The cell membrane (plasma membrane) is a thin, semi-permeable membrane that surrounds cells.

Function and role of the cell membrane

Its function is to protect the integrity of the interior by letting some essential substances into the cell and preventing others from entering.

It also serves as the basis for attachment to some organisms and to others. Thus, the plasma membrane also provides the shape of the cell. Another function of the membrane is to regulate cell growth through balance and.

In endocytosis, lipids and proteins are removed from the cell membrane as substances are absorbed. In exocytosis, vesicles containing lipids and proteins fuse with the cell membrane, increasing cell size. , and fungal cells have plasma membranes. Internal, for example, are also enclosed in protective membranes.

Cell membrane structure

The plasma membrane is mainly composed of a mixture of proteins and lipids. Depending on the location and role of the membrane in the body, lipids can make up 20 to 80 percent of the membrane, with the rest being proteins. While lipids help make the membrane flexible, proteins control and maintain the cell's chemistry and help transport molecules across the membrane.

Membrane lipids

Phospholipids are the main component of plasma membranes. They form a lipid bilayer in which the hydrophilic (water-attracted) "head" regions spontaneously organize to resist the aqueous cytosol and extracellular fluid, while the hydrophobic (water-repellent) "tail" regions face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only some molecules to diffuse across the membrane.

Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps keep cell membranes rigid by preventing phospholipids from being too tightly packed. Cholesterol is absent in plant cell membranes.

Glycolipids are located on the outer surface of cell membranes and are connected to them by a carbohydrate chain. They help the cell recognize other cells in the body.

Membrane proteins

The cell membrane contains two types of associated proteins. Peripheral membrane proteins are external and associated with it by interacting with other proteins. Integral membrane proteins are introduced into the membrane and most pass through it. Parts of these transmembrane proteins are located on both sides of it.

The proteins of the plasma membrane have a number various functions. Structural proteins provide support and shape to cells. Membrane receptor proteins help cells communicate with their external environment through the use of hormones, neurotransmitters, and other signaling molecules. Transport proteins, such as globular proteins, carry molecules across cell membranes by facilitated diffusion. Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane, helping in the exchange and transport of molecules.

The plasma membrane occupies a special position, as it limits the cell from the outside and is directly connected with the extracellular environment. It is about 10 nm thick and is the thickest of cell membranes. The main components are proteins (more than 60%), lipids (about 40%) and carbohydrates (about 1%). Like all other cell membranes, it is synthesized in the EPS channels.

Functions of the plasmalemma.

Transport.

The plasma membrane is semi-permeable, i.e. selectively different molecules pass through it at different speeds. There are two ways of transporting substances across a membrane: passive and active transport.

Passive transport. Passive transport or diffusion does not require energy. Uncharged molecules diffuse along the concentration gradient, the transport of charged molecules depends on the concentration gradient of hydrogen protons and the transmembrane potential difference, which are combined into an electrochemical proton gradient. As a rule, the inner cytoplasmic surface of the membrane carries a negative charge, which facilitates the penetration of positively charged ions into the cell. There are two types of diffusion: simple and facilitated.

Simple diffusion is typical for small neutral molecules (H 2 O, CO 2, O 2), as well as for hydrophobic low molecular weight organic matter. These molecules can pass without any interaction with membrane proteins through the pores or channels of the membrane as long as the concentration gradient is maintained.

Facilitated diffusion is characteristic of hydrophilic molecules that are transported through the membrane also along a concentration gradient, but with the help of special membrane carrier proteins according to the principle uniport.

Facilitated diffusion is highly selective, since the carrier protein has a binding center complementary to the transported substance, and the transfer is accompanied by conformational changes in the protein. One of the possible mechanisms of facilitated diffusion is as follows: a transport protein (translocase) binds a substance, then approaches opposite side membrane, releases this substance, takes the original conformation and is again ready to perform the transport function. Little is known about how the movement of the protein itself is carried out. Another possible mechanism of transfer involves the participation of several carrier proteins. In this case, the initially bound compound itself passes from one protein to another, sequentially binding to one or another protein until it is on the opposite side of the membrane.

active transport. Such transport occurs when the transfer occurs against a concentration gradient. It requires the expenditure of energy by the cell. Active transport serves to accumulate substances inside the cell. The source of energy is often ATP. For active transport, in addition to an energy source, the participation of membrane proteins is necessary. One of the active transport systems in the animal cell is responsible for the transfer of Na and K + ions across the cell membrane. This system is called Na + - K*-pump. It is responsible for maintaining the composition of the intracellular environment, in which the concentration of K + ions is higher than that of Na * ions.

The concentration gradient of both ions is maintained by transferring K + inside the cell, and Na + outside. Both transports occur against a concentration gradient. This distribution of ions determines the water content in cells, excitability nerve cells and muscle cells and other properties of normal cells. Na + -K + -pump is a protein - transport ATPase. The molecule of this enzyme is an oligomer and penetrates the membrane. During the full cycle of the pump, 3 Na + ions are transferred from the cell to the intercellular substance, and 2 K + ions in the opposite direction, while the energy of the ATP molecule is used. There are transport systems for the transfer of calcium ions (Ca 2+ -ATPase), proton pumps (H + -ATPase), etc.

