Capture and absorption of large particles by the cell. Non-cellular structures

Vesicular transfer can be divided into two types: exocytosis - the removal of macromolecular products from the cell, and endocytosis - the absorption of macromolecules by the cell.

During endocytosis, a certain section of the plasmalemma captures, as if enveloping the extracellular material, enclosing it in a membrane vacuole that has arisen due to the invagination of the plasma membrane. Any biopolymers, macromolecular complexes, parts of cells, or even whole cells can enter such a primary vacuole, or endosome, where they then decompose, depolymerize to monomers, which enter the hyaloplasm by transmembrane transfer.

The main biological significance of endocytosis is the acquisition of building blocks through intracellular digestion, which is carried out at the second stage of endocytosis after the fusion of the primary endosome with a lysosome, a vacuole containing a set of hydrolytic enzymes.

Endocytosis is formally divided into pinocytosis and phagocytosis.

Phagocytosis - the capture and absorption by a cell of large particles (sometimes even cells or their parts) - was first described by I.I. Mechnikov. Phagocytosis, the ability to capture large particles by a cell, is found among animal cells, both unicellular (for example, amoeba, some predatory ciliates) and specialized cells of multicellular animals. Specialized cells, phagocytes

characteristic of both invertebrates (amoebocytes of blood or cavity fluid) and vertebrates (neutrophils and macrophages). As well as pinocytosis, phagocytosis can be non-specific (for example, the absorption of particles of colloidal gold or dextran polymer by fibroblasts or macrophages) and specific, mediated by receptors on the surface of the plasma membrane

phagocytic cells. When phagocytosis occurs, the formation of large endocytic vacuoles - phagosomes, which then merge with lysosomes to form phagolysosomes.

Pinocytosis was originally defined as the absorption of water or aqueous solutions of various substances by the cell. It is now known that both phagocytosis and pinocytosis proceed very similarly, and therefore the use of these terms can only reflect differences in the volumes and mass of absorbed substances. What these processes have in common is that the absorbed substances on the surface of the plasma membrane are surrounded by a membrane in the form of a vacuole - an endosome, which moves inside the cell.

Endocytosis, including pinocytosis and phagocytosis, can be non-specific or constitutive, permanent and specific, mediated by receptors (receptor). Nonspecific endocytosis

(pinocytosis and phagocytosis), so called because it proceeds as if automatically and can often lead to the capture and absorption of substances that are completely alien or indifferent to the cell, for example,

particles of soot or dyes.

surface and extending deep into the cytoplasm. Both nonspecific and receptor endocytosis, leading to the cleavage of membrane vesicles, occurs in specialized regions of the plasma membrane. These are the so-called bordered pits. They are called so because

On the sides of the cytoplasm, the plasma membrane is covered, clothed, with a thin (about 20 nm) fibrous layer, which, on ultrathin sections, borders and covers small protrusions and pits. These holes are

in almost all animal cells, they occupy about 2% of the cell surface. The border layer consists mainly of the clathrin protein associated with a number of additional proteins.

These proteins bind to integral receptor proteins from the side of the cytoplasm and form a dressing layer along the perimeter of the emerging pinosome.

After the bordered vesicle separates from the plasmolemma and begins to move deep into the cytoplasm, the clathrin layer disintegrates, dissociates, and the endosome membrane (pinosomes) acquires its usual form. After the loss of the clathrin layer, the endosomes begin to fuse with each other.

Receptor-mediated endocytosis. The effectiveness of endocytosis increases significantly if it is mediated by membrane receptors that bind to the molecules of the absorbed substance or molecules located on the surface of the phagocytosed object - ligands (from Latin u^age - to bind). Later (after absorption of the substance), the receptor-ligand complex is cleaved, and the receptors can again return to the plasmalemma. An example of a receptor-mediated interaction is phagocytosis by a bacterial leukocyte.

Transcytosis(from lat. 1gash - through, through and Greek suUz - cell) a process characteristic of some types of cells, combining signs of endocytosis and exocytosis. An endocytic vesicle is formed on one cell surface, which is transferred to the opposite cell surface and, becoming an exocytic vesicle, releases its contents into the extracellular space.

Exocytosis

The plasma membrane takes part in the removal of substances from the cell using exocytosis, a process that is the reverse of endocytosis.

Exocytosis is associated with the release of various substances synthesized in the cell. Secreting, releasing substances into the external environment, cells can produce and release low molecular weight compounds (acetylcholine, biogenic amines, etc.), as well as in most cases macromolecules (peptides, proteins, lipoproteins, peptidoglycans, etc.). Exocytosis or secretion in most cases occurs in response to an external signal (nerve impulse, hormones, mediators, etc.). Although in some cases exocytosis occurs constantly (secretion of fibronectin and collagen by fibroblasts).

