Or transmembrane ion currents.

A substance that specifically binds to a receptor is called a ligand for that receptor. Inside the body, it is usually a hormone or neurotransmitter or their artificial substitutes used as drugs and poisons (agonists). Some ligands, on the contrary, block receptors (antagonists). When it comes to the senses, ligands are substances that act on the receptors of smell or taste. In addition, the molecules of visual receptors react to light, and in the organs of hearing and touch, receptors are sensitive to mechanical influences (pressure or stretching) caused by air vibrations and other stimuli. There are also thermosensitive receptor proteins and receptor proteins that respond to changes in membrane potential.

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  Cellular receptors can be divided into two main classes - membrane receptors and intracellular receptors.

  Membrane receptors

  The function of “antennas” is to recognize external signals. The recognition regions of two neighboring cells can provide cell adhesion by binding to each other. This allows cells to orient themselves and create tissues during the process of differentiation. Recognition sites are also present in some molecules that are in solution, due to which they are selectively taken up by cells that have complementary recognition sites (for example, LDL is taken up by LDL receptors).

  The two main classes of membrane receptors are metabotropic receptors and ionotropic receptors.

  Ionotropic receptors are membrane channels that open or close upon binding to a ligand. The resulting ionic currents cause changes in the transmembrane potential difference and, as a result, cell excitability, and also change intracellular ion concentrations, which can secondarily lead to activation of intracellular mediator systems. One of the most fully studied ionotropic receptors is the n-cholinergic receptor.

  Metabotropic receptors are associated with systems of intracellular messengers. Changes in their conformation upon binding to a ligand lead to the launch of a cascade of biochemical reactions and, ultimately, a change in the functional state of the cell. Main types of membrane receptors:

  1. Heterotrimeric G protein-coupled receptors (eg, vasopressin receptor).
  2. Receptors with intrinsic tyrosine kinase activity (for example, insulin receptor or epidermal growth factor receptor).

  G protein-coupled receptors are transmembrane proteins having 7 transmembrane domains, an extracellular N terminus and an intracellular C terminus. The ligand binding site is located on the extracellular loops, the G protein binding domain is located near the C-terminus in the cytoplasm.

  Activation of the receptor causes its α-subunit to dissociate from the βγ-subunit complex and thus become activated. After this, it either activates or, on the contrary, inactivates the enzyme that produces second messengers.

  Receptors with tyrosine kinase activity phosphorylate subsequent intracellular proteins, often also protein kinases, and thus transmit a signal into the cell. Structurally, these are transmembrane proteins with one membrane domain. As a rule, homodimers, the subunits of which are linked by disulfide bridges.

  Intracellular receptors

  Intracellular receptors are usually transcription factors (for example, glucocorticoid receptors) or proteins that interact with transcription factors. Most intracellular receptors bind to ligands in the cytoplasm, become active, are transported along with the ligand into the cell nucleus, where they bind to DNA and either induce or suppress the expression of a certain gene or group of genes.
  Nitric oxide (NO) has a special mechanism of action. Penetrating through the membrane, this hormone binds to soluble (cytosolic) guanylate cyclase, which is both a nitric oxide receptor and an enzyme that synthesizes the second messenger, cGMP.

  Basic systems of intracellular hormonal signal transmission

  Adenylate cyclase system

  The central part of the adenylate cyclase system is the enzyme adenylate cyclase, which catalyzes the conversion of ATP to cAMP. This enzyme can either be stimulated by the G s protein (from the English stimulating) or inhibited by the G i protein (from the English inhibiting). cAMP then binds to a cAMP-dependent protein kinase, also called protein kinase A, PKA. This leads to its activation and subsequent phosphorylation of effector proteins that perform some physiological role in the cell.

  Phospholipase-calcium system

  G q proteins activate the enzyme phospholipase C, which cleaves PIP2 (membrane phosphoinositol) into two molecules: inositol-3-phosphate (IP3) and diacylglyceride. Each of these molecules is a second messenger. IP3 further binds to its receptors on the membrane of the endoplasmic reticulum, which leads to the release of calcium into the cytoplasm and the initiation of many cellular reactions.

  Guanylate cyclase system

  The central molecule of this system is guanylate cyclase, which catalyzes the conversion of GTP to cGMP. cGMP modulates the activity of a number of enzymes and ion channels. There are several isoforms of guanylate cyclase. One of them is activated by nitric oxide NO, the other is directly associated with the atrial natriuretic factor receptor.

  cGMP controls water exchange and ion transport in the kidneys and intestines, and serves as a relaxation signal in the heart muscle.

