What is the structure of the plasma membrane? What are its functions? Functions, significance and structure of the plasma membrane.

Active

(with energy consumption)

Simple diffusion

(without proteins)

Energy source - ATP

Facilitated diffusion

(involving proteins)

Other types of sources

Through a channel in the protein

By way of a coup

protein with substance

Rice. 4. Classification of types of transport of substances through the membrane.

By active transfer, inorganic ions, amino acids and sugars, and almost all medicinal substances with polar molecules move through the membrane - para-aminobenzoic acid, sulfonamides, iodine, cardiac glycosides, B vitamins, corticosteroid hormones, etc.

To clearly illustrate the process of transfer of substances through the membrane, we present (with minor changes) Figure 5 taken from the book “Molecular Biology of the Cell” (1983) by B. Alberts and other scientists considered leaders in the development of the theory

Transported molecule

Channel Protein

protein transporter

Lipid Electrochemical

bilayer gradient

Simple diffusion Facilitated diffusion

Passive transport Active transport

Figure 5. Many small uncharged molecules pass freely through the lipid bilayer. Charged molecules, large uncharged molecules, and some small uncharged molecules pass through membranes through channels or pores or with the help of specific transporter proteins. Passive transport is always directed against the electrochemical gradient towards the establishment of equilibrium. Active transport occurs against an electrochemical gradient and requires energy.

transmembrane transport, reflects the main types of transfer of substances across the membrane. It should be noted that the proteins involved in transmembrane transport belong to integral proteins and are most often represented by one complex protein.

The transfer of high molecular weight protein molecules and other large molecules through the membrane into the cell is carried out by endocytosis (pinocytosis, phagocytosis and endocytosis), and from the cell by exocytosis. In all cases, these processes differ from above topics that the transported substance (particle, water, microorganisms, etc.) is first packaged into a membrane and in this form is transferred into the cell or released from the cell. The packaging process can occur both on the surface of the plasma membrane and inside the cell.

b. Transfer of information across the plasma membrane.

In addition to the proteins involved in the transfer of substances across the membrane, complex complexes of several proteins have been identified. Spatially separated, they are united by one finite function. Complex protein assemblies include a complex of proteins responsible for the production of a very powerful biologically active substance in the cell - cAMP (cyclic adenosine monophosphate). This ensemble of proteins contains both surface and integral proteins. For example, on the inner surface of the membrane there is a surface protein called G protein. This protein maintains the relationship between two adjacent integral proteins - a protein called the adrenaline receptor and an enzyme protein - adenylate cyclase. The adrenergic receptor is able to connect with adrenaline, which enters the intercellular space from the blood and become excited. This excitation is transmitted by the G-protein to adenylate cyclase, an enzyme capable of producing the active substance - cAMP. The latter enters the cytoplasm of the cell and activates a variety of enzymes in it. For example, an enzyme that breaks down glycogen into glucose is activated. The formation of glucose leads to an increase in mitochondrial activity and an increase in the synthesis of ATP, which enters all cellular compartments as an energy carrier, enhancing the work of the lysosome, sodium-potassium and calcium membrane pumps, ribosomes, etc. ultimately increasing the vital activity of almost all organs, especially muscles. This example, although very simplified, shows how the activity of the membrane is connected with the work of other elements of the cell. At the everyday level, this complex scheme looks quite simple. Imagine that a dog suddenly attacked a person. The resulting feeling of fear leads to the release of adrenaline into the blood. The latter binds to adrenergic receptors on the plasma membrane, thereby changing the chemical structure of the receptor. This, in turn, leads to a change in the structure of the G-protein. The altered G-protein becomes capable of activating adenylate cyclase, which enhances the production of cAMP. The latter stimulates the formation of glucose from glycogen. As a result, the synthesis of the energy-intensive ATP molecule is enhanced. The increased formation of energy in a person’s muscles leads to a quick and strong reaction to a dog’s attack (flight, defense, fight, etc.).

Lecture No. 4.

Number of hours: 2

Plasma membrane

1.

2.

3. Intercellular contacts.

1. Structure of the plasma membrane

Plasma membrane, or plasmalemma, is a superficial peripheral structure that limitscell outside and ensuring 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 large group organic matter, having poor solubility in water (hydrophobicity) and good solubility in organic solvents and fats (lipophilicity).Typical lipids found in the plasma membrane are phospholipids, sphingomyelins, and cholesterol. In plant cells, cholesterol is replaced by phytosterol. Based on 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).

