Schematic representation of the cell nucleus, endoplasmic reticulum and Golgi complex.
(1) Cell nucleus.
(2) The pores of the nuclear membrane.
(3) Granular endoplasmic reticulum.
(4) Agranular endoplasmic reticulum.
(5) Ribosomes on the surface of the granular endoplasmic reticulum.
(6) Macromolecules
(7) Transport vesicles.
(8) Golgi complex.
(9) Tsis-Golgi
(10) Trans-Golgi
(11) Golgi Cisterns

Discovery history

For the first time, the endoplasmic reticulum was discovered by the American scientist K. Porter in 1945 by means of electron microscopy.

Structure

The endoplasmic reticulum consists of an extensive network of tubules and pockets surrounded by a membrane. The area of ​​the membranes of the endoplasmic reticulum is more than half of the total area of ​​all cell membranes.

The EPR membrane is morphologically identical to the membrane of the cell nucleus and forms one whole with it. Thus, the cavities of the endoplasmic reticulum open into the intermembrane cavity of the nuclear envelope. EPS membranes provide active transport of a number of elements against the concentration gradient. The filaments that form the endoplasmic reticulum have a diameter of 0.05-0.1 microns (sometimes up to 0.3 microns), the thickness of the bilayer membranes forming the wall of the tubules is about 50 angstroms (5 nm, 0.005 microns). These structures contain unsaturated phospholipids, as well as some cholesterol and sphingolipids. They also contain proteins.

The tubes, the diameter of which ranges from 0.1 to 0.3 microns, are filled with homogeneous contents. Their function is to carry out communication between the contents of the EPS bubbles, external environment and the cell nucleus.

The endoplasmic reticulum is not a stable structure and is subject to frequent changes.

There are two types of EPR:

• granular (rough) endoplasmic reticulum;
• agranular (smooth) endoplasmic reticulum.

On the surface of the granular endoplasmic reticulum is a large number of ribosomes that are absent on the surface of agranular EPR.

Granular and agranular endoplasmic reticulum perform different functions in the cell.

Functions of the endoplasmic reticulum

With the participation of the endoplasmic reticulum, translation and transport of proteins, synthesis and transport of lipids and steroids occur. EPR is also characterized by the accumulation of synthesis products. The endoplasmic reticulum also takes part in the creation of a new nuclear envelope (for example, after mitosis). The endoplasmic reticulum contains an intracellular calcium store, which is, in particular, a mediator of muscle cell contraction. A special form of the endoplasmic reticulum is located in the cells of muscle fibers - sarcoplasmic reticulum.

Functions of the agranular endoplasmic reticulum

The agranular endoplasmic reticulum is involved in many metabolic processes. Also, the agranular endoplasmic reticulum plays important role in carbohydrate metabolism, neutralization of poisons and storage of calcium. Agranular endoplasmic reticulum enzymes are involved in the synthesis of various lipids and phospholipids, fatty acids and steroids. In particular, in this regard, agranular endoplasmic reticulum predominates in the cells of the adrenal glands and liver.

Synthesis of hormones

The hormones that are formed in the agranular EPS include, for example, the sex hormones of vertebrates and the steroid hormones of the adrenal glands. The cells of the testes and ovaries, which are responsible for the synthesis of hormones, contain a large amount of agranular endoplasmic reticulum.

Accumulation and conversion of carbohydrates

Carbohydrates in the body are stored in the liver as glycogen. Through glycogenolysis, glycogen in the liver is transformed into glucose, which is the most important process in maintaining blood glucose levels. One of the enzymes of agranular EPR cleaves a phospho group from the first product of glycogenolysis, glucose-6-phosphate, thus allowing glucose to leave the cell and increase blood sugar levels.

Neutralization of poisons

The smooth endoplasmic reticulum of liver cells takes an active part in neutralizing all kinds of poisons. Enzymes of smooth EPR attach hydrophilic radicals to the molecules of toxic substances, as a result of which the solubility of toxic substances in the blood and urine increases, and they are quickly eliminated from the body. In the case of a continuous intake of poisons, medicines or alcohol, a larger amount of agranular EPR is formed, which increases the dose of the active substance required to achieve the previous effect.

