Cytoplasm includes the liquid content of the cell or hyaloplasm and organelles. The plasma membrane is 80-90% water. The dense residue includes various electrolytes and organic substances. From the point of view of the content of substances and the concentration of enzymes, hyaloplasm can be divided into central and peripheral. The content of enzymes in the peripheral hyaloplasm is much higher, in addition, the concentration of ions is higher in it. The hyaloplasm is compartmentalized mainly due to thin filaments. Although all other components of COCA perform a structural function. Part of the organelles, for example, ribosomes, mitochondria, and the cell center interact with fibrillar structures, so we can say that the entire cytoplasm is structurally organized. Cell organelles are divided into membrane and non-membrane. Membrane organelles include: the Golgi complex, EPS, lysosomes, peroxisomes. Non-membrane organelles include: the cell center, ribosomes (in prokaryotes, only ribosomes are present from organoids).

E.P.S.

This is a structurally unified membrane system that permeates the entire cell and which is believed to have been the first to form in the process of becoming a eukaryotic cell. Exocytosis of the plasmalemma occurred, and such cells received a certain advantage, because. a compartment appeared in which certain enzymatic processes can be carried out, namely the cavity of the EPS. From a functional point of view, EPS can be divided into 3 departments:

rough or granular EPS. Represented by flattened membrane tanks, on which ribosomes are located.

intermediate EPS, also represented by flattened tanks, but they do not have ribosomes

smooth ER is represented by a network of branched anostomizing membrane tubules. There are no ribosomes on the membrane.

SHEPS functions.

The main function is associated with the synthesis and segregation of proteins. This is largely determined by the fact that the membrane contains special ribophorin proteins, with which most of the ribosomes are able to interact. That. EPS membrane can undergo elongation and termination of protein synthesis. In a number of cases, the ribosomes on which protein synthesis occurs in the hyaloplasm do not complete it and enter the so-called translational pause; then, with the help of special mooring proteins, such ribosomes attach to the sER membrane and leave the translational pause, completing protein synthesis. In addition to ribophorins, a special complex of integral proteins is formed on the sER membrane, which is called the translocation complex. It is involved in the transport of certain proteins through the sER membrane into its cavity. All proteins that are synthesized on ER ribosomes can be divided into two groups:

proteins that go to PAC and healoplasm

proteins that go into the cavity of the ER and which have a special peptide sequence at their end, it is recognized by the receptors of the translocation complex and is separated during the passage of the protein through the translocation complex.

The first stage of sigregation takes place on the sEPS membrane. In the sEPS cavity, proteins segregate into two streams:

proteins of the EPS itself, for example, ribophorins, proteins of the translocation complex, receptors, enzymes. These proteins have a special amino acid delay signal and are called resident proteins.

proteins that are excreted from the sER cavity into the intermediate ER do not have a delay signal and are still glycosylated in the sER cavity. Such proteins are called transit proteins.

On the inside, on the membrane of the intermediate EPS, there are receptors that recognize the hydrocarbon signaling part. Due to exocytosis, membrane vesicles are formed in the intermediate EPS, which contain glycosylated proteins and receptors that recognize them. These vesicles are sent to the Golgi complex.

In addition to the synthesis and segregation of proteins in sEPS, the final stages of the synthesis of some membrane lipids are carried out.

Functions of the intermediate EPS.

It consists in the budding of membrane vesicles with the help of clathrin-like proteins. These proteins greatly increase the rate of exocytosis.

Functions of smooth EPS.

there are enzymes on the HEPS membrane due to which almost all cellular lipids are synthesized. First of all, this applies to phospholipids and ceramide. In addition, smooth ER contains enzymes that are involved in the synthesis of cholesterol, which in turn is a precursor of steroid hormones. Cholesterol is mainly synthesized by hepatocytes, therefore, with various viral hepatitis, hypocholesteremia is observed. The result is anemia, as erythrocyte membranes are damaged. In some cells, such as the adrenal glands and gonads, steroid hormones are synthesized, and in the adrenal glands, female sex hormones are synthesized at the beginning, and then, based on them, male sex hormones.

