Main functions of membranes. What function does the cell membrane perform - its properties and functions

The cell membrane is the structure that covers the outside of the cell. It is also called cytolemma or plasmalemma.

This formation is built from a bilipid layer (bilayer) with proteins built into it. The carbohydrates that make up the plasmalemma are in a bound state.

The distribution of the main components of the plasmalemma is as follows: more than half of the chemical composition is proteins, a quarter is occupied by phospholipids, and a tenth is cholesterol.

Cell membrane and its types

The cell membrane is a thin film, the basis of which is made up of layers of lipoproteins and proteins.

According to localization, membrane organelles are distinguished, which have some features in plant and animal cells:

mitochondria;
core;
endoplasmic reticulum;
Golgi complex;
lysosomes;
chloroplasts (in plant cells).

There is also an inner and outer (plasmolemma) cell membrane.

Structure of the cell membrane

The cell membrane contains carbohydrates that cover it in the form of a glycocalyx. This is a supra-membrane structure that performs a barrier function. The proteins located here are in free state. Unbound proteins participate in enzymatic reactions, providing extracellular breakdown of substances.

Proteins of the cytoplasmic membrane are represented by glycoproteins. Based on their chemical composition, proteins that are completely included in the lipid layer (along its entire length) are classified as integral proteins. Also peripheral, not reaching one of the surfaces of the plasmalemma.

The former function as receptors, binding to neurotransmitters, hormones and other substances. Insertion proteins are necessary for the construction of ion channels through which the transport of ions and hydrophilic substrates occurs. The latter are enzymes that catalyze intracellular reactions.

Basic properties of the plasma membrane

The lipid bilayer prevents the penetration of water. Lipids are hydrophobic compounds represented in the cell by phospholipids. The phosphate group faces outward and consists of two layers: the outer one, directed to the extracellular environment, and the inner one, delimiting the intracellular contents.

Water-soluble areas are called hydrophilic heads. The fatty acid sites are directed into the cell, in the form of hydrophobic tails. The hydrophobic part interacts with neighboring lipids, which ensures their attachment to each other. The double layer has selective permeability in different areas.

So, in the middle the membrane is impermeable to glucose and urea; hydrophobic substances pass through here freely: carbon dioxide, oxygen, alcohol. Cholesterol is important; the content of the latter determines the viscosity of the plasmalemma.

Functions of the outer cell membrane

The characteristics of the functions are briefly listed in the table:

Membrane function	Description
Barrier role	The plasmalemma performs a protective function, protecting the contents of the cell from the effects of foreign agents. Thanks to the special organization of proteins, lipids, and carbohydrates, the semipermeability of the plasmalemma is ensured.
Receptor function	Biologically active substances are activated through the cell membrane in the process of binding to receptors. Thus, immune reactions are mediated through the recognition of foreign agents by the cell receptor apparatus localized on the cell membrane.
Transport function	The presence of pores in the plasmalemma allows you to regulate the flow of substances into the cell. The transfer process occurs passively (without energy consumption) for compounds with low molecular weight. Active transport is associated with the expenditure of energy released during the breakdown of adenosine triphosphate (ATP). This method takes place for the transfer of organic compounds.
Participation in digestive processes	Substances are deposited on the cell membrane (sorption). Receptors bind to the substrate, moving it into the cell. A bubble is formed, lying freely inside the cell. Merging, such vesicles form lysosomes with hydrolytic enzymes.
Enzymatic function	Enzymes are essential components of intracellular digestion. Reactions requiring the participation of catalysts occur with the participation of enzymes.

What is the importance of the cell membrane

The cell membrane is involved in maintaining homeostasis due to the high selectivity of substances entering and exiting the cell (in biology this is called selective permeability).

Outgrowths of the plasmalemma divide the cell into compartments (compartments) responsible for carrying out certain functions. Specifically designed membranes corresponding to the fluid-mosaic pattern ensure the integrity of the cell.

Plasma membrane , or plasmalemma,- the most permanent, basic, universal membrane for all cells. It is a thin (about 10 nm) film covering the entire cell. The plasmalemma consists of protein molecules and phospholipids (Fig. 1.6).

