The structure of the membrane and its functions table. Cell and cell membrane

9.5.1. One of the main functions of membranes is participation in the transport of substances. This process is provided by three main mechanisms: simple diffusion, facilitated diffusion and active transport (Figure 9.10). Remember the most important features of these mechanisms and examples of the transported substances in each case.

Figure 9.10. Mechanisms of transport of molecules across the membrane

simple diffusion- transfer of substances through the membrane without the participation of special mechanisms. Transport occurs along a concentration gradient without energy consumption. Small biomolecules - H2O, CO2, O2, urea, hydrophobic low molecular weight substances are transported by simple diffusion. The rate of simple diffusion is proportional to the concentration gradient.

Facilitated diffusion- the transfer of substances across the membrane using protein channels or special carrier proteins. It is carried out along the concentration gradient without energy consumption. Monosaccharides, amino acids, nucleotides, glycerol, some ions are transported. Saturation kinetics is characteristic - at a certain (saturating) concentration of the transferred substance, all carrier molecules take part in the transfer and the transport speed reaches the limit value.

active transport- also requires the participation of special carrier proteins, but the transfer occurs against a concentration gradient and therefore requires energy. With the help of this mechanism, Na+, K+, Ca2+, Mg2+ ions are transported through the cell membrane, and protons through the mitochondrial membrane. The active transport of substances is characterized by saturation kinetics.

9.5.2. An example of a transport system that performs active ion transport is Na+,K+ -adenosine triphosphatase (Na+,K+ -ATPase or Na+,K+ -pump). This protein is located in the thickness of the plasma membrane and is able to catalyze the reaction of ATP hydrolysis. The energy released during the hydrolysis of 1 ATP molecule is used to transfer 3 Na + ions from the cell to the extracellular space and 2 K + ions in the opposite direction (Figure 9.11). As a result of the action of Na + , K + -ATPase, a concentration difference is created between the cytosol of the cell and the extracellular fluid. Since the transport of ions is non-equivalent, a difference in electrical potentials arises. Thus, an electrochemical potential arises, which is the sum of the energy of the difference in electric potentials Δφ and the energy of the difference in the concentrations of substances ΔС on both sides of the membrane.

Figure 9.11. Scheme of Na+, K+ -pump.

9.5.3. Transfer through membranes of particles and macromolecular compounds

Along with the transport of organic substances and ions carried out by carriers, there is a very special mechanism in the cell designed to absorb and remove macromolecular compounds from the cell by changing the shape of the biomembrane. Such a mechanism is called vesicular transport.

Figure 9.12. Types of vesicular transport: 1 - endocytosis; 2 - exocytosis.

During the transfer of macromolecules, sequential formation and fusion of vesicles (vesicles) surrounded by a membrane occur. According to the direction of transport and the nature of the transferred substances, the following types of vesicular transport are distinguished:

Endocytosis(Figure 9.12, 1) - the transfer of substances into the cell. Depending on the size of the resulting vesicles, there are:

a) pinocytosis - absorption of liquid and dissolved macromolecules (proteins, polysaccharides, nucleic acids) using small bubbles (150 nm in diameter);

b) phagocytosis — absorption of large particles, such as microorganisms or cell debris. In this case, large vesicles are formed, called phagosomes with a diameter of more than 250 nm.

Pinocytosis is characteristic of most eukaryotic cells, while large particles are absorbed by specialized cells - leukocytes and macrophages. At the first stage of endocytosis, substances or particles are adsorbed on the membrane surface; this process occurs without energy consumption. At the next stage, the membrane with the adsorbed substance deepens into the cytoplasm; the resulting local invaginations of the plasma membrane are laced from the cell surface, forming vesicles, which then migrate into the cell. This process is connected by a system of microfilaments and is energy dependent. The vesicles and phagosomes that enter the cell can merge with lysosomes. Enzymes contained in lysosomes break down substances contained in vesicles and phagosomes to low molecular weight products (amino acids, monosaccharides, nucleotides), which are transported to the cytosol, where they can be used by the cell.

