Cell membrane. Cell membrane: structure and functions

Brief description:

Sazonov V.F. 1_1 Structure of the cell membrane [ Electronic resource] // Kinesiologist, 2009-2018: [website]. Update date: 02/06/2018..__.201_). _The structure and functioning of the cell membrane is described (synonyms: plasmalemma, plasmalemma, biomembrane, cell membrane, outer cell membrane, cell membrane, cytoplasmic membrane). This initial information is necessary both for cytology and for understanding the processes nervous activity: nervous excitation, inhibition, work of synapses and sensory receptors.

Cell membrane (plasma) A lemma or plasma O lemma)

Definition of the concept

The cell membrane (synonyms: plasmalemma, plasmalemma, cytoplasmic membrane, biomembrane) is a triple lipoprotein (i.e. “fat-protein”) membrane that separates the cell from environment and carrying out controlled exchange and communication between the cell and its environment.

The main thing in this definition is not that the membrane separates the cell from the environment, but precisely that it connects cell with the environment. The membrane is active the structure of the cell, it is constantly working.

A biological membrane is an ultrathin bimolecular film of phospholipids encrusted with proteins and polysaccharides. This cellular structure underlies the barrier, mechanical and matrix properties of a living organism (Antonov V.F., 1996).

A figurative representation of a membrane

To me cell membrane appears in the form of a lattice fence with many doors in it, which surrounds a certain territory. Any small living creature can move freely back and forth through this fence. But larger visitors can only enter through doors, and even then not all doors. Different visitors have keys only to their own doors, and they cannot go through other people's doors. So, through this fence there are constantly flows of visitors back and forth, because the main function of the membrane fence is twofold: to separate the territory from the surrounding space and at the same time connect it with the surrounding space. This is why there are many holes and doors in the fence - !

Membrane properties

1. Permeability.

2. Semi-permeability (partial permeability).

3. Selective (synonym: selective) permeability.

4. Active permeability (synonym: active transport).

5. Controlled permeability.

As you can see, the main property of a membrane is its permeability to various substances.

6. Phagocytosis and pinocytosis.

7. Exocytosis.

8. The presence of electrical and chemical potentials, or rather the potential difference between the inner and outer sides of the membrane. Figuratively we can say that “the membrane turns the cell into an “electric battery” by controlling ionic flows”. Details: .

9. Changes in electrical and chemical potential.

10. Irritability. Special molecular receptors located on the membrane can connect with signaling (control) substances, as a result of which the state of the membrane and the entire cell can change. Molecular receptors trigger bio chemical reactions in response to the connection of ligands (control substances) with them. It is important to note that the signaling substance acts on the receptor from the outside, and the changes continue inside the cell. It turns out that the membrane transferred information from the environment to internal environment cells.

11. Catalytic enzymatic activity. Enzymes can be embedded in the membrane or associated with its surface (both inside and outside the cell), and there they carry out their enzymatic activities.

12. Changing the shape of the surface and its area. This allows the membrane to form outgrowths outward or, conversely, invaginations into the cell.

13. The ability to form contacts with other cell membranes.

14. Adhesion - the ability to stick to hard surfaces.

Brief list of membrane properties

Permeability.
Endocytosis, exocytosis, transcytosis.
Potentials.
Irritability.
Enzyme activity.
Contacts.
Adhesion.

Membrane functions

1. Incomplete isolation of internal contents from the external environment.

2. The main thing in the functioning of the cell membrane is exchange various substances between the cell and the intercellular environment. This is due to the membrane property of permeability. In addition, the membrane regulates this exchange by regulating its permeability.

3. One more important function membranes - creating a difference in chemical and electrical potentials between its inner and outer sides. Due to this, the inside of the cell has a negative electrical potential - .

4. The membrane also carries out information exchange between the cell and its environment. Special molecular receptors located on the membrane can bind to control substances (hormones, mediators, modulators) and trigger biochemical reactions in the cell, leading to various changes in the functioning of the cell or in its structures.

Video:Cell membrane structure

Video lecture:Details about membrane structure and transport

Membrane structure

The cell membrane has a universal three-layer structure. Its middle fat layer is continuous, and the upper and lower protein layers cover it in the form of a mosaic of separate protein areas. The fat layer is the basis that ensures the isolation of the cell from the environment, isolating it from the environment. By itself, it allows water-soluble substances to pass through very poorly, but easily allows fat-soluble substances to pass through. Therefore, the permeability of the membrane for water-soluble substances (for example, ions) must be ensured by special protein structures - and.