deposition of calcium and regulation of Ca concentration in the hyaloplasm. This function is determined by the fact that there are Ca carriers on the membrane of the tubules of the HEPS, and Ca-binding proteins are located in the cavity of the HEPS. Due to active transport with the help of the Ca-th pump, it is pumped into the cavity of the ER and binds to proteins. With a decrease in the concentration of Ca in the cell, Ca is excreted by passive transport into the hyaloplasm. This function is especially developed in muscle cells, for example, in cardiomyocytes. Ca transport can be caused by the activation of the phospholipase system. The regulation of the Ca level in the cell is especially important under conditions of Ca overload. With an excess of Ca, Ca-dependent apoptosis is possible. Therefore, there is a protein in the ER membrane that prevents apoptosis.

detoxification. It is carried out mainly by liver cells, where drugs and various toxic substances from the intestines enter. In liver cells, toxic hydrophobic substances are converted into non-toxic hydrophobic substances using specific oxidoreductases.

smooth ER is involved in the metabolism of carbohydrates. This function is especially characteristic of liver cells, muscle cells, and intestinal cells. In these cells, the enzyme glucose-6-phosphatase is localized on the HEPS membrane, which is able to cleave the phosphate residue from glucose. Glucose can be excreted into the blood only after dephosphorylation; with hereditary defects in this enzyme, Gierke's disease is observed. This disease is characterized by the accumulation of excess glycogen in the liver and kidneys, as well as hypoglycemia. In addition, a large amount of lactic acid is formed, which leads to the development of acidosis.

GOLGI COMPLEX.

The universal function of the Golgi complex is that it is involved in:

formation of PAK components

formation of secretory granules

formation of lysosomes

in the Golgi complex, segregation of proteins is observed, which are transported here from the ER. (The proteins of the Golgi complex themselves are synthesized on ribosomes that are localized in the immediate vicinity of the complex. These proteins have a signal sequence and are transported into the cavity of the Golgi complex through the translocation complex.)

Membrane bubbles coming from the EPS merge with the rescue tank. The rescue tank performs the function of returning receptors and mooring proteins to the EPS. Proteins from the rescue cistern are transported to the adjacent cis cistern. Here, the segregation of proteins into two streams occurs. Some proteins are phosphorylated by a special enzyme called phosphoglycosydase, i.e. Phospholylation occurs at the carbohydrate moiety. After that, the proteins enter the medial section, where various chemical modifications occur: glycosylation, acetylation, sialylation, after which the proteins enter the trans section, where partial protein proteolysis is observed, further chemical modifications are possible, and then the proteins in the transdistribution section are segregated into three streams:

a constant or constitutive flow of proteins to PAK, due to which the components of the plasmolemma and glycocalyx are regenerated

flow of secretory granules. They can linger, either near the Golgi complex, or under the plasmalemma, this is the so-called induced exocytosis.

with the help of this flow, membrane vesicles with phosphorylated proteins are removed from the Golgi complex. This is the flow of so-called primary lysosomes, which then participate in the phage cycles of the cell. In addition, the synthesis of glycosaminoglycans occurs in the Golgi complex, many glycoproteins and glycolipids are synthesized, the final synthesis of sphingolipids occurs, and the condensation of dissolved substances occurs.

LYSOSOME.

These are universal organelles of the eukaryotic cell, which is represented by membrane vesicles with a diameter of 0.4 μm, which are involved in providing the cell with hydrolysis reactions. All lysosomes have a matrix consisting of mucopolysaccharides, to which inactive hydrolases are localized. Inhibition of hydrolases is carried out due to their glycosylation in EPS, due to phosphorylation in the Golgi complex, due to the fact that the pH of the matrix does not correspond to hydrolysis reactions. The functions of lysosomes are realized in two phage cycles:

autophagic cycle

heterophagic cycle

autophagic cycle.

With this loop, you can:

break down old cell components that have lost their functional activity (mitochondria). This ensures the physiological regeneration of the cell and the possibility of its existence much longer than any of its structures.

break down stored nutrients in the cell

break down excess secretory granules.