Role of EPS as a calcium depot

The concentration of calcium ions in EPS can reach 10–3 mol, while in the cytosol it is about 10–7 mol (at rest). Under the influence of inositol triphosphate and some other stimuli, calcium is released from the EPS through facilitated diffusion. The return of calcium to EPS is provided by active transport. In this case, the EPS membrane provides active transport of calcium ions against concentration gradients of large orders. Both the intake and release of calcium ions in the EPS is in a delicate relationship with physiological conditions.

The concentration of calcium ions in the cytosol affects many intracellular and intercellular processes, such as activation or inactivation of enzymes, gene expression, synaptic plasticity of neurons, contraction of muscle cells, and release of antibodies from cells of the immune system.

Sarcoplasmic reticulum

A special form of agranular endoplasmic reticulum, sarcoplasmic reticulum, is EPS in muscle cells, in which calcium ions are actively pumped from

Among the organelles of the cell, the most diverse are single-membrane organelles. It is surrounded by membranes of cytoplasm compartments in the form of vesicles, tubules, sacs. One membrane organelle includes the endoplasmic reticulum, the Golgi complex, lysosomes, vacuoles, peroxisomes, and the like. In general, they can occupy up to 17% of the cell volume. Single-membrane organelles form a system of synthesis, segregation (separation) and intracellular transport of macromolecules.

Endoplasmic reticulum , or endoplasmic reticulum (from lat. Reticulum - mesh) - single-membrane organelles of eukaryotic cells in the form closed system tubules and flat membrane sacs-tanks. The EPS was first discovered by the American scientist K. Porter in 1945 using an electron microscope. EPS is an organelle that divides the cytoplasm into compartments and is associated with the plasmalemma and nuclear membranes. With the participation of EPS, the nuclear envelope is formed in the period between cell divisions.

Structure . EPS form cisterns, tubular membrane tubules, membrane vesicles-vesicles(transport substances synthesized) and inner substance - matrix with big amount enzymes. The reticulum contains proteins and lipids, among which there are many phospholipids, as well as enzymes for the synthesis of lipids and carbohydrates. The membranes of the EPS, like the components of the cytoskeleton, are polar: at one end they grow, and at the other end they disintegrate into separate fragments... There are two types of endoplasmic reticulum: rough (granular) and smooth (agrarian). Rough EPS has ribosomes that form complexes with mRNA (polyriboss, or polysomes) and is present in all living eukaryotic cells (except for spermatozoa and mature erythrocytes), but the degree of its development is different and depends on the specialization of cells. So, glandular cells of the pancreas, hepatocytes, fibroblasts (cells of connective tissue that produce collagen protein), plasma cells (produce immunoglobulins) have a highly developed rough EPS. Smooth EPS has no ribosomes and is a derivative of rough. It predominates in the cells of the adrenal glands (carries out the synthesis of steroid hormones), in muscle cells (participates in calcium metabolism), the cells of the main glands of the stomach (participates in the release of hydrochloric acid).

Functions . Smooth and rough EPS perform joint functions: 1) delimiting - provides an ordered distribution of the cytoplasm; 2) transport - the necessary substances are transferred in the cell; 3) synthesizing - formation of membrane lipids. In addition, each of the types of EPS performs its own special functions.

EPS structure 1 - free ribosomes; 2 - EPS cavities; C - ribosomes on EPS membranes; 4 - smooth EPS

Types and functions of EPS

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 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 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 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 of the endoplasmic reticulum

Definition 1

Endoplasmic reticulum(EPS, endoplasmic reticulum) is a complex ultramicroscopic, very ramified, interconnected system of membranes, which 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, canals and tubules. Due to this structure, the endoplasmic reticulum significantly increases the area inner surface cells and divides the cell into sections. Inside she is filled matrix(moderately dense loose material (synthesis product)). Content of various chemical substances in the sections is not the same, therefore in the cell, both simultaneously and in a certain sequence, different chemical reactions in an insignificant cell volume. The endoplasmic reticulum opens in perinuclear space(the cavity between the two membranes of the karyolem).

