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 transfer of substances. This process is achieved through 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 substances transported 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. By simple diffusion, small biomolecules are transported - H2O, CO2, O2, urea, hydrophobic low-molecular substances. The rate of simple diffusion is proportional to the concentration gradient.

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

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

9.5.2. An example of a transport system that carries out active transport of ions is Na+,K+-adenosine triphosphatase (Na+,K+-ATPase or Na+,K+-pump). This protein is located deep in the plasma membrane and is capable of catalyzing 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 cell cytosol and the extracellular fluid. Since the transfer of ions is not equivalent, an electrical potential difference occurs. Thus, an electrochemical potential arises, which consists of the energy of the difference in electrical potentials Δφ and the energy of the difference in the concentrations of substances ΔC on both sides of the membrane.

Figure 9.11. Na+, K+ pump diagram.

9.5.3. Transport of particles and high molecular weight compounds across membranes

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 high-molecular compounds into the cell and remove high-molecular compounds from it by changing the shape of the biomembrane. This 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 membrane-surrounded vesicles (vesicles) occurs. Based on the direction of transport and the nature of the substances transported, the following types of vesicular transport are distinguished:

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

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 called phagosomes with a diameter of more than 250 nm are formed.

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 surface of the membrane; 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 detached 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 into low molecular weight products (amino acids, monosaccharides, nucleotides), which are transported into the cytosol, where they can be used by the cell.

Exocytosis(Figure 9.12, 2) - transfer of particles and large compounds from the cell. This process, like endocytosis, occurs 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 unspecialized cells and by cells of the endocrine glands, the mucous membrane 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 released 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 the mechanisms of facilitated diffusion and active transport.

b) excretion - removal from the cell of substances that cannot be used (for example, during erythropoiesis, removal from reticulocytes of the mesh substance, which is aggregated remains of organelles). The mechanism of excretion appears to be that the excreted particles are initially trapped in a cytoplasmic vesicle, which then fuses 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 electrical potential difference;
3) Specificity 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 capable of changing 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, which prevent the free movement of water and substances dissolved in it from the cytoplasm into them and vice versa. This creates conditions for the separation of biochemical processes occurring in different 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.

Basic 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 elongated 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 makes wave-like movements. Its sections, as well as proteins immersed in the lipid bilayer, are mixed from one part to another. The mobility (fluidity) of cell membranes facilitates the processes of transport of substances across the membrane.

Cell membrane proteins are represented mainly by glycoproteins. There are:

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 silk phosphatases, etc.). But some enzymes are also represented by integral proteins - ATPase.

Integral proteins provide selective exchange of ions through membrane channels between extracellular and intracellular fluid, and also act as proteins that transport large molecules.

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

Proteins adjacent to the membrane from the cytoplasmic side are classified as cell cytoskeleton . They can attach to membrane proteins.

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

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

Spectrin is a major cytoskeletal protein 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 flexible-elastic properties and 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 their significant sialic acid content (for example, the glycophorin molecule). This provides the surfaces of most cells with a negative charge, helping to repel other negatively charged objects. Carbohydrate protrusions of glycoproteins are carriers of blood group antigens, other antigenic determinants of the cell, and they act as receptors that bind hormones. Glycoproteins form adhesive molecules that cause cells to attach to one another, 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 or within their membrane. These include oxidative enzymes, which play an important role in the modification of 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 activation of phospholipid metabolism, thromboxanes and leukotrienes are formed in the membrane, which have a powerful effect on platelet adhesion, the process of inflammation, etc.

The processes of renewal of its components continuously occur in the membrane . 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. “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 are also characterized by a significant rate of exchange, which requires large amounts of fatty acids for the synthesis of these membrane components.
The specificity of the lipid composition of cell membranes is influenced 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, leading to a favorable change in the ratio of phospholipids to sphingomyelins and lipids to proteins 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 and reduces membrane microviscosity. This increases the deformability of red blood cells and facilitates their transport function (Chapter 6).

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

For example, fat deficiency disrupts the functions of the neutrophil membrane, which inhibits their ability to move and phagocytosis (the active capture and absorption of microscopic foreign living objects and particulate matter by single-celled 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 formed in the cell in conjunction with normally occurring metabolic reactions (microsomal oxidation, etc.).

Generated 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 the phospholipids of cell membranes (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, “quenching” 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 activity. Prostaglandins, by binding to specific receptors on cell membranes, stabilize the bilayer of the latter and reduce the loss of phospholipids by the membranes.

