Active

(with energy cost)

simple diffusion

(without the participation of proteins)

Energy source - ATP

Facilitated diffusion

(involving proteins)

Other types of sources

Through a channel in a protein

By coup

protein with substance

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

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

For a visual illustration of the process of transfer of substances through the membrane, we present (with minor changes) Figure 5 taken from the book "Molecular Biology of the Cell" (1983) by B. Alberts and other scientists who are considered leaders in the development of the theory

Transported molecule

Channel Protein

carrier protein

Lipid Electrochemical

bilayer gradient

Simple diffusion Facilitated diffusion

Passive transport Active transport

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

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

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

b. Transfer of information across the plasma membrane.

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

It consists of a bilipid layer, the lipids of which are strictly oriented - the hydrophobic part of the lipids (tail) is turned inside the layer, while the hydrophilic part (head) is outward. In addition to lipids, three types of membrane proteins take part in the construction of the plasma membrane: peripheral, integral, and semi-integral.

One of the current directions of membrane research is a detailed study of the properties of various structural and regulatory lipids, as well as individual integral and semi-integral proteins that make up membranes.

Integral membrane proteins

The main role in the organization of the membrane itself is played by integral and semi-integral proteins, which have a globular structure and are associated with the lipid phase by hydrophilic-hydrophobic interactions. Globules of integral proteins penetrate the entire thickness of the membrane, and their hydrophobic part is located in the middle of the globule and is immersed in the hydrophobic zone of the lipid phase.

semi-integral membrane proteins

In semi-integral proteins, hydrophobic amino acids are concentrated at one of the poles of the globule, and, accordingly, the globules are only half immersed in the membrane, protruding from one (external or internal) surface of the membrane.

Functions of membrane proteins

Integral and semi-integral proteins of the plasma membrane were previously assigned two functions: general structural and specific. Accordingly, structural and functional proteins were distinguished among them. However, the improvement of methods for isolating protein fractions of membranes and a more detailed analysis of individual proteins now indicate the absence of structural proteins that are universal for all membranes and do not carry any specific functions. On the contrary, membrane proteins with specific functions are very diverse. These are proteins that perform receptor functions, proteins that are active and passive carriers of various compounds, and finally, proteins that are part of numerous enzyme systems. material from the site

Properties of membrane proteins

A common property of all these integral and semi-integral membrane proteins, which differ not only in functional but also in chemical terms, is their fundamental ability to move, “swim” in the plane of the membrane in the liquid lipid phase. As noted above, the existence of such movements in the plasma membranes of some cells has been experimentally proven. But this is far from the only type of movement identified in membrane proteins. In addition to lateral displacement, individual integral and semi-integral proteins can rotate in the membrane plane in horizontal and even vertical directions, and can also change the degree of immersion of the molecule in the lipid phase.

Opsin. All these diverse and complex movements of protein globules are especially well illustrated by the example of the opsin protein, which is specific for the membranes of photoreceptor cells (Fig. 3). As is known, opsin in the dark is associated with the carotenoid retinal, which contains a double cis bond; the complex of retinal and opsin forms rhodopsin, or visual purple. The rhodopsin molecule is capable of lateral movement and rotation in the horizontal plane of the membrane (Fig. 3, A). Under the action of light, retinal undergoes photoisomerization and transforms into a trans form. In this case, the conformation of retinal changes and it separates from opsin, which, in turn, changes the plane of rotation from horizontal to vertical (Fig. 3b). The consequence of such transformations is a change in the permeability of membranes for ions, which leads to the emergence of a nerve impulse.

It is interesting that changes in the conformation of opsin globules induced by light energy not only can serve to generate a nerve impulse, as occurs in the cells of the retina, but are also the simplest photosynthetic system found in special purple bacteria.

PLASMATIC MEMBRANE, STRUCTURE AND FUNCTIONS. STRUCTURES FORMED BY THE PLASMATIC MEMBRANE

We will begin histology by studying the eukaryotic cell, which is the simplest system endowed with life. When examining a cell in a light microscope, we obtain information about its size, shape, and this information is associated with the presence of membrane-limited boundaries in cells. With the development of electron microscopy (EM), our understanding of the membrane as a clearly defined dividing line between the cell and the environment has changed, because it turned out that there is a complex structure on the cell surface, consisting of the following 3 components:

1. supramembrane component(glycocalix) (5 - 100 nm);

2. plasma membrane(8 - 10 nm);

3. Submembrane component(20 - 40 nm).

At the same time, components 1 and 3 are variable and depend on the type of cells; the structure of the plasma membrane seems to be the most static, which we will consider.