The active transport of a substance through a membrane, carried out due to the energy of the concentration gradient of another substance is called symport. The transport ATPase in this case has binding sites for both substances. Antiport is the movement of a substance against its concentration gradient. In this case, the other substance moves in the opposite direction along its concentration gradient. Symport and antiport (cotransport) can occur during the absorption of amino acids from the intestine and the reabsorption of glucose from primary urine, using the energy of the concentration gradient of Na + ions created by Na + , K + -ATPase.

Another 2 types of transport are endocytosis and exocytosis.

Endocytosis- the capture of large particles by the cell. There are several ways of endocytosis: pinocytosis and phagocytosis. Usually under pinocytosis understand the capture by the cell of liquid colloidal particles, under phagocytosis- capture of corpuscles (more dense and large particles up to other cells). The mechanism of pino- and phagocytosis is different.

IN general view the entry of solid particles or liquid droplets into the cell from the outside is called heterophagy. This process is most widespread in protozoa, but it is also very important in humans (as well as in other mammals). Heterophagy plays a significant role in protecting the body (segmented neutrophils - granulocytes; macrophagocytes), restructuring of bone tissue (osteoclasts), the formation of thyroxine by thyroid follicles, reabsorption of protein and other macromolecules in the proximal nephron and other processes.

Pinocytosis.

In order for external molecules to enter the cell, they must first be bound by glycocalyx receptors (a set of molecules associated with the surface proteins of the membrane) (Fig.).

At the site of such binding under the plasmalemma, clathrin protein molecules are found. The plasmalemma, together with molecules attached from the outside and lined with clathrin from the cytoplasm, begins to invaginate. The invagination becomes deeper, its edges approach and then close. As a result, a bubble is split off from the plasmalemma, carrying the trapped molecules. Clathrin on its surface looks like an uneven border on electron microphotographs; therefore, such bubbles are called bordered.

Clathrin prevents vesicles from attaching to intracellular membranes. Therefore, bordered vesicles can be freely transported in the cell to precisely those areas of the cytoplasm where their contents should be used. So, in particular, steroid hormones are delivered to the nucleus. However, usually bordered vesicles shed their border soon after detachment from the plasmalemma. Clathrin is transferred to the plasmalemma and can again participate in endocytosis reactions.

At the surface of the cell in the cytoplasm there are more permanent vesicles - endosomes. The bordered vesicles shed clathrin and fuse with endosomes, increasing the volume and surface of endosomes. Then the excess part of the endosomes is split off in the form of a new vesicle, in which there are no substances that have entered the cell, they remain in the endosome. The new vesicle travels to the cell surface and fuses with the membrane. As a result, the decrease in the plasmalemma that occurred when the bordered vesicle was cleaved off is restored, and its receptors also return to the plasmalemma.

Endosomes sink into the cytoplasm and fuse with lysosome membranes. Incoming substances inside such a secondary lysosome undergo various biochemical transformations. Upon completion of the process, the lysosome membrane can disintegrate into fragments, and the decay products and contents of the lysosome become available for intracellular metabolic reactions. For example, amino acids are bound by tRNA and delivered to ribosomes, while glucose can enter the Golgi complex or the tubules of the agranular ER.

Although endosomes do not have a clathrin border, not all of them fuse with lysosomes. Some of them are directed from one cell surface to another (if the cells form an epithelial layer). There, the endosome membrane fuses with the plasma membrane and the contents are expelled. As a result, substances are transferred through the cell from one environment to another without changes. This process is called transcytosis. Protein molecules, in particular immunoglobulins, can also be transferred by transcytosis.

Phagocytosis.

If a large particle has molecular groups on its surface that can be recognized by cell receptors, it binds. It is far from always that alien particles themselves possess such groupings. However, when they enter the body, they are surrounded by immunoglobulin molecules (opsonins), which are always found both in the blood and in the intercellular environment. Immunoglobulins are always recognized by phagocyte cells.

After the opsonins covering the foreign particle have bound to the receptors of the phagocyte, its surface complex is activated. Actin microfilaments begin to interact with myosin, and the configuration of the cell surface changes. Outgrowths of the cytoplasm of the phagocyte extend around the particle. They cover the surface of the particle and combine above it. The outer sheets of outgrowths merge, closing the surface of the cell.

Deep sheets of outgrowths form a membrane around the absorbed particle - is formed phagosome. The phagosome fuses with lysosomes, resulting in their complex - heterolysosome (heterosome, or phagolysosome). In it, the lysis of the trapped components of the particle occurs. Some of the lysis products are removed from the heterosome and utilized by the cell, while some may not be susceptible to the action of lysosomal enzymes. These residues form residual bodies.

Potentially all cells have the ability to phagocytosis, but in the body only a few specialize in this direction. These are neutrophilic leukocytes and macrophages.

Exocytosis.