Below are micrographs of real cell membranes of contacting cells obtained using an electron microscope, as well as a schematic drawing showing the three-layer structure of the membrane and the mosaic nature of its protein layers. To enlarge the image, click on it.

Single image the inner lipid (fat) layer of the cell membrane, permeated with integral built-in proteins. The top and bottom protein layers have been removed so as not to interfere with viewing the lipid bilayer

Figure above: Partial schematic representation of a cell membrane (cell membrane), given on Wikipedia.

Please note that the outer and inner protein layers have been removed from the membrane here so that we can better see the central fatty lipid bilayer. In a real cell membrane, large protein “islands” float above and below the fatty film (small balls in the figure), and the membrane turns out to be thicker, three-layered: protein-fat-protein . So it's actually like a sandwich of two protein "pieces of bread" with a fatty layer of "butter" in the middle, i.e. has a three-layer structure, not a two-layer one.

In this picture, the small blue and white balls correspond to the hydrophilic (wettable) “heads” of the lipids, and the “strings” attached to them correspond to the hydrophobic (non-wettable) “tails”. Of the proteins, only integral end-to-end membrane proteins (red globules and yellow helices) are shown. The yellow oval dots inside the membrane are cholesterol molecules. The yellow-green chains of beads on the outside of the membrane are chains of oligosaccharides that form the glycocalyx. A glycocalyx is a kind of carbohydrate (“sugar”) “fluff” on a membrane, formed by long carbohydrate-protein molecules sticking out of it.

Living is a small “protein-fat sac” filled with semi-liquid jelly-like contents, which are permeated with films and tubes.

The walls of this sac are formed by a double fatty (lipid) film, covered inside and outside with proteins - the cell membrane. Therefore they say that the membrane has three-layer structure : proteins-fat-proteins. Inside the cell there are also many similar fatty membranes that divide its internal space into compartments. The same membranes surround cellular organelles: nucleus, mitochondria, chloroplasts. So the membrane is a universal molecular structure common to all cells and all living organisms.

On the left is no longer a real, but an artificial model of a piece of a biological membrane: this is an instantaneous snapshot of a fatty phospholipid bilayer (i.e., a double layer) in the process of its molecular dynamics simulation. The calculation cell of the model is shown - 96 PC molecules ( f osphatidyl X olina) and 2304 water molecules, for a total of 20544 atoms.

On the right is a visual model of a single molecule of the same lipid from which the membrane lipid bilayer is assembled. At the top it has a hydrophilic (water-loving) head, and at the bottom there are two hydrophobic (water-afraid) tails. This lipid has a simple name: 1-steroyl-2-docosahexaenoyl-Sn-glycero-3-phosphatidylcholine (18:0/22:6(n-3)cis PC), but you don't need to remember it unless you you plan to make your teacher faint with the depth of your knowledge.

A more precise scientific definition of a cell can be given:

is an ordered, structured heterogeneous system of biopolymers bounded by an active membrane, participating in a single set of metabolic, energetic and information processes, and also maintaining and reproducing the entire system as a whole.

Inside the cell is also permeated with membranes, and between the membranes there is not water, but a viscous gel/sol of variable density. Therefore, interacting molecules in a cell do not float freely, as in a test tube with an aqueous solution, but mostly sit (immobilized) on the polymer structures of the cytoskeleton or intracellular membranes. And chemical reactions therefore take place inside the cell almost as if in a solid rather than in a liquid. The outer membrane surrounding the cell is also lined with enzymes and molecular receptors, making it a very active part of the cell.

The cell membrane (plasmalemma, plasmolemma) is an active membrane that separates the cell from the environment and connects it with the environment. © Sazonov V.F., 2016.

From this definition of a membrane it follows that it not only limits the cell, but actively working, connecting it with its environment.

The fat that makes up the membranes is special, so its molecules are usually called not just fat, but "lipids", "phospholipids", "sphingolipids". The membrane film is double, that is, it consists of two films stuck together. Therefore, in textbooks they write that the basis of the cell membrane consists of two lipid layers (or " bilayer", i.e. a double layer). For each individual lipid layer, one side can be wetted with water, but the other cannot. So, these films stick to each other precisely with their non-wettable sides.