Plasma membrane. The study of the plasmolemma under EM conditions led to the conclusion that its structural organization is uniform, in which it has the form of a trilaminar line, where the inner and outer layers are electron-dense, and the wider layer located between them appears to be electron-transparent. This type of structural organization of the membrane indicates its chemical heterogeneity. Without touching the discussion on this issue, we will stipulate that the plasmalemma consists of three types of substances: lipids, proteins and carbohydrates.

Lipids, which are part of the membranes, have amphiphilic properties due to the presence of both hydrophilic and hydrophobic groups in their composition. The amphipathic nature of membrane lipids promotes the formation of a lipid bilayer. At the same time, two domains are distinguished in membrane phospholipids:

a) phosphate - the head of the molecule, the chemical properties of this domain determine its solubility in water and it is called hydrophilic;

b) acyl chains, which are esterified fatty acids hydrophobic domain.

Types of membrane lipids: The main class of lipids in biological membranes are phospholipids, they form the framework of a biological membrane. See fig.1

Rice. 1: Types of membrane lipids

Biomembranes is a double layer amphiphilic lipids (lipid bilayer). In an aqueous medium, such amphiphilic molecules spontaneously form a bilayer, in which the hydrophobic parts of the molecules are oriented towards each other, and the hydrophilic parts are oriented towards water. See fig. 2

Rice. 2: Diagram of the structure of a biomembrane

The composition of membranes includes lipids of the following types:

1. Phospholipids;

2. Sphingolipids- “heads” + 2 hydrophobic “tails”;

3. Glycolipids.

Cholesterol (CL)- is located in the membrane mainly in the middle zone of the bilayer, it is amphiphilic and hydrophobic (with the exception of one hydroxyl group). The lipid composition affects the properties of the membranes: the ratio of protein/lipids is close to 1:1, however, the myelin sheaths are enriched in lipids, and the inner membranes are enriched in proteins.

Packing methods for amphiphilic lipids:

1. Bilayers(lipid membrane);

2. Liposomes- this is a bubble with two layers of lipids, while both the inner and outer surfaces are polar;

3. Micelles- the third variant of the organization of amphiphilic lipids - a bubble, the wall of which is formed by a single layer of lipids, while their hydrophobic ends are facing the center of the micelle and their internal environment is not aqueous, but hydrophobic.

The most common form of packaging of lipid molecules is their formation flat membrane bilayer. Liposomes and micelles are fast transport forms that ensure the transfer of substances into and out of the cell. In medicine, liposomes are used to transport water-soluble substances, while micelles are used to transport fat-soluble substances.

Membrane proteins

1. Integral (included in lipid layers);

2. Peripheral. See fig. 3

Integral (transmembrane proteins):

1. Monotopic- (for example, glycophorin. They cross the membrane 1 time), and are receptors, while their outer - extracellular domain - refers to the recognizing part of the molecule;

2.Polytopic- repeatedly penetrate the membrane - these are also receptor proteins, but they activate the signal transmission pathway into the cell;

3.Membrane proteins associated with lipids;

4. Membrane proteins, associated with carbohydrates.

Rice. 3: Membrane proteins

Peripheral proteins:

Not immersed in the lipid bilayer and not covalently linked to it. They are held together by ionic interactions. Peripheral proteins are associated with integral proteins in the membrane through interaction - protein-protein interactions.

1. Spectrin, which is located on the inner surface of the cell;

2.fibronectin, located on the outer surface of the membrane.

Squirrels - usually make up to 50% of the mass of the membrane. Wherein integral proteins perform the following functions:

a) ion channel proteins;

b) receptor proteins.

BUT peripheral membrane proteins (fibrillar, globular) perform the following functions:

a) external (receptor and adhesion proteins);

b) internal - cytoskeletal proteins (spectrin, ankyrin), proteins of the system of second mediators.

ion channels are channels formed by integral proteins; they form a small pore through which ions pass along the electrochemical gradient. The most well-known channels are channels for Na, K, Ca, Cl.