That. the autophagic cycle provides the cell with monomers that are necessary for the synthesis of new biopolymers characteristic of the cell. In some cases, when there is no exogenous nutrition of the cell, it becomes the only source of monomers; the cell switches to exogenous nutrition. With prolonged starvation, this leads to cell lysis. There are 2 types of autophagic cycle:

macroautophagy or typical autophagy. It begins with the formation of membrane vesicles, which enclose the old cell organelle. This vesicle is called an autophagosome. The primary lysosome, formed in the Golgi complex and containing inactive hydrolases, fuses with the autophagosome. The fusion process activates protol pumps or pumps on the membrane of the secondary lysosome. Protons are pumped into the lysosome, which leads to a Ph shift, the acid phosphatase enzyme is activated on the membrane, which cleaves the phosphate residue from hydrolases. Hydrolases become active and begin to split off complex molecules, and monomers enter the cytoplasm. Autophagosomes and primary lysosomes can fuse with the secondary lysosome until the hydrolases lose their activity and the secondary lysosomes become telolisosomes. Telolisosomes are either removed from the cell or accumulate in it.

microautophagy. In this case, the substances to be cleaved enter the primary lysosome not in the form of an autophagic vesicle, but directly through the lysosome membrane. In this case, phosphorylation of certain proteins of the primary lysosome is observed.

Pathologies. The causes of pathologies may be the destabilization of the membrane of the primary lysosome. There is a massive release of hydrolases into the cytoplasm and uncontrolled cleavage of cell components. Such a destabilizing agent is ionizing radiation, toxins of some fungi, vitamins A, D, E, intense physical activity, hyper- and hypothermia. Stress factors cause such an output of hydrolases, because. on the cells of the body begins to act by increasing the amount of adrenaline, which destabilizes the membrane. Variants of superstabilization of the lysosomal membrane are possible. In this case, lysosomes cannot enter the phage cycle. In case of violation of the structure of lysosome enzymes, various diseases are observed, which most often lead to the death of the body. If the proteins in the Golgi complex are not phosphorylated, then the hydrolases are found not in the primary lysosomes, but in the secretory streams that are excreted from the cell. One of the pathologies is Y-cell disease, characteristic of fibroblasts, connective tissue cells. There, lysosomes do not contain hydrolases. They are excreted into the blood plasma. Various substances accumulate in fibroblasts, which leads to the development of storage disease (Tay-Sachs syndrome). Neurons accumulate a large amount of complex carbohydrates - glycosides, and lysosomes occupy a very large volume. The child loses his emotionality, stops smiling, stops recognizing his parents, lags behind in psychomotor development, loses his sight and dies by the age of 4-5. Storage diseases can be associated with abnormal development of lysosomal enzymes, but are usually fatal. Variants of normal cell lysis during the autophagic cycle are possible. This mainly concerns cell lysis in different organisms during embryonic development. In humans, the membranes between the fingers undergo autolysis. In the tadpole, the tail undergoes autolysis. Insects with complete metamorphosis undergo autolysis to the greatest extent.

heterophagic cycle.

It consists in the breakdown of substances entering the cell from the external environment. Due to any of the types of endocytosis, a heterophagosome is formed, which is able to merge with the primary lysosome. The entire further heterophagic cycle is carried out in the same way as the autophagic one.

Functions of the heterophagic cycle.

Trophic in unicellular

Protective. Characteristic of neutrophils and macrophages.

There are variants of the heterophagic cycle, in which hydrolases are excreted from the cell into the external environment. For example, parietal digestion, acrosome reaction of the sperm. The modification hetephagic cycle is observed in bone fractures, in the places of fractures the inter-fragmentary gap is filled with cartilage tissue, then due to the activity of special osteoblast cells. Cartilage is destroyed and callus is formed. Pathologies of the heterophagic cycle are various immunodeficiencies.

PEROXISOMS.

This is a universal membrane cell organoid, with a diameter of approximately 0.15-0.25 nm. The main function of peroxisomes is the breakdown of long-radical fatty acids. Although in general they can perform other functions. Peroxisomes in a cell are formed only due to the division of maternal peroxisomes, therefore, if peroxisomes do not enter the cell for some reason, the cell dies due to the accumulation of fatty acids. The membrane of peroxisomes has a typical fluid-mosaic structure and can increase due to complex lipids and proteins carried here by special carrier proteins.

Functions.

