The structure of the cell membrane and its functions. What is the function of the cell membrane - its properties and functions

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Cells are separated from the internal environment of the body by a cell or plasma membrane.

The membrane provides:

1) Selective penetration into and out of the cell of molecules and ions necessary to perform specific cell functions;
2) Selective transport of ions across the membrane, maintaining a transmembrane electric potential difference;
3) The specifics of intercellular contacts.

Due to the presence in the membrane of numerous receptors that perceive chemical signals - hormones, mediators and other biologically active substances, it is able to change the metabolic activity of the cell. Membranes provide the specificity of immune manifestations due to the presence of antigens on them - structures that cause the formation of antibodies that can specifically bind to these antigens.
The nucleus and organelles of the cell are also separated from the cytoplasm by membranes that prevent the free movement of water and substances dissolved in it from the cytoplasm to them and vice versa. This creates conditions for the separation of biochemical processes occurring in different compartments (compartments) inside the cell.

cell membrane structure

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The cell membrane is an elastic structure, with a thickness of 7 to 11 nm (Fig. 1.1). It consists mainly of lipids and proteins. From 40 to 90% of all lipids are phospholipids - phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin and phosphatidylinositol. An important component of the membrane are glycolipids, represented by cerebrosides, sulfatides, gangliosides and cholesterol.

Rice. 1.1 Organization of the membrane.

The main structure of the cell membrane is a double layer of phospholipid molecules. Due to hydrophobic interactions, the carbohydrate chains of lipid molecules are held near each other in an extended state. Groups of phospholipid molecules of both layers interact with protein molecules immersed in the lipid membrane. Due to the fact that most of the lipid components of the bilayer are in a liquid state, the membrane has mobility and undulates. Its sections, as well as proteins immersed in the lipid bilayer, will mix from one part to another. Mobility (fluidity) of cell membranes facilitates the transport of substances through the membrane.

cell membrane proteins represented mainly by glycoproteins. Distinguish:

integral proteins penetrating through the entire thickness of the membrane and
peripheral proteins attached only to the surface of the membrane, mainly to its inner part.

Peripheral proteins almost all function as enzymes (acetylcholinesterase, acid and alkaline phosphatases, etc.). But some enzymes are also represented by integral proteins - ATPase.

integral proteins provide a selective exchange of ions through the membrane channels between the extracellular and intracellular fluid, and also act as proteins - carriers of large molecules.

Membrane receptors and antigens can be represented by both integral and peripheral proteins.

Proteins adjacent to the membrane from the cytoplasmic side belong to cell cytoskeleton . They can attach to membrane proteins.

So, protein strip 3 (band number during protein electrophoresis) of erythrocyte membranes is combined into an ensemble with other cytoskeleton molecules - spectrin through the low molecular weight protein ankyrin (Fig. 1.2).

Rice. 1.2 Scheme of the arrangement of proteins in the membrane cytoskeleton of erythrocytes.
1 - spectrin; 2 - ankyrin; 3 - protein band 3; 4 - protein band 4.1; 5 - protein band 4.9; 6 - actin oligomer; 7 - protein 6; 8 - gpicophorin A; 9 - membrane.

Spectrin is the main protein of the cytoskeleton, constituting a two-dimensional network to which actin is attached.

actin forms microfilaments, which are the contractile apparatus of the cytoskeleton.

cytoskeleton allows the cell to exhibit flexibly elastic properties, provides additional strength to the membrane.

Most integral proteins are glycoproteins. Their carbohydrate part protrudes from the cell membrane to the outside. Many glycoproteins have a large negative charge due to the significant content of sialic acid (for example, the glycophorin molecule). This provides the surface of most cells with a negative charge, helping to repel other negatively charged objects. Carbohydrate protrusions of glycoproteins carry blood group antigens, other antigenic determinants of the cell, and act as hormone-binding receptors. Glycoproteins form adhesive molecules that cause cells to attach to each other, i.e. close intercellular contacts.

Features of metabolism in the membrane

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Membrane components are subject to many metabolic transformations under the influence of enzymes located on their membrane or inside it. These include oxidative enzymes that play an important role in modifying the hydrophobic elements of membranes - cholesterol, etc. In membranes, when enzymes - phospholipases are activated, biologically active compounds - prostaglandins and their derivatives - are formed from arachidonic acid. As a result of the activation of phospholipid metabolism in the membrane, thromboxanes and leukotrienes are formed, which have a powerful effect on platelet adhesion, inflammation, etc.

The membrane constantly undergoes renewal processes of its components. . Thus, the lifetime of membrane proteins ranges from 2 to 5 days. However, there are mechanisms in the cell that ensure the delivery of newly synthesized protein molecules to membrane receptors, which facilitate the incorporation of the protein into the membrane. The "recognition" of this receptor by the newly synthesized protein is facilitated by the formation of a signal peptide, which helps to find the receptor on the membrane.