Functions of membrane receptors

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

Transmembrane signal transmission in a cell can be schematically represented as follows:

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

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

The precursor of one second 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 turning on 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 lobe of the pituitary gland by thyrogropin-releasing factor, the response of lymphocytes to antigen, etc.
In some cells, the role of a second messenger is played by cGMP, 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 the walls of blood vessels. cAMP serves as a secondary 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 makes it possible to examine their structure and study their key properties without being tied to a specific 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 life activity is inextricably linked with the environment with which it exchanges energy, information, and matter. Thus, the nutritional energy necessary for the functioning of the cell comes from the outside and is spent on 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 cell activity

The biological membrane plays a pivotal role in the structure and functioning of an elementary living system. Only a cell surrounded by a protective shell can rightfully 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 disrupted, this leads to a change in the functional state of the body 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 specificity of intercellular contacts and immune manifestations, and maintains the transmembrane difference in electrical potential. It contains receptors that can perceive chemical signals - hormones, mediators and other biological 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. Transfer through 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 the carrier.

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

Cell membrane structure

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.

Roofing membrane: definition and typology

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

As a rule, the membrane as the main material when covering the roof is ideal for flat roofs. For those with a large slope it is less suitable.

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

The service life is about 60 years. Just imagine, only the warranty period of use, which is established by the manufacturer, reaches 20 years.
Easy to install. For comparison: installing a bitumen roof takes 1.5 times longer than installing a membrane roof.
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 aggressive environments;
frost resistance;
fire resistance.

There are three types of roofing membrane. The main classification feature is the type of polymer 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, or simply put, Advantages: high strength, elasticity, water resistance, environmental friendliness, low cost. Disadvantages: adhesive technology for joining sheets using a special tape, low strength of joints. Scope of application: used as a waterproofing material for tunnel floors, water sources, waste storage facilities, 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, wide range of colors of membrane fabrics. Disadvantages: low resistance to bituminous materials, oils, solvents; releases harmful substances into the atmosphere; The color of the canvas fades over time.
TPO. Made from thermoplastic olefins. They can be reinforced or unreinforced. The former are equipped with a polyester mesh or fiberglass fabric. Advantages: environmental friendliness, durability, high elasticity, temperature resistance (both at high and low temperatures), welded joints of fabric seams. Disadvantages: high price category, lack of manufacturers in the domestic market.

Profiled membrane: characteristics, functions and advantages

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

The substance used in production is polyethylene. The latter comes in two types: high-density polyethylene (HDPE) and low-density polyethylene (LDPE).

Technical characteristics of LDPE and HDPE membranes

Index
Tensile strength (MPa)
Tensile elongation (%)
Density (kg/cu.m.)
Compressive Strength (MPa)
Impact strength (notched) (KJ/sq.m)
Flexural modulus of elasticity (MPa)
Hardness (MRa)
Operating temperature (˚С)	from -60 to +80	from -60 to +80
Daily water absorption rate (%)

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 allows for trouble-free bending of building materials.

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

Advantages of profiled membranes:

increased strength;
heat resistance;
resistance to chemical and biological influences;
long service life (more than 50 years);
ease of installation and maintenance;
affordable price.

Profiled membranes come in three types:

with single-layer fabric;
with two-layer fabric = geotextile + drainage membrane;
with three-layer fabric = 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 installation. A three-layer protective one is used on soil that is susceptible to frost heaving and on deep soil.

Areas of use of drainage membranes

The profiled membrane finds its application in the following areas:

Basic waterproofing of the foundation. Provides reliable protection against the destructive influence of groundwater, plant root systems, soil subsidence, and mechanical damage.
Foundation wall drainage. Neutralizes the effects of groundwater and atmospheric precipitation by transporting them to drainage systems.
Horizontal type - protection against deformation due to structural features.
Analogous to concrete preparation. It is used in the case of construction work on the construction of buildings in an area of low groundwater, in cases where horizontal waterproofing is used to protect against capillary moisture. Also, the functions of the profiled membrane include preventing the passage of cement laitance into the ground.
Ventilation of wall surfaces with high humidity levels. Can be installed both on the inside and outside of the room. In the first case, air circulation is activated, and in the second, optimal humidity and temperature are ensured.
Inversion roofing used.

Superdiffusion membrane

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

The production of protective material is based on the use of non-woven substances, dense fibers of high quality. Three-layer and four-layer membranes are popular in the domestic market. Reviews from experts and consumers confirm that the more layers a structure is based on, 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, and climatic conditions, manufacturers recommend giving preference to one or another type of diffusion membrane. So, they exist for pitched roofs of complex and simple structures, for pitched roofs with a minimum slope, for roofs with seam covering, etc.