  Receptor pharmacology

  As a rule, receptors are able to bind not only to the main endogenous ligands, but also to other structurally similar molecules. This fact allows the use of exogenous substances that bind to receptors and change their state as drugs or poisons.

  For example, receptors for endorphins, neuropeptides that play an important role in the modulation of pain and emotional state, also bind to drugs of the morphine group. A receptor may have, in addition to the main site, or “site” of binding to a hormone or mediator specific to this receptor, also additional allosteric regulatory sites to which other chemical substances bind, modulating (changing) the receptor’s response to the main hormonal signal - enhancing or weakening it , or replacing the main signal. A classic example of such a receptor with multiple binding sites for different substances is the gamma-aminobutyric acid subtype A (GABA) receptor. In addition to the binding site for GABA itself, it also has a binding site for benzodiazepines (“benzodiazepine site”), a binding site for barbiturates (“barbiturate site”), and a binding site for neurosteroids such as allopregnenolone (“steroid site”).

  Many types of receptors can recognize several different chemical substances with the same binding site, and depending on the specific attached substance, be in more than two spatial configurations - not only “on” (hormone on the receptor) or “off” (no hormone on the receptor) ), and also in several intermediate ones.

  A substance that is 100% likely to cause a transition of the receptor to the “100% on” configuration upon binding to a receptor is called a full receptor agonist. A substance that, with 100% probability, when binding to a receptor causes it to switch to the “100% off” configuration is called an inverse receptor agonist. A substance that causes a receptor to transition to one of the intermediate configurations or causes a change in the state of the receptor not with 100% probability (that is, some receptors, when bound to this substance, will turn on or off, but some will not), is called a partial receptor agonist. The term agonist-antagonist is also used in relation to such substances. A substance that does not change the state of the receptor upon binding and only passively prevents the binding of a hormone or mediator to the receptor is called a competitive antagonist, or receptor blocker (antagonism is not based on turning off the receptor, but on blocking the binding of its natural ligand to the receptor).

  As a rule, if some exogenous substance has receptors inside the body, then the body also has endogenous ligands for this receptor. For example, endogenous ligands of benzodiazepine

  Protective function

  Blood and other fluids contain proteins that can kill or help neutralize germs. The composition of blood plasma includes antibodies - proteins, each of which recognizes a certain type of microorganisms or other foreign agents - as well as protective proteins of the complement system. There are several classes of antibodies (these proteins are also called immunoglobulins), the most common of them is immunoglobulin G. Saliva and tears contain the protein lysozyme, an enzyme that breaks down murein and destroys the cell walls of bacteria. When infected with a virus, animal cells secrete a protein called interferon, which prevents the virus from multiplying and the formation of new viral particles.

  A protective function for microorganisms is also performed by proteins that are unpleasant for us, such as microbial toxins - cholera toxin, botulism toxin, diphtheria toxin, etc. By damaging the cells of our body, they protect microbes from us.

  Receptor function

  Proteins serve to perceive and transmit signals. In physiology there is the concept of a receptor cell, i.e. a cell that perceives a certain signal (for example, visual receptor cells are located in the retina of the eye). But in receptor cells this work is carried out by receptor proteins. Thus, the protein rhodopsin, contained in the retina of the eye, captures light quanta, after which a cascade of events begins in the retinal cells, which leads to the emergence of a nerve impulse and transmission of the signal to the brain.

  Receptor proteins are found not only in receptor cells, but also in other cells. Hormones play a very important role in the body - substances secreted by some cells and regulating the function of other cells. Hormones bind to special proteins - hormone receptors on the surface or inside target cells.

  Regulatory function

  Many (though by no means all) hormones are proteins - for example, all the hormones of the pituitary gland and hypothalamus, insulin, etc. Another example of proteins that perform this function are intracellular proteins that regulate gene function.

  Many proteins can perform multiple functions.

  Protein macromolecules consist of b-amino acids. If the composition of polysaccharides usually includes the same “unit” (sometimes two), repeated many times, then proteins are synthesized from 20 different amino acids. Once a protein molecule is assembled, some of the amino acid residues within the protein can undergo chemical changes, so that more than 30 different amino acid residues can be found in “mature” proteins. This diversity of monomers also provides a variety of biological functions performed by proteins.

  b-amino acids have the following structure:

  

  here R is different groups of atoms (radicals) for different amino acids. The carbon atom closest to the carboxyl group is designated by the Greek letter b; it is with this atom that the amino group in b-amino acid molecules is connected.