Membrane lipids also have a significant metabolic rate., which requires a large amount of fatty acids for the synthesis of these membrane components.
The specifics of the lipid composition of cell membranes are affected by changes in the human environment and the nature of his diet.

For example, an increase in dietary fatty acids with unsaturated bonds increases the liquid state of lipids in cell membranes of various tissues, leads to a change in the ratio of phospholipids to sphingomyelins and lipids to proteins that is favorable for the function of the cell membrane.

Excess cholesterol in membranes, on the contrary, increases the microviscosity of their bilayer of phospholipid molecules, reducing the rate of diffusion of certain substances through cell membranes.

Food enriched with vitamins A, E, C, P improves lipid metabolism in erythrocyte membranes, reduces membrane microviscosity. This increases the deformability of erythrocytes, facilitates their transport function (Chapter 6).

Deficiency of fatty acids and cholesterol in food disrupts the lipid composition and function of cell membranes.

For example, a fat deficiency disrupts the function of the neutrophil membrane, which inhibits their ability to move and phagocytosis (active capture and absorption of microscopic foreign living objects and solid particles by unicellular organisms or some cells).

In the regulation of the lipid composition of membranes and their permeability, regulation of cell proliferation an important role is played by reactive oxygen species, which are formed in the cell in conjunction with normal metabolic reactions (microsomal oxidation, etc.).

Formed reactive oxygen species- superoxide radical (O 2), hydrogen peroxide (H 2 O 2), etc. are extremely reactive substances. Their main substrate in free radical oxidation reactions are unsaturated fatty acids that are part of cell membrane phospholipids (the so-called lipid peroxidation reactions). The intensification of these reactions can cause damage to the cell membrane, its barrier, receptor and metabolic functions, modification of nucleic acid molecules and proteins, which leads to mutations and inactivation of enzymes.

Under physiological conditions, the intensification of lipid peroxidation is regulated by the antioxidant system of cells, represented by enzymes that inactivate reactive oxygen species - superoxide dismutase, catalase, peroxidase and substances with antioxidant activity - tocopherol (vitamin E), ubiquinone, etc. A pronounced protective effect on cell membranes (cytoprotective effect) with various damaging effects on the body, prostaglandins E and J2 have, "extinguishing" the activation of free radical oxidation. Prostaglandins protect the gastric mucosa and hepatocytes from chemical damage, neurons, neuroglial cells, cardiomyocytes - from hypoxic damage, skeletal muscles - during heavy physical exertion. Prostaglandins, binding to specific receptors on cell membranes, stabilize the bilayer of the latter, reduce the loss of phospholipids by membranes.

Membrane receptor functions

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A chemical or mechanical signal is first perceived by cell membrane receptors. The consequence of this is the chemical modification of membrane proteins, which leads to the activation of "second messengers" that ensure the rapid propagation of the signal in the cell to its genome, enzymes, contractile elements, etc.

Schematically, transmembrane signaling in a cell can be represented as follows:

1) Excited by the perceived signal, the receptor activates the γ-proteins of the cell membrane. This occurs when they bind guanosine triphosphate (GTP).

2) The interaction of the "GTP-y-proteins" complex, in turn, activates the enzyme - the precursor of secondary messengers, located on the inner side of the membrane.

The precursor of one secondary messenger - cAMP, formed from ATP, is the enzyme adenylate cyclase;
The precursor of other secondary messengers - inositol triphosphate and diacylglycerol, formed from membrane phosphatidylinositol-4,5-diphosphate, is the enzyme phospholipase C. In addition, inositol triphosphate mobilizes another secondary messenger in the cell - calcium ions, which are involved in almost all regulatory processes in the cell. For example, the resulting inositol triphosphate causes the release of calcium from the endoplasmic reticulum and an increase in its concentration in the cytoplasm, thereby including various forms of cellular response. With the help of inositol triphosphate and diacylglycerol, the function of smooth muscles and B-cells of the pancreas is regulated by acetylcholine, the anterior pituitary thyropin-releasing factor, the response of lymphocytes to antigen, etc.
In some cells, the role of the second messenger is performed by cGMP, which is formed from GTP with the help of the enzyme guanylate cyclase. It serves, for example, as a second messenger for natriuretic hormone in the smooth muscle of blood vessel walls. cAMP serves as a second messenger for many hormones - adrenaline, erythropoietin, etc. (Chapter 3).

Nature has created many organisms and cells, but despite this, the structure and most of the functions of biological membranes are the same, which allows us to consider their structure and study their key properties without being tied to a particular type of cell.

What is a membrane?

Membranes are a protective element that is an integral part of the cell of any living organism.

The structural and functional unit of all living organisms on the planet is the cell. Its vital activity is inextricably linked with the environment with which it exchanges energy, information, matter. So, the nutritional energy necessary for the functioning of the cell comes from outside and is spent on the implementation of its various functions.

The structure of the simplest structural unit of a living organism: organelle membrane, various inclusions. It is surrounded by a membrane, inside which the nucleus and all organelles are located. These are mitochondria, lysosomes, ribosomes, endoplasmic reticulum. Each structural element has its own membrane.