The superdiffusion membrane is laid directly on the thermal insulation layer, the flooring made of boards. There is no need for a ventilation gap. The material is secured with special staples or steel nails. The edges of the diffusion sheets are joined and 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

PFC membranes are a roofing material made from polyvinyl chloride and have elastic properties. Such modern roofing material has completely replaced bitumen 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 this material is easy to use, and its installation can be done on any type of surface, at any time of the year and in any weather conditions. The 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 their unique properties that PVC membranes will serve you faithfully for many years. The lifespan of such a roof is equal to the lifespan of the building itself, while roll roofing materials require regular repairs, and in some cases, complete dismantling and installation of a new floor.

PVC membrane sheets are connected to each other by hot 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:

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

Membrane fabric

Membrane fabric has been known to the textile industry for a long time. Shoes and clothing are made from this material: adults and children. Membrane is the basis of membrane fabric, presented in the form of a thin polymer film and having such characteristics as waterproofness and vapor permeability. To produce this material, this film is coated with outer and inner protective layers. Their structure is determined by the membrane itself. This is done in order to preserve all beneficial properties even in the event 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 allows steam to pass 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 clothing is made from such fabric. The membrane at the base of the fabric can be:

with pores;
without pores;
combined.

The membranes, which have many micropores, contain Teflon. The dimensions of such pores do not reach the dimensions of even 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 the sweat and 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 from membrane fabrics and intended for wear in the winter season are durable, but lightweight, and provide excellent protection from frost, moisture, and dust. They are simply irreplaceable for many active types of winter recreation and mountaineering.

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

Regulate the exchange between the cell and the environment;

They divide cells into compartments, or compartments, intended for certain specialized metabolic pathways ( dividing);

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 location of receptor sites that recognize external stimuli.

Transport of substances through the membrane - one of the leading functions of the membrane, ensuring the exchange of substances between the cell and the external environment. Depending on the energy consumption for the transfer of substances, they are distinguished:

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 released into the cytoplasm. The vesicle is delimited from the cytoplasm by a single membrane, which is part of the outer cytoplasmic membrane. There are phagocytosis and pinocytosis. Phagocytosis is the absorption of large particles that are quite hard. For example, phagocytosis of lymphocytes, protozoa, etc. Pinocytosis is the process of capturing and absorbing droplets of liquid 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, fuses with the outer cytoplasmic membrane. The contents of the vesicle are removed beyond the cell surface, and the membrane is included in the outer cytoplasmic membrane.

At the core passive transport of uncharged molecules lies in 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 speed of transport depends on the difference in gradients.

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 the bilayer because its molecule is small and electrically neutral. The diffusion of water through membranes is called osmosis.

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

Facilitated diffusion is the transport of substances using 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 carrier proteins (ATPase) against an electrochemical gradient, with energy consumption. Its source is ATP molecules. For example, sodium is a 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 through the water pores of the membrane along a concentration gradient. This is explained by the fact that the permeability of the membrane for potassium ions is higher than for sodium ions. Accordingly, potassium diffuses out of the cell faster than sodium into the cell. However, for normal cell functioning a certain ratio of 3 potassium and 2 sodium ions is necessary. Therefore, there is a sodium-potassium pump in the membrane that 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 and sodium ions (antiport). The process is energy intensive:

From the inside of the membrane, sodium ions and an ATP molecule enter the pump protein, and potassium ions come 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 attaches to the protein, i.e. phosphorylates the protein.

Phosphorylation causes conformational changes in the protein; it becomes unable to retain sodium ions. They are released and move outside the cell.

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

The addition of potassium ions causes dephosphorylation of the protein. It changes its conformation again.

A 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 out 3 sodium ions from the cell and pumps in 2 potassium ions.

Cytoplasm– an obligatory component of the cell, located between the surface apparatus of the cell and the nucleus. This is a complex heterogeneous structural complex consisting of:

hyaloplasma

organelles (permanent components of the cytoplasm)

inclusions are temporary components of the cytoplasm.

Cytoplasmic matrix(hyaloplasm) is the internal contents of the cell - a colorless, thick and transparent colloidal solution. The components of the cytoplasmic matrix carry out biosynthesis processes in the cell and contain enzymes necessary for energy production, 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 a highly heterogeneous or multiphase colloidal system.