Currently it is 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, i.e. they seem to float in a “lipid lake”. Therefore, protein molecules are able to move along the membrane, assemble into groups or, conversely, scatter on the surface of the membrane. This suggests that the plasma membrane is not a static, frozen formation.

Outside the plasmalemma there is a supra-membrane 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 plasmalemma glycoprotein complex. Carbohydrates form long, branching chains of polysaccharides associated with proteins and lipids of the plasma membrane. The glycocalyx may contain enzyme proteins involved in extracellular breakdown various substances. Products of enzymatic activity (amino acids, nucleotides, fatty acids, etc.) are transported across the plasma membrane and absorbed by cells.

The plasma membrane is constantly renewed. This occurs by detaching small bubbles from its surface into the cell and embedding vacuoles from inside the cell into the membrane. Thus, there is a constant flow of membrane elements in the cell: from the plasma membrane into the cytoplasm (endocytosis) and the flow of membrane structures from the cytoplasm to the cell surface (exocytosis). In membrane turnover, the leading role is played by 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 essential functions:

1) Barrier.The barrier function of the plasma membrane is tolimiting the free diffusion of substances from cell to cell, preventingrotating leakage of water-soluble cell contents. But sincethe cell must receive the necessary nutrients, youdivide end products of metabolism, regulate intracellularIf the concentration of ions is high, then special mechanisms for the transfer of substances through the cell membrane have been formed.

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

There are several mechanisms for transporting 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 of energysubstances from an area where its concentration is high to an area with low concentrationtion. During diffusion transport, the membrane functions as an osmotic barrier. The rate of diffusion depends on the magnitudemolecules and their relative solubility in fats. The fewer timesmeasures of molecules and the more fat-soluble (lipophilic) they are, the faster they will move through the lipid bilayer.Diffusion can be neutral(transfer of unchargedmolecules) and lightweight(with the help of special proteinscarriers). The rate of facilitated diffusion is higher than that of neutral diffusion.Maximum penetrationWater has the ability tohow its molecules are small and uncharged. Diffusion of water through cellsthe membrane is called osmo catfishIt is assumed that in cellsmembrane for penetrationwater and some ions arethere are special “pores”. Their numberis small, and the diameter isabout 0.3-0.8 nm. Diffuses most quickly through the membrane well, easily soluble in lipids bilayer of a molecule, such as O, and uncharged polar moleculessmall diameter lyes (SO, mo chevina).

Transfer of polar molecules (withsugars, amino acids), especiallymanufactured using special membrane transportproteins are called facilitated diffusion. Such proteins are foundarmed in all types biological membranes, and each specific This protein is designed to transport 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: proteins- carriers (transporters) And channel-forming proteins (whiteki channels). Carrier proteins transport molecules across the membrane, first changing their configuration. Channel-forming proteins form filled membranes water pores. When the pores are open, molecules of specific substances(usually inorganic ions of 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 its transport, in addition to the gradient, depends concentration, influences electric charge membranes (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 transfer of substances against an electrochemical gradient. It is always carried out by trans proteinsporters and closely related zan with energy sourcegii. In protein transfer there are plots binding to transporttitrated substance. The more such studies tkov contacts the thingthe higher the rategrowth of transport. Selective transfer of one substance is called uniport. Transfer of several substances is carried out Kotran sports systems. If the transfer goes in one direction -This simport, if in opposite – antiport. So,for example, glucose is transferred from the extracellular fluid into the cell uniportally. The transfer of glucose and Na 4 from the intestinal cavity orkidney tubules, respectively, into intestinal cells or blood is carried out symportally, and the transfer of C1~ and HCO is antiportor. It is assumed that during transfer, reversible conformational changes arise. changes in the transporter, which allows the movement of substances connected to it.