41 .Endoplasmic reticulum (reticulum).

In a light microscope in fibriblasts after fixation and staining, it can be seen that the periphery of the cells (ectoplasm) stains weakly, while the central part of the cells (endoplasm) perceives dyes well. So K. Porter in 1945 saw in an electron microscope that the endoplasmic zone is filled with a large number of small vacuoles and channels that connect with each other and form something like a loose network (reticulum). It was seen that the stacks of these vacuoles and tubules were limited by thin membranes. So it was discovered endoplasmic reticulum, or endoplasmic reticulum. Later, in the 1950s, using the method of ultrathin sections, it was possible to elucidate the structure of this formation and to detect its heterogeneity. The most important thing turned out to be that the endoplasmic reticulum (ER) is found in almost all eukaryotes.

Such an electron microscopic analysis made it possible to distinguish two types of ER: granular (rough) and smooth.

1. Hooke discovered the existence of cells 2. The existence of unicellular organisms discovered Leeuwenhoek

4. Cells containing a nucleus are called eukaryotes

5. The structural components of a eukaryotic cell include the nucleus, ribosomes, plastids, mitochondria, the golgi complex, the endoplasmic reticulum

6. The intracellular structure in which the main hereditary information is stored is called the nucleus

7. The nucleus consists of a nuclear matrix and 2 membranes

8. The number of nuclei in one cell is usually 1

9. Compact intranuclear structure called chromatin

10. The biological membrane that covers the entire cell is called the cytoplasmic membrane

11. The basis of all biological membranes is polysaccharides

12. Biological membranes must contain proteins

13. A thin layer of carbohydrates on the outer surface of the plasma membrane is called the glycocalyx

14. The main property of biological membranes is their selective permeability

15. Plant cells are protected by a membrane, which consists of cellulose

16. Absorption of large particles by a cell is called phagocytosis.

17. The absorption of liquid droplets by the cell is called pinocytosis.

18. Part of a living cell without a plasma membrane and a nucleus is called cytoplasm 19. The composition of the cytoplasm includes the protoplast and the nucleus

20. The main substance of the cytoplasm, soluble in water, is called glucose.

21. Part of the cytoplasm, represented by support-contractile structures (complexes), is called vacuoles

22. Intracellular structures that are not its mandatory components are called inclusions

23. Non-membrane organelles that provide the biosynthesis of proteins with a genetically determined structure are called ribosomes.

24. A complete ribosome consists of 2 subunits

25. The composition of the ribosome includes ... .

26. The main function of ribosomes is protein synthesis

27. Complexes of one molecule of mRNA (mRNA) and dozens of ribosomes associated with it are called ....

28. The basis of the cell center is microtubules

29. A single centriole is ... .

30. Organelles of movement include flagella, cilia

31. The system of tanks and tubules interconnected into a single intracellular space, delimited from the rest of the cytoplasm by a closed intracellular membrane, is called EPS

32. The main function of EPS is the synthesis of organic substances.

33. Ribosomes are located on the surface of the rough ER

34. Part of the endoplasmic reticulum, on the surface of which ribosomes are located, is called rough EPS
35. The main function of the granular ER is the synthesis of proteins.

36. Part of the endoplasmic reticulum, on the surface of which there are no ribosomes, is called smooth eps

37. Synthesis of sugars and lipids occurs in the cavity of the agranular ER

38. The system of flattened single-membrane cisterns is called the Golgi complex

39. Accumulation of substances, their modification and sorting, packaging of final products into single-membrane vesicles, excretion of secretory vacuoles outside the cell and formation of primary lysosomes - these are the functions of the Golgi complex

40. Single-membrane vesicles containing hydrolytic enzymes are called the Goljilysosome complex.

41. Liquid-filled large single-membrane cavities are called vacuoles.

42. The content of vacuoles is called cell sap

43. Two-membrane organelles (which include outer and inner membranes) include plastids and mitochondria

44. Organelles that contain their own DNA, all types of RNA, ribosomes and are able to synthesize some proteins are plastids and mitochondria.
45. The main function of mitochondria is to obtain energy in the process of cellular respiration.

