home Secrets of medicine The outer cell membrane ensures the cell's constant shape. Cell membrane: its structure and functions

The outer cell membrane ensures the cell's constant shape. Cell membrane: its structure and functions

Image cell membrane. The small blue and white balls correspond to the hydrophilic heads of the lipids, and the lines attached to them correspond to the hydrophobic tails. The figure shows only integral membrane proteins (red globules and yellow helices). Yellow oval dots inside the membrane - cholesterol molecules Yellow-green chains of beads on the outside of the membrane - chains of oligosaccharides forming the glycocalyx

A biological membrane also includes various proteins: integral (penetrating the membrane through), semi-integral (immersed at one end into the outer or inner lipid layer), surface (located on the outer or adjacent to internal sides membranes). Some proteins are the points of contact between the cell membrane and the cytoskeleton inside the cell, and the cell wall (if there is one) outside. Some of the integral proteins function as ion channels, various transporters and receptors.

Functions of biomembranes

barrier - ensures regulated, selective, passive and active metabolism with environment. For example, the peroxisome membrane protects the cytoplasm from peroxides that are dangerous to the cell. Selective permeability means that the permeability of the membrane to different atoms or molecules depends on their size, electrical charge and chemical properties. Selective permeability ensures that the cell and cellular compartments are separated from the environment and supplied with the necessary substances.

transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery of nutrients, removal of metabolic end products, secretion of various substances, creation of ion gradients, maintenance of the appropriate pH and ionic concentration in the cell, which are necessary for the functioning of cellular enzymes.

Particles that for some reason are not able to cross the phospholipid bilayer (for example, due to hydrophilic properties, since the membrane inside is hydrophobic and does not allow hydrophilic substances to pass through, or due to their large size), but necessary for the cell, can penetrate the membrane through special carrier proteins (transporters) and channel proteins or by endocytosis.

During passive transport, substances cross the lipid bilayer without energy consumption, by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance pass through the membrane. This molecule may have a channel that allows only one type of substance to pass through.

Active transport requires energy as it occurs against a concentration gradient. There are special pump proteins on the membrane, including ATPase, which actively pumps potassium ions (K+) into the cell and pumps sodium ions (Na+) out of it.

matrix - ensures a certain relative position and orientation of membrane proteins, their optimal interaction;

mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play a major role in ensuring mechanical function, and in animals, the intercellular substance.

energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate;

receptor - some proteins located in the membrane are receptors (molecules with the help of which the cell perceives certain signals).

For example, hormones circulating in the blood act only on target cells that have receptors corresponding to these hormones. Neurotransmitters (chemical substances that ensure the conduction of nerve impulses) also bind to special receptor proteins in target cells.

enzymatic - membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.

implementation of generation and conduction of biopotentials.

With the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K+ ion inside the cell is much higher than outside, and the concentration of Na+ is much lower, which is very important, since this ensures the maintenance of the potential difference on the membrane and the generation of a nerve impulse.

cell marking - there are antigens on the membrane that act as markers - “labels” that allow the cell to be identified. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of “antennas”. Because of the myriad configurations of side chains, it is possible to make a specific marker for each cell type. With the help of markers, cells can recognize other cells and act in concert with them, for example, in the formation of organs and tissues. This also allows the immune system to recognize foreign antigens.

Structure and composition of biomembranes

Membranes are composed of three classes of lipids: phospholipids, glycolipids and cholesterol. Phospholipids and glycolipids (lipids with carbohydrates attached) consist of two long hydrophobic hydrocarbon tails that are connected to a charged hydrophilic head. Cholesterol gives the membrane rigidity by occupying the free space between the hydrophobic tails of lipids and preventing them from bending. Therefore, membranes with a low cholesterol content are more flexible, and those with a high cholesterol content are more rigid and fragile. Cholesterol also serves as a “stopper” that prevents the movement of polar molecules from the cell and into the cell. An important part of the membrane consists of proteins that penetrate it and are responsible for the various properties of membranes. Their composition and orientation differ in different membranes.

Cell membranes are often asymmetrical, that is, the layers differ in lipid composition, the transition of an individual molecule from one layer to another (the so-called flip flop) is difficult.

Membrane organelles

These are closed single or interconnected sections of the cytoplasm, separated from the hyaloplasm by membranes. Single-membrane organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes; to double membranes - nucleus, mitochondria, plastids. The outside of the cell is bounded by the so-called plasma membrane. The structure of the membranes of various organelles differs in the composition of lipids and membrane proteins.

Selective permeability

Cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, and the membranes themselves, to a certain extent, actively regulate this process - some substances pass through, but others do not. There are four main mechanisms for the entry of substances into the cell or their removal from the cell to the outside: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive in nature, that is, they do not require energy expenditure; the last two are active processes associated with energy consumption.

The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane through, forming a kind of passage. The elements K, Na and Cl have their own channels. Relative to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open and a sudden influx of sodium ions into the cell occurs. In this case, an imbalance of membrane potential occurs. After which the membrane potential is restored. Potassium channels are always open, allowing potassium ions to slowly enter the cell.