Exocytosis(Figure 9.12, 2) - the transfer of particles and large compounds from the cell. This process, like endocytosis, proceeds with the absorption of energy. The main types of exocytosis are:

a) secretion - removal from the cell of water-soluble compounds that are used or affect other cells of the body. It can be carried out both by non-specialized cells and cells of the endocrine glands, the mucosa of the gastrointestinal tract, adapted for the secretion of the substances they produce (hormones, neurotransmitters, proenzymes), depending on the specific needs of the body.

Secreted proteins are synthesized on ribosomes associated with the membranes of the rough endoplasmic reticulum. These proteins are then transported to the Golgi apparatus, where they are modified, concentrated, sorted, and then packaged into vesicles, which are cleaved into the cytosol and subsequently fuse with the plasma membrane so that the contents of the vesicles are outside the cell.

Unlike macromolecules, small secreted particles, such as protons, are transported out of the cell using facilitated diffusion and active transport mechanisms.

b) excretion - removal from the cell of substances that cannot be used (for example, removal of a reticular substance from reticulocytes during erythropoiesis, which is an aggregated remnant of organelles). The mechanism of excretion, apparently, consists in the fact that at first the released particles are in the cytoplasmic vesicle, which then merges with the plasma membrane.

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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

Index
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.

delimitative ( barrier) - separate the cellular contents from the external environment;

Regulate the exchange between the cell and the environment;

Divide cells into compartments, or compartments, designed for certain specialized metabolic pathways ( separating);

It is the site of some chemical reactions (light reactions of photosynthesis in chloroplasts, oxidative phosphorylation during respiration in mitochondria);

Provide communication between cells in the tissues of multicellular organisms;

Transport- carries out transmembrane transport.

Receptor- are the site of localization of receptor sites that recognize external stimuli.

Transport of substances through the membrane is one of the leading functions of the membrane, which ensures the exchange of substances between the cell and the external environment. Depending on the energy costs for the transfer of substances, there are:

passive transport, or facilitated diffusion;

active (selective) transport with the participation of ATP and enzymes.

transport in membrane packaging. There are endocytosis (into the cell) and exocytosis (out of the cell) - mechanisms that transport large particles and macromolecules through the membrane. During endocytosis, the plasma membrane forms an invagination, its edges merge, and a vesicle is laced into the cytoplasm. The vesicle is delimited from the cytoplasm by a single membrane, which is part of the outer cytoplasmic membrane. Distinguish between phagocytosis and pinocytosis. Phagocytosis is the absorption of large particles, rather solid. For example, phagocytosis of lymphocytes, protozoa, etc. Pinocytosis is the process of capturing and absorbing liquid droplets with substances dissolved in it.

Exocytosis is the process of removing various substances from the cell. During exocytosis, the membrane of the vesicle or vacuole merges with the outer cytoplasmic membrane. The contents of the vesicle are removed from the cell surface, and the membrane is included in the outer cytoplasmic membrane.

At the core passive transport of uncharged molecules is the difference between the concentrations of hydrogen and charges, i.e. electrochemical gradient. Substances will move from an area with a higher gradient to an area with a lower one. The transport speed depends on the gradient difference.

Simple diffusion is the transport of substances directly through the lipid bilayer. Characteristic of gases, non-polar or small uncharged polar molecules, soluble in fats. Water quickly penetrates through the bilayer, because. its molecule is small and electrically neutral. The diffusion of water across membranes is called osmosis.

Diffusion through membrane channels is the transport of charged molecules and ions (Na, K, Ca, Cl) that penetrate the membrane due to the presence in it of special channel-forming proteins that form water pores.

Facilitated diffusion is the transport of substances with the help of special transport proteins. Each protein is responsible for a strictly defined molecule or group of related molecules, interacts with it and moves through the membrane. For example, sugars, amino acids, nucleotides and other polar molecules.

active transport carried out by proteins - carriers (ATPase) against an electrochemical gradient, with the expenditure of energy. Its source is ATP molecules. For example, the sodium-potassium pump.