Role in the life of the cell

The biological membrane plays a culminating role in the structure and functioning of an elementary living system. Only a cell surrounded by a protective shell can rightly be called an organism. A process such as metabolism is also carried out due to the presence of a membrane. If its structural integrity is violated, this leads to a change in the functional state of the organism as a whole.

Cell membrane and its functions

It separates the cytoplasm of the cell from the external environment or from the membrane. The cell membrane ensures the proper performance of specific functions, the specifics of intercellular contacts and immune manifestations, and supports the transmembrane difference in electrical potential. It contains receptors that can perceive chemical signals - hormones, mediators and other biologically active components. These receptors give it another ability - to change the metabolic activity of the cell.

Membrane functions:

1. Active transfer of substances.

2. Passive transfer of substances:

2.1. Diffusion is simple.

2.2. transport through the pores.

2.3. Transport carried out by diffusion of a carrier along with a membrane substance or by relaying a substance along the molecular chain of a carrier.

3. Transfer of non-electrolytes due to simple and facilitated diffusion.

The structure of the cell membrane

The components of the cell membrane are lipids and proteins.

Lipids: phospholipids, phosphatidylethanolamine, sphingomyelin, phosphatidylinositol and phosphatidylserine, glycolipids. The proportion of lipids is 40-90%.

Proteins: peripheral, integral (glycoproteins), spectrin, actin, cytoskeleton.

The main structural element is a double layer of phospholipid molecules.

Roof membrane: definition and typology

Some statistics. On the territory of the Russian Federation, the membrane has been used as a roofing material not so long ago. The share of membrane roofs from the total number of soft roof slabs is only 1.5%. Bituminous and mastic roofs have become more widespread in Russia. But in Western Europe, membrane roofs account for 87%. The difference is palpable.

As a rule, the membrane as the main material in the roof overlap is ideal for flat roofs. For those with a large bias, it is less suitable.

The volumes of production and sales of membrane roofs in the domestic market have a positive growth trend. Why? The reasons are more than clear:

The service life is about 60 years. Imagine, only the warranty period of use, which is set by the manufacturer, reaches 20 years.
Ease of installation. For comparison: the installation of a bituminous roof takes 1.5 times more time than the installation of a membrane floor.
Ease of maintenance and repair work.

The thickness of roofing membranes can be 0.8-2 mm, and the average weight of one square meter is 1.3 kg.

Properties of roofing membranes:

elasticity;
strength;
resistance to ultraviolet rays and other aggressor media;
frost resistance;
fire resistance.

There are three types of roofing membrane. The main classification feature is the type of polymeric material that makes up the base of the canvas. So, roofing membranes are:

belonging to the EPDM group, are made on the basis of polymerized ethylene-propylene-diene monomer, in other words, Advantages: high strength, elasticity, water resistance, environmental friendliness, low cost. Disadvantages: adhesive technology for joining canvases using a special tape, low strength joints. Scope of application: used as a waterproofing material for tunnel ceilings, water sources, waste storages, artificial and natural reservoirs, etc.
PVC membranes. These are shells, in the production of which polyvinyl chloride is used as the main material. Advantages: UV resistance, fire resistance, extensive color range of membrane sheets. Disadvantages: low resistance to bituminous materials, oils, solvents; emits harmful substances into the atmosphere; the color of the canvas fades over time.
TPO. Made from thermoplastic olefins. They can be reinforced and non-reinforced. The first are equipped with a polyester mesh or fiberglass cloth. Advantages: environmental friendliness, durability, high elasticity, temperature resistance (both at high and low temperatures), welded joints of the seams of the canvases. Disadvantages: high price category, lack of manufacturers in the domestic market.

Profiled membrane: characteristics, functions and benefits

Profiled membranes are an innovation in the construction market. Such a membrane is used as a waterproofing material.

The material used in the manufacture is polyethylene. The latter is of two types: high pressure polyethylene (LDPE) and low pressure polyethylene (HDPE).

Technical characteristics of the membrane from LDPE and HDPE

Indicator
Tensile strength (MPa)
Tensile elongation (%)
Density (kg / m3)
Compressive strength (MPa)
Impact strength (notched) (KJ/sqm)
Flexural modulus (MPa)
Hardness (MPa)
Operating temperature (˚С)	-60 to +80	-60 to +80
Daily rate of water absorption (%)

The profiled membrane made of high pressure polyethylene has a special surface - hollow pimples. The height of these formations can vary from 7 to 20 mm. The inner surface of the membrane is smooth. This enables trouble-free bending of building materials.

A change in the shape of individual sections of the membrane is excluded, since the pressure is evenly distributed over its entire area due to the presence of all the same protrusions. Geomembrane can be used as ventilation insulation. In this case, free heat exchange inside the building is ensured.

Benefits of profiled membranes:

increased strength;
heat resistance;
stability of chemical and biological influence;
long service life (more than 50 years);
ease of installation and maintenance;
affordable cost.