Bacteria membrane

The prokaryotic cell wall of gram-negative bacteria consists of several layers, shown in the figure below.
Layers of the shell of gram-negative bacteria:
1. Internal three-layer cytoplasmic membrane, which is in contact with the cytoplasm.
2. Cell wall, which consists of murein.
3. The outer three-layer cytoplasmic membrane, which has the same system of lipids with protein complexes as the inner membrane.
Gram-negative communication bacterial cells with the outside world through such a complex three-stage structure does not give them an advantage in survival in harsh conditions compared to gram-positive bacteria, which have a less powerful shell. They don't tolerate it just as well high temperatures, increased acidity and pressure changes.

Video lecture:Plasma membrane. E.V. Cheval, Ph.D.

Video lecture:Membrane as a cell boundary. A. Ilyaskin

Importance of Membrane Ion Channels

It is easy to understand that only fat-soluble substances can penetrate the cell through the membrane fat film. These are fats, alcohols, gases. For example, in red blood cells, oxygen and oxygen easily pass in and out directly through the membrane. carbon dioxide. But water and water-soluble substances (for example, ions) simply cannot pass through the membrane into any cell. This means that they require special holes. But if you simply make a hole in the fatty film, it will immediately close back. What to do? A solution was found in nature: it is necessary to make special protein transport structures and stretch them through the membrane. This is exactly how channels are formed for the passage of fat-insoluble substances - ion channels of the cell membrane.

So, to give its membrane additional properties of permeability to polar molecules (ions and water), the cell synthesizes special proteins in the cytoplasm, which are then integrated into the membrane. They come in two types: transport proteins (for example, transport ATPases) and channel-forming proteins (channel builders). These proteins are embedded in the fatty double layer of the membrane and form transport structures in the form of transporters or in the form of ion channels. Various water-soluble substances that cannot otherwise pass through the fatty membrane film can now pass through these transport structures.

In general, proteins embedded in the membrane are also called integral, precisely because they seem to be included in the membrane and penetrate it through. Other proteins, not integral, form islands, as it were, “floating” on the surface of the membrane: either on its outer surface or on its inner surface. After all, everyone knows that fat is a good lubricant and it’s easy to glide over it!

Conclusions

1. In general, the membrane turns out to be three-layer:

1) outer layer of protein “islands”,

2) fatty two-layer “sea” (lipid bilayer), i.e. double lipid film,

3) an inner layer of protein “islands”.

But there is also a loose outer layer - the glycocalyx, which is formed by glycoproteins protruding from the membrane. They are molecular receptors to which signaling control substances bind.

2. Special protein structures are built into the membrane, ensuring its permeability to ions or other substances. We must not forget that in some places the sea of fat is permeated through and through with integral proteins. And it is the integral proteins that form special transport structures cell membrane (see section 1_2 Membrane transport mechanisms). Through them, substances enter the cell and are also removed from the cell to the outside.

3. On any side of the membrane (outer and inner), as well as inside the membrane, enzyme proteins can be located, which affect both the state of the membrane itself and the life of the entire cell.

So the cell membrane is an active, variable structure that actively works in the interests of the entire cell and connects it with the outside world, and is not just a “protective shell”. This is the most important thing you need to know about the cell membrane.

In medicine, membrane proteins are often used as “targets” for medicines. Such targets include receptors, ion channels, enzymes, and transport systems. IN lately In addition to the membrane, genes hidden in the cell nucleus also become targets for drugs.

Video:Introduction to the biophysics of the cell membrane: Membrane structure 1 (Vladimirov Yu.A.)

Video:History, structure and functions of the cell membrane: Membrane structure 2 (Vladimirov Yu.A.)

Based on its functional characteristics, the cell membrane can be divided into 9 functions it performs.
Functions of the cell membrane:
1. Transport. Transports substances from cell to cell;
2. Barrier. Has selective permeability, ensures the necessary metabolism;
3. Receptor. Some proteins found in the membrane are receptors;
4. Mechanical. Ensures the autonomy of the cell and its mechanical structures;
5. Matrix. Ensures optimal interaction and orientation of matrix proteins;
6. Energy. Membranes contain energy transfer systems during cellular respiration in mitochondria;
7. Enzymatic. Membrane proteins are sometimes enzymes. For example, intestinal cell membranes;
8. Marking. The membrane contains antigens (glycoproteins) that allow cell identification;
9. Generating. Carries out the generation and conduction of biopotentials.