The concentration of potassium inside the cell is much higher than outside it, and sodium - vice versa. Therefore, potassium and sodium cations passively diffuse along the concentration gradient through the water pores of the membrane. This is due to the fact that the permeability of the membrane for potassium ions is higher than for sodium ions. Accordingly, potassium diffuses faster out of the cell than sodium into the cell. However, for the normal functioning of the cell, a certain ratio of 3 potassium and 2 sodium ions is necessary. Therefore, there is a sodium-potassium pump in the membrane, which actively pumps sodium out of the cell, and potassium into the cell. This pump is a transmembrane membrane protein capable of conformational rearrangements. Therefore, it can attach to itself both potassium ions and sodium ions (antiport). The process is energy intensive:

Sodium ions and an ATP molecule enter the pump protein from the inside of the membrane, and potassium ions from the outside.

Sodium ions combine with a protein molecule, and the protein acquires ATPase activity, i.e. the ability to cause ATP hydrolysis, which is accompanied by the release of energy that drives the pump.

The phosphate released during ATP hydrolysis is attached to the protein, i.e. phosphorylates a protein.

Phosphorylation causes a conformational change in the protein, it is unable to retain sodium ions. They are released and go outside the cell.

The new conformation of the protein promotes the addition of potassium ions to it.

The addition of potassium ions causes dephosphorylation of the protein. He again changes his conformation.

The change in protein conformation leads to the release of potassium ions inside the cell.

The protein is again ready to attach sodium ions to itself.

In one cycle of operation, the pump pumps 3 sodium ions out of the cell and pumps 2 potassium ions.

Cytoplasm- an obligatory component of the cell, enclosed between the surface apparatus of the cell and the nucleus. It is a complex heterogeneous structural complex, consisting of:

hyaloplasm

organelles (permanent components of the cytoplasm)

inclusions - temporary components of the cytoplasm.

cytoplasmic matrix(hyaloplasm) is the inner contents of the cell - a colorless, thick and transparent colloidal solution. The components of the cytoplasmic matrix carry out the processes of biosynthesis in the cell, contain the enzymes necessary for the formation of energy, mainly due to anaerobic glycolysis.

Basic properties of the cytoplasmic matrix.

Determines the colloidal properties of the cell. Together with the intracellular membranes of the vacuolar system, it can be considered as a highly heterogeneous or multiphase colloidal system.

Provides a change in the viscosity of the cytoplasm, the transition from a gel (thicker) to a sol (more liquid), which occurs under the influence of external and internal factors.

Provides cyclosis, amoeboid movement, cell division and movement of pigment in chromatophores.

Determines the polarity of the location of intracellular components.

Provides mechanical properties of cells - elasticity, ability to merge, rigidity.

Organelles- permanent cellular structures that ensure the performance of specific functions by the cell. Depending on the features of the structure, there are:

membranous organelles - have a membrane structure. They can be single-membrane (ER, Golgi apparatus, lysosomes, vacuoles of plant cells). Double membrane (mitochondria, plastids, nucleus).

Non-membrane organelles - do not have a membrane structure (chromosomes, ribosomes, cell center, cytoskeleton).

General purpose organelles - characteristic of all cells: nucleus, mitochondria, cell center, Golgi apparatus, ribosomes, ER, lysosomes. If organelles are characteristic of certain types of cells, they are called special organelles (for example, myofibrils that contract a muscle fiber).

Endoplasmic reticulum- a single continuous structure, the membrane of which forms many invaginations and folds that look like tubules, microvacuoles and large cisterns. EPS membranes, on the one hand, are associated with the cellular cytoplasmic membrane, and on the other hand, with the outer shell of the nuclear membrane.

There are two types of EPS - rough and smooth.

In rough or granular ER, cisterns and tubules are associated with ribosomes. is the outer side of the membrane. There is no connection with ribosomes in a smooth or agranular EPS. This is the inside of the membrane.

The structure of the biomembrane. The cell-bounding membranes and membrane organelles of eukaryotic cells share a common chemical composition and structure. They include lipids, proteins and carbohydrates. Membrane lipids are mainly represented by phospholipids and cholesterol. Most membrane proteins are complex proteins such as glycoproteins. Carbohydrates do not occur on their own in the membrane, they are associated with proteins and lipids. The thickness of the membranes is 7-10 nm.