46. The main substance that is the source of energy in the cell is ATP

Large molecules of biopolymers are practically not transported through membranes, and yet they can get inside the cell as a result of endocytosis. It is divided into phagocytosis and pinocytosis. These processes are associated with vigorous activity and mobility of the cytoplasm. Phagocytosis is the capture and absorption of large particles by a cell (sometimes even whole cells and their parts). Phagocytosis and pinocytosis proceed very similarly, therefore these concepts reflect only the difference in the volumes of absorbed substances. What they have in common is that the absorbed substances on the cell surface are surrounded by a membrane in the form of a vacuole, which moves inside the cell (either a phagocytic or pinocytic vesicle. These processes are associated with energy expenditure; the cessation of ATP synthesis completely inhibits them. , for example, the walls of the intestines, numerous microvilli, significantly increasing the surface through which absorption occurs. The plasma membrane is also involved in the removal of substances from the cell, this occurs in the process exocytosis. This is how hormones, polysaccharides, proteins, fat droplets and other cell products are excreted. They are enclosed in membrane-bound vesicles and approach the plasmalemma. Both membranes fuse and the contents of the vesicle are released into the environment surrounding the cell.

Cells are also able to absorb macromolecules and particles using a similar exocytosis mechanism, but in reverse order. Absorbed matter is gradually surrounded by a small area plasma membrane, which first invaginates and then splits off, forming intracellular vesicle containing material captured by the cell. This process of formation of intracellular vesicles around the material absorbed by the cell is called endocytosis.

Depending on the size of the vesicles formed, two types of endocytosis are distinguished:

1) pinocytosis- absorption of liquid and solutes through small bubbles, and

2) phagocytosis- absorption of large particles such as microorganisms or cell debris. In this case, large bubbles are formed, called vacuoles and absorption of corpuscular material: bacteria, large viruses, dying own cells of the body or foreign cells, such as, for example, erythrocytes of various types, is carried out by cells ( macrophages ,neutrophils)

Fluid and solutes are continuously taken up by most cells through pinocytosis, while large particles are taken up mainly by specialized cells - phagocytes. Therefore, the terms "pinocytosis" and "endocytosis" are usually used in the same sense.

pinocytosis characterized by the absorption and intracellular destruction of macromolecular compounds, such as proteins and protein complexes, nucleic acids, polysaccharides, lipoproteins. The object of pinocytosis as a factor of nonspecific immune defense are, in particular, the toxins of microorganisms. Adhesion of substances on the cell surface leads to local invagination (invagination) of the membrane, culminating in the formation of a very small pinocytic vesicle (approximately 0.1 micron). Several merged bubbles form a larger formation - pinosome. In the next step, the pinosomes fuse with lysosomes containing hydrolytic enzymes that break down polymer molecules into monomers. In those cases when the process of pinocytosis is realized through the receptor apparatus, in pinosomes, before merging with lysosomes, the detachment of captured molecules from receptors is observed, which, as part of the daughter vesicles, return to the cell surface.

Macromolecules such as proteins, nucleic acids, polysaccharides, lipoprotein complexes and others do not pass through cell membranes, in contrast to how ions and monomers are transported. The transport of micromolecules, their complexes, particles into and out of the cell occurs in a completely different way - through vesicular transfer. This term means that various macromolecules, biopolymers, or their complexes cannot enter the cell through the plasma membrane. And not only through it: any cell membranes are not capable of transmembrane transfer of biopolymers, with the exception of membranes that have special protein complex carriers - porins (membranes of mitochondria, plastids, peroxisomes). In a cell or from one membrane compartment to another, macromolecules enter enclosed within vacuoles or vesicles. Such vesicular transfer can be divided into two types: exocytosis- removal of macromolecular products from the cell, and endocytosis- absorption of macromolecules by the cell (Fig. 133).

During endocytosis, a certain section of the plasmalemma captures, as if enveloping the extracellular material, enclosing it in a membrane vacuole that has arisen due to the invagination of the plasma membrane. In such a primary vacuole, or in endosome, any biopolymers, macromolecular complexes, parts of cells or even whole cells can enter, where they then decompose, depolymerize to monomers, which enter the hyaloplasm by transmembrane transfer. The main biological significance of endocytosis is the acquisition of building blocks through intracellular digestion, which is carried out at the second stage of endocytosis after the fusion of the primary endosome with the lysosome, a vacuole containing a set of hydrolytic enzymes (see below).