Profiled membranes are of three types:

with a single layer;
with a two-layer canvas = geotextile + drainage membrane;
with a three-layer canvas = slippery surface + geotextile + drainage membrane.

A single-layer profiled membrane is used to protect the main waterproofing, installation and dismantling of concrete preparation of walls with high humidity. A two-layer protective one is used during equipment. A three-layer one is used on soil that lends itself to frost heaving and deep soil.

Areas of use for drainage membranes

The profiled membrane finds its application in the following areas:

Basic foundation waterproofing. Provides reliable protection against the destructive influence of groundwater, plant root systems, soil subsidence, and mechanical damage.
Foundation wall drainage. Neutralizes the impact of groundwater, precipitation by transferring them to drainage systems.
Horizontal type - protection against deformation due to structural features.
An analogue of concrete preparation. It is used in the case of construction work on the construction of buildings in the zone of low groundwater, in cases where horizontal waterproofing is used to protect against capillary moisture. Also, the functions of the profiled membrane include the impermeability of cement laitance into the soil.
Ventilation of wall surfaces with a high level of humidity. It can be installed both on the inside and on the outside of the room. In the first case, air circulation is activated, and in the second, optimal humidity and temperature are ensured.
Used inverted roof.

Super diffusion membrane

The superdiffusion membrane is a material of a new generation, the main purpose of which is to protect the elements of the roof structure from wind phenomena, precipitation, and steam.

The production of protective material is based on the use of nonwovens, high quality dense fibers. In the domestic market, a three-layer and four-layer membrane is popular. Reviews of experts and consumers confirm that the more layers underlie the design, the stronger its protective functions, and therefore the higher the energy efficiency of the room as a whole.

Depending on the type of roof, its design features, climatic conditions, manufacturers recommend giving preference to one or another type of diffusion membranes. So, they exist for pitched roofs of complex and simple structures, for pitched roofs with a minimum slope, for folded roofs, etc.

The superdiffusion membrane is laid directly on the heat-insulating layer, flooring from the boards. There is no need for a ventilation gap. The material is fastened with special brackets or steel nails. The edges of the diffusion sheets are connected. Work can be carried out even under extreme conditions: in strong gusts of wind, etc.

In addition, the coating in question can be used as a temporary roof covering.

PVC membranes: essence and purpose

PVC membranes are a roofing material made from polyvinyl chloride and have elastic properties. Such a modern roofing material completely replaced bituminous roll analogues, which have a significant drawback - the need for systematic maintenance and repair. Today, the characteristic features of PVC membranes make it possible to use them when carrying out repair work on old flat roofs. They are also used when installing new roofs.

A roof made of such material is easy to use, and its installation is possible on any type of surface, at any time of the year and under any weather conditions. PVC membrane has the following properties:

strength;
stability when exposed to UV rays, various types of precipitation, point and surface loads.

It is thanks to its unique properties that PVC membranes will serve you faithfully for many years. The period of use of such a roof is equal to the period of operation of the building itself, while rolled roofing materials need regular repairs, and in some cases even dismantling and installing a new floor.

Between themselves, PVC membrane sheets are connected by hot breath welding, the temperature of which is in the range of 400-600 degrees Celsius. This connection is completely sealed.

Advantages of PVC membranes

Their advantages are obvious:

the flexibility of the roofing system, which is most consistent with the construction project;
durable, airtight connecting seam between the membrane sheets;
ideal tolerance to climate change, weather conditions, temperature, humidity;
increased vapor permeability, which contributes to the evaporation of moisture accumulated in the under-roof space;
many color options;
fire-fighting properties;
the ability to maintain the original properties and appearance for a long period;
PVC membrane is an absolutely environmentally friendly material, which is confirmed by the relevant certificates;
the installation process is mechanized, so it will not take much time;
operating rules allow the installation of various architectural additions directly on top of the PVC membrane roof itself;
single-layer styling will save you money;
ease of maintenance and repair.

Membrane fabric

Membrane fabric has been known to the textile industry for a long time. Shoes and clothes are made from this material: for adults and children. Membrane - the basis of membrane fabric, presented in the form of a thin polymer film and having such characteristics as water resistance and vapor permeability. For the production of this material, this film is covered with outer and inner protective layers. Their structure is determined by the membrane itself. This is done in order to preserve all useful properties even in case of damage. In other words, membrane clothing does not get wet when exposed to precipitation in the form of snow or rain, but at the same time it perfectly passes steam from the body into the external environment. This throughput allows the skin to breathe.

Considering all of the above, we can conclude that ideal winter clothes are made from such a fabric. The membrane, which is at the base of the fabric, can be:

with pores;
without pores;
combined.

Teflon is included in the composition of membranes with many micropores. The dimensions of such pores do not even reach the dimensions of a drop of water, but are larger than a water molecule, which indicates water resistance and the ability to remove sweat.