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 operates on the principle of antiport, pumping vaya Na "out of the cell and K t into the cell against their electrochemical gradients. Gradient Na+ creates osmotic pressure, maintains cell volume and ensures transport of sugars and amino acidsnoacids The operation of this pump consumes a third of all the energy necessary for the functioning of cells.When studying the mechanism of action Na+ - K+ the pump was installedIt is shown that it is an ATPase enzyme and a transmembrane protein. integral protein. In the presence Na+ and ATP under the influence of ATP-The terminal phosphate is separated from ATP and added to the remainderaspartic acid on an ATPase molecule. ATPase phos moleculeforylates, changes its configuration and Na+ is removed from cells. Following the withdrawal Na K" is always transported from the cell into the cell. To do this, 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 can be reversibly phosphorylated and dephosphorizedto be realised. Large subunit on the cytoplasmic sidedoes not have areas for binding Na+ and ATP, and on the outside -binding sites for 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 Naf from the cage and brings twoion K As a result, a current flows through the membrane, forming an electroderic potential with negative value in the inner part of the cell in relation to its outer surface. Na"-K+ the pump regulates cellular volume, controls the concentration of substancesinside the cell, maintains osmotic pressure, participates in the creation of membrane potential.

Transport in membrane packaging. Transfer of macromolecules (proteins, nucleic acids) through the membranelot, polysaccharides, lipoproteins) and other particles are carried out through the sequential formation and fusion of surroundedmembrane-bound vesicles (vesicles). Vesicular transport processit takes place in two stages. In the beginningvesicle membrane and plasmalemmastick together and then merge.For stage 2 to take place it is necessaryI wish you were water moleculesare crowded by interacting lipid bilayers, which approach to a distance of 1-5 nm. Counts Xia that this process is being activatedspecial fusion proteins(They isolated so far only from viruses). Vesicular transport hasimportant feature - absorbed or secreted macromolecules,located in bubbles, usually notmix with other macromolcules or organelles of the cell. Pu pimples can merge with the specifics chemical membranes, which providefacilitates the exchange of macromolecules betweenbetween the extracellular space andcontents of the cell. Likewisemacromolecules are transferred from one cell compartment to another.

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

Pinocytosisensures absorption of liquid and dissolvedsubstances in the form of small bubbles ( d =150 nm). Phagocytosis -this is the absorption of large particles, microorganismscall or fragments of organelles, cells. In this case they formthere are large vesicles, phagosomes or vacuoles ( d -250 nm or more). U protozoa phagocytic function - form of nutrition. In mammals, the phagocytic function is carried out by macrophages androfils, which protect the body from infection by absorbing invading microbes. Macrophages are also involved in the recyclingtions of old or damaged cells and their debris (in the bodyhuman macrophages daily absorb more than 100 old eritisrocytes). Phagocytosis begins only when the engulfed particlebinds to the surface of the phagocyte and activates specializedny receptor cells. Binding of particles to specific substancesmembrane receptors causes the formation of pseudopodia, whichThey envelop the particle and, merging at the edges, form a bubble -phagosome.The formation of a phagosome and phagocytosis itself occurmoves only if, during the enveloping process, the particleconstantly contacts the plasmalemma receptors, as if “stagnating” flashing lightning."

A significant portion of the material absorbed by the cell via endocytosis, ends its journey in lysosomes. Large particles includedare looking forward to phagosomes, which then fuse with lysosomes and form phagolysosomes. Liquid and macromolecules absorbed duringpinocytosis, are initially transferred to endosomes, which arethey fuse with lysosomes to form endolysosomes. I'm present various hydrolytic enzymes present in lysosomes quicklyro destroy macromolecules. Hydrolysis products (amino acidslots, 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 via exocytosis to the plasma membrane and are redistributed thereare lysed. The main biological significance of endocytosis is it is possible to obtain building blocks due to intracellular digestion of macromolecules in lysosomes.

The absorption of substances in eukaryotic cells begins in thecialized areas of the plasma membrane, the so-calledours bordered pits. In electron micrographspits look like invaginations of the plasma membrane, cytoplasmthe matte side of which is covered with a fibrous layer. Layer aswould border the small pits of the plaza Malemmas. Pits occupy about 2% vol.the surface of the cell membraneus eukaryotes. Within a minute the holes grow, they dig deeper and deeper Xia, are drawn into the cell and then, tapering at the base, split off,forming bordered bubbles.It has been established that from the plazafibroblast mat membraneComrade within one minute flakeabout a quarter is pouredmembranes in the form of bordered PU Zyrkov. Bubbles disappear quickly their border and acquire a wayability to fuse with the lysosome.