Endocytosis is formally divided into pinocytosis and phagocytosis(Fig. 134). Phagocytosis - the capture and absorption of large particles by a cell (sometimes even cells or their parts) - was first described by I.I. Mechnikov. Phagocytosis, the ability to capture large particles by a cell, is found among animal cells, both unicellular (for example, amoeba, some predatory ciliates) and specialized cells of multicellular animals. Specialized cells, phagocytes, are characteristic of both invertebrates (amoebocytes of blood or cavity fluid) and vertebrates (neutrophils and macrophages). Pinocytosis was originally defined as the absorption of water or aqueous solutions of various substances by the cell. It is now known that both phagocytosis and pinocytosis proceed very similarly, and therefore the use of these terms can only reflect differences in the volumes and mass of absorbed substances. What these processes have in common is that the absorbed substances on the surface of the plasma membrane are surrounded by a membrane in the form of a vacuole - an endosome, which moves inside the cell.

Endocytosis, including pinocytosis and phagocytosis, can be non-specific or constitutive, permanent and specific, mediated by receptors (receptor). Nonspecific endocyto h (pinocytosis and phagocytosis), so called because it proceeds as if automatically and can often lead to the capture and absorption of substances completely alien or indifferent to the cell, for example, particles of soot or dyes.

Nonspecific endocytosis is often accompanied by initial sorption of the entrapping material by the plasma membrane glycocalyx. The glycocalyx, due to the acidic groups of its polysaccharides, has a negative charge and binds well to various positively charged groups of proteins. With such adsorption nonspecific endocytosis, macromolecules and small particles (acidic proteins, ferritin, antibodies, virions, colloidal particles) are absorbed. Liquid-phase pinocytosis leads to the absorption together with the liquid medium of soluble molecules that do not bind to the plasmalemma.

At the next stage, a change in the morphology of the cell surface occurs: it is either the appearance of small invaginations of the plasma membrane, invagination, or it is the appearance on the cell surface of outgrowths, folds or “frills” (rafl - in English), which, as it were, overlap, fold, separating small volumes of the liquid medium (Fig. 135, 136). The first type of occurrence of a pinocytic vesicle, pinosomes, is characteristic of cells of the intestinal epithelium, endothelium, amoeba, the second - for phagocytes and fibroblasts. These processes depend on the supply of energy: respiration inhibitors block these processes.

Following this rearrangement of the surface, the process of adhesion and fusion of the contacting membranes follows, which leads to the formation of a penicytic vesicle (pinosome), which detaches from the cell surface and goes deep into the cytoplasm. Both nonspecific and receptor endocytosis, leading to the cleavage of membrane vesicles, occurs in specialized regions of the plasma membrane. These are the so-called bordered pits. They are called so because from the side of the cytoplasm, the plasma membrane is covered, clothed, with a thin (about 20 nm) fibrous layer, which, on ultrathin sections, borders and covers small protrusions, pits (Fig. 137). Almost all animal cells have these pits; they occupy about 2% of the cell surface. Surrounding layer composed mainly of protein clathrin associated with a number of additional proteins. Three molecules of clathrin, together with three molecules of a low molecular weight protein, form the structure of a triskelion, resembling a three-beam swastika (Fig. 138). Clathrin triskelions on the inner surface of the pits of the plasma membrane form a loose network consisting of pentagons and hexagons, generally resembling a basket. The clathrin layer covers the entire perimeter of the separating primary endocytic vacuoles, bordered by vesicles.

Clathrin belongs to one of the so-called species. "clothing" proteins (COP - coated proteins). These proteins bind to integral receptor proteins from the side of the cytoplasm and form a dressing layer along the perimeter of the emerging pinosome, the primary endosomal vesicle - the “bordered” vesicle. in the separation of the primary endosome, proteins are also involved - dynamins, which polymerize around the neck of the separating vesicle (Fig. 139).

After the bordered vesicle separates from the plasmolemma and begins to be transferred deep into the cytoplasm, the clathrin layer disintegrates, dissociates, and the endosome membrane (pinosomes) acquires its usual form. After the loss of the clathrin layer, the endosomes begin to fuse with each other.

It was found that the membranes of the bordered pits contain relatively little cholesterol, which can determine the decrease in membrane stiffness and contribute to the formation of bubbles. The biological meaning of the appearance of a clathrin “coat” along the periphery of the vesicles may be that it provides adhesion of the bordered vesicles to the elements of the cytoskeleton and their subsequent transport in the cell, and prevents them from merging with each other.

The intensity of liquid-phase nonspecific pinocytosis can be very high. So the epithelial cell of the small intestine forms up to 1000 pinosomes per second, and macrophages form about 125 pinosomes per minute. The size of pinosomes is small, their lower limit is 60-130 nm, but their abundance leads to the fact that during endocytosis, the plasmolemma is quickly replaced, as if “spent” on the formation of many small vacuoles. So in macrophages, the entire plasma membrane is replaced in 30 minutes, in fibroblasts - in two hours.