Membranes that do not have pores are usually made from polyurethane. Their inner layer concentrates all sweat-fat secretions of the human body and pushes them out.

The structure of the combined membrane implies the presence of two layers: porous and smooth. This fabric has high quality characteristics and will last for many years.

Thanks to these advantages, clothes and shoes made of membrane fabrics and designed to be worn in the winter season are durable, but light, and perfectly protect against frost, moisture, and dust. They are simply indispensable for many active types of winter recreation, mountaineering.

Outer cell membrane (plasmalemma, cytolemma, plasma membrane) of animal cells covered on the outside (i.e., on the side not in contact with the cytoplasm) with a layer of oligosaccharide chains covalently attached to membrane proteins (glycoproteins) and, to a lesser extent, to lipids (glycolipids). This carbohydrate coating of the membrane is called glycocalyx. The purpose of the glycocalyx is not yet very clear; there is an assumption that this structure takes part in the processes of intercellular recognition.

In plant cells on top of the outer cell membrane is a dense cellulose layer with pores through which communication is carried out between neighboring cells through cytoplasmic bridges.

Cells mushrooms on top of the plasmalemma - a dense layer chitin.

At bacteria – mureina.

Properties of biological membranes

1. Ability to self-assemble after destructive impacts. This property is determined by the physicochemical characteristics of phospholipid molecules, which in an aqueous solution come together so that the hydrophilic ends of the molecules turn outward, and the hydrophobic ends inward. Proteins can be incorporated into ready-made phospholipid layers. The ability to self-assemble is essential at the cellular level.

2. Semi-permeability(selectivity in the transmission of ions and molecules). Ensures the maintenance of the constancy of the ionic and molecular composition in the cell.

3. Membrane fluidity. Membranes are not rigid structures; they constantly fluctuate due to the rotational and oscillatory movements of lipid and protein molecules. This provides a high rate of enzymatic and other chemical processes in the membranes.

4. Fragments of membranes do not have free ends, as they are closed in bubbles.

Functions of the outer cell membrane (plasmalemma)

The main functions of the plasmalemma are as follows: 1) barrier, 2) receptor, 3) exchange, 4) transport.

1. barrier function. It is expressed in the fact that the plasmalemma limits the contents of the cell, separating it from the external environment, and intracellular membranes divide the cytoplasm into separate reactionary compartments.

2. receptor function. One of the most important functions of the plasmalemma is to ensure communication (connection) of the cell with the external environment through the receptor apparatus present in the membranes, which has a protein or glycoprotein nature. The main function of the receptor formations of the plasmalemma is the recognition of external signals, due to which cells are correctly oriented and form tissues in the process of differentiation. The activity of various regulatory systems, as well as the formation of an immune response, is associated with the receptor function.

exchange function is determined by the content of enzyme proteins in biological membranes, which are biological catalysts. Their activity varies depending on the pH of the medium, temperature, pressure, the concentration of both the substrate and the enzyme itself. Enzymes determine the intensity of key reactions metabolism, as well as orientation.

Transport function of membranes. The membrane provides selective penetration into the cell and from the cell into the environment of various chemicals. The transport of substances is necessary to maintain the appropriate pH in the cell, the proper ionic concentration, which ensures the efficiency of cellular enzymes. Transport supplies nutrients that serve as a source of energy, as well as material for the formation of various cellular components. It determines the removal of toxic waste from the cell, the secretion of various useful substances and the creation of ionic gradients necessary for nerve and muscle activity. Changes in the rate of transfer of substances can lead to disturbances in bioenergetic processes, water-salt metabolism, excitability and other processes. Correction of these changes underlies the action of many drugs.

There are two main ways in which substances enter the cell and out of the cell into the external environment;

passive transport,

active transport.

Passive transport goes along the gradient of chemical or electrochemical concentration without the expenditure of ATP energy. If the molecule of the transported substance has no charge, then the direction of passive transport is determined only by the difference in the concentration of this substance on both sides of the membrane (chemical concentration gradient). If the molecule is charged, then its transport is affected by both the chemical concentration gradient and the electrical gradient (membrane potential).

Both gradients together constitute an electrochemical gradient. Passive transport of substances can be carried out in two ways: simple diffusion and facilitated diffusion.

With simple diffusion salt ions and water can penetrate through the selective channels. These channels are formed by some transmembrane proteins that form end-to-end transport pathways that are open permanently or only for a short time. Through the selective channels, various molecules penetrate, having the size and charge corresponding to the channels.

There is another way of simple diffusion - this is the diffusion of substances through the lipid bilayer, through which fat-soluble substances and water easily pass. The lipid bilayer is impermeable to charged molecules (ions), and at the same time, uncharged small molecules can freely diffuse, and the smaller the molecule, the faster it is transported. The rather high rate of water diffusion through the lipid bilayer is precisely due to the small size of its molecules and the absence of a charge.