  In a neutral environment, the amino group exhibits weak basic properties and attaches the H+ ion, and the carboxyl group exhibits weakly acidic properties and dissociates with the release of this ion, so that although in general the total charge of the molecule will not change, it will simultaneously carry a positively and negatively charged group.

  Depending on the nature of the R radical, hydrophobic (non-polar), hydrophilic (polar), acidic and alkaline amino acids are distinguished.

  Acidic amino acids have a second carboxyl group. It is slightly stronger than the carboxyl group of acetic acid: in aspartic acid, half of the carboxyls are dissociated at pH 3.86, in glutamic acid - at pH 4.25, and in acetic acid - only at 4.8. Among the alkaline amino acids, arginine is the strongest: half of its side radicals retain a positive charge at pH 11.5. Lysine has a side radical that is a typical primary amine and remains half-ionized at pH 9.4. The weakest of the alkaline amino acids is histidine, its imidazole ring is half protonated at pH 6.

  Among the hydrophilic (polar) there are also two amino acids that can ionize at physiological pH - cysteine, in which the SH group can donate an H+ ion like hydrogen sulfide, and tyrosine, which has a weakly acidic phenolic group. However, this ability is very weakly expressed in them: at pH 7, cysteine ​​is ionized by 8%, and tyrosine by 0.01%.

  To detect b-amino acids, the ninhydrin reaction is usually used: when an amino acid reacts with ninhydrin, a brightly colored blue product is formed. In addition, individual amino acids give their own specific qualitative reactions. Thus, aromatic amino acids give a yellow color with nitric acid (during the reaction, nitration of the aromatic ring occurs). When the medium is alkalized, the color changes to orange (a similar color change occurs in indicators, for example, methyl orange). This reaction, called xanthoprotein reaction, is also used for protein detection, since most proteins contain aromatic amino acids; gelatin does not give this reaction, since it contains almost no tyrosine, phenylalanine, or tryptophan. When heated with sodium plumbite Na2PbO2, cysteine ​​forms a black precipitate of lead sulfide PbS.

  Plants and many microbes can synthesize amino acids from simple inorganic substances. Animals can synthesize only some amino acids, but others must be obtained from food. Such amino acids are called essential. Essential for humans are phenylalanine, tryptophan, threonine, methionine, lysine, leucine, isoleucine, histidine, valine and arginine. Unfortunately, cereals contain very little lysine and tryptophan, but these amino acids are found in significantly larger quantities in legumes. It is no coincidence that traditional diets of agricultural peoples usually contain both cereals and legumes: wheat (or rye) and peas, rice and soybeans, corn and beans are classic examples of such a combination among peoples of different continents.

  b-The carbon atom of all 20 amino acids is in a state of sp3 hybridization. All 4 of its bonds are located at an angle of about 109°, so that the amino acid formula can be inscribed in a tetrahedron.

  It is easy to see that there can be two types of amino acids that are mirror images of each other. No matter how we move and rotate them in space, it is impossible to combine them - they differ like the right and left hand.

  This type of isomerism is called optical isomerism. It is only possible if the central carbon atom (called the asymmetric center) has different groups on all 4 sides (therefore, glycine does not have optical isomers, but the other 19 amino acids do). Of the two different isomeric forms of amino acids, the one in Fig. 1 located on the right is called D-shape, and on the left is called L-shape.

  The basic physical and chemical properties of D- and L-isomers of amino acids are the same, but their optical properties differ: their solutions rotate the plane of polarization of light in opposite directions. The speed of their reactions with other optically active compounds is also different.

  Interestingly, the proteins of all living organisms, from viruses to humans, contain only L-amino acids. D-forms are found in some antibiotics synthesized by fungi and bacteria. Proteins can form an ordered structure only if they contain only isomers of amino acids of the same type.

  Short review:

  Glycocalyx- This is a layer external to the lipoprotein membrane containing polysaccharide chains of membrane integral proteins - glycoproteins.

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

  Main part:

  Such receptors on the cell surface can be membrane proteins or elements of the glycocalyx - glycoproteins. Such areas sensitive to individual substances can be scattered over the surface of the cell or collected in small zones.

  Different cells of animal organisms may have different sets of receptors or different sensitivity of the same receptor.