The further fate of endosomes can be different, some of them can return to the cell surface and merge with it, but most of them enter the process of intracellular digestion. Primary endosomes contain mostly foreign molecules trapped in the liquid medium and do not contain hydrolytic enzymes. endosomes can fuse with each other while increasing in size. They then fuse with primary lysosomes (see below), which introduce enzymes into the endosome cavity that hydrolyze various biopolymers. The action of these lysosomal hydrolases causes intracellular digestion - the breakdown of polymers to monomers.

As already mentioned, during phagocytosis and pinocytosis, cells lose a large area of the plasmolemma (see macrophages), which, however, is quickly restored during membrane recycling, due to the return of vacuoles and their incorporation into the plasmolemma. This is due to the fact that small vesicles can separate from endosomes or vacuoles, as well as from lysosomes, which again merge with the plasma membrane. With such recyclization, a kind of “shuttle” transfer of membranes occurs: plasmolemma - pinosome - vacuole - plasmolemma. This leads to the restoration of the original area of the plasma membrane. It was found that with such a return, membrane recycling, all absorbed material is retained in the remaining endosome.

Specific or receptor-mediated endocytosis has a number of differences from nonspecific. The main thing is that molecules are absorbed for which there are specific receptors on the plasma membrane that are associated only with this type of molecules. Often such molecules that bind to receptor proteins on the surface of cells are called ligands.

Receptor-mediated endocytosis was first described in the accumulation of proteins in avian oocytes. Proteins of yolk granules, vitellogenins, are synthesized in various tissues, but then they enter the ovaries with the blood flow, where they bind to special membrane receptors of oocytes and then enter the cell with the help of endocytosis, where yolk granules are deposited.

Another example of selective endocytosis is the transport of cholesterol into the cell. This lipid is synthesized in the liver and, in combination with other phospholipids and a protein molecule, forms the so-called. low-density lipoprotein (LDL), which is secreted by liver cells and carried throughout the body by the circulatory system (Fig. 140). Special receptors of the plasma membrane, diffusely located on the surface of various cells, recognize the protein component of LDL and form a specific receptor-ligand complex. Following this, such a complex moves to the zone of bordered pits and is internalized - surrounded by a membrane and immersed deep into the cytoplasm. It has been shown that mutant receptors can bind LDL, but do not accumulate in the area of bordered pits. In addition to LDL receptors, more than two dozen other substances involved in receptor endocytosis of various substances have been found, all of which use the same internalization pathway through the bordered pits. Probably, their role is in the accumulation of receptors: one and the same bordered pit can collect about 1000 receptors of different classes. However, in fibroblasts, LDL receptor clusters are located in the zone of bordered pits even in the absence of a ligand in the medium.

The further fate of the absorbed LDL particle is that it undergoes decay in the composition secondary lysosome. After immersion in the cytoplasm of a bordered vesicle loaded with LDL, there is a rapid loss of the clathrin layer, membrane vesicles begin to merge with each other, forming an endosome - a vacuole containing absorbed LDL particles still associated with receptors on the membrane surface. Then the dissociation of the ligand-receptor complex occurs, small vacuoles are split off from the endosome, the membranes of which contain free receptors. These vesicles are recycled, incorporated into the plasma membrane, and thus the receptors return to the cell surface. The fate of LDL is that after fusion with lysosomes, they are hydrolyzed to free cholesterol, which can be incorporated into cell membranes.

Endosomes are characterized by a lower pH value (pH 4-5), a more acidic environment than other cell vacuoles. This is due to the presence in their membranes of proton pump proteins that pump in hydrogen ions with the simultaneous consumption of ATP (H + -dependent ATPase). The acidic environment within endosomes plays a critical role in the dissociation of receptors and ligands. In addition, an acidic environment is optimal for the activation of hydrolytic enzymes in lysosomes, which are activated upon fusion of lysosomes with endosomes and lead to the formation endolysosomes, in which the splitting of absorbed biopolymers occurs.

In some cases, the fate of dissociated ligands is not related to lysosomal hydrolysis. Thus, in some cells, after binding of plasmolemma receptors to certain proteins, clathrin-coated vacuoles sink into the cytoplasm and are transferred to another area of the cell, where they fuse again with the plasma membrane, and the bound proteins dissociate from the receptors. This is how the transfer, transcytosis, of some proteins through the wall of the endothelial cell from the blood plasma into the intercellular environment is carried out (Fig. 141). Another example of transcytosis is the transfer of antibodies. So in mammals, the antibodies of the mother can be transmitted to the cub through milk. In this case, the receptor-antibody complex remains unchanged in the endosome.