Phospholipid molecules are arranged in two rows - with hydrophobic ends inward, hydrophilic heads towards the internal and external aqueous environment. In some places, the bilayer (double layer) of phospholipids is penetrated through and through by protein molecules (integral proteins). Inside such protein molecules there are channels - pores through which water-soluble substances pass. Other protein molecules penetrate the lipid bilayer halfway on one side or the other (semi-integral proteins). There are peripheral proteins on the surface of the membranes of eukaryotic cells. Lipid and protein molecules are held together due to hydrophilic-hydrophobic interactions.

Properties and functions of membranes. All cell membranes are mobile fluid structures, since lipid and protein molecules are not interconnected covalent bonds and are capable of moving quite quickly in the plane of the membrane. Thanks to this, membranes can change their configuration, i.e., they have fluidity.

Membranes are very dynamic structures. They quickly recover from damage and also stretch and contract with cellular movements.

Membranes of different types of cells differ significantly both in chemical composition and in the relative content of proteins, glycoproteins, lipids in them, and, consequently, in the nature of the receptors they contain. Each cell type is therefore characterized by an individuality, which is determined mainly glycoproteins. Branched chains of glycoproteins protruding from cell membrane, participate in factor recognition external environment, as well as in mutual recognition of related cells. For example, an egg and a sperm recognize each other by cell surface glycoproteins, which fit together as separate elements of a whole structure. Such mutual recognition is a necessary stage preceding fertilization.

A similar phenomenon is observed in the process of tissue differentiation. In this case, cells similar in structure, with the help of recognition areas of the plasmalemma, are correctly oriented relative to each other, thereby ensuring their adhesion and tissue formation. Associated with recognition transport regulation molecules and ions through the membrane, as well as an immunological response in which glycoproteins play the role of antigens. Sugars can thus function as information molecules (like proteins and nucleic acids). The membranes also contain specific receptors, electron carriers, energy converters, and enzyme proteins. Proteins are involved in ensuring the transport of certain molecules into or out of the cell, provide a structural connection between the cytoskeleton and cell membranes, or serve as receptors for receiving and converting chemical signals from environment.

The most important property of the membrane is also selective permeability. This means that molecules and ions pass through it at different speeds, and the larger the size of the molecules, the slower the speed at which they pass through the membrane. This property defines the plasma membrane as osmotic barrier. Water and gases dissolved in it have the maximum penetrating ability; Ions pass through the membrane much more slowly. The diffusion of water through a membrane is called by osmosis.

There are several mechanisms for transporting substances across the membrane.

Diffusion-penetration of substances through a membrane along a concentration gradient (from an area where their concentration is higher to an area where their concentration is lower). Diffuse transport of substances (water, ions) is carried out with the participation of membrane proteins, which have molecular pores, or with the participation of the lipid phase (for fat-soluble substances).

With facilitated diffusion special membrane transport proteins selectively bind to one or another ion or molecule and transport them across the membrane along a concentration gradient.

Active transport involves energy costs and serves to transport substances against their concentration gradient. He carried out by special carrier proteins that form the so-called ion pumps. The most studied is the Na - / K - pump in animal cells, which actively pumps Na + ions out while absorbing K - ions. Due to this, a higher concentration of K - and a lower concentration of Na + is maintained in the cell compared to the environment. This process requires ATP energy.

As a result of active transport using a membrane pump in the cell, the concentration of Mg 2- and Ca 2+ is also regulated.

During the process of active transport of ions into the cell, various sugars, nucleotides, and amino acids penetrate through the cytoplasmic membrane.

Protein macromolecules, nucleic acids, polysaccharides, lipoprotein complexes, etc. do not pass through cell membranes, unlike ions and monomers. Transport of macromolecules, their complexes and particles into the cell occurs in a completely different way - through endocytosis. At endocytosis (endo...- inward) a certain area of the plasmalemma captures and, as it were, envelops extracellular material, enclosing it in a membrane vacuole that arises as a result of invagination of the membrane. Subsequently, such a vacuole connects with a lysosome, the enzymes of which break down macromolecules into monomers.

The reverse process of endocytosis is exocytosis (exo...- out). Thanks to it, the cell removes intracellular products or undigested residues enclosed in vacuoles or pu-

zyryki. The vesicle approaches the cytoplasmic membrane, merges with it, and its contents are released into the environment. This is how digestive enzymes, hormones, hemicellulose, etc. are removed.

Thus, biological membranes, as the main structural elements of a cell, serve not just as physical boundaries, but are dynamic functional surfaces. Numerous biochemical processes take place on the membranes of organelles, such as active absorption of substances, energy conversion, ATP synthesis, etc.