You can see what a cell membrane looks like using the example of the structure of an animal cell or plant cell.

The figure shows the structure of the cell membrane.
The components of the cell membrane include various cell membrane proteins (globular, peripheral, surface), as well as cell membrane lipids (glycolipid, phospholipid). Also in the structure of the cell membrane there are carbohydrates, cholesterol, glycoprotein and protein alpha helix.

Cell membrane composition

The main composition of the cell membrane includes:
1. Proteins - responsible for various properties of the membrane;
2. Lipids three types(phospholipids, glycolipids and cholesterol) responsible for membrane rigidity.
Cell membrane proteins:
1. Globular protein;
2. Surface protein;
3. Peripheral protein.

The main purpose of the cell membrane

The main purpose of the cell membrane:
1. Regulate the exchange between the cell and the environment;
2. Separate the contents of any cell from the external environment, thereby ensuring its integrity;
3. Intracellular membranes divide the cell into specialized closed compartments - organelles or compartments in which certain environmental conditions are maintained.

Cell membrane structure

The structure of the cell membrane is a two-dimensional solution of globular integral proteins dissolved in a liquid phospholipid matrix. This model of membrane structure was proposed by two scientists Nicholson and Singer in 1972. Thus, the basis of the membranes is a bimolecular lipid layer, with an ordered arrangement of molecules, as you could see in.

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 in both chemical composition their proteins and lipids, and by 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
Delimiting (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 conduction.
Motor	Carries out the movement of the cell or its individual parts.

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, carbohydrates, the semi-permeability 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 leaving 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.

Cell membrane also called plasma (or cytoplasmic) membrane and plasmalemma. This structure not only separates the internal contents of the cell from the external environment, but is also part of most cellular organelles and the nucleus, in turn separating them from the hyaloplasm (cytosol) - the viscous-liquid part of the cytoplasm. Let's agree to call cytoplasmic membrane the one that separates the contents of the cell from the external environment. The remaining terms denote all membranes.

The structure of the cellular (biological) membrane is based on a double layer of lipids (fats). The formation of such a layer is associated with the characteristics of their molecules. Lipids do not dissolve in water, but condense in it in their own way. One part of a single lipid molecule is a polar head (it is attracted to water, i.e. hydrophilic), and the other is a pair of long non-polar tails (this part of the molecule is repelled by water, i.e. hydrophobic). This structure of molecules causes them to “hide” their tails from the water and turn their polar heads towards the water.

The result is a lipid bilayer in which the nonpolar tails are inward (facing each other) and the polar heads are outward (towards external environment and cytoplasm). The surface of such a membrane is hydrophilic, but inside it is hydrophobic.

In cell membranes, phospholipids predominate among the lipids (they belong to complex lipids). Their heads contain a phosphoric acid residue. In addition to phospholipids, there are glycolipids (lipids + carbohydrates) and cholesterol (related to sterols). The latter imparts rigidity to the membrane, being located in its thickness between the tails of the remaining lipids (cholesterol is completely hydrophobic).

Due to electrostatic interaction, some protein molecules are attached to the charged lipid heads, which become surface membrane proteins. Other proteins interact with nonpolar tails, are partially buried in the bilayer, or penetrate through it.

Thus, the cell membrane consists of a bilayer of lipids, surface (peripheral), embedded (semi-integral) and permeating (integral) proteins. In addition, some proteins and lipids on the outside of the membrane are associated with carbohydrate chains.

This fluid mosaic model of membrane structure was put forward in the 70s of the XX century. Previously, a sandwich model of structure was assumed, according to which the lipid bilayer is located inside, and on the inside and outside the membrane is covered with continuous layers of surface proteins. However, the accumulation of experimental data refuted this hypothesis.

Membrane thickness different cells is about 8 nm. Membranes (even different sides one) differ from each other in percentage various types lipids, proteins, enzymatic activity, etc. Some membranes are more liquid and more permeable, others are more dense.