Phagocytosis

As already mentioned, phagocytosis is a variant of endocytosis and is associated with the absorption by the cell of large aggregates of macromolecules up to living or dead cells. As well as pinocytosis, phagocytosis can be non-specific (for example, the absorption of particles of colloidal gold or dextran polymer by fibroblasts or macrophages) and specific, mediated by receptors on the surface of the plasma membrane of phagocytic cells. During phagocytosis, large endocytic vacuoles are formed - phagosome, which then merge with lysosomes to form phagolysosomes.

On the surface of cells capable of phagocytosis (in mammals, these are neutrophils and macrophages), there is a set of receptors that interact with ligand proteins. Thus, in bacterial infections, antibodies to bacterial proteins bind to the surface of bacterial cells, forming a layer in which the F c -regions of the antibodies look outward. This layer is recognized by specific receptors on the surface of macrophages and neutrophils, and at the sites of their binding, absorption of the bacterium begins by enveloping it with the plasma membrane of the cell (Fig. 142).

Exocytosis

The plasma membrane is involved in the removal of substances from the cell with the help of exocytosis- the reverse process of endocytosis (see Fig. 133).

In the case of exocytosis, intracellular products enclosed in vacuoles or vesicles and separated from the hyaloplasm by a membrane approach the plasma membrane. At their points of contact, the plasma membrane and the vacuole membrane merge, and the bubble is emptied into the environment. With the help of exocytosis, the process of recycling of membranes involved in endocytosis occurs.

Most secreted substances are used by other cells of multicellular organisms (secretion of milk, digestive juices, hormones, etc.). But often cells secrete substances for their own needs. For example, the growth of the plasma membrane is carried out due to the incorporation of sections of the membrane as part of exocytic vacuoles, some of the elements of the glycocalyx are secreted by the cell in the form of glycoprotein molecules, etc.

Hydrolytic enzymes isolated from cells by exocytosis can be sorbed in the glycocalyx layer and provide membrane-bound extracellular cleavage of various biopolymers and organic molecules. Membrane non-cellular digestion is of great importance for animals. It was found that in the intestinal epithelium of mammals in the area of the so-called brush border of the absorbing epithelium, which is especially rich in glycocalyx, a huge number of various enzymes are found. Some of these enzymes are of pancreatic origin (amylase, lipases, various proteinases, etc.), and some are secreted by the epithelial cells themselves (exohydrolases, which break down mainly oligomers and dimers with the formation of transported products).

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Vesicular transport: endocytosis and exocytosis

vesicular transfer exocytosis endocytosis

endosome

pinocytosis and phagocytosis

Nonspecific endocyto

bordered pits clathrin

Specific or receptor-mediated ligands.

secondary lysosome

endolysosomes

Phagocytosis

phagosome phagolysosomes.

Exocytosis

exocytosis

The receptor role of the plasmalemma

We have already met with this feature of the plasma membrane when getting acquainted with its transport functions. Carrier proteins and pumps are also receptors that recognize and interact with certain ions. Receptor proteins bind to ligands and participate in the selection of molecules entering cells.

Membrane proteins or glycocalyx elements - glycoproteins can act as such receptors on the cell surface. Such sensitive sites to individual substances can be scattered over the surface of the cell or collected in small zones.

Different cells of animal organisms may have different sets of receptors or different sensitivity of the same receptor.

The role of many cell receptors is not only in the binding of specific substances or the ability to respond to physical factors, but also in the transmission of intercellular signals from the surface into the cell. At present, the system of signal transmission to cells with the help of certain hormones, which include peptide chains, has been well studied. These hormones have been found to bind to specific receptors on the surface of the cell's plasma membrane. Receptors, after binding to the hormone, activate another protein, which is already in the cytoplasmic part of the plasma membrane, adenylate cyclase. This enzyme synthesizes the cyclic AMP molecule from ATP. The role of cyclic AMP (cAMP) is that it is a secondary messenger - an activator of enzymes - kinases that cause modifications of other enzyme proteins. So, when the pancreatic hormone glucagon, produced by A-cells of the islets of Langerhans, acts on the liver cell, the hormone binds to a specific receptor, which stimulates the activation of adenylate cyclase. Synthesized cAMP activates protein kinase A, which in turn activates a cascade of enzymes that ultimately break down glycogen (animal storage polysaccharide) to glucose. The action of insulin is the opposite - it stimulates the entry of glucose into the liver cells and its deposition in the form of glycogen.