This is the removal of substances from the cell. First, macromolecular compounds are segregated in the Golgi complex in the form of transport vesicles. The latter, with the participation of microtubules, are directed to the cell surface. The membrane of the vesicle is built into the plasmalemma, and the contents of the vesicle are outside the cell (Fig.). The fusion of the vesicle with the plasmalemma can occur without any additional signals. This exocytosis is called constitutive. This is how most of the products of its own metabolism are removed from the cells. A number of cells, however, are intended for the synthesis of special compounds - secrets that are used in the body in other parts of it. In order for the transport bubble with the secret to merge with the plasmalemma, signals from the outside are necessary. Only then will the merge occur and the secret be released. This exocytosis is called regulated. Signaling molecules that promote the excretion of secretions are called liberins (releasing factors), and those that prevent removal - statins.

receptor functions.

They are mainly provided by glycoproteins located on the surface of the plasmalemma and capable of binding to their ligands. The ligand corresponds to its receptor like a key to a lock. Binding of the ligand to the receptor causes a change in the conformation of the polypeptide. With such a change in the transmembrane protein, a message is established between the extra- and intracellular environment.

types of receptors.

Receptors associated with protein ion channels. They interact with a signal molecule that temporarily opens or closes the channel for the passage of ions. (For example, the acetylcholine neurotransmitter receptor is a protein consisting of 5 subunits that form an ion channel. In the absence of acetylcholine, the channel is closed, and after attachment it opens and allows sodium ions to pass through).

catalytic receptors. They consist of an extracellular part (the receptor itself) and an intracellular cytoplasmic part that functions as the enzyme prolinkinase (for example, growth hormone receptors).

Receptors associated with G-proteins. These are transmembrane proteins consisting of a ligand-interacting receptor and a G-protein (guanosine triphosphate-related regulatory protein) that transmits a signal to a membrane-bound enzyme (adenylate cyclase) or to an ion channel. As a result, cyclic AMP or calcium ions are activated. (This is how the adenylate cyclase system works. For example, there is a receptor for the hormone insulin in the liver cells. The supracellular part of the receptor binds to insulin. This causes the activation of the intracellular part, the enzyme adenylate cyclase. It synthesizes cyclic AMP from ATP, which regulates the rate of various intracellular processes, causing activation or inhibition of those or other metabolic enzymes).

Receptors that perceive physical factors. For example, the photoreceptor protein rhodopsin. When light is absorbed, it changes its conformation and excites a nerve impulse.

The kernel is responsible for storing genetic material, written on DNA, and also controls all cell processes. The cytoplasm contains organelles, each of which has its own functions, such as, for example, the synthesis of organic substances, digestion, etc. And we will talk about the last component in more detail in this article.

in biology?

talking plain language, it's a shell. However, it is not always completely impenetrable. Transport of certain substances across the membrane is almost always allowed.

In cytology, membranes can be divided into two main types. The first is the plasma membrane that covers the cell. The second is the membranes of organelles. There are organelles that have one or two membranes. Single-membrane cells include the endoplasmic reticulum, vacuoles, and lysosomes. Plastids and mitochondria belong to the two-membrane ones.

Also, membranes can be inside organelles. Usually these are derivatives of the inner membrane of two-membrane organelles.

How are the membranes of two-membrane organelles arranged?

Plastids and mitochondria have two membranes. The outer membrane of both organelles is smooth, but the inner one forms the structures necessary for the functioning of the organoid.

So, the shell of mitochondria has protrusions inward - cristae or ridges. They cycle through chemical reactions required for cellular respiration.

Derivatives of the inner membrane of chloroplasts are disk-shaped sacs - thylakoids. They are collected in piles - grains. Separate grana are combined with each other with the help of lamellae - long structures also formed from membranes.

The structure of the membranes of single-membrane organelles

These organelles have only one membrane. It is usually a smooth membrane composed of lipids and proteins.

Features of the structure of the plasma membrane of the cell

The membrane is made up of substances such as lipids and proteins. The structure of the plasma membrane provides for its thickness of 7-11 nanometers. The bulk of the membrane is made up of lipids.

The structure of the plasma membrane provides for the presence of two layers in it. The first is a double layer of phospholipids, and the second is a layer of proteins.

Plasma membrane lipids

The lipids that make up the plasma membrane are divided into three groups: steroids, sphingophospholipids, and glycerophospholipids. The molecule of the latter has in its composition the residue of the trihydric alcohol glycerol, in which the hydrogen atoms of two hydroxyl groups are replaced by chains of fatty acids, and the hydrogen atom of the third hydroxyl group is replaced by a phosphoric acid residue, to which, in turn, the residue of one of the nitrogenous bases is attached.

The glycerophospholipid molecule can be divided into two parts: the head and tails. The head is hydrophilic (that is, it dissolves in water), and the tails are hydrophobic (they repel water, but dissolve in organic solvents). Due to this structure, the molecule of glycerophospholipids can be called amphiphilic, that is, both hydrophobic and hydrophilic at the same time.

Sphingophospholipids are chemically similar to glycerophospholipids. But they differ from those mentioned above in that in their composition, instead of a glycerol residue, they have a sphingosine alcohol residue. Their molecules also have heads and tails.

The picture below clearly shows the structure of the plasma membrane.

Plasma membrane proteins

As for the proteins that make up the structure of the plasma membrane, these are mainly glycoproteins.

Depending on their location in the shell, they can be divided into two groups: peripheral and integral. The first are those that are on the surface of the membrane, and the second are those that penetrate the entire thickness of the membrane and are inside the lipid layer.