Cell membrane breaks easily merge due to the physicochemical properties of the lipid bilayer. In the plane of the membrane, lipids and proteins (unless they are anchored by the cytoskeleton) move.

Functions of the cell membrane

Most proteins immersed in the cell membrane perform an enzymatic function (they are enzymes). Often (especially in the membranes of cell organelles) enzymes are located in a certain sequence so that the reaction products catalyzed by one enzyme move to the second, then the third, etc. A conveyor is formed that stabilizes surface proteins, because they do not allow the enzymes to float along the lipid bilayer.

The cell membrane performs a delimiting (barrier) function from the environment and at the same time transport functions. We can say that this is its most important purpose. The cytoplasmic membrane, having strength and selective permeability, maintains the constancy of the internal composition of the cell (its homeostasis and integrity).

In this case, the transport of substances occurs in various ways. Transport along a concentration gradient involves the movement of substances from an area with a higher concentration to an area with a lower one (diffusion). For example, gases (CO 2 , O 2 ) diffuse.

There is also transport against a concentration gradient, but with energy consumption.

Transport can be passive and facilitated (when it is assisted by some carrier). Passive diffusion across the cell membrane is possible for fat-soluble substances.

There are special proteins that make membranes permeable to sugars and other water-soluble substances. Such carriers bind to transported molecules and pull them through the membrane. This is how glucose is transported inside red blood cells.

Threading proteins combine to form a pore for the movement of certain substances across the membrane. Such carriers do not move, but form a channel in the membrane and work similarly to enzymes, binding a specific substance. Transfer occurs due to a change in protein conformation, resulting in the formation of channels in the membrane. An example is the sodium-potassium pump.

The transport function of the eukaryotic cell membrane is also realized through endocytosis (and exocytosis). Thanks to these mechanisms, large molecules of biopolymers, even whole cells, enter the cell (and out of it). Endo- and exocytosis are not characteristic of all eukaryotic cells (prokaryotes do not have it at all). Thus, endocytosis is observed in protozoa and lower invertebrates; in mammals, leukocytes and macrophages absorb harmful substances and bacteria, i.e. endocytosis performs a protective function for the body.

Endocytosis is divided into phagocytosis(cytoplasm envelops large particles) and pinocytosis(capturing droplets of liquid with substances dissolved in it). The mechanism of these processes is approximately the same. Absorbed substances on the surface of cells are surrounded by a membrane. A vesicle (phagocytic or pinocytic) is formed, which then moves into the cell.

Exocytosis is the removal of substances from the cell by the cytoplasmic membrane (hormones, polysaccharides, proteins, fats, etc.). These substances are contained in membrane vesicles that approach the cell membrane. Both membranes merge and the contents appear outside the cell.

The cytoplasmic membrane performs a receptor function. To do this, structures are located on its outer side that can recognize a chemical or physical stimulus. Some of the proteins that penetrate the plasmalemma are connected from the outside to polysaccharide chains (forming glycoproteins). These are peculiar molecular receptors that capture hormones. When a particular hormone binds to its receptor, it changes its structure. This in turn triggers the cellular response mechanism. In this case, channels can open, and certain substances can begin to enter or exit the cell.

The receptor function of cell membranes has been well studied based on the action of the hormone insulin. When insulin binds to its glycoprotein receptor, the catalytic intracellular part of this protein (adenylate cyclase enzyme) is activated. The enzyme synthesizes cyclic AMP from ATP. Already it activates or suppresses various enzymes of cellular metabolism.

The receptor function of the cytoplasmic membrane also includes recognition of neighboring cells of the same type. Such cells are attached to each other by various intercellular contacts.

In tissues, with the help of intercellular contacts, cells can exchange information with each other using specially synthesized low-molecular substances. One example of such an interaction is contact inhibition, when cells stop growing after receiving information that free space is occupied.

Intercellular contacts can be simple (the membranes of different cells are adjacent to each other), locking (invaginations of the membrane of one cell into another), desmosomes (when the membranes are connected by bundles of transverse fibers that penetrate the cytoplasm). In addition, there is a variant of intercellular contacts due to mediators (intermediaries) - synapses. In them, the signal is transmitted not only chemically, but also electrically. Synapses transmit signals between nerve cells, as well as from nervous to muscular.