Breakdown of fatty acids. Peroxisomes contain enzymes belonging to the group of oxidoreductase enzymes, which begin the breakdown of fatty acids from the elimination of acetic acid residues and form a double bond inside the fatty acid radical, and hydrogen peroxide is formed as a by-product. Peroxide is broken down by a special enzyme catalase to H 2 O and O 2. such a process of splitting fatty acids is called β-oxidation, it takes place not only in peroxisomes, but also in mitochondria. In mitochondria, short-radical acids are broken down. In any case, the cleavage proceeds with the formation of acetic acid or acetate residues. Acetate reacts with coenzymes A to form acetylCoA. This substance is a key metabolic product, to which all organic compounds are broken down. AcCoA can be used in energy metabolism and new fatty acids are formed on the basis of AcCoA. When β-oxidation of fatty acids is disturbed, Bowman-Zelweger Syndrome is observed. It is characterized by the absence of peroxisomes in the cells. Newborns are born with very low weight and with pathological development of some internal organs, such as the brain, liver, kidneys. They lag far behind in development, die early (up to 1 year), and a large number of long-radical acids are found in the cells.

Peroxisomes are involved in the detoxification of many harmful substances, such as alcohols, aldehydes, and acids. This function is characteristic of liver cells, and peroxisomes in the liver are larger. Detoxification of poisonous substances occurs due to their oxidation. For example, ethanol is oxidized to H 2 O and acetaldehyde. In peroxisomes, 50% ethanol is oxidized. The resulting acetaldehyde enters the mitochondria, where acetyl-CoA is formed from it. With chronic alcohol consumption, the amount of acetyl-CoA in hepatocytes increases dramatically. This leads to a decrease in β-oxidation of fatty acids and to the synthesis of new fatty acids. Consequently, fats begin to be synthesized, which are deposited in the liver cells and this leads to the occurrence of fatty degeneration of the liver (cirrhosis)

Peroxisomes are able to catalyze the oxidation of urates, because they contain the enzyme urate oxidase. However, in higher primates and humans, this enzyme is inactive, so a large amount of dissolved urate circulates in the blood. They are well filtered in the renal glomeruli and excreted in the secondary urine. The concentration of urates in the blood contributes to the development of certain diseases, for example, hereditary pathologies of purine metabolism lead to an increase in the concentration of urates tenfold. As a result, gout develops, which consists in the deposition of urate in the joints and some tissues, as well as the occurrence of urate stones in the kidneys.

The endoplasmic reticulum or EPS is a collection of membranes relatively evenly distributed throughout the cytoplasm of eukaryotic cells. EPS has a huge number of branches and is a complex structured system of relationships.

EPS is one of the components of the cell membrane. It itself includes channels, tubules and tanks, allowing you to distribute the internal space of the cell to certain areas, as well as significantly expand it. The entire place inside the cell is filled with a matrix - a dense synthesized substance, and each of its sections has a different chemical composition. Therefore, several chemical reactions can take place in the cell cavity at once, covering only a certain area, and not the entire system. Ends EPS perinuclear space.

Lipids and proteins are the main substances in the membrane of the endoplasmic reticulum. Often there are also various enzymes.

Types of EPS:

• Agranular (aPS) - in essence - a system of fastened tubules that does not contain ribosomes. The surface of such EPS, due to the absence of anything on it, is smooth.
• Granular (grES) - the same as the previous one, but it has ribosomes on the surface, due to which roughness is observed.

In some cases, this list includes the transient endoplasmic reticulum (tER). Its second name is passing. It is located at the junction of two types of networks.

Rough ES can be observed inside all living cells, excluding spermatozoa. However, in each organism it is developed to varying degrees.

For example, HRES is quite highly developed in plasma cells that produce immunoglobulins, in fibroblasts, collagen producers, and in glandular epithelial cells. The latter are found in the pancreas, where enzymes are synthesized, and in the liver, producing albumins.

Smooth ES is represented by cells of the adrenal glands, which are known to create hormones. It can also be found in the muscles, where calcium is exchanged, and in the fundic gastric glands, which secrete chlorine.

There are also two types of internal EPS membranes. The first is a system of tubules with numerous branches, they are saturated with a variety of enzymes. The second type - vesicles - small vesicles with their own membrane. They perform a transport function for synthesized substances.

EPS functions

First of all, the endoplasmic reticulum is a synthesizing system. But it is also no less involved in the transport of cytoplasmic compounds, which makes the entire cell capable of more complex functional features.

The above features of EPS are typical for any of its types. Thus, this organelle is a universal system.