Depending on the functions that proteins perform, they can be divided into four groups: enzymes, structural, transport and receptor.

All proteins that are in the structure of the plasma membrane are not chemically associated with phospholipids. Therefore, they can move freely in the main layer of the membrane, gather in groups, etc. That is why the structure of the plasma membrane of the cell cannot be called static. It is dynamic, as it changes all the time.

What is the role of the cell membrane?

The structure of the plasma membrane allows it to cope with five functions.

The first and main one is the restriction of the cytoplasm. Due to this, the cell has a constant shape and size. This function is ensured by the fact that the plasma membrane is strong and elastic.

The second role is provision Due to their elasticity, plasma membranes can form outgrowths and folds at their junctions.

The next function of the cell membrane is transport. It is provided by special proteins. Thanks to them, the necessary substances can be transported into the cell, and unnecessary substances can be disposed of from it.

In addition, the plasma membrane performs an enzymatic function. It is also carried out thanks to proteins.

And the last function is signaling. Due to the fact that proteins under the influence of certain conditions can change their spatial structure, the plasma membrane can send signals to cells.

Now you know everything about membranes: what is a membrane in biology, what they are, how the plasma membrane and organoid membranes are arranged, what functions they perform.

Lecture number 4.

Number of hours: 2

plasma membrane

1.

2.

3. Intercellular contacts.

1. The structure of the plasma membrane

The plasma membrane, or plasmalemma, is a surface peripheral structure that limitsthe cell from the outside and providing its connection with other cells and the extracellular environment. It has a thicknessabout 10 nm. Among other cell membranes, the plasmalemma is the thickest. Chemically, the plasma membrane is lipoprotein complex. The main components are lipids (about 40%), proteins (more than 60%) and carbohydrates (about 2-10%).

Lipids include a large group of organic substances that have poor solubility in water (hydrophobicity) and good solubility in organic solvents and fats (lipophilicity).Representative lipids found in the plasma membrane are phospholipids, sphingomyelins, and cholesterol. In plant cells, cholesterol is replaced by phytosterol. According to their biological role, plasmalemma proteins can be divided into enzyme proteins, receptor and structural proteins. Plasmalemma carbohydrates are part of the plasmalemma in a bound state (glycolipids and glycoproteins).

It is currently generally accepted fluid-mosaic model of the structure of a biological membrane. According to this model, the structural basis of the membrane is formed by a double layer of phospholipids encrusted with proteins. The tails of the molecules face each other in a double layer, while the polar heads remain outside, forming hydrophilic surfaces. Protein molecules do not form a continuous layer, they are located in the lipid layer, plunging to different depths (there are peripheral proteins, some proteins penetrate the membrane through, some are immersed in the lipid layer). Most proteins are not associated with membrane lipids; they seem to float in a "lipid lake". Therefore, protein molecules are able to move along the membrane, gather in groups, or, conversely, disperse on the membrane surface. This suggests that the plasma membrane is not a static, frozen formation.

Outside of the plasmalemma is the epimembrane layer - glycocalyx. The thickness of this layer is about 3-4 nm. Glycocalyx is found in almost all animal cells. It is associated with the plasma membrane glycoprotein complex. Carbohydrates form long, branching chains of polysaccharides associated with proteins and lipids of the plasma membrane. The glycocalyx can contain enzyme proteins involved in the extracellular breakdown of various substances. Products of enzymatic activity (amino acids, nucleotides, fatty acid etc.) are transported through the plasma membrane and absorbed by cells.

The plasma membrane is constantly being renewed. This occurs by lacing off small bubbles from its surface into the cell and embedding vacuoles from inside the cell into the membrane. Thus, in the cell there is a constant flow of membrane elements: from the plasma membrane into the cytoplasm (endocytosis) and the flow of membrane structures from the cytoplasm to the cell surface (exocytosis). In the circulation of membranes, the leading role is assigned to the system of membrane vacuoles of the Golgi complex.

4. Functions of the plasma membrane. Mechanisms of transport of substances through the plasmalemma. Receptor function of the plasmalemma

The plasma membrane performs a number of important functions:

1) Barrier.The barrier function of the plasma membrane is tolimiting the free diffusion of substances from cell to cell, preventingrotation leakage of the water-soluble contents of the cell. But sincehow the cell must receive the necessary nutrients, youdivide metabolic end products, regulate intracellularion concentrations, then it formed special mechanisms for the transfer of substances through the cell membrane.

2) Transport.The transport function is Ensuring the entry and exit of various substances into and out of the cell. An important property of the membrane is selective permeability, or semipermeability. It easily passes water and water-solublegases and repels polar molecules such as glucose or amino acids.

There are several mechanisms for the transport of substances across the membrane:

passive transport;

active transport;

transport in membrane packaging.

Passive transport. Diffusion -this is the movement of particles of the medium, leading to the transfer ofsubstances from an area where its concentration is high to an area with a low concentrationtion. During diffusion transport, the membrane functions as an osmotic barrier. The diffusion rate depends on the valuemolecules and their relative solubility in fats. The less timesmeasures of molecules and the more they are fat-soluble (lipophilic), the faster they will move through the lipid bilayer.Diffusion can be neutral(transfer of unchargedmolecules) and lightweight(with the help of special proteinsnoses). Facilitated diffusion is faster than neutral diffusion.Maximum penetratingwater has the abilityhow its molecules are small and uncharged. Diffusion of water through cellsmembrane is called osmo catfishIt is assumed that in the cellmembrane for penetrationwater and some ions essentiallythere are special "pores". Their numbersmall, and the diameter isabout 0.3-0.8 nm. Diffuse most rapidly through the membrane well, easily soluble in lipid bilayer of a molecule, for example O, and uncharged polar moleculesly of small diameter (SO, mo chevin).