Endocytosis may be nonspecific(constitutive)And specific(receptor).At nonspecific endocytosis the cell takes over andabsorbs substances completely foreign to it, for example, soot particles,dyes. First, particles are deposited on the glycocalyx. plasmalemmas. They are especially well deposited (adsorbed) by positively charged groups of proteins, since the glycocalyx carries negative charge. Then the morphology of the cell changesmembranes. It can either sink, forming invaginations(invaginations), or, conversely, to form outgrowths, which seem to fold, separating small volumes liquid medium. The formation of intussusceptions is more typical for intestinal epithelial cells, amoebas, and outgrowths - for phagocytes and fibroblasts. These processes can be blocked with inhibitorsbreathing. The resulting vesicles are primary endosomes and can drain exchanging with each other, increasing in size. Later they will connect interact with lysosomes, turning into an endolysosome - digestive new vacuole. The intensity of liquid-phase nonspecific pinocytosis up toquite high. Macrophages form up to 125, and epithelial cells thinlyth intestines up to a thousand pinos per minute. The abundance of pinosomes leads to the fact that the plasmalemma is quickly spent on the formation of manyof small vacuoles. Membrane restoration is quite fasttro during recycling during exocytosis due to the return of vacuoles and their integration into the plasmalemma. Macrophages have all plasmaThe chemical membrane is replaced in 30 minutes, and in fibroblasts in 2 hours.

More in an efficient way absorption from extracellular fluidbone specific macromolecules is specific en docytosis(receptor-mediated). At the same time, macromoleculesbind to complementary receptors on the surfacecells accumulate in the bordered pit, 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 with receptorstorus are called ligands. Using the receptor endocytosis in many animal cells absorption occurscholesterol from extracellular 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 plasma membrane and the vacuole membrane merge and the contents of the vacuole enter the environment.Some protozoa have places on cell membrane for exocytosis are predetermined. So, in the plasma membrane Some ciliated ciliates have certain areas with the correct arrangement of large globules of integral proteins. Umucocysts and trichocysts of ciliates are completely ready for secretion; on the upper part of the plasmalemma there is a rim of integral globulesproteins. These areas of the membrane of mucocysts and trichocysts are adjacentadhere to the cell surface.A kind of exocytosis is observed in neutrophils. They aremay, under certain conditions, be released into the environmentdo your lysosomes. In some cases, small outgrowths of the plasmalemma containing lysosomes are formed, which then break off and move into the medium. In other cases, invagination of the plasmalemma deep into the cell and its capture of lysosomes is observed, located located far from the cell surface.

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

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

The whole variety of informational intercellular interactions can be schematically represented as a chain of sequentialsignal-receptor-second messenger-response reactions (concept signal-response).Signals transmit information from cell to cellmolecules that are produced in certain cells and specialphysically influence other signal-sensitive cells (cells) sheni). Signal molecule - primary intermediary tying interacts with receptors located on target cells, reacts transmitting only to certain signals. Signal molecules - ligands- fits its receptor like a key to a lock. Ligand-for membrane receptors (plasmalemma receptors) arehydrophilic molecules, peptide hormones, neuromedia- tors, cytokines, antibodies, and for nuclear receptors - fat Roman molecules, steroid and thyroid hormones, vitamin DAs receptors on topprotein may act as a cellmembranes or glycocalyx elementsca - polysaccharides and glycoproteins.It is believed that they are sensitive toareas, scatteredsan on the surface of the cell or withbranes into small zones. Yes, onsurface of prokaryotic cellsand animal cells there are limitslimited number of places with which they canbind viral particles. Memeswear proteins (transporters and canaly) recognize, interact and transfercarry only certain substances.Cellular receptors are involved intransmitting signals from the surface of the cell into it.Diversity and specificitymoat of receptors on the cell surfaceleads to the creation of a very complex systemwe have markers that allow us to distinguishyour cells from others. Similar cellsinteract with each other, their surfaces can stick together (conjugationprotozoa, tissue formation in multicellular organisms). I don't perceive cellscommon markers, as well as those that differ inboron of determinant markerscling to or reject.Upon formation of the receptor-ligand complex, they are activatedtransmembrane proteins: transformer protein, enhancer protein.As a result, the receptor changes its conformation and interactionexists with the precursor of the second messenger located in the cell ka - messenger.Messengers can be ionized calcium, phospholipidfor C, adenylate cyclase, guanylate cyclase. Under the influence of the messengerenzymes involved in the synthesis are activated cyclic monophosphates - AMP or GMF. The latter change the assetthe presence of two types of protein kinase enzymes in the cell cytoplasm, leading to the phosphorylation of numerous intracellular proteins.