In general, the chain of events unfolds as follows: the hormone interacts specifically with the receptor part of this system and, without penetrating into the cell, activates adenylate cyclase, which synthesizes cAMP, which activates or inhibits an intracellular enzyme or a group of enzymes. Thus, the command, the signal from the plasma membrane is transmitted inside the cell. The efficiency of this adenylate cyclase system is very high. Thus, the interaction of one or several hormone molecules can lead, due to the synthesis of many cAMP molecules, to a signal amplification thousands of times. In this case, the adenylate cyclase system serves as a converter of external signals.

There is another way in which other secondary messengers are used - this is the so-called. phosphatidylinositol pathway. Under the action of the appropriate signal (some nerve mediators and proteins), the enzyme phospholipase C is activated, which cleaves the phosphatidylinositol diphosphate phospholipid, which is part of the plasma membrane. The hydrolysis products of this lipid, on the one hand, activate protein kinase C, which activates the kinase cascade, which leads to certain cellular reactions, and on the other hand, leads to the release of calcium ions, which regulates a number of cellular processes.

Another example of receptor activity is the receptors for acetylcholine, an important neurotransmitter. Acetylcholine, being released from the nerve ending, binds to the receptor on the muscle fiber, causes an impulsive flow of Na + into the cell (membrane depolarization), immediately opening about 2000 ion channels in the area of the neuromuscular ending.

The diversity and specificity of sets of receptors on the surface of cells leads to the creation of a very complex system of markers that make it possible to distinguish one's own cells (of the same individual or of the same species) from those of others. Similar cells enter into interactions with each other, leading to adhesion of surfaces (conjugation in protozoa and bacteria, the formation of tissue cell complexes). In this case, cells that differ in the set of determinant markers or do not perceive them are either excluded from such interaction, or are destroyed in higher animals as a result of immunological reactions (see below).

The plasma membrane is associated with the localization of specific receptors that respond to physical factors. So, in the plasma membrane or its derivatives in photosynthetic bacteria and blue-green algae, receptor proteins (chlorophylls) interacting with light quanta are localized. In the plasma membrane of light-sensitive animal cells, there is a special system of photoreceptor proteins (rhodopsin), with the help of which the light signal is converted into a chemical one, which in turn leads to the generation of an electrical impulse.

Intercellular recognition

In multicellular organisms, due to intercellular interactions, complex cellular ensembles are formed, the maintenance of which can be carried out in different ways. In germinal, embryonic tissues, especially in the early stages of development, cells remain connected to each other due to the ability of their surfaces to stick together. This property adhesion(connection, adhesion) of cells can be determined by the properties of their surface, which specifically interact with each other. The mechanism of these connections is well studied, it is provided by the interaction between glycoproteins of plasma membranes. With such intercellular interaction of cells between plasma membranes, there always remains a gap about 20 nm wide, filled with glycocalyx. Treatment of tissue with enzymes that violate the integrity of the glycocalyx (mucases that act hydrolytically on mucins, mucopolysaccharides) or damage the plasma membrane (proteases) leads to the separation of cells from each other, to their dissociation. However, if the dissociation factor is removed, the cells can reassemble and reaggregate. So it is possible to dissociate cells of sponges of different colors, orange and yellow. It turned out that two types of aggregates are formed in the mixture of these cells: those consisting of only yellow and only of orange cells. In this case, mixed cell suspensions self-organize, restoring the original multicellular structure. Similar results were obtained with separated cell suspensions of amphibian embryos; in this case, there is a selective spatial separation of ectoderm cells from the endoderm and from the mesenchyme. Moreover, if tissues of late stages of embryonic development are used for reaggregation, then various cell ensembles with tissue and organ specificity independently assemble in a test tube, epithelial aggregates similar to renal tubules are formed, etc.

It was found that transmembrane glycoproteins are responsible for the aggregation of homogeneous cells. Directly for the connection, adhesion, cells are responsible for the molecules of the so-called. CAM proteins (cell adhesion molecules). Some of them connect cells with each other due to intermolecular interactions, others form special intercellular connections or contacts.

Interactions between adhesive proteins can be homophilic when neighboring cells bind to each other with the help of homogeneous molecules, heterophilic when different kinds of CAMs on neighboring cells are involved in adhesion. Intercellular binding occurs through additional linker molecules.

There are several classes of CAM proteins. These are cadherins, immunoglobulin-like N-CAM (nerve cell adhesion molecules), selectins, integrins.