With facilitated diffusion proteins are involved in the transport of substances - carriers that work on the principle of "ping-pong". In this case, the protein exists in two conformational states: in the “pong” state, the binding sites of the transported substance are open on the outside of the bilayer, and in the “ping” state, the same sites open on the other side. This process is reversible. From which side the binding site of a substance will be open at a given time depends on the concentration gradient of this substance.

In this way, sugars and amino acids pass through the membrane.

With facilitated diffusion, the rate of transport of substances increases significantly in comparison with simple diffusion.

In addition to carrier proteins, some antibiotics, such as gramicidin and valinomycin, are involved in facilitated diffusion.

Because they provide ion transport, they are called ionophores.

Active transport of substances in the cell. This type of transport always comes with the cost of energy. The source of energy needed for active transport is ATP. A characteristic feature of this type of transport is that it is carried out in two ways:

with the help of enzymes called ATPases;

transport in membrane packaging (endocytosis).

AT the outer cell membrane contains enzyme proteins such as ATPases, whose function is to provide active transport ions against a concentration gradient. Since they provide the transport of ions, this process is called an ion pump.

There are four main ion transport systems in the animal cell. Three of them provide transfer through biological membranes. Na + and K +, Ca +, H +, and the fourth - the transfer of protons during the operation of the mitochondrial respiratory chain.

An example of an active ion transport mechanism is sodium-potassium pump in animal cells. It maintains a constant concentration of sodium and potassium ions in the cell, which differs from the concentration of these substances in the environment: normally, there are less sodium ions in the cell than in the environment, and more potassium.

As a result, according to the laws of simple diffusion, potassium tends to leave the cell, and sodium diffuses into the cell. In contrast to simple diffusion, the sodium-potassium pump constantly pumps out sodium from the cell and injects potassium: for three molecules of sodium thrown out, there are two molecules of potassium introduced into the cell.

This transport of sodium-potassium ions is ensured by the ATP-dependent enzyme, which is localized in the membrane in such a way that it penetrates its entire thickness. Sodium and ATP enter this enzyme from the inside of the membrane, and potassium from the outside.

The transfer of sodium and potassium across the membrane occurs as a result of conformational changes that the sodium-potassium-dependent ATPase undergoes, which is activated when the concentration of sodium inside the cell or potassium in the environment increases.

ATP hydrolysis is required to power this pump. This process is provided by the same enzyme sodium-potassium-dependent ATP-ase. At the same time, more than one third of the ATP consumed by the animal cell at rest is spent on the work of the sodium - potassium pump.

Violation of the proper functioning of the sodium - potassium pump leads to various serious diseases.

The efficiency of this pump exceeds 50%, which is not achieved by the most advanced machines created by man.

Many active transport systems are driven by energy stored in ionic gradients rather than by direct hydrolysis of ATP. All of them work as cotransport systems (facilitating the transport of low molecular weight compounds). For example, the active transport of certain sugars and amino acids into animal cells is determined by the sodium ion gradient, and the higher the sodium ion gradient, the greater the rate of glucose absorption. Conversely, if the concentration of sodium in the intercellular space decreases markedly, glucose transport stops. In this case, sodium must join the sodium - dependent glucose carrier protein, which has two binding sites: one for glucose, the other for sodium. Sodium ions penetrating into the cell contribute to the introduction of the carrier protein into the cell along with glucose. Sodium ions that have entered the cell along with glucose are pumped back out by the sodium-potassium-dependent ATPase, which, by maintaining the sodium concentration gradient, indirectly controls glucose transport.

Transport of substances in membrane packaging. Large molecules of biopolymers practically cannot penetrate through the plasmalemma by any of the above-described mechanisms of transport of substances into the cell. They are captured by the cell and absorbed in the membrane package, which is called endocytosis. The latter is formally divided into phagocytosis and pinocytosis. The capture of solid particles by the cell is phagocytosis, and liquid - pinocytosis. During endocytosis, the following stages are observed:

reception of the absorbed substance due to receptors in the cell membrane;

invagination of the membrane with the formation of a bubble (vesicles);

separation of the endocytic vesicle from the membrane with the expenditure of energy - phagosome formation and restoration of membrane integrity;

Fusion of phagosome with lysosome and formation phagolysosomes (digestive vacuole) in which the digestion of absorbed particles occurs;

removal of undigested material in the phagolysosome from the cell ( exocytosis).

In the animal kingdom endocytosis is a characteristic way of feeding many unicellular organisms (for example, in amoebas), and among multicellular organisms this type of digestion of food particles is found in endodermal cells in coelenterates. As for mammals and humans, they have a reticulo-histio-endothelial system of cells with the ability to endocytosis. Examples are blood leukocytes and liver Kupffer cells. The latter line the so-called sinusoidal capillaries of the liver and capture various foreign particles suspended in the blood. Exocytosis- this is also a way of removing from the cell of a multicellular organism the substrate secreted by it, which is necessary for the function of other cells, tissues and organs.