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

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 influence of cGMP, the secretion of insulin by the pancreas, histamine by mast cells, and serotonin by the throm increasesbocytes, smooth muscle tissue contracts.

In many cases, when a receptor-ligand complex is formedthere 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.

Specific receptors are located on the plasma membrane tors that respond to physical factors. Thus, in photosynthetic bacteria, chlorophylls are located on the cell surface,responsive to light. In photosensitive animals in plasmachelic membrane is located the whole system phogoreceptor proteinsrhodopsins, with the help of which the light stimulus transforms converted into a chemical signal and then an electrical impulse.

3. Intercellular contacts

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

§ Simple contact.Simple contact occurs among most adjacent cells of different origins. It 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 (closed) contact. With this connection, the outer layers of the two plasma membranes are as close as possible. The rapprochement is so close that it is as if the plasmalemma sections of two neighboring cells are merging. Membrane fusion does not occur over the entire area of ​​tight contact, but represents a series of point-like convergences of membranes. The role of the tight junction is to mechanically connect cells to each other. This area is impermeable to macromolecules and ions and, therefore, it closes and delimits intercellular gaps (and with them the actual internal environment body) from external environment.

§ Cohesion spot, or desmosome. The desmosome is a small area with a diameter of up to 0.5 microns. In the desmosome zone on the cytoplasmic side there is an area of ​​thin fibrils. The functional role of desmosomes is mainly mechanical communication between cells.

§ Gap junction, or nexus. With this type of contact, the plasmalemmas of neighboring cells are separated by a gap of 2-3 nm over a distance of 0.5-3 µm. The structure of the plasma membranes contains special protein complexes (connexons). One connexon on the plasma membrane of a cell is exactly opposed by a connexon on the plasma membrane of an 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 transport ions and small molecules from cell to cell. Thus, in the cardiac muscle, excitation, which is based on the process of changing ionic permeability, is transmitted from cell to cell through the nexus.

§ Synaptic contact, or synapse. Synapses are areas of contact between two cells specialized for the unilateral 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 transmitter are detected. At the moment of passage of a nerve impulse, synaptic vesicles release the transmitter 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 nerve impulses, synapses provide a rigid connection between the surfaces of two interacting cells.

§ Plasmodesmata.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 large (up to 1000 per cell). As cells age, their number decreases due to ruptures as the thickness of the cell wall increases. The functional role of plasmodesmata is to ensure intercellular circulation of solutions containing nutrients, ions and other compounds. Through plasmodesmata, cells are infected 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 diameter of microvilli is about 100 nm. Their number and length are different different types cells. The significance of microvilli is to significantly increase the cell surface area. This is especially important for cells involved in absorption. Thus, in the intestinal epithelium there are up to 2x10 8 microvilli per 1 mm 2 of surface.

The article is a summary of a lesson-study and primary consolidation of new knowledge (course “General Biology”, grade 10, according to the program of V.B. Zakharov).

Tasks:

1. formation of knowledge about the structure, properties and functions of the inner layer of the cell membrane - the plasma membrane (and, using its example, other cell membranes), using a soap bubble as a model.
2. development of the concept of the correspondence of the structure to the functions performed.
3. primary consolidation of acquired knowledge using tasks in the Unified State Exam format.

Equipment:

1. table “Structure of plant and animal cells according to light and electron microscopes.”
2. detergent solution (to produce soap bubbles), plastic tube, thin sewing needle.
3. blackboard drawing: molecular models<Figure 1 >.
4. didactic materials with tasks in the Unified State Exam format.

Lesson progress

Teacher: In the last lesson, we conducted laboratory work “Plasmolysis and deplasmolysis in onion skin cells,” during which we became acquainted with interesting phenomena. What is their essence?

Students: When plant tissue (the epidermis of onion scales) was placed in a hypertonic solution of sodium chloride (NaCl), there was no diffusion of this solution into the cells, but a release of water from the cell vacuoles towards the hypertonic NaCl solution in order to balance the concentrations of ions on both sides of the cell membrane. At the same time, the volume of vacuoles and the entire cytoplasm as a whole decreased, which led to the separation of the cytoplasm from the cell wall - plasmolysis. When returning the tissue under study to clean water, we also did not observe the release of solutes from the vacuoles, but only the flow of water from the surrounding space into the cell, into the vacuoles with cell sap, which led to the restoration of the cell volume to its previous boundaries - deplasmolysis.