Cadherins are integral fibrillar membrane proteins that form parallel homodimers. Separate domains of these proteins are associated with Ca 2+ ions, which gives them a certain rigidity. There are more than 40 species of cadherins. Thus, E-cadherin is characteristic of cells of preimplanted embryos and epithelial cells of adult organisms. P-cadherin is characteristic of trophoblast, placenta, and epidermis cells; N-cadherin is located on the surface of nerve cells, lens cells, and on cardiac and skeletal muscles.

Nerve cell adhesion molecules(N-CAM) belong to the immunoglobulin superfamily, they form connections between nerve cells. Some of the N-CAMs are involved in the connection of synapses, as well as in the adhesion of cells of the immune system.

selectins also, integral proteins of the plasma membrane are involved in the adhesion of endothelial cells, in the binding of platelets, leukocytes.

Integrins are heterodimers, with a and b chains. Integrins primarily connect cells with extracellular substrates, but they can also participate in cell adhesion to each other.

Recognition of foreign proteins

As already mentioned, foreign macromolecules (antigens) that have entered the body develop a complex complex reaction - an immune reaction. Its essence lies in the fact that some of the lymphocytes produce special proteins - antibodies that specifically bind to antigens. For example, macrophages recognize antigen-antibody complexes with their surface receptors and absorb them (for example, the absorption of bacteria during phagocytosis).

In the body of all vertebrates, in addition, there is a system of reception of foreign cells or their own, but with altered plasma membrane proteins, for example, during viral infections or mutations, often associated with tumor degeneration of cells.

Proteins are located on the surface of all vertebrate cells, the so-called. major histocompatibility complex(major histocompatibility complex - MHC). These are integral proteins, glycoproteins, heterodimers. It is very important to remember that each individual has a different set of these MHC proteins. This is due to the fact that they are very polymorphic, because each individual has a large number of alternating forms of the same gene (more than 100), in addition, there are 7-8 loci encoding MHC molecules. This leads to the fact that each cell of a given organism, having a set of MHC proteins, will be different from the cells of an individual of the same species. A special form of lymphocytes, T-lymphocytes, recognize the MHC of their body, but the slightest change in the structure of the MHC (for example, association with a virus, or the result of a mutation in individual cells), causes T-lymphocytes to recognize such changed cells and destroy them, but not by phagocytosis. They secrete specific perforin proteins from secretory vacuoles, which are embedded in the cytoplasmic membrane of the altered cell, form transmembrane channels in it, making the plasma membrane permeable, which leads to the death of the altered cell (Fig. 143, 144).

Special intercellular connections

In addition to these relatively simple adhesive (but specific) bonds (Fig. 145), there are a number of special intercellular structures, contacts or connections that perform certain functions. These are locking, anchoring and communication connections (Fig. 146).

Locking or tight connection characteristic of single-layered epithelium. This is the zone where the outer layers of the two plasma membranes are as close as possible. The three-layer membrane is often seen in this contact: the two outer osmophilic layers of both membranes seem to merge into one common layer 2–3 nm thick. The fusion of membranes does not occur over the entire area of tight contact, but is a series of point convergence of membranes (Fig. 147a, 148).

On planar preparations of plasma membrane fractures in the zone of tight contact, using the freezing and shearing method, it was found that the points of contact of the membranes are rows of globules. These are the proteins occludin and claudin, special integral proteins of the plasma membrane, built in rows. Such rows of globules or strips can intersect in such a way that they form, as it were, a lattice or network on the cleavage surface. This structure is very typical for epithelia, especially glandular and intestinal. In the latter case, tight contact forms a continuous zone of fusion of plasma membranes, encircling the cell in its apical (upper, looking into the intestinal lumen) part (Fig. 148). Thus, each cell of the layer is, as it were, surrounded by a tape of this contact. Such structures can also be seen with special stains in a light microscope. They received the name from morphologists end plates. It turned out that in this case the role of the closing tight contact is not only in the mechanical connection of cells with each other. This contact area is poorly permeable to macromolecules and ions, and thus it locks, blocks the intercellular cavities, isolating them (and with them the internal environment of the body) from the external environment (in this case, the intestinal lumen).

This can be demonstrated using electron dense contrasters such as lanthanum hydroxide solution. If the lumen of the intestine or duct of some gland is filled with a solution of lanthanum hydroxide, then on sections under an electron microscope, the zones where this substance is located have a high electron density and will be dark. It turned out that neither the zone of tight contact nor the intercellular spaces below it darken. If the tight junctions are damaged (by light enzymatic treatment or removal of Ca ++ ions), then lanthanum also penetrates into the intercellular regions. Similarly, tight junctions have been shown to be impermeable to hemoglobin and ferritin in the tubules of the kidneys.