Provides a change in the viscosity of the cytoplasm, a 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 cell performs specific functions. Depending on the structural features, they are distinguished:

membrane 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 are characteristic of all cells: nucleus, mitochondria, cell center, Golgi apparatus, ribosomes, EPS, lysosomes. When organelles are characteristic of certain cell types, they are called specialty 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. The ER membranes are, on the one hand, connected to the cell cytoplasmic membrane, and on the other, to 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. Smooth or agranular ER has no connection with ribosomes. This is the inner side of the membrane.

The structure of the biomembrane. The cell-bounding membranes and membrane organelles of eukaryotic cells have 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 independently in the membrane; they are associated with proteins and lipids. The thickness of the membranes is 7-10 nm.

According to the currently generally accepted fluid mosaic model of membrane structure, lipids form a double layer, or lipid bilayer, in which the hydrophilic “heads” of lipid molecules face 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 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 submerged 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, chemical composition and 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, which are part of complex proteins-glycoproteins and complex lipids-glycolipids, protrude above the surface of the membrane. Plasmalemma glycoproteins are extremely specific. For example, they are used for mutual recognition of cells, including sperm and egg.

On the surface of animal cells, carbohydrate chains form a thin surface layer - glycocalyx. It is detected in almost all animal cells, but its degree of expression varies (10-50 µm). The glycocalyx provides direct communication between the cell and the external environment, where extracellular digestion occurs; Receptors are located in the glycocalyx. In addition to the plasmalemma, the cells of bacteria, plants and fungi are also surrounded by cell membranes.

Internal membranes eukaryotic cells delimit different parts of the cell, forming peculiar “compartments” - compartments, which promotes the separation of various metabolic and energy processes. They may differ in chemical composition and functions, but their general structural plan remains the same.

Membrane functions:

1. Limiting. The idea is that they separate the internal space of the cell from the external environment. The membrane is semi-permeable, that is, only those substances that the cell needs can freely pass through it, and there are mechanisms for transporting the necessary substances.

2. Receptor. It is primarily associated 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 occur on them.

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

5. Compartmentalization. Membranes also delimit the space inside the cell, thereby separating the starting materials of the reaction and the enzymes that can carry out the corresponding reactions.

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

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 - membrane transport is required (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 energy consumption from a higher concentration to a lower one. Lipid-soluble small non-polar molecules (0 2, C0 2) easily penetrate the cell by simple diffusion. Those 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 through a membrane is called by 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 the 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 transport occurs in the opposite direction - into the cell. However, there are limits to the extensibility of the cytoplasmic membrane, and an animal cell eventually ruptures, while a plant cell does not allow this to happen due to its strong cell wall. The phenomenon of filling the entire internal space of a cell with cellular contents is called deplasmolysis. The intracellular concentration of salts should be taken into account when preparing medications, especially for intravenous administration, as this can lead to damage to blood cells (for this, saline solution with a concentration of 0.9% sodium chloride is used). This is no less important when cultivating cells and tissues, as well as animal and plant organs.

Active transport proceeds with the expenditure of ATP energy from a lower concentration of a substance to a higher one. It is carried out using special pump proteins. Proteins pump K + , Na + , Ca 2+ and other ions through the membrane, which promotes the transport of essential 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 contained. Then the primary lysosome fuses with the phagosome and forms secondary lysosome, or phagolysosome, or digestive vacuole. The contents of the vesicle are digested by lysosome enzymes, and the breakdown 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 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 (unicellular animals, human leukocytes), it provides their nutrition, and often protection of the body (Fig. 2.28).

By pinocytosis, proteins, antigen-antibody complexes are absorbed during immune reactions, etc. However, many viruses also enter the cell by pinocytosis or phagocytosis. In plant and fungal cells, phagocytosis is practically impossible, as they are surrounded by durable cell membranes.

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

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

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

Plasmodesmata carry out the transport of substances between cells. However, 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 wall of plants, which leads to lignification or suberization of the cell wall itself, displacement of water and death of cellular contents. Between the cell walls of neighboring plant cells there are jelly-like spacers - middle plates that hold them together and cement the plant body as a whole. They are destroyed only during the process of fruit ripening and when the leaves fall.

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

In bacteria, the cell wall contains carbohydrates with peptide fragments - murein, however, its content varies significantly among different groups of bacteria. Other polysaccharides can also be released outside the cell wall, forming a mucous capsule that protects bacteria from external influences.

The membrane 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 rupture of the cell, which increases due to the entry 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 through the cell walls faster than through the cytoplasm.