There are also water channels aquoporins (erythrocytes, kidney, eye).

supramembrane component - glycocalyx, thickness 50 nm. These are carbohydrate regions of glycoproteins and glycolipids that provide a negative charge. Under EM is a loose layer of moderate density covering the outer surface of the plasmalemma. The composition of the glycocalyx, in addition to carbohydrate components, includes peripheral membrane proteins (semi-integral). Their functional areas are located in the supra-membrane zone - these are immunoglobulins. See fig. 4

Function of the glycocalyx:

1. Play a role receptors;

2. Intercellular recognition;

3. Intercellular interactions(adhesive interactions);

4. Histocompatibility receptors;

5. Enzyme adsorption zone(parietal digestion);

6. Hormone receptors.

Rice. 4: Glycocalyx and submembrane proteins

Submembrane component - the outermost zone of the cytoplasm, usually has a relative rigidity and this zone is especially rich in filaments (d = 5-10 nm). It is assumed that the integral proteins that make up the cell membrane are directly or indirectly associated with actin filaments lying in the submembrane zone. At the same time, it was experimentally proved that during the aggregation of integral proteins, actin and myosin located in this zone also aggregate, which indicates the participation of actin filaments in the regulation of the cell shape.

Cell— self-regulating structural and functional unit of tissues and organs. The cellular theory of the structure of organs and tissues was developed by Schleiden and Schwann in 1839. Subsequently, using electron microscopy and ultracentrifugation, it was possible to elucidate the structure of all the main organelles of animal and plant cells (Fig. 1).

Rice. 1. Scheme of the structure of the cell of animal organisms

The main parts of the cell are the cytoplasm and the nucleus. Each cell is surrounded by a very thin membrane that limits its contents.

The cell membrane is called plasma membrane and is characterized by selective permeability. This property allows the necessary nutrients and chemical elements to penetrate into the cell, and excess products to leave it. The plasma membrane consists of two layers of lipid molecules with the inclusion of specific proteins in it. The main membrane lipids are phospholipids. They contain phosphorus, a polar head, and two non-polar long-chain fatty acid tails. Membrane lipids include cholesterol and cholesterol esters. In accordance with the fluid mosaic model of the structure, membranes contain inclusions of protein and lipid molecules that can mix relative to the bilayer. Each type of membrane of any animal cell is characterized by its relatively constant lipid composition.

Membrane proteins are divided into two types according to their structure: integral and peripheral. Peripheral proteins can be removed from the membrane without destroying it. There are four types of membrane proteins: transport proteins, enzymes, receptors, and structural proteins. Some membrane proteins have enzymatic activity, while others bind certain substances and facilitate their transfer into the cell. Proteins provide several pathways for the movement of substances across membranes: they form large pores consisting of several protein subunits that allow water molecules and ions to move between cells; form ion channels specialized for the movement of certain types of ions across the membrane under certain conditions. Structural proteins are associated with the inner lipid layer and provide the cytoskeleton of the cell. The cytoskeleton gives mechanical strength to the cell membrane. In various membranes, proteins account for 20 to 80% of the mass. Membrane proteins can move freely in the lateral plane.

Carbohydrates are also present in the membrane, which can covalently bind to lipids or proteins. There are three types of membrane carbohydrates: glycolipids (gangliosides), glycoproteins and proteoglycans. Most membrane lipids are in a liquid state and have a certain fluidity, i.e. the ability to move from one area to another. On the outer side of the membrane there are receptor sites that bind various hormones. Other specific sections of the membrane can> t recognize and bind some proteins alien to these cells and various biologically active compounds.

The inner space of the cell is filled with cytoplasm, in which most enzyme-catalyzed reactions of cellular metabolism take place. The cytoplasm consists of two layers: the inner, called the endoplasm, and the peripheral, the ectoplasm, which has a high viscosity and is devoid of granules. The cytoplasm contains all the components of a cell or organelle. The most important of the cell organelles are the endoplasmic reticulum, ribosomes, mitochondria, the Golgi apparatus, lysosomes, microfilaments and microtubules, peroxisomes.

Endoplasmic reticulum is a system of interconnected channels and cavities penetrating the entire cytoplasm. It provides transport of substances from the environment and inside cells. The endoplasmic reticulum also serves as a depot for intracellular Ca 2+ ions and serves as the main site for lipid synthesis in the cell.