  The role of many cellular receptors is not only the binding of specific substances or the ability to respond to physical factors, but also the transmission of intercellular signals from the surface into the cell. Currently, the system of signal transmission to cells using certain hormones, which include peptide chains, has been well studied. The hormone interacts specifically with the receptor part of this system and, without penetrating into the cell, activates adenylate cyclase (a protein already located in the cytoplasmic part of the plasma membrane), which synthesizes cyclic AMP. The latter activates or inhibits an intracellular enzyme or group of enzymes. Thus, the command (signal from the plasma membrane) is transmitted inside the cell. The efficiency of this adenylate cyclase system is very high. Thus, the interaction of one or several hormone molecules can lead, due to the synthesis of many cAMP molecules, to amplify the signal thousands of times. In this case, the adenylate cyclase system serves as a transducer of external signals.

  The diversity and specificity of sets of receptors on the surface of cells lead to the creation of a very complex system of markers that make it possible to distinguish one's cells (of the same individual or the same species) from foreign ones. Similar cells enter into interactions with each other, leading to the adhesion of surfaces (conjugation in protozoa and bacteria, the formation of tissue cell complexes). In this case, cells that differ in the set of determinant markers or do not perceive them are either excluded from such interaction, or (in higher animals) are destroyed as a result of immunological reactions.

  The localization of specific receptors that respond to physical factors is associated with the plasma membrane. Thus, receptor proteins (chlorophylls) that interact with light quanta are localized in the plasma membrane or its derivatives in photosynthetic bacteria and blue-green algae. In the plasma membrane of light-sensitive animal cells there is a special system of photoreceptor proteins (rhodopsin), with the help of which the light signal is converted into a chemical signal, which in turn leads to the generation of an electrical impulse.

  Types of active transport across the plasma membrane

  Briefly:

  
  

  • primary active transport - carried out by transport ATPases, which are called ion pumps.
  • secondary active transport is the transfer of a substance across a membrane against its concentration gradient due to the energy of the concentration gradient of another substance created in the process of active transport.

  Full:
  Active transport is carried out by transport adenosine triphosphatases (ATPases) and occurs due to the energy of ATP hydrolysis.
  Types of active transport of substances:

  • primary active transport,
  • secondary active transport.

  Primary active transport

  The transport of substances from an environment with a low concentration to an environment with a higher concentration cannot be explained by movement along a gradient, i.e. diffusion. This process is carried out due to the energy of ATP hydrolysis or energy due to the concentration gradient of any ions, most often sodium. If the source of energy for the active transport of substances is the hydrolysis of ATP, and not the movement of some other molecules or ions through the membrane, the transport is called primary active.

  Primary active transfer is carried out by transport ATPases, which are called ion pumps. In animal cells, the most common is Na+,K+ - ATPase (sodium pump), which is an integral protein of the plasma membrane and Ca2+ - ATPase contained in the plasma membrane of the sarco-(endo)-plasmic reticulum. All three proteins have a common property - the ability to be phosphorylated and form an intermediate phosphorylated form of the enzyme. In the phosphorylated state, the enzyme can be in two conformations, which are usually designated E1 and E2. The conformation of an enzyme is the method of spatial orientation (laying) of the polypeptide chain of its molecule. The two indicated conformations of the enzyme are characterized by different affinities for the transferred ions, i.e. different ability to bind transported ions.

  Secondary active transport

  Secondary active transport is the transfer of a substance across a membrane against its concentration gradient due to the energy of the concentration gradient of another substance created in the process of active transport. In animal cells, the main source of energy for secondary active transport is the energy of the concentration gradient of sodium ions, which is created due to the work of Na+/K+ - ATPase. For example, the membrane of the cells of the mucous membrane of the small intestine contains a protein that transports (symports) glucose and Na+ into epithelial cells. Glucose transport occurs only if Na+, simultaneously with glucose binding to the specified protein, is transported along an electrochemical gradient. The electrochemical gradient for Na+ is maintained by the active transport of these cations out of the cell.

  In the brain, the work of the Na+ pump is associated with the reverse absorption (reabsorption) of mediators - physiologically active substances that are released from nerve endings under the action of stimulating factors.

  In cardiomyocytes and smooth muscle cells, the functioning of Na+, K+-ATPase is associated with the transport of Ca2+ through the plasma membrane, due to the presence in the cell membrane of a protein that countertransports (antiports) Na+ and Ca2+. Calcium ions are transported across the cell membrane in exchange for sodium ions and due to the energy of the concentration gradient of sodium ions.