Transport of polar molecules (withsugars, amino acids), implementeddelivered with the help of special membrane transportproteins is called facilitated diffusion. Such proteins areare found in all types of biological membranes, and each specific ny protein is designed to transfer molecules of a certain class sa. Transport proteins are transmembrane, their polypeptide chain crosses the lipid bilayer several times, forming it has through passages. This ensures the transfer of specificsubstances through the membrane without direct contact with it.There are two main classes of transport proteins: squirrels- carriers (transporters) And channel-forming proteins (whiteki channels). Carrier proteins carry molecules across the membrane by first changing their configuration. Channel-forming proteins form filled membranes pore water. When the pores are open, molecules of specific substances(usually inorganic ions of a suitable size and charge) pass through them. If the molecule of the transported substance has no charge, then the direction of transport is determined by the concentration gradient. If the molecule is charged, then on its transport, in addition to the gradient, con concentration, the electric charge of the membrane also affects (membranepotential). The inner side of the plasmalemma is usually charged from negative in relation to the outside. The membrane potential facilitates the penetration of positively charged ions into the cell and prevents the passage of negatively charged ions.

active transport. Active transport is the movement of substances against an electrochemical gradient. It is always carried out by trans proteins.porters and closely related zan with energy sourcegee. In proteins-transfer chiki have plots binding with transporttiable substance. The more such tkov communicates with thingsthe higher the speedtransport growth. The selective transfer of one substance is called uniport. The transfer of several substances is carried out kotran sports systems. If the transfer goes in one direction -This symport, if in opposite antiport. So,for example, glucose is transported from the extracellular fluid into the cell in a uniportal manner. The transfer of glucose and Na 4 from the intestinal tract ortubules of the kidneys, respectively, into the cells of the intestine or blood is carried out symportally, and the transfer of C1 ~ and HCO "is antiport. Presumably It is assumed that during the transfer, reversible conformational changes in the conveyor, which allows the substances connected to it to move.

An example of a carrier protein used for transportsubstances, the energy released during the hydrolysis of ATP isNa + -K + pump, found in the plasma membrane of all cells. Na+-K The pump works on the principle of anti-port, pumping wai Na "from the cell and to the inside of the cell against their electrochemical gradients. Gradient Na+ creates osmotic pressure, maintains cell volume and ensures the transport of sugars and amino acidsnoacids. The operation of this pump consumes a third of all the energy necessary for the vital activity of cells.When studying the mechanism of action Na + - K + pump has been installedit is known that it is an ATPase enzyme and a transmembrane enzyme tegral protein. In the presence Na+ and ATP under the action of ATP-terminal phosphate is separated from ATP and attached to the residueaspartic acid on the ATPase molecule. Phos ATPase moleculephorylated, changes its configuration and Na+ is derived from cells. Following the withdrawal Na K" is always transported from the cell into the cell. To do this, the previously attached phosphate is cleaved from ATPase in the presence of K. The enzyme is dephosphorylated, restores its configuration, and K 1 is "pumped" into the cell.

ATPase is formed by two subunits, large and small.The large subunit consists of thousands of amino acid residues,crossing the bilayer several times. It has a catalytic activity and is capable of being reversibly phosphorylated and dephosphorilled. Large subunit on the cytoplasmic sidedoes not have binding sites Na+ and ATP, and on the outside -sites for binding K + and ouabain. The small subunit isglycoprotein and its function is not yet known.

Na+-K the pump has an electrogenic effect. He removes threepositively charged ion Na f out of the cell and introduces twoion K As a result, a current flows through the membrane, forming an electronric potential with a negative value in the inner part of the cell in relation to its outer surface. Na "-K + the pump regulates cell volume, controls the concentration of substancesinside the cell, maintains osmotic pressure, participates in the creation of membrane potential.

Transport in membrane packaging. Transfer across the membrane of macromolecules (proteins, nucleic acids)lot, polysaccharides, lipoproteins) and other particles is carried out through the sequential formation and fusion of surroundedmembrane-bound vesicles (vesicles). Vesicular transport processit goes through two stages. At the beginningvesicle membrane and plasmalemmastick together and then merge.For the passage of stage 2, it is necessarydimo so that the water molecules are youare crowded by interacting lipid bilayers, which approach each other up to a distance of 1-5 nm. Considers that this process is activatedspecial fusion proteins(They isolated so far only from viruses). Vesicular transport hasimportant feature - absorbed or secreted macromolecules,contained in vesicles, usually notmixable with other macromolesculae or cell organelles. Pu vesicles can merge with specific membranes, which providechivaet exchange of macromolecules betweendu extracellular space andthe contents of the cell. Similarlymacromolecules are transferred from one cell compartment to another.