Functions of biological membranes the following:

Separate the contents of the cell from external environment and the contents of organelles from the cytoplasm.

They ensure the transport of substances into and out of the cell, from the cytoplasm to organelles and vice versa.

They act as receptors (receiving and converting chemicals from the environment, recognizing cell substances, etc.).

They are catalysts (providing near-membrane chemical processes).

Participate in energy conversion.

Membranes perform a large number of different functions:

membranes determine the shape of an organelle or cell;

barrier: control the exchange soluble substances(for example, Na +, K +, Cl - ions) between the internal and external compartments;

energy: ATP synthesis on the inner membranes of mitochondria and photosynthesis in the membranes of chloroplasts; form a surface for flow chemical reactions(phosphorylation on mitochondrial membranes);

are a structure that ensures the recognition of chemical signals (hormone and neurotransmitter receptors are located on the membrane);

play a role in intercellular interaction and promote cell movement.

Transport through the membrane. The membrane has selective permeability to soluble substances, which is necessary for:

separation of the cell from the extracellular environment;

ensuring the penetration into the cell and retention of necessary molecules (such as lipids, glucose and amino acids), as well as the removal of metabolic products (including unnecessary ones) from the cell;

maintaining a transmembrane ion gradient.

Intracellular organelles may also have a selectively permeable membrane. For example, in lysosomes the membrane maintains a concentration of hydrogen ions (H+) 1000-10000 times higher than in the cytosol.

Transport across the membrane can be passive, lightened or active.

Passive transport- this is the movement of molecules or ions along a concentration or electrochemical gradient. This may be simple diffusion, as in the case of penetration of gases (for example O 2 and CO 2) or simple molecules (ethanol) through the plasma membrane. In simple diffusion, small molecules dissolved in the extracellular fluid are successively dissolved in the membrane and then in the intracellular fluid. This process is nonspecific, and the rate of penetration through the membrane is determined by the degree of hydrophobicity of the molecule, that is, its fat solubility. The rate of diffusion through the lipid bilayer is directly proportional to hydrophobicity as well as to the transmembrane concentration gradient or electrochemical gradient.

Facilitated diffusion is the rapid movement of molecules across a membrane with the help of specific membrane proteins called permeases. This process is specific; it proceeds faster than simple diffusion, but has a transport speed limitation.

Facilitated diffusion is usually characteristic of water-soluble substances. Most (if not all) membrane transporters are proteins. The specific mechanism of transporter functioning during facilitated diffusion has not been sufficiently studied. They can, for example, mediate transport by rotational motion in the membrane. IN lately Information has appeared that carrier proteins, upon contact with the transported substance, change their conformation, as a result of which a kind of “gate” or channel opens in the membrane. These changes occur due to the energy released when the transported substance binds to the protein. Relay-type transfers are also possible. In this case, the carrier itself remains motionless, and ions migrate along it from one hydrophilic bond to another.

The antibiotic gramicidin can serve as a model for this type of vector. In the lipid layer of the membrane, its long linear molecule takes the shape of a helix and forms a hydrophilic channel through which the K ion can migrate along a gradient.

Experimental evidence has been obtained for the existence of natural channels in biological membranes. Transport proteins are highly specific for the substance transported through the membrane, resembling enzymes in many properties. They exhibit greater sensitivity to pH, are competitively inhibited by compounds similar in structure to the transported substance, and non-competitively by agents that change specifically functional groups of proteins.

Facilitated diffusion differs from ordinary diffusion not only in speed, but also in its ability to saturate. The increase in the rate of transfer of substances occurs in proportion to the increase in the concentration gradient only up to certain limits. The latter is determined by the “power” of the carrier.

Active transport is the movement of ions or molecules across a membrane against a concentration gradient due to the energy of ATP hydrolysis. There are three main types of active ion transport:

sodium-potassium pump - Na + /K + -adenosine triphosphatase (ATPase), which transports Na + out and K + in;

calcium (Ca 2+) pump - Ca 2+ -ATPase, which transports Ca 2+ from the cell or cytosol to the sarcoplasmic reticulum;

proton pump - H + -ATPase. The ion gradients created by active transport can be used for the active transport of other molecules, such as some amino acids and sugars (secondary active transport).