  A protein has been discovered in cells that exchanges extracellular sodium ions for intracellular protons - the Na+/H+ exchanger. This transporter plays an important role in maintaining a constant intracellular pH. The rate at which Na+/Ca2+ and Na+/H+ exchange occurs is proportional to the electrochemical gradient of Na+ across the membrane. With a decrease in the extracellular concentration of Na+, inhibition of Na+, K+-ATPase by cardiac glycosides or in a potassium-free environment, the intracellular concentration of calcium and protons is increased. This increase in intracellular Ca2+ concentration upon inhibition of Na+, K+-ATPase underlies the clinical use of cardiac glycosides to enhance heart contractions.

  Various transport ATPases, localized in cell membranes and involved in the mechanisms of substance transfer, are the main element of molecular devices - pumps that ensure the selective absorption and pumping out of certain substances (for example, electrolytes) by the cell. Active specific transport of non-electrolytes (molecular transport) is realized using several types of molecular machines - pumps and carriers. The transport of non-electrolytes (monosaccharides, amino acids and other monomers) can be coupled with symport - the transport of another substance, the movement of which along the concentration gradient is a source of energy for the first process. Symport can be provided by ion gradients (for example, sodium) without the direct participation of ATP.

  Transport ATPases are high-molecular transport proteins capable of breaking down ATP to release energy. This process serves as the engine of active transport. This is how protons (proton pump) or inorganic ions (ion pump) are transferred.

  Active transport is carried out by endo- and exocytosis.
  Endocytosis is the formation of vesicles by invagination of the plasma membrane during the absorption of solid particles (phagocytosis) or solutes (pinocytosis). The smooth or bordered vesicles that arise are called phagosomes or pinosomes. By endocytosis, eggs absorb yolk proteins, leukocytes absorb foreign particles and immunoglobulins, and renal tubules absorb proteins from primary urine.
  Exocytosis is a process opposite to endocytosis. Various vesicles from the Golgi apparatus and lysosomes merge with the plasma membrane, releasing their contents to the outside. In this case, the vesicle membrane can either be embedded in the plasma membrane or return to the cytoplasm in the form of a vesicle.

  The receptor function of the cell is provided by receptors that implement responses in certain ways.

  The method of influence is associated with the transfer of information that occurs when substances coming from outside with membrane receptor complexes are added into the cell.

  Ionotropic receptor complexes form complex molecular or supramolecular compounds that contain ion channels. When combined with a biologically active substance, the opening or opening of ion channels occurs. The rate of cell excitation is high. Ionotropic receptors are located predominantly in the area of ​​synapses and are involved in the transmission of excitatory and inhibitory influences.

  Metabotropic receptor complexes are associated with integral intermediary proteins that transmit information to the internal surface. First of all, these are G-proteins and membrane tyrosine kinases. Intermediary proteins excite enzymes on the inner surface of the cell membrane, and they, in turn, synthesize second intermediaries - low molecular weight substances that trigger biological reactions of the cell. These receptors are sometimes called slow receptors. Most hormones and mediators that do not penetrate cells well act through similar mechanisms.

  Receptors that regulate the entry of molecules into cells, such as lipids in low-density lipoproteins. This group of receptors is capable of changing the permeability of biological membranes, thus affecting the chemical composition inside the cell.

  Adhesion receptors (families of integrins, cadherins, immunoglobulins, selectins, etc.) connect neighboring cells or a cell with structures of the intercellular environment, for example, with the basement membrane. The possibility of adhesive interactions is essential in the life of the cell and the entire organism as a whole. The loss of a cell's ability to adhere is accompanied by its uncontrolled migration (metastasis) and impaired differentiation. Pathological dysfunction of adhesive receptors is characteristic of malignant tumor cells.

  The actual reception process occurs with the help of special glycoproteins - receptors. They are located in the supra-membrane layer - the glycocalyx of the cell.

  Receptors provide the perception of specific stimuli: hormones, biologically active substances, membranes of neighboring cells, adhesive molecules of intercellular substances, etc. Receptors are highly specialized cell structures. They can be highly specific (high affinity) or less specific (low affinity). The degree of specificity determines the degree of sensitivity of the cell. The receptors for hormones have the highest affinity.

  Receptor complexes are also characteristic of the inner layer of the membrane. They are located on membrane and non-membrane organelles, the inner and outer layers of the karyolemma, etc.

  In response to the action of a signal (the connection of a receptor with a regulatory substance), a chain of biochemical reactions occurs, leading to the formation of biological responses - excitation or inhibition of the cell. Receptors for polypeptides, amino acid derivatives, antigenic complexes, glycoproteins, etc. are located on the cell membrane. Some receptors have connections with proteins that provide the formation of second messengers, as well as with ion channel proteins. Such receptor systems are called metabotropic.