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

There are two types of endoplasmic reticulum:

• Smooth (agranular, nuclear power plant), represented by tubules that anastamose with each other and do not have ribosomes on the surface;
• Grungy (granular, GRES), which also consists of interconnected tanks, but they are covered with ribosomes.

Remark 1

Sometimes they also allocate carryover, or transitory(TPP) endoplasmic reticulum, which is located in the area of ​​transition from one type of ES to another.

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

A highly developed hydroelectric power station of epithelial glandular cells (pancreas, producing digestive enzymes, liver - synthesizing serum albumin), fibroblasts (connective tissue cells that produce collagen protein), plasma cells (production of immunoglobulins).

Agranular ES predominates in the cells of the adrenal glands (synthesis of steroid hormones), in muscle cells (calcium metabolism), and in the cells of the fundic glands of the stomach (secretion of chlorine ions).

Another type of EPS membrane is branched membrane tubules containing a large number of specific enzymes inside, and vesicles are small vesicles surrounded by a membrane, mainly located next to 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 distributed in an orderly manner and does not mix, and also prevents random substances from entering the organelle;
• transmembrane transport, due to which the necessary substances are transported 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 arising 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 characteristic only for it:

• calcium depot... In many cells (in skeletal muscles, in the 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

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

organelle enzymes take part in glycogen synthesis(in liver cells).

Functions of the rough (granular) endoplasmic reticulum

For the granular endoplasmic reticulum, in addition to the listed general functions, special ones are also inherent:

• protein synthesis at the hydroelectric power station has some peculiarities. It begins with free polysomes, which are later linked to the ES Mebrans.
• 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 inner phase of membrane organelles, as well as secretory proteins that are transported through the cell and enter the extracellular space.
• post-translational protein modification: hydroxylation, sulfation, phosphorylation. An important process is glycosylation, which occurs under the action of a membrane bound enzyme, glycosyltransferase. Glycosylation occurs before the secretion or transport of substances to certain parts of the cell (Golgi complex, lysosomes, or plasmolemma).
• transport of substances along the intramembrane part of the network. The synthesized proteins move along the ES intervals to the Golgi complex, which removes substances from the cell.
• due to the participation 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. The synthesized proteins enter the EPS, twist and acquire a tertiary structure.

The protein that is transported to the cisterns changes significantly along the way. It can, for example, be phosphorylated or converted to a glycoprotein. The usual path for a protein is through the granular EPS into the Golgi apparatus, from where it either leaves the cell, or enters other organelles of the same cell, for example, lysosomes), or is deposited as spare 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 channels of the reticulum is not the same. This is important 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.

Endoplasmic reticulum formation

The lipid components of the membranes of the endoplasmic reticulum are synthesized by the enzymes of the reticulum itself, the protein component comes from the ribosomes located on its membranes. In the smooth (agranular) endoplasmic reticulum, there are no own factors of protein synthesis, therefore it is believed that this organelle is formed as a result of the loss of ribosomes by the granular endoplasmic reticulum.

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

The EPS membranes extend from the outer membrane of the nucleus and are structurally similar 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" occurs). 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.

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

Ribosomes located on the rough EPS synthesize proteins that enter the EPS 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 a specific EPS receptor. As a result, the ribosome and the endoplasmic reticulum are linked. In this case, the receptor forms a channel for the passage of the synthesized protein into the EPS cisterns.

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 EPS, the protein acquires a number of other changes (phosphorylation, the formation of a bond with a carbohydrate, i.e., conversion into a glycoprotein).

Most of the proteins trapped in the rough EPS, 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 storage granules.

It should be borne in mind that not all cell proteins are synthesized on rough EPS. Part (usually a smaller one) is synthesized by free ribosomes in the hyaloplasm, such proteins are used by the cell itself. Their signal sequence is not synthesized because it is unnecessary.

The main function of the smooth endoplasmic reticulum is lipid synthesis(fat). For example, EPS of intestinal epithelium synthesizes them from fatty acids and glycerol absorbed from the intestine. Then lipids enter the Golgi complex. In addition to intestinal cells, smooth EPS 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 EPS. In baking, the endoplasmic reticulum is involved in detoxification processes. A special form of smooth EPS - 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 a cell are not constant throughout the cell cycle, but are subject to various changes.