES membranes, it is possible to ensure the conduction of excitation pulses.

In addition, each type of network has its own specific functions.

Functions of the smooth (agranular) endoplasmic reticulum

The agranular endoplasmic reticulum, in addition to the named functions common to both types of ES, also performs functions peculiar only to it:

• calcium depot. In many cells (skeletal muscle, heart, eggs, neurons) there are mechanisms that can change the concentration of calcium ions. Striated muscle tissue contains a specialized endoplasmic reticulum called the sarcoplasmic reticulum. This is a reservoir of calcium ions, and the membranes of this network contain powerful calcium pumps capable of ejecting a large amount of calcium into the cytoplasm or transporting it into the cavities of the network channels in hundredths of a second;
• lipid synthesis, substances such as cholesterol and steroid hormones. Steroid hormones are synthesized mainly in the endocrine cells of the gonads and adrenal glands, in the cells of the kidneys and liver. Intestinal cells synthesize lipids, which are excreted into the lymph, and then into the blood;

detoxification function– neutralization of exogenous and endogenous toxins;

Example 1

Kidney cells (hepatocytes) contain oxidase enzymes that can destroy phenobarbital.

organelle enzymes are involved in glycogen synthesis(in liver cells).

Functions of the rough (granular) endoplasmic reticulum

For granular endoplasmic reticulum, other than those listed common functions, there are also special ones:

• protein synthesis at the TPP has some peculiarities. It begins on free polysomes, which subsequently bind to ES membranes.
• The granular endoplasmic reticulum synthesizes: all proteins of the cell membrane (except for some hydrophobic proteins, proteins of the inner membranes of mitochondria and chloroplasts), specific proteins of the internal phase of membrane organelles, as well as secretory proteins that are transported through the cell and enter the extracellular space.
• post-translational modification of proteins: hydroxylation, sulfation, phosphorylation. An important process is glycosylation, which occurs under the action of the membrane-bound enzyme glycosyltransferase. Glycosylation occurs before the secretion or transport of substances to certain parts of the cell (Golgi complex, lysosomes or plasmalemma).
• transport of substances along the intramembrane part of the network. Synthesized proteins move along the intervals of ES to the Golgi complex, which removes substances from the cell.
• due to the involvement of the granular endoplasmic reticulum the Golgi complex is formed.

The functions of the granular endoplasmic reticulum are associated with the transport of proteins that are synthesized in ribosomes and located on its surface. Synthesized proteins enter the ER, twist and acquire a tertiary structure.

The protein that is transported to the tanks changes significantly along the way. It can, for example, be phosphorylated or converted to a glycoprotein. The usual route for a protein is through the granular ER to the Golgi apparatus, from where it either exits the cell, or enters other organelles of the same cell, such as lysosomes), or is deposited as storage granules.

In liver cells, both granular and non-granular endoplasmic reticulum take part in the processes of detoxification of toxic substances, which are then removed from the cell.

Like the outer plasma membrane, the endoplasmic reticulum has selective permeability, as a result of which the concentration of substances inside and outside the reticulum channels is not the same. It matters for the function of the cell.

Example 2

There are more calcium ions in the endoplasmic reticulum of muscle cells than in its cytoplasm. Leaving the channels of the endoplasmic reticulum, calcium ions start the process of contraction of muscle fibers.

Formation of the endoplasmic reticulum

The lipid components of the membranes of the endoplasmic reticulum are synthesized by the enzymes of the network itself, the protein comes from the ribosomes located on its membranes. The smooth (agranular) endoplasmic reticulum does not have its own protein synthesis factors, therefore it is believed that this organelle is formed as a result of the loss of ribosomes by the granular endoplasmic reticulum.

A bit of history

The cell is considered the smallest structural unit of any organism, however, it also consists of something. One of its components is the endoplasmic reticulum. Moreover, EPS is a mandatory component of any cell in principle (except for some viruses and bacteria). It was discovered by the American scientist K. Porter back in 1945. It was he who noticed the systems of tubules and vacuoles, which, as it were, accumulated around the nucleus. Porter also noted that the sizes of EPS in the cells of different creatures and even organs and tissues of the same organism are not similar to each other. He came to the conclusion that this is due to the functions of a particular cell, the degree of its development, as well as the stage of differentiation. For example, in humans, EPS is very well developed in the cells of the intestines, mucous membranes and adrenal glands.

concept

EPS is a system of tubules, tubules, vesicles and membranes that are located in the cytoplasm of the cell.

Endoplasmic reticulum: structure and functions

Brief information about the other most important components of the cell

Cytoplasm: structure and functions

Cell membrane: structure and functions

Core: structure and functions

Ribosomes

They are organelles that provide basic protein synthesis. They can be located both in the free space of the cytoplasm of the cell, and in combination with other organelles (endoplasmic reticulum, for example). If the ribosomes are located on the membranes of the rough EPS (being on the outer walls of the membranes, the ribosomes create roughness) , the efficiency of protein synthesis increases several times. This has been proven by numerous scientific experiments.

Golgi complex

An organoid consisting of several cavities that constantly secrete bubbles of various sizes. The accumulated substances are also used for the needs of the cell and the body. The Golgi complex and the endoplasmic reticulum are often located side by side.

Lysosomes

Organelles surrounded by a special membrane and performing the digestive function of the cell are called lysosomes.

Mitochondria

Organelles surrounded by several membranes and performing an energy function, that is, providing the synthesis of ATP molecules and distributing the energy received throughout the cell.

Plastids. Types of plastids

Chloroplasts (function of photosynthesis);

Chromoplasts (accumulation and preservation of carotenoids);

Leukoplasts (accumulation and storage of starch).

Organelles designed for locomotion

They also make some movements (flagella, cilia, long processes, etc.).

Cell center: structure and functions