  Excitation in metabotropic receptors caused by a signal can be transmitted deep into the cell in several ways. In one case, the interaction of the receptor with a signaling molecule changes the stereological configuration of the receptor, which changes the structure of the so-called G protein, which, in turn, activates the formation of cytoplasmic signaling molecules (second messengers).

  There are Gs proteins that activate adenylate cyclase with the formation of cAMP, Gi proteins that inhibit adenylate cyclase, Gp proteins that activate phospholipase C and increase the content of calcium ions in the cytosol. There are also Gt proteins that activate cyclic guanosine monophosphate (cGMP) phosphodiesterase and reduce the cGMP content, which leads to inhibition (membrane hyperpolarization) of the cell. Cyclic AMP (cAMP) activates protein kinases and accelerates biochemical reactions in the cell.

  In the second case, the receptor is associated with tyrosine kinases, which activate the Ras-G protein and trigger the Ras cascade. As a result of this process, inositol 1,4,5-triphosphate, diacylglycerol, is formed. This triggers a chain of catalytic reactions, including transcription.

  Receptors can be associated with ion channels, change their permeability, cause membrane depolarization, penetration of calcium ions into the cell, etc. Ionotropic receptor complexes contain several molecules - these are receptor proteins that perceive a signal molecule. They attach to proteins of the effector device - ion channels. The inactivation enzyme breaks the connection between the receptor and the signaling molecule of the mediator or other signaling substances.

  Along with signaling functions, some receptors play an important role in adhesion and aggregation - the adherence of cells to similar and/or intercellular structures. “Recognition” of related cells by the glycocalyx receptor is accompanied by simultaneous aggregation. It is important that such receptors have individual, organ and tissue specificity. Examples include selectins, integrins and cadherins. They give cells antigenic properties and allow them to “recognize” each other.

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  The receptor function of the membrane plays an important role in the life of the cell. It is associated with the localization on the plasma membrane of special structures (receptor proteins) associated with the specific recognition of chemical or physical factors. Many piercing proteins are glycoproteins—on the outside of the cell they contain polysaccharide side chains. Some of these glycoproteins, covering the cell with a “forest” of molecular antennas, act as hormone receptors. When a certain hormone binds to its receptor, it changes the structure of the glycoprotein, which leads to the initiation of a cellular response. Channels open through which certain substances enter or exit the cell. The cell surface has a large set of receptors that make specific reactions with various agents possible. The role of many cellular receptors is to transmit signals from outside to inside the cell.

  22. Cell receptors: concept, location, varieties, structure.

  Signaling molecules - proteins called receptors - are located on the plasma membranes of cells. Cell receptors bind the molecule and initiate a response. They are represented by transmembrane proteins that have a special site for binding physiologically active molecules - hormones and neurotransmitters. Many receptor proteins, in response to the binding of certain molecules, change the transport properties of membranes. As a result, the polarity of the membranes may change, a nerve impulse may be generated, or metabolism may change.

  There are intracellular receptors and receptors located on the cell surface in the plasma membrane. Among them, two types of receptors are distinguished: cells associated with channels and cells not associated with channels. They differ from each other in the speed and selectivity of the signal’s impact on certain targets. Receptors associated with channels, after interacting with chemicals (hormone, neurotransmitter), promote the formation of an open channel in the membrane, as a result of which its permeability immediately changes. Receptors not associated with channels also interact with chemicals, but of a different nature, mainly enzymes. Here the effect is indirect, relatively slow, but longer lasting. The function of these receptors underlies learning and memory.

  23. Transport of substances through the cell membrane: concept, varieties, examples.

  Membrane transport is the transport of substances through the cell membrane into or out of the cell, carried out using various mechanisms - simple diffusion, facilitated diffusion and active transport. Types of transport are described in answers 16 and 17.

  24. Intercellular contacts: concept, varieties, meaning.

  Intercellular contacts are connections between cells formed with the help of proteins. They provide direct communication between cells. In addition, cells interact with each other at a distance using signals (mainly signaling substances) transmitted through the intercellular substance.

  Each type of intercellular contacts is formed by specific proteins, the vast majority of which are transmembrane proteins. Special adapter proteins can connect proteins of intercellular contacts with the cytoskeleton, and special “skeletal” proteins can connect individual molecules of these proteins into a complex supramolecular structure. In many cases, intercellular connections are destroyed when Ca2+ ions are removed from the environment.