Endoplasmic reticulum: structure, types and functions. Smooth endoplasmic reticulum
Endoplasmic reticulum (endoplasmic reticulum) was discovered by K.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 (EPS) is found in almost all eukaryotes. It binds the 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 cisterns, tubules, and transport vesicles. 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 cisterna expands sharply, then 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 ensure attachment of ribosomes to the membrane, translate polypeptide chains of secretory and lysosomal proteins, cytolemma 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 in electron microscopy are visible as small (about 20 nm in diameter), dark, almost round 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 arranged in the form of clusters - polisomes, which form rosettes, bunches 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 information is read and synthesized polypeptide chains. These ribosomes attach to the EPS membrane using one of the regions of the large subunit.
In some cells, the granular endoplasmic reticulum (group EPS) consists of rare scattered cisterns, but can form large local (focal) clusters. Poorly developed gr. EPS in poorly differentiated cells or in cells with low protein secretion. Clusters c. 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 cell poles, while numerous ribosomes stain this area basophilically. For example, in plasma cells (plasma cells), a well-developed gr. EPS determines 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 cisterns, which, under light microscopy, manifests itself in the form of basophilic granularity in the cytoplasm (chromatophilic substance of the cytoplasm, or tigroid).
In most cases, gr. EPS are synthesized proteins that are not used by the cell itself, but are secreted during 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 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 along the 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. In 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 the nucleoli increases, the number of nuclear pores increases. Often in such cells there are numerous secretory inclusions (granules) containing secretory proteins, and 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 the 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 ribosomes gr. EPS are synthesized membrane integral proteins that are embedded in the thickness of the membrane. Here, from the side of the hyaloplasm, lipids are synthesized and incorporated into the membrane. 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 ribosomes, isolation from the contents of the hyaloplasm inside membrane cavities and the transport of these proteins to other parts of the cell, chemical modification or local condensation, as well as the synthesis of structural components of cell membranes.
In the process of translation, ribosomes are attached to the membrane of c. EPS in the form of a chain (polysome). The ability to bind to the membrane is provided by signaling sites that attach to special EPS 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 are opened by receptors. As a result, the subunits of proteins appear in the intermembrane space of gr. EPS. An oligosaccharide (glycosylation) can be attached to the resulting polypeptides, which is cleaved from dolichol phosphate attached to the inner surface of the membrane. Subsequently, the contents of the lumen of the tubules and cisterns of gr. The EPS is transported by transport vesicles to the cis-compartment of the Golgi complex, where it undergoes further transformation.
Smooth (agranular) EPS... She may be associated with c. EPS is a transitional zone, but, nevertheless, it is an independent organelle with its own system of receptor and enzymatic complexes. It consists of a complex network of tubules, flat and expanded cisterns and transport vesicles, but if in gr. EPS is dominated by cisterns, then in the smooth endoplasmic reticulum (smooth EPS) there are more tubules with a diameter of about 50 ... 100 nm.
To membranes smooth. EPS does not attach ribosomes, which is due to the absence of receptors to these organelles. Thus, smooth. EPS, although it is a morphological continuation of the granular, is not just the endoplasmic reticulum, on which in this moment there are 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 medicinal substances (in particular, barbiturates). In glad. EPS pass final stages synthesis of all lipids of cell membranes. On membranes smooth. EPS are lipid-transforming enzymes - flippases, moving fat molecules and maintaining the asymmetry of 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 interconnecting networks of L-tubules and marginal cisterns. They braid special contractile muscle organelles - myofibrils. In striated muscle tissues, the organelle contains a protein called 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 cell excitation during 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 a contraction of cells and muscle fibers due to the interaction of actomyosin or actomyosin complexes of myofibrils. At rest, Ca 2+ is reabsorbed into the lumen of the smooth tubules. 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 poisoning, 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 reproductive glands, there is a smooth surface. EPS is involved in the synthesis of steroid hormones, and key enzymes of steroidogenesis are located on its membranes. In such endocrinocytes smooth. 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.