Ribosomes - microscopic spherical particles with a diameter of 10-25 nm. Ribosomes are freely located in the cytoplasm or attached to the outer surface of the membranes of the endoplasmic reticulum and the nuclear membrane. They interact with informational and transport RNA, and protein synthesis is carried out in them. They synthesize proteins that enter the cisterns or the Golgi apparatus and are then released outside. Ribosomes that are free in the cytoplasm synthesize protein for use by the cell itself, and ribosomes associated with the endoplasmic reticulum produce protein that is excreted from the cell. Various functional proteins are synthesized in ribosomes: carrier proteins, enzymes, receptors, cytoskeletal proteins.

golgi apparatus formed by a system of tubules, cisterns and vesicles. It is associated with the endoplasmic reticulum, and the biologically active substances that have entered here are stored in a compacted form in secretory vesicles. The latter are constantly separated from the Golgi apparatus, transported to the cell membrane and merge with it, and the substances contained in the vesicles are removed from the cell in the process of exocytosis.

Lysosomes - particles surrounded by a membrane with a size of 0.25-0.8 microns. They contain numerous enzymes involved in the breakdown of proteins, polysaccharides, fats, nucleic acids, bacteria and cells.

Peroxisomes formed from a smooth endoplasmic reticulum, resemble lysosomes and contain enzymes that catalyze the decomposition of hydrogen peroxide, which is cleaved under the influence of peroxidases and catalase.

Mitochondria contain outer and inner membranes and are the "energy station" of the cell. Mitochondria are round or elongated structures with a double membrane. The inner membrane forms folds protruding into the mitochondria - cristae. ATP is synthesized in them, the substrates of the Krebs cycle are oxidized, and many biochemical reactions are carried out. ATP molecules formed in mitochondria diffuse into all parts of the cell. Mitochondria contain a small amount of DNA, RNA, ribosomes, and with their participation, renewal and synthesis of new mitochondria takes place.

Microfilaments are thin protein filaments, consisting of myosin and actin, and form the contractile apparatus of the cell. Microfilaments are involved in the formation of folds or protrusions of the cell membrane, as well as in the movement of various structures inside cells.

microtubules form the basis of the cytoskeleton and provide its strength. The cytoskeleton gives the cells a characteristic appearance and shape, serves as a site for attachment of intracellular organelles and various bodies. In nerve cells, bundles of microtubules are involved in the transport of substances from the cell body to the ends of axons. With their participation, the functioning of the mitotic spindle during cell division is carried out. They play the role of motor elements in the villi and flagella in eukaryotes.

Core is the main structure of the cell, is involved in the transmission of hereditary traits and in the synthesis of proteins. The nucleus is surrounded by a nuclear membrane containing many nuclear pores through which various substances are exchanged between the nucleus and the cytoplasm. Inside it is the nucleolus. The important role of the nucleolus in the synthesis of ribosomal RNA and histone proteins has been established. The rest of the nucleus contains chromatin, consisting of DNA, RNA, and a number of specific proteins.

Functions of the cell membrane

Cell membranes play an important role in the regulation of intracellular and intercellular metabolism. They are selective. Their specific structure makes it possible to provide barrier, transport and regulatory functions.

barrier function It manifests itself in limiting the penetration of compounds dissolved in water through the membrane. The membrane is impermeable to large protein molecules and organic anions.

Regulatory function membrane is the regulation of intracellular metabolism in response to chemical, biological and mechanical influences. Various influences are perceived by special membrane receptors with a subsequent change in the activity of enzymes.

transport function through biological membranes can be carried out passively (diffusion, filtration, osmosis) or with the help of active transport.

Diffusion - the movement of a gas or solute along a concentration and electrochemical gradient. The diffusion rate depends on the permeability of the cell membrane, as well as the concentration gradient for uncharged particles, electric and concentration gradients for charged particles. simple diffusion occurs through the lipid bilayer or through channels. Charged particles move along the electrochemical gradient, while uncharged particles follow the chemical gradient. For example, oxygen, steroid hormones, urea, alcohol, etc. penetrate through the lipid layer of the membrane by simple diffusion. Various ions and particles move through the channels. Ion channels are formed by proteins and are divided into gated and uncontrolled channels. Depending on the selectivity, there are ion-selective ropes that allow only one ion to pass through, and channels that do not have selectivity. Channels have a mouth and a selective filter, and controlled channels have a gate mechanism.

Facilitated diffusion - a process in which substances are transported across a membrane by special membrane carrier proteins. In this way, amino acids and monosugars enter the cell. This mode of transport is very fast.

Osmosis - movement of water across a membrane from a solution with a lower osmotic pressure to a solution with a higher osmotic pressure.

Active transport - transfer of substances against a concentration gradient using transport ATPases (ion pumps). This transfer occurs with the expenditure of energy.