General functions for granular and agranular network:

• Synthesizing - the production of membrane fats (lipids) with the help of enzymes. They allow EPS to reproduce independently.
• Structuring - organizing areas of the cytoplasm and preventing unwanted substances from entering it.
• Conductive - the occurrence of exciting impulses due to the reaction between the membranes.
• Transport - removal of substances even through the membrane walls.

In addition to the main features, each kind of endoplasmic reticulum has its own specific functions.

Functions of the smooth (agranular) endoplasmic reticulum

NPP, in addition to the features inherent in all types of EPS, has its own following functions:

• Detoksikatsionnaya - the elimination of toxins both inside and outside the cell.

Phenobarbital is destroyed in the cells of the kidneys, namely, in hepatocytes, due to the action of oxidase enzymes.

• Synthesizing - the production of hormones and cholesterol. The latter is excreted in several places at once: the gonads, kidneys, liver and adrenal glands. And in the intestines, fats (lipids) are synthesized, which enter the blood through the lymph.

AES promotes the synthesis of glycogen in the liver, due to the action of enzymes.

• Transport - sarcoplasmic reticulum, it is also a special EPS in striated muscles, serves as a storage place for calcium ions. And thanks to specialized calcium pumps, it throws calcium directly into the cytoplasm, from where it instantly sends it to the channel area. The muscle ER is engaged in this, due to a change in the amount of calcium by special mechanisms. They are found mainly in the cells of the heart, skeletal muscle, as well as in neurons and the egg.

Functions of the rough (granular) endoplasmic reticulum

As well as agranular, the power plant has functions peculiar only to itself:

• Transport - the movement of substances along the intramembrane section, for example, the produced proteins on the surface of the EPS pass into the Golgi complex, and then exit the cell.
• Synthesizing - everything is the same as before: the production of proteins. But it begins on free polysomes, and only after that the substances bind to the EPS.
• Thanks to the granular endoplasmic reticulum, literally all types of proteins are synthesized: secretory proteins that go inside the cell itself, specific in the internal phase of organelles, as well as all substances in the cell membrane, with the exception of mitochondria, chloroplasts and some types of proteins.
• The generatrix - the Golgi complex is being created, among other things, thanks to the hydroelectric power station.
• Modification - includes phosphorylation, sulfation and hydroxylation of proteins. A special enzyme glycosyltransferase ensures the process of glycosylation. Basically, it precedes the transport of substances to the exit from the cytoplasm or occurs before cell secretion.

It can be seen that the functions of GRES are mainly aimed at regulating the transport of proteins synthesized on the surface of the endoplasmic reticulum in ribosomes. They are converted into a tertiary structure, twisting, namely in the EPS.

The typical behavior of a protein is to enter the granular endoplasmic reticulum, then to the Golgi apparatus and, finally, to exit to other organelles. He can also be postponed as a spare. But often, in the process of moving, it is able to radically change the composition and appearance: to be phosphorylated, for example, or to be converted into a glycoprotein.

Both types of the endoplasmic reticulum contribute to the detoxification of liver cells, that is, the removal of toxic compounds from it.

EPS does not allow substances to pass through itself in all areas, due to which the number of connections in the tubules and outside them is different. The permeability of the outer membrane works on the same principle. This feature plays a certain role in the life of the cell.

In the cell cytoplasm of muscles, there are much fewer calcium ions than in its endoplasmic reticulum. The consequence of this is a successful muscle contraction, because it is calcium that provides this process when leaving the EPS channels.

Formation of the endoplasmic reticulum

The main components of EPS are proteins and lipids. The former are transported from membrane ribosomes, the latter are synthesized by the endoplasmic reticulum itself with the help of its enzymes. Since smooth ER (aPS) does not have ribosomes on the surface, and is not capable of synthesizing protein itself, it is formed when ribosomes are discarded by a granular-type network.

In the endoplasmic reticulum, many substances are produced, processed and transported, which are used by the cell or released from it. There are granular (granular, rough) and smooth endoplasmic reticulum (reticulum). The cisternae of the granular and smooth endoplasmic reticulum do not communicate. Cells specialized in protein production have a more developed granular endoplasmic reticulum. Cells that produce lipids and steroid hormones contain a pronounced smooth endoplasmic reticulum.