Endoplasmic reticulum
The endoplasmic reticulum (EPS) is a system of communicating or separate tubular channels and flattened cisterns located throughout the cytoplasm of a cell. They are delimited by membranes (membrane organelles). Sometimes the cisterns have bubble-shaped expansions. The EPS channels can connect with the surface or nuclear membranes, contact with the Golgi complex.
In this system, smooth and rough (granular) EPS can be distinguished.
Rough EPS
Ribosomes are located on the channels of the rough EPS in the form of polysomes. Here, the synthesis of proteins occurs, mainly produced by the cell for export (removal from the cell), for example, the secretions of glandular cells. Here lipids and proteins of the cytoplasmic membrane are formed and assembled.
Smooth EPS
There are no ribosomes on the membranes of smooth EPS. Here, mainly the synthesis of fats and similar substances (for example, steroid hormones), as well as carbohydrates takes place. Through the channels of the smooth EPS, the finished material also moves to the place of its packing in granules (to the zone of the Golgi complex).
Golgi complex
The lamellar Golgi complex is the packing center of the cell. It is a collection of dictyosomes (from several tens to hundreds and thousands per cell). Dictyosome- a stack of 3-12 flattened oval-shaped cisterns, along the edges of which small vesicles (vesicles) are located. Larger expansion of the cisterns give vacuoles that contain a water reserve in the cell and are responsible for maintaining turgor. The lamellar complex gives rise to secretory vacuoles, which contain substances intended for removal from the cell. At the same time, the secretion entering the vacuole from the synthesis zone (EPS, mitochondria, ribosomes) undergoes some chemical transformations here.
The Golgi complex gives rise to primary lysosomes. Polysaccharides, glycoproteins and glycolipids are also synthesized in dictyosomes, which are then used to build cytoplasmic membranes.
The structure and function of non-membrane cell structures
This group of organelles includes ribosomes, microtubules and microfilaments, and the cell center.
Ribosome
Ribosomes (Fig. 1) are present in the cells of both eukaryotes and prokaryotes, since they perform an important function in biosynthesis of proteins. Each cell contains tens, hundreds of thousands (up to several million) of these small rounded organelles. It is a rounded ribonucleoprotein particle. Its diameter is 20-30 nm. The ribosome consists of large and small subunits, which combine in the presence of a strand of m-RNA (messenger, or messenger, RNA). A complex of a group of ribosomes united by one mRNA molecule like a string of beads is called polysome... These structures are either freely located in the cytoplasm, or are attached to the membranes of the granular EPS (in both cases, protein synthesis is actively proceeding on them).
Fig. 1. Diagram of the structure of the ribosome sitting on the membrane of the endoplasmic reticulum: 1 - small subunit; 2 mRNA; 3 - aminoacyl-tRNA; 4 - amino acid; 5 - large subunit; 6 - - membrane of the endoplasmic reticulum; 7 - synthesized polypeptide chain
Granular EPS polysomes form proteins that are removed from the cell and used for the needs of the whole body (for example, digestive enzymes, proteins of human breast milk). In addition, ribosomes are present on the inner surface of mitochondrial membranes, where they are also actively involved in the synthesis of protein molecules.
Microtubules
These are tubular hollow formations devoid of a membrane. The outer diameter is 24 nm, the lumen width is 15 nm, and the wall thickness is about 5 nm. In a free state, they are presented in the cytoplasm; structural elements flagella, centrioles, division spindles, cilia. Microtubules are built from stereotyped protein subunits by polymerization. In any cell, polymerization processes run parallel to depolymerization processes. Moreover, their ratio is determined by the number of microtubules. Microtubules have different resistance to factors destroying them, for example, to colchicine (this Chemical substance causing depolymerization). Microtubule functions:
1) are the supporting apparatus of the cell;
2) determine the shape and size of the cell;
3) are factors of directed movement of intracellular structures.