Cotransport is the transport of an ion or molecule coupled with the transfer of another ion. Simport- simultaneous transfer of both molecules in one direction; antiport- simultaneous transfer of both molecules into opposite directions. If transport is not associated with the transfer of another ion, this process is called uniport. Cotransport is possible both during facilitated diffusion and during active transport.

Glucose can be transported by facilitated diffusion using the symport type. Cl - and HCO 3 - ions are transported across the red blood cell membrane by facilitated diffusion by a carrier called band 3, an antiport type. In this case, Cl - and HCO 3 - are transferred in opposite directions, and the direction of transfer is determined by the prevailing concentration gradient.

Active transport of ions against a concentration gradient requires energy released during the hydrolysis of ATP to ADP: ATP ADP + P (inorganic phosphate). Active transport, as well as facilitated diffusion, is characterized by: specificity, limitation of the maximum speed (that is, the kinetic curve reaches a plateau) and the presence of inhibitors. An example is the primary active transport carried out by Na + /K + - ATPase. For the functioning of this enzyme antiport system, the presence of Na +, K + and magnesium ions is necessary. It is present in virtually all animal cells, and its concentration is especially high in excitable tissues (for example, nerves and muscles) and in cells that receive active participation in movement carried out by Na + across the plasma membrane (for example, in the renal cortex and salivary glands).

The ATPase enzyme itself is an oligomer consisting of 2 -subunits of 110 kDa and 2 glycoprotein -subunits of 55 kDa each.. during ATP hydrolysis, a certain aspartate residue on the -subunit is reversibly phosphorylated to form -aspartamyl phosphate.. Phosphorylation requires Na + and Mg 2+ , but not K + , whereas dephosphorylation requires K + , but not Na + or Mg 2+ . Two conformational states of the protein complex with different energy levels have been described, which are usually designated E 1 and E 2, therefore ATPase is also called type E vector 1 - E 2 . Cardiac glycosides, e.g. digoxin And ouabain, inhibit ATPase activity. Ouabain, due to its excellent solubility in water, is widely used in experimental studies to study the sodium pump.

The generally accepted idea of how Na + /K + - ATPase works is as follows. Na and ATP ions join the ATPase molecule in the presence of Mg 2+. The binding of Na ions triggers the hydrolysis reaction of ATP, which results in the formation of ADP and the phosphorylated form of the enzyme. Phosphorylation induces a transition of the enzymatic protein to a new conformational state and the Na-bearing region or regions become exposed to the external environment. Here, Na + is exchanged for K + , since the phosphorylated form of the enzyme is characterized by a high affinity for K ions. The reverse transition of the enzyme to its original conformation is initiated by the hydrolytic elimination of the phosphoryl group in the form of inorganic phosphate and is accompanied by the release of K + into the internal space of the cell. The dephosphorylated active site of the enzyme is able to attach a new ATP molecule, and the cycle repeats.

The amounts of K and Na ions entering the cell as a result of the pump are not equal. For three removed Na ions, there are two introduced K ions with the simultaneous hydrolysis of one ATP molecule. The opening and closing of the channel on opposite sides of the membrane and the alternating change in the efficiency of Na and K binding are provided by the energy of ATP hydrolysis. The transported ions - Na and K - are cofactors of this enzymatic reaction. Theoretically, one can imagine a variety of pumps operating on this principle, although only a few are currently known.

Glucose transport. Glucose transport can occur by type of either facilitated diffusion or active transport, and in the first case it occurs as uniport, in the second - as symport. Glucose can be transported into red blood cells by facilitated diffusion. The Michaelis constant (Km) for the transport of glucose into red blood cells is approximately 1.5 mmol/L (that is, at this glucose concentration, about 50% of the available permease molecules will be bound to glucose molecules). Since the concentration of glucose in human blood is 4-6 mmol/l, its absorption by red blood cells occurs almost immediately maximum speed. The specificity of the permease is already manifested in the fact that the L-isomer is almost not transported into erythrocytes, unlike D-galactose and D-mannose, but higher concentrations are required to achieve half-saturation of the transport system. Once inside the cell, glucose undergoes phosphorylation and is no longer able to leave the cell. Glucose permease is also called D-hexose permease. It is an integral membrane protein with a molecular weight of 45 kDa.

Glucose can also be transported by Na+ -dependent system symport found in the plasma membranes of a number of tissues, including renal tubules and intestinal epithelium. In this case, one glucose molecule is transported by facilitated diffusion against the concentration gradient, and one Na ion is transported along the concentration gradient. The entire system ultimately functions through the pumping function of Na + /K + - ATPase. Thus, symport is a secondary active transport system. Amino acids are transported in a similar way.