Teacher: What conclusion can be drawn from the experiment?

Students: Probably, the cell surface freely allows water to pass in both directions, but retains Na + and Cl - ions that are part of table salt.

Teacher: The property we discovered is called selective permeability or semipermeability of the plasma membrane.

What is a plasma membrane (or plasma membrane), what is its structure, properties and functions, we must understand in today's lesson. As we agreed, the lesson will be taught by your comrades who prepared a lecture on cell membranes. Your task is to write down basic information about cell membranes while listening. You will have to apply the acquired knowledge by answering the test question at the end of the lesson.

Lecturer 1. Membrane structure.

The plasma membrane is present in all cells (under the glycocalyx in animals and under the cell wall in other organisms), it ensures the interaction of the cell with its environment. The plasmalemma forms a mobile surface of the cell, which can have outgrowths and invaginations, performs wave-like oscillatory movements, and macromolecules constantly move in it.

Despite these continuous changes, the cell always remains surrounded by a tight membrane. The plasma membrane is a thin film less than 10 nm thick. Even if its thickness is increased by 1 million times, we will get a value of only about 1 cm, while if the entire cell is increased by 1 million times, its size will be comparable to a fairly large audience.

The membrane contains two main types of molecules: phospholipids, forming bilayer in the thickness of the membrane, and squirrels on its surfaces. These molecules are held together by non-covalent interactions. This sandwich-like membrane model was proposed by American scientists Danieli and Dawson in 1935. With the advent of the electron microscope, it was confirmed and somewhat modified. Currently accepted fluid mosaic membrane model, according to which protein molecules floating in a liquid lipid bilayer form a kind of mosaic in it. A diagram of this modern model, proposed in 1972 by Singer and Nicholson, is given in the textbook.

Some proteins have carbohydrates covalently attached to their outer surface, forming glycoproteins– peculiar molecular antennas that are receptors. Glycoproteins are involved in the recognition of external signals coming from the environment or from other parts of the body itself, and in the response of cells to their influence. Such mutual recognition is a necessary stage preceding fertilization, as well as the adhesion of cells in the process of tissue differentiation. Recognition is also associated with the regulation of the transport of molecules and ions through the membrane, as well as the immune response, in which glycoproteins play the role of antigens.

Lecturer 2.Membrane properties.

To understand what properties these microscopic structures have, let's take a soap bubble as a model. The fact is that the molecules of soap and phospholipids that make up the membranes have a similar structure<Figure 1>. Soaps (salts of fatty acids) have in their structure hydrophilic head(from a charged carboxyl group) and long hydrophobic tail. Phospholipids that make up membranes also have a hydrophobic tail (of two fatty acid chains) and a large hydrophilic head containing a negatively charged phosphoric acid group.

Rice. 1. Models of molecules.

When substances of a similar structure are mixed with water, their molecules spontaneously take on the following configuration: the hydrophilic heads are immersed in water, and the hydrophobic tails do not come into contact with water, contacting only each other and with other hydrophobic substances that may be around, for example, with air . Finding themselves at the boundary between two environments of a similar nature, both soap molecules and phospholipid molecules are capable of forming a bilayer. Some of the important properties of biological membranes (like soap bubbles), listed below, are explained by the structure of the lipid bilayer.

A) Mobility.

A lipid bilayer is essentially a liquid formation, within the plane of which molecules can move freely - “flow” without losing contacts due to mutual attraction ( “lecturer” demonstrates the flow of liquid in the wall of a soap bubble hanging on a plastic tube). The hydrophobic tails can slide freely past each other.

b) Self-locking ability.

“The lecturer” demonstrates how when a soap bubble is pierced and the needle is subsequently removed, the integrity of its wall is immediately restored. Thanks to this ability, cells can fuse by fusing their plasma membranes (for example, during the development of muscle tissue). The same effect is observed when cutting a cell into two parts with a microknife, after which each part is surrounded by a closed plasma membrane.

V) Selective permeability.

That is, impermeability to water-soluble molecules due to the oily film formed by the hydrophobic tails of phospholipid molecules. To physically penetrate such a film, the substance itself must be hydrophobic, or it can squeeze through random gaps formed as a result of molecular movements (small molecules, such as water molecules).