It's no secret to anyone that all living beings on our planet are composed of their cells, these countless "" organic matter. Cells, in turn, are surrounded by a special protective shell - a membrane that plays a very important role in the life of the cell, and the functions of the cell membrane are not limited to protecting the cell, but represent the most complex mechanism involved in cell reproduction, nutrition, and regeneration.

What is a cell membrane

The word “membrane” itself is translated from Latin as “film”, although the membrane is not just a kind of film in which the cell is wrapped, but a combination of two films interconnected and having different properties. In fact, the cell membrane is a three-layer lipoprotein (fat-protein) shell that separates each cell from neighboring cells and the environment, and carries out a controlled exchange between cells and the environment, this is the academic definition of what a cell membrane is.

The value of the membrane is simply enormous, because it not only separates one cell from another, but also ensures the interaction of the cell, both with other cells and with the environment.

History of cell membrane research

An important contribution to the study of the cell membrane was made by two German scientists Gorter and Grendel back in 1925. It was then that they managed to conduct a complex biological experiment on red blood cells - erythrocytes, during which scientists received the so-called "shadows", empty shells of erythrocytes, which were folded into one pile and measured the surface area, and also calculated the amount of lipids in them. Based on the amount of lipids obtained, the scientists came to the conclusion that they are just enough for the double layer of the cell membrane.

In 1935, another pair of cell membrane researchers, this time the Americans Daniel and Dawson, after a series of long experiments, determined the protein content in the cell membrane. Otherwise, it was impossible to explain why the membrane has such a high surface tension. Scientists cleverly presented a model of the cell membrane in the form of a sandwich, in which the role of bread is played by homogeneous lipid-protein layers, and between them instead of butter is emptiness.

In 1950, with the advent of the electronic theory of Daniel and Dawson, it was already possible to confirm practical observations - on micrographs of the cell membrane, layers of lipid and protein heads and also an empty space between them were clearly visible.

In 1960, the American biologist J. Robertson developed a theory about the three-layer structure of cell membranes, which for a long time was considered the only true one, but with the further development of science, doubts about its infallibility began to appear. So, for example, from the point of view of cells, it would be difficult and laborious to transport the necessary useful substances through the entire “sandwich”

And only in 1972, the American biologists S. Singer and G. Nicholson were able to explain the inconsistencies of Robertson's theory with the help of a new fluid-mosaic model of the cell membrane. In particular, they found that the cell membrane is not homogeneous in composition, moreover, it is asymmetric and filled with liquid. In addition, cells are in constant motion. And the notorious proteins that make up the cell membrane have different structures and functions.

Properties and functions of the cell membrane

Now let's look at what functions the cell membrane performs:

The barrier function of the cell membrane - the membrane, as a real border guard, stands guard over the boundaries of the cell, delaying, not letting through harmful or simply inappropriate molecules

The transport function of the cell membrane - the membrane is not only a border guard at the gates of the cell, but also a kind of customs checkpoint, through which the exchange of useful substances with other cells and the environment constantly passes.

Matrix function - it is the cell membrane that determines the location relative to each other, regulates the interaction between them.

Mechanical function - is responsible for the restriction of one cell from another and in parallel for the correct connection of cells with each other, for their formation into a homogeneous tissue.

The protective function of the cell membrane is the basis for building a protective shield of the cell. In nature, this function can be exemplified by hard wood, a dense skin, a protective shell, all due to the protective function of the membrane.

The enzymatic function is another important function performed by some cell proteins. For example, due to this function, the synthesis of digestive enzymes occurs in the intestinal epithelium.

Also, in addition to all this, cell metabolism is carried out through the cell membrane, which can take place by three different reactions:

Phagocytosis is a cellular exchange in which phagocytic cells embedded in the membrane capture and digest various nutrients.
Pinocytosis - is the process of capture by the cell membrane, fluid molecules in contact with it. To do this, special tendrils are formed on the surface of the membrane, which seem to surround a drop of liquid, forming a bubble, which is subsequently “swallowed” by the membrane.
Exocytosis - is the reverse process, when the cell releases secretory functional fluid through the membrane to the surface.

The structure of the cell membrane

There are three classes of lipids in the cell membrane:

phospholipids (they are a combination of fats and phosphorus),
glycolipids (combination of fats and carbohydrates),
cholesterol.

Phospholipids and glycolipids, in turn, consist of a hydrophilic head, into which two long hydrophobic tails extend. Cholesterol, on the other hand, occupies the space between these tails, preventing them from bending, all this in some cases makes the membrane of certain cells very rigid. In addition to all this, cholesterol molecules regulate the structure of the cell membrane.

But be that as it may, the most important part of the structure of the cell membrane is protein, or rather different proteins that play various important roles. Despite the diversity of proteins contained in the membrane, there is something that unites them - annular lipids are located around all membrane proteins. Annular lipids are special structured fats that serve as a kind of protective shell for proteins, without which they simply would not work.