Microfilaments
These are thin and long formations that are found throughout the cytoplasm. Sometimes they form bundles. Types of micro-filaments:
1) actin. They contain contractile proteins (actin), provide cellular forms of movement (for example, amoeboid), play the role of a cell framework, participate in organizing the movement of organelles and sections of the cytoplasm inside the cell;
2) intermediate (10 nm thick). Their bundles are found along the periphery of the cell under the plasmalemma and around the circumference of the nucleus. They perform a supporting (frame) role. The different cells (epithelial, muscle, nerve, fibroblasts) are built from different proteins.
Microfilaments, like microtubules, are built of subunits; therefore, their number is determined by the ratio of polymerization and depolymerization processes.
The cells of all animals, some fungi, algae, and higher plants are characterized by the presence of a cell center.
Cell center usually located near the core.
It consists of two centrioles, each of which is a hollow cylinder about 150 nm in diameter and 300-500 nm in length.
The centrioles are mutually perpendicular. The wall of each centriole is formed by 27 microtubules composed of tubulin protein. Microtubules are grouped into 9 triplets.
During cell division, filaments of the division spindle are formed from the centrioles of the cell center.
Centrioles polarize the process of cell division, thereby achieving a uniform divergence of sister chromosomes (chromatids) in anaphase of mitosis.
Cellular inclusions. This is the name of the unstable components in the cell, which are present in the basic substance of the cytoplasm in the form of grains, granules or droplets. The inclusions may or may not be surrounded by a membrane.
Functionally, three types of inclusions are distinguished: reserve nutrients (starch, glycogen, fats, proteins), secretory inclusions (substances characteristic of glandular cells produced by them - hormones of endocrine glands, etc.) and inclusions special purpose(in highly specialized cells, for example, hemoglobin in erythrocytes).
Endoplasmic reticulum (EPS) , or endoplasmic reticulum (ER), is a system consisting of membrane cisterns, channels and vesicles. ER accounts for about half of all cell membranes.
Morphofunctionally, EPS is differentiated into 3 sections: rough (granular), smooth (agranular), and intermediate. The granular EPS contains ribosomes (PC), the smooth and intermediate ones are devoid of them. Granular ER is mainly represented by cisterns, while smooth and intermediate ER is mainly represented by canals. The membranes of tanks, channels and bubbles can merge into each other. ER contains a semi-liquid matrix characterized by a special chemical composition.
ER functions:
- compartmentalization;
- synthetic;
- transport;
- detoxification;
- regulation of the concentration of calcium ions.
Compartmentalization function
associated with cell division into compartments (compartments) using ER membranes. Such a division allows to isolate part of the cytoplasmic content from the hyaloplasm and enables the cell to separate and localize certain processes, as well as to make them proceed more efficiently and directed.
Synthetic function.
Almost all lipids are synthesized on smooth ER, with the exception of two mitochondrial lipids, the synthesis of which occurs in the mitochondria themselves. On the membranes of smooth ER, cholesterol is synthesized (in humans, up to 1 g per day, mainly in the liver; with liver damage, the amount of cholesterol in the blood falls, the shape and function of erythrocytes changes and anemia develops).
Protein synthesis occurs on a rough ER:
- internal phase of ER, Golgi complex, lysosomes, mitochondria;
- secretory proteins such as hormones, immunoglobulins;
- membrane proteins.
Protein synthesis begins on free ribosomes in the cytosol. After chemical transformations, proteins are packed into membrane vesicles, which are cleaved from the ER and transported to other areas of the cell, for example, to the Golgi complex.
Proteins synthesized on ER can be conditionally subdivided into two streams:
- internal, which remain in the ER;
- external, which do not remain in the ER.
Internal proteins, in turn, can also be divided into two streams:
- resident, not leaving the ER;
- transit, leaving the ER.
In ER there is detoxification of harmful substances
trapped in the cage or formed in the cage itself. Most of the harmful substances are
hydrophobic substances, which therefore cannot be excreted in the urine. In the membranes of the ER, there is a protein called cytochrome P450, which converts hydrophobic substances into hydrophilic ones, and after that they are removed from the body in the urine.
The structure and functions of the endoplasmic reticulum are associated with the synthesis organic matter (proteins, fats and carbohydrates) and their transport inside the cell. It is a membrane organoid of a cell that occupies a significant part of it and looks like a system of tubes, tubules, etc., branching (originating) from the shell of the nucleus, more precisely from its outer membrane.