Proteins that penetrate the entire thickness of the membrane, or are located on its outer and inner surfaces, help the cell exchange substances with environment. Protein molecules provide selective transport of substances across the membrane, being enzymes; in addition, pores are formed inside protein molecules or between neighboring molecules, through which water and some ions passively enter the cells.

Lecturer 3. Functions of the plasma membrane.

What does a structure with such a structure and properties serve for a cell? It turns out that she:

1. Gives the cell shape and protects from physical and chemical damage.
2. Thanks to its mobility, the ability to form outgrowths and protrusions, it carries out contact and interaction of cells in tissues and organs.
3. Separates the cellular environment from the external environment and maintains their differences.
4. It is a kind of indicator of cell type due to the fact that proteins and carbohydrates on the surface of membranes and different cells are not the same.
5. Regulates the exchange between the cell and the environment, selectively ensuring the transport of nutrients into the cell and the removal of final metabolic products to the outside.

Lecturer 4. I want to tell you how it happens transport across the plasma membrane, and similarly through other cell membranes. Transport can be passive, which does not require energy, and active, energy-dependent, during which energy is consumed resulting from the hydrolysis of ATP molecules.

1. Diffusion.

This is a passive process; the movement of substances occurs from an area of ​​high concentration to an area of ​​low concentration. Gases and lipophilic (fat-soluble) molecules diffuse quickly, ions and small polar molecules (glucose, amino acids, fatty acids) diffuse slowly. Diffusion is accelerated by pores in protein molecules.

A type of diffusion is osmosis– movement of water through the membrane.

2. Endocytosis.

This is the active transport of substances across the membrane into the cell (exocytosis - out of the cell). Depending on the nature of the substance transferred through the membrane, two types of these processes are distinguished: if a dense substance is transferred - phagocytosis(from the Greek “phagos” - devour and “cytos” - cell), if drops of liquid containing various substances in a dissolved or suspended state, then - pinocytosis(from the Greek “pino” - drink and “cytos” - cell).

The principle of transfer in both cases is identical: in the place where the surface of the cell comes into contact with a particle or drop of a substance, the membrane bends, forms a depression and surrounds the particle or drop of liquid, which is immersed in a “membrane package” inside the cell. A digestive vacuole is formed here, and organic substances entering the cell are digested in it. Phagocytosis is widespread in animals, and pinocytosis is carried out by cells of animals, plants, fungi, bacteria and blue-green algae.

3. Active transport using membrane-embedded enzymes.

The transfer occurs against the concentration gradient with energy expenditure, for example, potassium ions enter (“pumped”) into the cell, and sodium ions are removed (“pumped”) out of the cell. This work is accompanied by the accumulation of an electrical potential difference on the membrane. Such cellular transport systems are usually called “ pumps" The transport of amino acids and sugars is carried out similarly.

Conclusions:

1. Plasmalemma is a thin, about 10 nm thick, film on the surface of the cell. It includes lipoprotein structures (lipids and proteins).
2. Some surface molecules of proteins have carbohydrate molecules attached (they are associated with the recognition mechanism).
3. Membrane lipids spontaneously form a bilayer. This determines the selective permeability of the membrane.
4. Membrane proteins perform a variety of functions and significantly facilitate transport across the membrane.
5. Membrane lipids and proteins are able to move in the plane of the membrane, due to which the cell surface is not perfectly smooth.

To consolidate the information received in the lesson, students are offered tasks in the Unified State Exam format.

Part "A"

Choose one correct answer.

A1. The structure and functions of the plasma membrane are determined by the molecules that make up it:

1) glycogen and starch
2) DNA and ATP
3) proteins and lipids
4) fiber and glucose

A2. The plasma membrane does not perform the function of:

1) transport of substances
2) cell protection
3) interaction with other cells
4) protein synthesis

A3. Carbohydrates included in the structure of the cell membrane perform the following functions:

1) transport of substances
2) receptor
3) formation of a double layer of membrane
4) photosynthesis

A4. Proteins included in the structure of the cell membrane perform the following functions:

1) construction
2) protective
3) transport
4) all specified functions

A5. Phagocytosis is:

1) absorption of fluid by the cell
2) capture of solid particles
3) transport of substances across the membrane
4) bio acceleration chemical reactions