The structure of the cell membrane has three layers: the basis of the cell membrane is a homogeneous liquid lipid layer. Proteins cover it on both sides like a mosaic. It is proteins, in addition to the functions described above, also play the role of peculiar channels through which substances pass through the membrane that are unable to penetrate the liquid layer of the membrane. These include, for example, potassium and sodium ions; for their penetration through the membrane, nature provides special ion channels of cell membranes. In other words, proteins provide the permeability of cell membranes.

If we look at the cell membrane through a microscope, we will see a layer of lipids formed by small spherical molecules on which proteins float like on the sea. Now you know what substances are part of the cell membrane.

Cell membrane, video

And finally, an educational video about the cell membrane.

Cell membrane.
The cell membrane separates the contents of any cell from the external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments - compartments or organelles, in which certain environmental conditions are maintained.
Structure.
The cell membrane is a double layer (bilayer) of molecules of the class of lipids (fats), most of which are the so-called complex lipids - phospholipids. Lipid molecules have a hydrophilic (“head”) and a hydrophobic (“tail”) part. During the formation of membranes, the hydrophobic portions of the molecules turn inward, while the hydrophilic portions turn outward. Membranes are very similar structures in different organisms. The membrane thickness is 7-8 nm. (10-9 meters)
hydrophilicity- the ability of a substance to be wetted by water.
hydrophobicity- the inability of a substance to be wetted by water.
The biological membrane also includes various proteins:
- integral (penetrating the membrane through)
- semi-integral (immersed at one end into the outer or inner lipid layer)
- superficial (located on the outer or adjacent to the inner sides of the membrane).
Some proteins are the points of contact of the cell membrane with the cytoskeleton inside the cell, and the cell wall (if any) outside.
cytoskeleton- cell scaffold inside the cell.
Functions.
1) Barrier- provides a regulated, selective, passive and active metabolism with the environment.
2) Transport- through the membrane there is a transport of substances into and out of the cell. matrix - provides a certain relative position and orientation of membrane proteins, their optimal interaction.
3) Mechanical- ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). The intercellular substance plays a large role in ensuring the mechanical function.
4) Receptor- some proteins in the membrane are receptors (molecules by which the cell perceives certain signals).
For example, hormones circulating in the blood only act on target cells that have receptors corresponding to those hormones. Neurotransmitters (chemicals that conduct nerve impulses) also bind to specific receptor proteins on target cells.
Hormones- biologically active signaling chemicals.
5) Enzymatic Membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.
6) Implementation of the generation and conduction of biopotentials.
With the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K + ion inside the cell is much higher than outside, and the concentration of Na + is much lower, which is very important, since this maintains the potential difference across the membrane and generates a nerve impulse.
nerve impulse – a wave of excitation transmitted along a nerve fiber.
7) Cell labeling- there are antigens on the membrane that act as markers - "labels" that allow you to identify the cell. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of "antennas". Due to the myriad of side chain configurations, it is possible to make a specific marker for each cell type. With the help of markers, cells can recognize other cells and act in concert with them, for example, when forming organs and tissues. It also allows the immune system to recognize foreign antigens.
permeability features.
Cell membranes have selective permeability: they slowly penetrate through them in various ways:
Glucose is the main source of energy.

Amino acids are the building blocks that make up all the proteins in the body.

Fatty acids - structural, energy and other functions.

Glycerol - makes the body retain water and reduces the production of urine.

Ions are enzymes for reactions.

Moreover, the membranes themselves actively regulate this process to a certain extent - some substances pass through, while others do not. There are four main mechanisms for the entry of substances into the cell or their removal from the cell to the outside:
Passive permeability mechanisms:
1) Diffusion.
A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance to pass through the membrane. This molecule may have a channel that allows only one type of substance to pass through.
Diffusion- the process of mutual penetration of molecules of one substance between the molecules of another.
Osmosis– the process of one-way diffusion through a semipermeable membrane of solvent molecules towards a higher concentration of a solute.
The membrane surrounding a normal blood cell is permeable only to water molecules, oxygen, some of the nutrients dissolved in the blood, and cellular waste products.
Active permeability mechanisms:
1) Active transport.
active transport– the transfer of a substance from an area of low concentration to an area of high concentration.
Active transport requires energy, as it moves from an area of low concentration to an area of high concentration. There are special pump proteins on the membrane that actively pump potassium ions (K +) into the cell and pump sodium ions (Na +) out of it, ATP serves as energy.
ATP– universal source of energy for all biochemical processes. .(more later)
2) Endocytosis.
Particles that for some reason are not able to cross the cell membrane, but are necessary for the cell, can penetrate the membrane by endocytosis.
Endocytosis– the process of uptake of external material by the cell.
The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane through and through, forming a kind of passage. The elements K, Na and Cl have their own channels. With respect to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open, and there is a sharp influx of sodium ions into the cell. This results in an imbalance in the membrane potential. After that, the membrane potential is restored. Potassium channels are always open, through which potassium ions slowly enter the cell.
Membrane structure
Permeability
active transport
Osmosis
Endocytosis