Na + /K + -, Ca 2+ - and H + pumps have been studied to a greater extent. Pumps are located on cell membranes.

A type of active transport is endocytosis and exocytosis. With the help of these mechanisms, larger substances (proteins, polysaccharides, nucleic acids) that cannot be transported through the channels are transported. This transport is more common in the epithelial cells of the intestine, renal tubules, and vascular endothelium.

At In endocytosis, cell membranes form invaginations into the cell, which, when laced, turn into vesicles. During exocytosis, vesicles with contents are transferred to the cell membrane and merge with it, and the contents of the vesicles are released into the extracellular environment.

The structure and functions of the cell membrane

To understand the processes that ensure the existence of electrical potentials in living cells, it is first of all necessary to understand the structure of the cell membrane and its properties.

At present, the fluid-mosaic model of the membrane, proposed by S. Singer and G. Nicholson in 1972, enjoys the greatest recognition. The basis of the membrane is a double layer of phospholipids (bilayer), the hydrophobic fragments of the molecule of which are immersed in the thickness of the membrane, and the polar hydrophilic groups are oriented outward, those. into the surrounding aquatic environment (Fig. 2).

Membrane proteins are localized on the membrane surface or can be embedded at different depths in the hydrophobic zone. Some proteins penetrate the membrane through and through, and different hydrophilic groups of the same protein are found on both sides of the cell membrane. Proteins found in the plasma membrane play a very important role: they participate in the formation of ion channels, play the role of membrane pumps and carriers of various substances, and can also perform a receptor function.

The main functions of the cell membrane: barrier, transport, regulatory, catalytic.

The barrier function is to limit the diffusion of water-soluble compounds through the membrane, which is necessary to protect cells from foreign, toxic substances and to maintain a relatively constant content of various substances inside the cells. So, the cell membrane can slow down the diffusion of various substances by 100,000-10,000,000 times.

Rice. 2. Three-dimensional scheme of the fluid-mosaic model of the Singer-Nicolson membrane

Globular integral proteins embedded in a lipid bilayer are shown. Some proteins are ion channels, others (glycoproteins) contain oligosaccharide side chains involved in the recognition of each other by cells and in the intercellular tissue. Cholesterol molecules are closely adjacent to the phospholipid heads and fix the adjacent areas of the "tails". The inner regions of the tails of the phospholipid molecule are not limited in their movement and are responsible for the fluidity of the membrane (Bretscher, 1985)

There are channels in the membrane through which ions penetrate. Channels are potential dependent and potential independent. Potential-gated channels open when the potential difference changes, and potential-independent(hormone-regulated) open when the receptors interact with substances. Channels can be opened or closed thanks to gates. Two types of gates are built into the membrane: activation(in the depth of the channel) and inactivation(on the surface of the channel). The gate can be in one of three states:

• open state (both types of gate are open);
• closed state (activation gate closed);
• inactivation state (inactivation gates are closed).

Another characteristic feature of membranes is the ability to selectively transfer inorganic ions, nutrients, and various metabolic products. There are systems of passive and active transfer (transport) of substances. Passive transport is carried out through ion channels with or without the help of carrier proteins, and its driving force is the difference in the electrochemical potentials of ions between the intra- and extracellular space. The selectivity of ion channels is determined by its geometric parameters and the chemical nature of the groups lining the channel walls and mouth.

At present, channels with selective permeability for Na + , K + , Ca 2+ ions and also for water (the so-called aquaporins) are the most well studied. The diameter of ion channels, according to various studies, is 0.5-0.7 nm. The throughput of the channels can be changed; 10 7 - 10 8 ions per second can pass through one ion channel.

Active transport occurs with the expenditure of energy and is carried out by the so-called ion pumps. Ion pumps are molecular protein structures embedded in the membrane and carrying out the transfer of ions towards a higher electrochemical potential.

The operation of the pumps is carried out due to the energy of ATP hydrolysis. Currently, Na + / K + - ATPase, Ca 2+ - ATPase, H + - ATPase, H + / K + - ATPase, Mg 2+ - ATPase, which ensure the movement of Na +, K +, Ca 2+ ions, respectively , H+, Mg 2+ isolated or conjugated (Na+ and K+; H+ and K+). The molecular mechanism of active transport has not been fully elucidated.

What is the structure of the plasma membrane? What are its functions?

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

Membrane functions:

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

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

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

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

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