The transport of macromolecules and particles into a cell is called endo cytosis.In this case, the transported substances are enveloped in teathe plasma membrane, a vesicle (vacuole) is formed, whichwhich moves into the cell. Depending on the image sizebubbles, there are two types of endocytosis - pinocytosis and phagocytosis.

pinocytosisprovides absorption of liquid and dissolvedsubstances in the form of small bubbles ( d =150 nm). Phagocytosis -is the absorption of large particles, microorganismcall or fragments of organelles, cells. At the same time, they formXia large vesicles, phagosomes or vacuoles ( d -250 nm or more). At protozoan phagocytic function - a form of nutrition. In mammals, the phagocytic function is carried out by macrophages and neutprofiles that protect the body from infection by absorbing invading microbes. Macrophages are also involved in the utilizationtion of old or damaged cells and their fragments (in the bodyhuman macrophages ingest more than 100 old erythritides dailyrocytes). Phagocytosis begins only when the ingested particlebinds to the surface of the phagocyte and activates specializednye receptor cells. Associating particles with specific remembrane receptors causes the formation of pseudopodia, whichrye envelop the particle and, merging at the edges, form a bubble -phagosome.Phagosome formation and phagocytosis properwalks only if in the process of enveloping the particleis constantly in contact with plasmalemma receptors, as if "stagnant flashing lightning."

A significant part of the material absorbed by the cell by endocytosis, ends up in lysosomes. Large particles includinghope in phagosomes which then fuse with lysosomes to form phagolysosomes. Liquid and macromolecules absorbed duringpinocytosis, are initially transferred to endosomes, whichfuse with lysosomes to form endolysosomes. I am present various hydrolytic enzymes present in lysosomesro destroy macromolecules. hydrolysis products (amino acidlots, sugars, nucleotides) are transported from lysosomes to the cytosol, where they are used by the cell. Most membrane components endocytic vesicles from phagosomes and endosomes return by exocytosis to the plasma membrane and there re-disappearlyse. The main biological significance of endocytosis is there is a receipt of building blocks due to intracellular digestion of macromolecules in lysosomes.

Absorption of substances in eukaryotic cells begins in thecialized regions of the plasma membrane, the so-calledwe are X bordered pits. On electron micrographspits look like invaginations of the plasma membrane, cytoplasmthe mat side of which is covered with a fibrous layer. layer likewould border small pits plaz malemmas. The pits occupy about 2% of theover the surface of the cell membranewe are eukaryotes. Within a minute the fossae grow deeper and deeper Xia, are drawn into the cage and then, narrowing at the base, split off,forming fringed vesicles.It has been established that frommatic membrane fibroblasttov within one minute flakeabout a quarter ofmembranes in the form of bordered pu zyrkov. Bubbles lose fast their border and acquire a wayability to fuse with the lysosome.

Endocytosis may be non-specific(constitutive)And specific(receptor).At nonspecific endocytosis the cell takes overabsorbs substances completely alien to it, for example, soot particles,dyes. First, particles are deposited on the glycocalyx plasmalemma. Especially well precipitated (adsorbed) on positively charged groups of proteins, since the glycocalyx carries negative charge. Then the cell morphology changesmembranes. It can either sink, forming invaginations(invagination), or, conversely, form outgrowths, which seem to add up, separating small volumes liquid environment. The formation of invaginations is more characteristic for cells of the intestinal epithelium, amoebae, and outgrowths - for phagocytes and fibroblasts. These processes can be blocked by inhibitorsbreathing. The resulting vesicles are primary endosomes that can merge each other, increasing in size. In the future, they will join mingle with lysosomes, turning into an endolysosome - a digester new vacuole. The intensity of liquid-phase nonspecific pinocytosis up towildly high. Macrophages form up to 125, and epithelial cells finelyth intestine up to a thousand pinosom per minute. The abundance of pinosomes leads to the fact that the plasmalemma is quickly spent on the formation of multiplethe presence of small vacuoles. Membrane recovery is quite fast.tro during recycling in the process of exocytosis due to the return of wakuoles and their incorporation into the plasmalemma. In macrophages, all plasmaThe classical membrane is replaced in 30 minutes, and in fibroblasts in 2 hours.

More effective way absorption from extracellular fluidbone-specific macromolecules is specific en docytosis(mediated by receptors). The macromolecules in thisbind to complementary receptors on the surfacecells accumulate in the bordered fossa, and then, forming an endosome, are immersed in the cytosol. Receptor endocytosis ensures the accumulation of specific macromolecules at its receptor.Molecules that bind on the surface of the plasmalemma to the receptortorus are called ligands. With the help of the receptor endocytosis in many animal cells is the absorptionextracellular cholesterol environment.

The plasma membrane takes part in the removal of substances from the cell (exocytosis). In this case, the vacuoles approach the plasmalemma. At the points of contact, the plasmolemma and the vacuole membrane merge and the contents of the vacuole enter the environment.In some protozoa, sites on the cell membrane for exocytosis are predetermined. So, in the plasma membrane some ciliary ciliates have certain areas with the correct arrangement of large globules of integral proteins. Atmucocysts and trichocysts of ciliates completely ready for secretion, on the upper part of the plasmalemma there is a corolla of integral globulesproteins. These sections of the membrane of mucocysts and trichocysts of soprikaadhere to the surface of the cell.A peculiar exocytosis is observed in neutrophils. They spocapable of being released into the environment under certain conditionsblow your lysosomes. In some cases, small outgrowths of the plasmalemma containing lysosomes are formed, which then break off and pass into the environment. In other cases, there is invagination of the plasmalemma deep into the cell and its capture of lysosomes, located nyh far from the cell surface.