According to the currently accepted fluid mosaic model of membrane structure, lipids form a double layer, or lipid bilayer, in which the hydrophilic "heads" of lipid molecules are turned outward, and the hydrophobic "tails" are hidden inside the membrane (Fig. 2.24). These “tails”, due to their hydrophobicity, ensure the separation of the aqueous phases of the internal environment of the cell and its environment. Proteins are associated with lipids through various types of interactions. Some of the proteins are located on the surface of the membrane. Such proteins are called peripheral, or superficial. Other proteins are partially or completely immersed in the membrane - these are integral, or immersed proteins. Membrane proteins perform structural, transport, catalytic, receptor, and other functions.

Membranes are not like crystals, their components are constantly in motion, as a result of which gaps appear between lipid molecules - pores through which various substances can enter or leave the cell.

Biological membranes differ in their location in the cell, their chemical composition, and their functions. The main types of membranes are plasma and internal.

plasma membrane(Fig. 2.24) contains about 45% lipids (including glycolipids), 50% proteins and 5% carbohydrates. Chains of carbohydrates that make up complex proteins-glycoproteins and complex lipids-glycolipids protrude above the surface of the membrane. Plasmalemmal glycoproteins are extremely specific. So, for example, through them there is a mutual recognition of cells, including sperm and eggs.

On the surface of animal cells, carbohydrate chains form a thin surface layer - glycocalyx. It has been found in almost all animal cells, but its severity is not the same (10-50 microns). The glycocalyx provides a direct connection of the cell with the external environment; extracellular digestion occurs in it; receptors are located in the glycocalyx. The cells of bacteria, plants and fungi, in addition to the plasmalemma, are also surrounded by cell membranes.

Internal membranes eukaryotic cells delimit various parts of the cell, forming a kind of "compartments" - compartments, which contributes to the separation of various processes of metabolism and energy. They may differ in chemical composition and functions, but they retain the general plan of the structure.

Membrane functions:

1. Limiting. It consists in the fact that they separate the internal space of the cell from the external environment. The membrane is semi-permeable, that is, only those substances that are necessary for the cell can freely overcome it, while there are mechanisms for transporting the necessary substances.

2. Receptor. It is associated primarily with the perception of environmental signals and the transfer of this information into the cell. Special receptor proteins are responsible for this function. Membrane proteins are also responsible for cellular recognition according to the “friend or foe” principle, as well as for the formation of intercellular connections, the most studied of which are the synapses of nerve cells.

3. catalytic. Numerous enzyme complexes are located on the membranes, as a result of which intensive synthetic processes take place on them.

4. Energy transforming. Associated with the formation of energy, its storage in the form of ATP and expenditure.

5. Compartmentalization. The membranes also delimit the space inside the cell, thereby separating the initial substances of the reaction and the enzymes that can carry out the corresponding reactions.

6. Formation of intercellular contacts. Despite the fact that the membrane thickness is so small that it cannot be distinguished with the naked eye, on the one hand, it serves as a fairly reliable barrier for ions and molecules, especially water-soluble ones, and on the other hand, it ensures their transfer into the cell and out.

membrane transport. Due to the fact that cells, as elementary biological systems, are open systems, to ensure metabolism and energy, maintain homeostasis, growth, irritability, and other processes, the transfer of substances through the membrane is required - membrane transport (Fig. 2.25). Currently, the transport of substances across the cell membrane is divided into active, passive, endo- and exocytosis.

Passive transport- this is a type of transport that occurs without the expenditure of energy from a higher concentration to a lower one. Small non-polar molecules (0 2 , CO 2 ) soluble in lipids easily penetrate the cell by simple diffusion. Insoluble in lipids, including charged small particles, are picked up by carrier proteins or pass through special channels (glucose, amino acids, K +, PO 4 3-). This type of passive transport is called facilitated diffusion. Water enters the cell through pores in the lipid phase, as well as through special channels lined with proteins. The transport of water across a membrane is called osmosis(Fig. 2.26).

Osmosis is extremely important in the life of a cell, because if it is placed in a solution with a higher concentration of salts than in a cell solution, then water will begin to leave the cell, and the volume of living contents will begin to decrease. In animal cells, the cell as a whole shrinks, and in plant cells, the cytoplasm lags behind the cell wall, which is called plasmolysis(Fig. 2.27).