In addition to the term "endoplasmic reticulum", the term "endoplasmic reticulum" is used. This is the same thing, "reticulum" is translated from English as "network". In the literature, you can find the following abbreviations for this cellular structure: EPS, EPR, ES, ER.
If we take any part of the endoplasmic reticulum, then in its structure it will represent an internal space limited by a membrane (cavity, channel). In this case, the channel is somewhat flattened, in different parts of the EPS to a different degree. In their own way chemical structure the EPS membranes are close to the membrane of the nuclear envelope.
Distinguish smooth and rough endoplasmic reticulum... Rough is different in that ribosomes are attached to its membranes from the outside, and its channels are more flattened.
A bit of history
The cell is considered the smallest structural unit 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. Also, Porter noticed that the sizes of EPS in the cells of different creatures and even organs and tissues of one 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 intestine, mucous membranes and adrenal glands.
Concept
EPS is a system of tubules, tubules, vesicles and membranes located in the cytoplasm of a cell.
Endoplasmic reticulum: structure and function
Structure | |
First, it is transport function... Like the cytoplasm, the endoplasmic reticulum ensures the exchange of substances between organelles. Secondly, the EPS performs structuring and grouping of the contents of the cell, dividing it into certain sections. Thirdly, the most important function is protein synthesis, which is carried out in the ribosomes of the rough endoplasmic reticulum, as well as the synthesis of carbohydrates and lipids, which occurs on the membranes of smooth EPS. |
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EPS structure In total, there are 2 types of endoplasmic reticulum: granular (rough) and smooth. The functions performed by this component depend precisely on the type of the cell itself. On the membranes of the smooth network, there are sections that produce enzymes, which are then involved in metabolism. The rough endoplasmic reticulum contains ribosomes on its membranes. |
Summary of the other most important constituents of the cell
Cytoplasm: structure and function
| Picture | Structure | Functions |
| It is a liquid in the cell. It is in it that all organelles are located (including the Golgi apparatus, and the endoplasmic reticulum, and many others) and the nucleus with its contents. Refers to mandatory components and is not an organoid as such. | The main function is transport. It is thanks to the cytoplasm that the interaction of all organelles occurs, their ordering (folded into a single system) and the course of all chemical processes. |
Cell membrane: structure and function
| Picture | Structure | Functions |
| Molecules of phospholipids and proteins, forming two layers, make up a membrane. It is the thinnest film that envelops the entire cell. Polysaccharides are also an integral component of it. And in plants, the outside is still covered with a thin layer of fiber. | The main function of the cell membrane is to restrict the internal contents of the cell (cytoplasm and all organelles). Since it contains the smallest pores, it provides transport and metabolism. It can also be a catalyst in the implementation of some chemical processes and a receptor in the event of an external hazard. |
The core: structure and function
| Picture | Structure | Functions |
| Has either oval or spherical shape. It contains special DNA molecules, which in turn carry the hereditary information of the whole organism. The core itself is covered with a special shell on the outside, in which there are pores. Also contains nucleoli (small bodies) and liquid (juice). The endoplasmic reticulum is located around this center. | It is the nucleus that regulates absolutely all processes occurring in the cell (metabolism, synthesis, etc.). And it is this component that is the main carrier hereditary information the whole organism. In the nucleoli, protein and RNA molecules are synthesized. |
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 a complex 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
Organoid, consisting of several cavities, constantly secreting 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 nearby.
Lysosomes
Organoids, surrounded by a special membrane and performing the digestive function of a cell, are called lysosomes.
Mitochondria
Organoids, surrounded by several membranes and performing an energetic function, that is, providing the synthesis of ATP molecules and distributing the received energy throughout the cell.
Plastids. Types of plastids
Chloroplasts (photosynthetic function);
Chromoplasts (storage and preservation of carotenoids);
Leukoplasts (accumulation and storage of starch).
Organoids designed for locomotion
They also perform some kind of movement (flagella, cilia, long processes, etc.).
Cell center: structure and function