The processes of endocytosis and exocytosis are carried out with the participation of the system of fibrillar components of the cytoplasm associated with the plasmolemma.

Receptor function of the plasmalemma. This one of the main, universal for all cells, is rereceptor function of the plasmalemma. It defines interactioncells with each other and with the environment.

The whole variety of informational intercellular interactions can be schematically represented as a chain of successivesignal-receptor-second messenger-response reactions (concept signal-response).Signals transmit information from cell to cellnye molecules that are produced in some cells and specialphysically affect others sensitive to the signal (cells-mi sheni). Signal molecule - primary intermediary binding etsya with receptors located on target cells, react only for certain signals. Signal molecules - ligands- approach their receptor like a key to a lock. Ligand-mi for membrane receptors (plasmalemma receptors) ishydrophilic molecules, peptide hormones, neuromedia tori, cytokines, antibodies, and for nuclear receptors - fat soluble molecules, steroid and thyroid hormones, vitamin DAs receptors on topcells can act as proteinsmembranes or glycocalytic elementsca - polysaccharides and glycoproteins.It is believed that sensitive toto separate substances areas, scattersan along the surface of the cell orbranes into small bands. Yes, onsurface of prokaryotic cellsand animal cells there are limitsa fixed number of places with which they canbind viral particles. memesquirrels (carriers and canaly) learn, interact and transferwear only certain substances.Cell receptors are involved intransmission of signals from the surface of the cell to the inside.Diversity and specificitya ditch of receptors on the cell surfaceleads to a very complex systemwe are markers to distinguishown cells from strangers. Similar cellsinteract with each other, their surfaces can stick together (conjugation atprotozoa, tissue formation in multicellular organisms). Cells do not perceivecommon markers, as well as those differing inboron of determinant markersare torn apart or rejected.Upon formation of the receptor-ligand complex,transmembrane proteins: protein converter, protein enhancer.As a result, the receptor changes its conformation and interactionno with cellular precursor of the second messenger ka - messenger.Messengers can be ionized calcium, phospholipfor C, adenylate cyclase, guanylate cyclase. Influenced by messengeractivation of enzymes involved in the synthesis cyclic monophosphates - AMP or HMF. The latter change the assetThe presence of two types of protein kinase enzymes in the cytoplasm of the cell, leading to the phosphorylation of numerous intracellular proteins.

The most common formation of cAMP, under the action ofthe secretion of a number of hormones - thyroxine, cortisone, progesterone, increases, the breakdown of glycogen in the liver and muscles increases,heart rate and force, osteodestruction, reverse absorption of water in the tubules of the nephron.

The activity of the adenylate cyclase system is very high - the synthesis of cAMP leads to a ten thousandth increase in the signal.

Under the action of cGMP, the secretion of insulin by the pancreas, histamine by mast cells, serotoninbocytes, smooth muscle tissue is reduced.

In many cases, upon formation of the receptor-ligand complexthere is a change in the membrane potential, which in turn leads to a change in the permeability of the plasmalemma and metabolicsome processes in the cell.

The plasma membrane contains specific receptors tori responding to physical factors. So, in photosynthetic bacteria, chlorophylls are located on the surface of the cell,reacting to light. In photosensitive animals in plasmaThere is a whole system of phogoreceptor proteins in the caustic membrane -rhodopsins, with the help of which the light stimulus transforms is converted into a chemical signal and then an electrical impulse.

3. Intercellular contacts

In multicellular animals, the plasmolemma takes part in the formation intercellular connections providing intercellular interactions. There are several types of such structures.

§ Simple contact.A simple contact is found among the majority of cells of various origins adjacent to each other. Represents the convergence of the plasma membranes of neighboring cells at a distance of 15-20 nm. In this case, the interaction of the glycocalyx layers of neighboring cells occurs.

§ Tight (closing) contact. With such a connection, the outer layers of the two plasma membranes are as close as possible. The rapprochement is so dense that there is a kind of merging of sections of the plasma membranes of two neighboring cells. The fusion of membranes does not occur over the entire area of ​​tight contact, but is a series of point convergence of membranes. The role of tight contact is to mechanically connect cells to each other. This area is impenetrable for macromolecules and ions and, therefore, it locks, delimits the intercellular gaps (and, together with them, the internal environment of the body) from the external environment.

§ The patch of adhesion, or desmosome. The desmosome is a small area up to 0.5 µm in diameter. In the zone of the desmosome on the side of the cytoplasm, there is an area of ​​thin fibrils. The functional role of desmosomes is mainly in the mechanical connection between cells.