Functions of the endoplasmic reticulum:❖ supply of lipids to other organelles (smooth); ❖ Ca2+ homeostasis (smooth); ❖ organelle biogenesis (granular); ❖ formation of a spatial (three-dimensional) structure (folding) of proteins (granular); ❖ post-translational protein quality control (granular).

Granular endoplasmic reticulum

The granular endoplasmic reticulum is a system of flat membrane tanks with ribosomes located on their outer surface (see Fig. 2-22). In the rough endoplasmic reticulum, proteins are synthesized for the plasma membrane, lysosomes, peroxisomes, as well as the synthesis of proteins for export, i.e. intended for secretion. The membranes of the granular endoplasmic reticulum are associated with the outer membrane of the nucleus envelope and the perinuclear cisterna. The granular endoplasmic reticulum is located in close proximity to the nucleus and the Golgi complex. It is involved in the synthesis and processing of proteins, mainly intended for release from the cell. Ribosomes are connected with the outer (facing the cytosol) surface of the network with the help of ribophorins. Their number (for example, in a hepatocyte) reaches 13 million. Proteins assembled on ribosomes enter the tank for subsequent processing. The protein concentration here can exceed 100 mg/ml. This is where proteins are laid and the correct three-dimensional structure is formed. In the cisterns of the network, carbohydrates are attached to proteins to form glycoproteins, and protein complexes with metals are also formed. From the endoplasmic reticulum, many proteins enter all compartments of the cell to perform their functions or are sent to the Golgi complex for subsequent modification. Resident proteins and chaperones. Along with the proteins leaving the network, there are resident proteins that are constantly present in the lumen of the cisterns and are needed to maintain the function of the network, namely, to recognize the proteins formed here, process them, and hold them for the necessary time before sending them to the desired address. An example of a resident protein is the BiP protein, an immunoglobulin-binding protein chaperone belonging to the Hsp70 family of heat shock proteins. Chaperones are involved in protein quality control. In the protein matrix of the endoplasmic reticulum, chaperones prevent protein aggregation and enable efficient folding.

Smooth endoplasmic reticulum

The smooth reticulum (smooth ER) - a system of anastomosing membrane channels, vesicles and tubules - does not contain ribophorins and for this reason is not associated with ribosomes.

The functions of the smooth endoplasmic reticulum are diverse: the synthesis of lipids and steroid hormones, detoxification and deposition of calcium ions.

Detoxification. One of the most important functions of a smooth ER is detoxification (with the help of hepatocyte oxidases) of both cellular metabolism products and substances coming from outside, including ethanol and barbiturates. With the participation of the smooth ER, substances are converted into water-soluble compounds, which contributes to their excretion from the body. For effective detoxification, a smooth ER can double its total surface area within a few days.

Synthesis of steroid hormones. In steroid-producing cells (adrenal cortex, gonads), the smooth ER serves to metabolize steroids and form (with the participation of mitochondria) the final forms of steroid hormones.

depot calcium. The cisterns of the smooth endoplasmic reticulum of many cells are specialized for the accumulation of Ca2+ in them by constantly pumping Ca2+ from the cytoplasm, where the Ca2+ content normally does not exceed 10-7 M. Similar depots exist in skeletal and cardiac muscles, neurons, chromaffin cells, the egg, endocrine cells, etc. .d. Various signals (eg, hormones, neurotransmitters, growth factors) affect cell function by changing the concentration of intracellular mediator Ca2+ in the cytosol. For example, the condition for contraction of muscle elements is a sharp increase in the concentration of Ca2+ in the cytosol. To do this, it is necessary to constantly pump out calcium ions from the cytosol and accumulate them in special depots formed by Ca2+-storing cisterns of the smooth endoplasmic reticulum. Inside the cisterns are Ca2+-binding proteins. Ca2+ pumps (Ca2+-ATPase) are built into the membrane of the tanks - the Ca2+ depot, constantly pumping Ca2+ into the tanks, and Ca2+ channels through which Ca2+ is released from the depot when a signal is received.

The endoplasmic reticulum is one of the most important organelles in the eukaryotic cell. Its second name is the endoplasmic reticulum. There are two types of EPS: smooth (agranular) and rough (granular). The more active the metabolism in the cell, the greater the amount of EPS there.