Ca 2+ pump is an active transport system of the E 1 - E 2 type, consisting of an integral membrane protein, which, during the transfer of Ca 2+, is phosphorylated at the aspartate residue. During the hydrolysis of each ATP molecule, two Ca 2+ ions are transferred. In eukaryotic cells, Ca 2+ can bind to a calcium-binding protein called calmodulin, and the entire complex binds to the Ca 2+ pump. Ca 2+ -binding proteins also include troponin C and parvalbumin.

Ca ions, like Na ions, are actively removed from cells by Ca 2+ -ATPase. The membranes of the endoplasmic reticulum contain particularly large amounts of calcium pump protein. The chain of chemical reactions leading to ATP hydrolysis and Ca 2+ transfer can be written in the form of the following equations:

2Ca n + ATP + E 1 Ca 2 - E - P + ADP

Ca 2 - E - P 2Ca ext + PO 4 3- + E 2

Where is San - Ca2+ located outside;

Ca ext - Ca 2+ located inside;

E 1 and E 2 are different conformations of the transporter enzyme, the transition of which from one to another is associated with the use of ATP energy.

The system for the active removal of H + from the cytoplasm is supported by two types of reactions: the activity of the electron transport chain (redox chain) and ATP hydrolysis. Both redox and hydrolytic H + pumps are located in membranes capable of converting light or chemical energy into H + energy (that is, the plasma membranes of prokaryotes, the conjugating membranes of chloroplasts and mitochondria). As a result of the work of H + ATPase and/or the redox chain, protons are translocated, and a proton motive force (H +) appears on the membrane. The electrochemical gradient of hydrogen ions, as studies show, can be used for coupled transport (secondary active transport) large number metabolites - anions, amino acids, sugars, etc.

Associated with the activity of the plasma membrane are those that ensure the absorption of solid and liquid substances with a large molecular weight by the cell, - phagocytosis And pinocytosis(from Gerch. phagos- There is , pinos- drink, cytos- cell). The cell membrane forms pockets, or invaginations, that draw in substances from the outside. Then such invaginations are detached and surround a droplet of the external environment (pinocytosis) or solid particles (phagocytosis) with a membrane. Pinocytosis is observed in a wide variety of cells, especially in those organs where absorption processes occur.

The outside of the cage is covered plasma membrane(or outer cell membrane) about 6-10 nm thick.

The cell membrane is a dense film of proteins and lipids (mainly phospholipids). Lipid molecules are arranged in an orderly manner - perpendicular to the surface, in two layers, so that their parts that interact intensively with water (hydrophilic) are directed outward, and their parts inert to water (hydrophobic) are directed inward.

Protein molecules are located in a non-continuous layer on the surface of the lipid framework on both sides. Some of them are immersed in the lipid layer, and some pass through it, forming areas permeable to water. These proteins perform various functions - some of them are enzymes, others are transport proteins involved in the transfer of certain substances from the environment to the cytoplasm and in the opposite direction.

Basic functions of the cell membrane

One of the main properties of biological membranes is selective permeability (semi-permeability)- some substances pass through them with difficulty, others easily and even towards higher concentrations. Thus, for most cells, the concentration of Na ions inside is significantly lower than in the environment. The opposite relationship is typical for K ions: their concentration inside the cell is higher than outside. Therefore, Na ions always tend to penetrate the cell, and K ions always tend to exit. The equalization of the concentrations of these ions is prevented by the presence in the membrane of a special system that plays the role of a pump, which pumps Na ions out of the cell and simultaneously pumps K ions inside.

The tendency of Na ions to move from outside to inside is used to transport sugars and amino acids into the cell. With the active removal of Na ions from the cell, conditions are created for the entry of glucose and amino acids into it.

In many cells, substances are also absorbed by phagocytosis and pinocytosis. At phagocytosis the flexible outer membrane forms a small depression into which the captured particle falls. This recess increases, and, surrounded by a section of the outer membrane, the particle is immersed in the cytoplasm of the cell. The phenomenon of phagocytosis is characteristic of amoebas and some other protozoa, as well as leukocytes (phagocytes). Cells absorb liquids containing substances necessary for the cell in a similar way. This phenomenon was called pinocytosis.