§ Gap contact, or nexus. With this type of contact, the plasma membranes of neighboring cells are separated by a gap of 2-3 nm over a distance of 0.5–3 µm. In the structure of plasmolemms, special protein complexes (connexons) are located. One connexon on the cell's plasma membrane is precisely opposed by a connexon on the plasma membrane of the adjacent cell. As a result, a channel is formed from one cell to another. Connexons can contract, changing the diameter of the internal channel, and thereby participate in the regulation of the transport of molecules between cells. This type of connection is found in all tissue groups. The functional role of the gap junction is to carry ions and small molecules from cell to cell. So, in the heart muscle, excitation, which is based on the process of changing ion permeability, is transmitted from cell to cell through the nexus.

§ Synaptic contact, or synapse. Synapses are areas of contact between two cells specialized for one-way transmission of excitation or inhibition from one element to another. This type of connection is characteristic of nervous tissue and occurs both between two neurons and between a neuron and some other element. The membranes of these cells are separated by an intercellular space - a synaptic cleft about 20-30 nm wide. The membrane in the area of ​​synaptic contact of one cell is called presynaptic, the other - postsynaptic. Near the presynaptic membrane, a huge number of small vacuoles (synaptic vesicles) containing the neurotransmitter are revealed. At the time of passing nerve impulse synaptic vesicles release neurotransmitters into the synaptic cleft. The mediator interacts with the receptor sites of the postsynaptic membrane, which ultimately leads to the transmission of a nerve impulse. In addition to transmitting a nerve impulse, synapses provide a rigid connection between the surfaces of two interacting cells.

§ Plasmodesma.This type of intercellular communication is found in plants. Plasmodesmata are thin tubular channels that connect two adjacent cells. The diameter of these channels is usually 40-50 nm. Plasmodesmata pass through the cell wall that separates the cells. In young cells, the number of plasmodesmata can be very high (up to 1000 per cell). With aging of cells, their number decreases due to ruptures with increasing thickness. cell wall. The functional role of plasmodesmata is to ensure intercellular circulation of solutions containing nutrients, ions and other compounds. Plasmodesmata infect cells with plant viruses.

Specialized structures of the plasma membrane

The plasmalemma of many animal cells forms outgrowths of various structures (microvilli, cilia, flagella). Most often found on the surface of many animal cells microvilli. These outgrowths of the cytoplasm, bounded by the plasmalemma, have the shape of a cylinder with a rounded top. Microvilli are characteristic of epithelial cells, but are also found in cells of other tissues. The microvilli are about 100 nm in diameter. Their number and length are different in different cell types. The significance of microvilli lies in a significant increase in the area of ​​the cell surface. This is especially important for cells involved in absorption. So, in the intestinal epithelium, there are up to 2x10 8 microvilli per 1 mm 2 of the surface.

Biological membranes form the basis of the structural organization of the cell. The plasma membrane (plasmalemma) is the membrane that surrounds the cytoplasm of a living cell. Membranes are made up of lipids and proteins. Lipids (mainly phospholipids) form a double layer in which the hydrophobic "tails" of the molecules face inside the membrane, and the hydrophilic tails - to its surfaces. Protein molecules can be located on the outer and inner surface of the membrane, they can be partially immersed in the lipid layer or penetrate it through. Most of the immersed membrane proteins are enzymes. This is a fluid-mosaic model of the structure of the plasma membrane. Protein and lipid molecules are mobile, which ensures the dynamism of the membrane. The membranes also contain carbohydrates in the form of glycolipids and glycoproteins (glycocalix) located on the outer surface of the membrane. The set of proteins and carbohydrates on the surface of the membrane of each cell is specific and is a kind of indicator of the cell type.

Membrane functions:

1. Dividing. It consists in the formation of a barrier between the internal contents of the cell and the external environment.
2. Ensuring the exchange of substances between the cytoplasm and the external environment. Water, ions, inorganic and organic molecules(transport function). Products formed in the cell (secretory function) are excreted into the external environment.
3. Transport. Transport across the membrane can take place in different ways. Passive transport is carried out without energy expenditure, by simple diffusion, osmosis or facilitated diffusion with the help of carrier proteins. Active transport is by carrier proteins and requires energy input (eg sodium-potassium pump). material from the site

Large molecules of biopolymers enter the cell as a result of endocytosis. It is divided into phagocytosis and pinocytosis. Phagocytosis is the capture and absorption of large particles by the cell. The phenomenon was first described by I.I. Mechnikov. First, substances adhere to the plasma membrane, to specific receptor proteins, then the membrane sags, forming a depression.

A digestive vacuole is formed. It digests the substances that have entered the cell. In humans and animals, leukocytes are capable of phagocytosis. Leukocytes engulf bacteria and other solid particles.

Pinocytosis is the process of capturing and absorbing liquid droplets with substances dissolved in it. Substances adhere to membrane proteins (receptors), and a drop of solution is surrounded by a membrane, forming a vacuole. Pinocytosis and phagocytosis occur with the expenditure of ATP energy.

1. Secretory. Secretion - the release by the cell of substances synthesized in the cell into the external environment. Hormones, polysaccharides, proteins, fat droplets are enclosed in membrane-bound vesicles and approach the plasmalemma. The membranes merge, and the contents of the vesicle are released into the environment surrounding the cell.
2. Connection of cells in tissue (due to folded outgrowths).
3. Receptor. The membranes have big number receptors are special proteins whose role is to transmit signals from the outside into the cell.