Structure

It is a vast labyrinth of channels, cavities, vesicles, "cisterns" that are closely connected and communicate with each other. This organelle is covered by a membrane that communicates with both the cytoplasm and the cell outer membrane. The volume of the cavities is different, but they all contain a homogeneous liquid, which allows interaction between the cell nucleus and the external environment. Sometimes there are branches from the main network in the form of single bubbles. Rough ER differs from smooth ER by the presence of a large number of ribosomes on the outer surface of the membrane.

Functions

• Functions of agranular EPS. It takes part in the formation of steroid hormones (for example, in the cells of the adrenal cortex). EPS, contained in liver cells, is involved in the destruction of certain hormones, drugs and harmful substances, and in the conversion of glucose, which is formed from glycogen. Also, the agranular network produces phospholipids necessary for the construction of membranes of all cell types. And in the reticulum of muscle tissue cells, calcium ions are deposited, which are necessary for muscle contraction. This type of smooth endoplasmic reticulum is also called the sarcoplasmic reticulum.
• Functions of granular EPS. First of all, in the granular reticulum, the production of proteins occurs, which will subsequently be removed from the cell (for example, the synthesis of secretion products of glandular cells). And also in the rough ER, synthesis and assembly of phospholipids and multi-chain proteins take place, which are then transported to the Golgi apparatus.
• Common functions, both for the smooth endoplasmic reticulum and for the rough one, are the delimiting function. Due to these organelles, the cell is divided into compartments (compartments). And additionally, these organelles are transporters of substances from one part of the cell to another.

Endoplasmic reticulum (ER) or endoplasmic reticulum (ER) discovered only with the advent of the electron microscope. EPS is found only in eukaryotic cells and is a complex system of membranes that form flattened cavities and tubules. All together it looks like a network. EPS refers to single-membrane cell organelles.

EPS membranes extend from the outer membrane of the nucleus and are similar in structure to it.

The endoplasmic reticulum is divided into smooth (agranular) and rough (granular). The latter is dotted with ribosomes attached to it (because of this, “roughness” arises). The main function of both types is associated with the synthesis and transport of substances. Only the rough one is responsible for protein, and the smooth one is responsible for carbohydrates and fats.

According to its structure, EPS is a set of paired parallel membranes penetrating almost the entire cytoplasm. A pair of membranes forms a plate (the cavity inside has a different width and height), however, the smooth endoplasmic reticulum has a tubular structure to a greater extent. Such flattened membrane sacs are called EPS tanks.

Ribosomes located on the rough ER synthesize proteins that enter the ER channels., mature (acquire a tertiary structure) there and are transported. These proteins first synthesize a signal sequence (consisting mainly of non-polar amino acids), the configuration of which corresponds to the specific EPS receptor. As a result, the ribosome and the endoplasmic reticulum bind. In this case, the receptor forms a channel for the transition of the synthesized protein into the EPS tanks.

After the protein is in the channel of the endoplasmic reticulum, the signal sequence is separated from it. After that, it folds into its tertiary structure. When transported along the ER, the protein acquires a number of other changes (phosphorylation, bond formation with a carbohydrate, i.e., transformation into a glycoprotein).

Most of the proteins trapped in the rough ER then enter the Golgi apparatus (complex). From there, proteins are either secreted from the cell, or enter other organelles (usually lysosomes), or are deposited as granules of reserve substances.

It should be kept in mind that not all cell proteins are synthesized on the rough ER. A part (usually smaller) is synthesized by free ribosomes in the hyaloplasm, such proteins are used by the cell itself. Their signal sequence is not synthesized as unnecessary.

The main function of the smooth endoplasmic reticulum is lipid synthesis.(fats). For example, ER of the intestinal epithelium synthesizes them from fatty acids and glycerol absorbed from the intestine. The lipids then enter the Golgi complex. In addition to intestinal cells, smooth ER is well developed in cells that secrete steroid hormones (steroids are lipids). For example, in the cells of the adrenal glands, interstitial cells of the testes.

The synthesis and transport of proteins, fats and carbohydrates are not the only functions of the ER. In baking, the endoplasmic reticulum is involved in detoxification processes. A special form of smooth EPS - the sarcoplasmic reticulum - is present in muscle cells and provides contraction due to the pumping of calcium ions.

The structure, volume and functionality of the endoplasmic reticulum of the cell is not constant throughout the cell cycle, but is subject to certain changes.