The outer membranes of different cells differ significantly both in the chemical composition of their proteins and lipids, and in their relative content. It is these features that determine the diversity in the physiological activity of the membranes of various cells and their role in the life of cells and tissues.

Associated with the outer membrane endoplasmic reticulum cells. With the help of outer membranes they are carried out various types intercellular contacts, i.e. communication between individual cells.

Many types of cells are characterized by the presence on their surface large quantity protrusions, folds, microvilli. They contribute to both a significant increase in cell surface area and improved metabolism, as well as stronger connections of individual cells with each other.

Plant cells have thick membranes on the outside of the cell membrane, clearly visible under an optical microscope, consisting of fiber (cellulose). They provide a strong support plant tissues(wood).

Some animal cells also have a number of external structures, located on top of the cell membrane and having a protective nature. An example is the chitin of insect integumentary cells.

Functions of the cell membrane (briefly)

Function	Description
Protective Barrier	Separates internal cell organelles from the external environment
Regulatory	Regulates the metabolism between the internal contents of the cell and the external environment
Dividing (compartmentalization)	Division of the internal space of the cell into independent blocks (compartments)
Energy	- Energy accumulation and transformation; - light reactions of photosynthesis in chloroplasts; - Absorption and secretion.
Receptor (informational)	Participates in the formation of arousal and its conduct.
Motor	Carries out the movement of the cell or its individual parts.

Among The main functions of the cell membrane can be distinguished: barrier, transport, enzymatic and receptor. The cellular (biological) membrane (also known as plasmalemma, plasma or cytoplasmic membrane) protects the contents of the cell or its organelles from the environment, provides selective permeability for substances, enzymes are located on it, as well as molecules that can “catch” various chemical and physical signals.

This functionality is ensured by the special structure of the cell membrane.

In the evolution of life on Earth, a cell could generally form only after the appearance of a membrane, which separated and stabilized the internal contents and prevented them from disintegrating.

In terms of maintaining homeostasis (self-regulation of relative constancy internal environment) the barrier function of the cell membrane is closely related to transport.

Small molecules are able to pass through the plasmalemma without any “helpers”, along a concentration gradient, i.e., from an area with a high concentration of a given substance to an area with a low concentration. This is the case, for example, for gases involved in respiration. Oxygen and carbon dioxide diffuse through the cell membrane in the direction where their concentration is in at the moment less.

Since the membrane is mostly hydrophobic (due to the lipid double layer), polar (hydrophilic) molecules, even small ones, often cannot penetrate through it. Therefore, a number of membrane proteins act as carriers of such molecules, binding to them and transporting them through the plasmalemma.

Integral (membrane-permeating) proteins often operate on the principle of opening and closing channels. When any molecule approaches such a protein, it combines with it, and the channel opens. This substance or another passes through the protein channel, after which its conformation changes, and the channel closes to this substance, but can open to allow the passage of another. The sodium-potassium pump works on this principle, pumping potassium ions into the cell and pumping sodium ions out of it.

Enzymatic function of the cell membrane V to a greater extent implemented on the membranes of cell organelles. Most proteins synthesized in the cell perform an enzymatic function. “Sitting” on the membrane in in a certain order, they organize a conveyor belt as the product of a reaction catalyzed by one enzyme protein moves on to the next. This “conveyor” is stabilized by surface proteins of the plasmalemma.

Despite the universality of the structure of all biological membranes (they are built according to a single principle, almost identical in all organisms and in different membrane cell structures), they chemical composition may still differ. There are more liquid and more solid ones, some have more of certain proteins, others have less. In addition, they differ different sides(internal and external) of the same membrane.

The membrane that surrounds the cell (cytoplasmic) has on its outer side many carbohydrate chains attached to lipids or proteins (resulting in the formation of glycolipids and glycoproteins). Many of these carbohydrates serve receptor function, being susceptible to certain hormones, detecting changes in physical and chemical indicators in the environment.

If, for example, a hormone connects with its cellular receptor, then the carbohydrate part of the receptor molecule changes its structure, followed by a change in the structure of the associated protein part that penetrates the membrane. At the next stage, various biochemical reactions are started or suspended in the cell, i.e. its metabolism changes, and a cellular response to the “stimulus” begins.

In addition to the listed four functions of the cell membrane, others are also distinguished: matrix, energy, marking, formation of intercellular contacts, etc. However, they can be considered as “subfunctions” of those already considered.