When a cell is placed in a solution less concentrated than the cytoplasm, water is transported in the opposite direction - into the cell. However, there are limits to the extensibility of the cytoplasmic membrane, and the animal cell eventually ruptures, while in the plant cell this is not allowed by a strong cell wall. The phenomenon of filling the entire internal space of the cell with cellular contents is called deplasmolysis. The intracellular salt concentration should be taken into account when preparing drugs, especially for intravenous administration, as this can lead to damage to blood cells (for this, a saline solution with a concentration of 0.9% sodium chloride is used). This is no less important in the cultivation of cells and tissues, as well as organs of animals and plants.

active transport proceeds with the expenditure of ATP energy from a lower concentration of a substance to a higher one. It is carried out with the help of special proteins-pumps. Proteins pump ions K +, Na +, Ca 2+ and others through the membrane, which contributes to the transport of the most important organic substances, as well as the emergence of nerve impulses, etc.

Endocytosis- this is an active process of absorption of substances by the cell, in which the membrane forms invaginations, and then forms membrane vesicles - phagosomes in which the absorbed objects are enclosed. The primary lysosome then fuses with the phagosome to form secondary lysosome, or phagolysosome, or digestive vacuole. The contents of the vesicle are cleaved by lysosome enzymes, and the cleavage products are absorbed and assimilated by the cell. Undigested residues are removed from the cell by exocytosis. There are two main types of endocytosis: phagocytosis and pinocytosis.

Phagocytosis- this is the process of capture by the cell surface and absorption of solid particles by the cell, and pinocytosis- liquids. Phagocytosis occurs mainly in animal cells (single-celled animals, human leukocytes), it provides their nutrition, and often the protection of the body (Fig. 2.28).

Through pinocytosis, the absorption of proteins, antigen-antibody complexes in the process of immune reactions, etc. occurs. However, many viruses also enter the cell through pinocytosis or phagocytosis. In the cells of plants and fungi, phagocytosis is practically impossible, as they are surrounded by strong cell membranes.

Exocytosis is the reverse process of endocytosis. Thus, undigested food residues are released from the digestive vacuoles, the substances necessary for the life of the cell and the organism as a whole are removed. For example, the transmission of nerve impulses occurs due to the release of chemical mediators by the neuron that sends the impulse - mediators, and in plant cells, auxiliary carbohydrates of the cell membrane are released in this way.

Cell walls of plant cells, fungi and bacteria. Outside of the membrane, the cell can secrete a strong framework - cell membrane, or cell wall.

In plants, the cell wall is made up of cellulose, packed in bundles of 50-100 molecules. The gaps between them are filled with water and other carbohydrates. The shell of a plant cell is permeated with channels - plasmodesmata(Fig. 2.29), through which the membranes of the endoplasmic reticulum pass.

The plasmodesmata transport substances between cells. However, the transport of substances, such as water, can also occur along the cell walls themselves. Over time, various substances, including tannins or fat-like substances, accumulate in the cell membrane of plants, which leads to lignification or corking of the cell wall itself, the displacement of water and the death of cellular contents. Between the cell walls of neighboring plant cells there are jelly-like pads - middle plates that fasten them together and cement the body of the plant as a whole. They are destroyed only in the process of fruit ripening and when the leaves fall.

The cell walls of fungal cells are formed chitin- carbohydrate containing nitrogen. They are strong enough and are the outer skeleton of the cell, but still, like in plants, they prevent phagocytosis.

In bacteria, the cell wall contains carbohydrate with fragments of peptides - murein, however, its content varies significantly in different groups of bacteria. Outside of the cell wall, other polysaccharides can also be released, forming a mucous capsule that protects bacteria from external influences.

The shell determines the shape of the cell, serves as a mechanical support, performs a protective function, provides the osmotic properties of the cell, limiting the stretching of the living contents and preventing the rupture of the cell, which increases due to the influx of water. In addition, water and substances dissolved in it overcome the cell wall before entering the cytoplasm or, conversely, when leaving it, while water is transported along the cell walls faster than through the cytoplasm.