A6. The hydrophilic surfaces of the membranes are formed:

1) non-polar tails of lipids
2) polar heads of lipids
3) proteins
4) carbohydrates

A7. The passage of Na + and K + ions through the membrane occurs through:

1) diffusion
2) osmosis
3) active transfer
4) not implemented

A8. The following passes freely through the lipid layer of the membrane:

1) water
2) broadcast
3) glucose
4) starch

Part "B"

1) active transport consumes energy
2) phagocytosis is a type of endocytosis
3) diffusion is a type of active transport
4) the cell wall of plants consists of cellulose
5) osmosis is the diffusion of water
6) pinocytosis is a type of phagocytosis
7) plasmalemma consists of three layers of lipids
8) at animal cell no cell wall
9) plasmalemma provides communication between the cell and its environment

Part "C"

Tasks with free detailed answer

C1. What is the meaning of endocytosis:

a) for protozoa and lower invertebrates?
b) for highly organized animals and humans?

C2. What is the physical basis of vacuolar transport in a cell?

C3. What's it like biological significance irregularities in the surface of the plasmalemma of some cells (microvilli, cilia, etc.)?

C4. The electric stingray and electric eel stun their prey with discharges of several hundred volts. What properties of cell plasma membranes support the possibility of creating such discharges?

C5. How does the plasmalemma function to provide the cell with an “identity card”?

Answers to assignments.

Part "A".

1–3, 2–4, 3–2, 4–4, 5–2, 6–2, 7–3, 8–2.

Part "B".

1, 2, 4, 5, 8, 9 – “yes”; 3, 6, 7 – “no”

Part "C".

1a. The possibility of food entering the cells and further digestion in lysosomes.

1b. The phagocytic activity of leukocytes is of great importance in protecting the body from pathogenic bacteria and other unwanted particles. Pinocytosis in renal tubular cells leads to the absorption of proteins from primary urine.

2. The main properties of lipid bilayers are the ability of membranes to close.

3. Increasing the surface area of ​​the cell for exchange between the cell and its environment.

4. The presence of enzyme systems that carry out active transport (“pumps”) leads to a redistribution of charges on the plasmalemma and the creation of a membrane potential difference.

5. For this, there are a number of specific chemical groups on the surface of the membrane - “antennas”, which are, most often, glycoproteins.

The kernel is responsible for storage 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?

Speaking in simple language, this is the shell. However, it is not always completely impenetrable. Transport of certain substances through the membrane is almost always allowed.

In cytology, membranes can be divided into two main types. The first is the plasma membrane, which 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 double-membrane group.

Membranes can also be present inside organelles. These are usually derivatives of the inner membrane of double-membrane organelles.

How are the membranes of double-membrane organelles arranged?

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

Thus, the mitochondrial membrane has inward projections - cristae or ridges. A cycle of chemical reactions necessary for cellular respiration occurs on them.

Derivatives of the inner membrane of chloroplasts are disc-shaped sacs - thylakoids. They are collected in stacks - grana. The individual granae are united with each other using lamellae - long structures also formed from membranes.

Structure of membranes of single-membrane organelles

Such organelles have one membrane. It is usually a smooth shell consisting of lipids and proteins.

Features of the structure of the cell plasma membrane

The membrane consists of substances such as lipids and proteins. The structure of the plasma membrane provides for its thickness to be 7-11 nanometers. The bulk of the membrane consists of lipids.

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

Plasma membrane lipids

Lipids that make up the plasma membrane are divided into three groups: steroids, sphingophospholipids and glycerophospholipids. The molecule of the latter contains a 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 the tails. The head is hydrophilic (i.e., dissolves in water), and the tails are hydrophobic (they repel water, but dissolve in organic solvents). Due to this structure, the glycerophospholipid molecule can be called amphiphilic, i.e., both hydrophobic and hydrophilic at the same time.

Sphingophospholipids are similar in chemical structure to glycerophospholipids. But they differ from those mentioned above in that instead of a glycerol residue they contain 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 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 located 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 located 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. This is why the structure of the plasma membrane of a cell cannot be called static. It is dynamic because 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 thing is the limitation of the cytoplasm. Thanks to this, the cell has a constant shape and size. This function is achieved due to 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 can change their spatial structure under the influence of certain conditions, the plasma membrane can send signals to cells.

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