Properties of biological membranes

1. Self-assembly ability after destructive influences. This property is determined by the physicochemical properties of phospholipid molecules, which in an aqueous solution come together so that the hydrophilic ends of the molecules unfold outward, and the hydrophobic ends inward. Proteins can be built into ready-made phospholipid layers. The ability to self-assemble is important at the cellular level.

2. Semi-permeable(selectivity in the transmission of ions and molecules). Ensures the maintenance of constancy of the ionic and molecular composition in the cell.

3. Membrane fluidity. Membranes are not rigid structures; they constantly fluctuate due to the rotational and vibrational movements of lipid and protein molecules. This ensures a higher rate of enzymatic and other chemical processes in membranes.

4. Membrane fragments do not have free ends, as they close into bubbles.

Functions of the outer cell membrane (plasmalemma)

The main functions of the plasmalemma are the following: 1) barrier, 2) receptor, 3) exchange, 4) transport.

1. Barrier function. It is expressed in the fact that the plasma membrane limits the contents of the cell, separating it from the external environment, and intracellular membranes divide the cytoplasm into separate reaction cells. compartments.

2. Receptor function. One of the most important functions of the plasmalemma is to ensure communication (connection) of the cell with 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.

Exchange function determined by the content of enzyme proteins in biological membranes, which are biological catalysts. Their activity varies depending on the pH of the environment, temperature, pressure, and the concentration of both the substrate and the enzyme itself. Enzymes determine the intensity of key reactions metabolism, as well as their direction.

Transport function of membranes. The membrane allows for selective penetration of various chemicals into the cell and out of the cell into the environment. Transport of substances is necessary to maintain the appropriate pH and proper ionic concentration in the cell, which ensures the efficiency of cellular enzymes. Transport supplies nutrients, which serve as a source of energy, as well as material for the formation of various cellular components. The removal of toxic waste from the cell, the secretion of various useful substances and the creation of ion gradients necessary for nervous and muscle activity depend on it. Changes in the rate of transfer of substances can lead to disturbances in bioenergetic processes, water-salt metabolism, excitability and other processes. Correction of these changes underlies the action of many medications.

There are two main ways for substances to enter the cell and exit the cell into the external environment;

passive transport,

active transport.

Passive transport follows a chemical or electrochemical concentration gradient without the expenditure of ATP energy. If the molecule of the transported substance has no charge, then the direction of passive transport is determined only by the difference in the concentration of this substance on both sides of the membrane (chemical concentration gradient). If the molecule is charged, then its transport is affected by both the chemical concentration gradient and the electrical gradient (membrane potential).

Both gradients together constitute the electrochemical gradient. Passive transport of substances can be carried out in two ways: simple diffusion and facilitated diffusion.

With simple diffusion salt ions and water can penetrate through selective channels. These channels are formed by certain transmembrane proteins that form end-to-end transport pathways that are open permanently or only for a short time. Penetrate through selective channels various molecules, having the size and charge corresponding to the channels.

There is another way of simple diffusion - this is the diffusion of substances through the lipid bilayer, through which fat-soluble substances and water easily pass. The lipid bilayer is impermeable to charged molecules (ions), and at the same time, uncharged small molecules can diffuse freely, and the smaller the molecule, the faster it is transported. The rather high rate of diffusion of water through the lipid bilayer is precisely explained by the small size of its molecules and the lack of charge.

With facilitated diffusion Transport of substances involves proteins - carriers that work on the “ping-pong” principle. The protein exists in two conformational states: in the “pong” state, the binding sites for the transported substance are open on the outside of the bilayer, and in the “ping” state, the same sites are open on the other side. This process is reversible. From which side the binding site of a substance will be open at a given moment depends on the concentration gradient of this substance.

In this way, sugars and amino acids pass through the membrane.

With facilitated diffusion, the rate of transport of substances increases significantly compared to simple diffusion.

In addition to carrier proteins, some antibiotics are involved in facilitated diffusion, for example, gramicidin and valinomycin.

Because they provide ion transport, they are called ionophores.

Active transport of substances in the cell. This type of transport always costs energy. The source of energy required for active transport is ATP. A characteristic feature of this type of transport is that it is carried out in two ways:

using enzymes called ATPases;

transport in membrane packaging (endocytosis).

IN The outer cell membrane contains enzyme proteins such as ATPases, whose function is to provide active transport ions against a concentration gradient. Since they provide ion transport, this process is called an ion pump.

There are four main known ion transport systems in animal cells. Three of them provide transfer through biological membranes: Na + and K +, Ca +, H +, and the fourth - transfer of protons during the functioning of the mitochondrial respiratory chain.

An example of an active ion transport mechanism is sodium-potassium pump in animal cells. It maintains a constant concentration of sodium and potassium ions in the cell, which differs from the concentration of these substances in the environment: normally, there are fewer sodium ions in the cell than in the environment, and more potassium ions.

As a result, according to the laws of simple diffusion, potassium tends to leave the cell, and sodium diffuses into the cell. In contrast to simple diffusion, the sodium-potassium pump constantly pumps sodium out of the cell and introduces potassium: for every three molecules of sodium released out, there are two molecules of potassium introduced into the cell.

This transport of sodium-potassium ions is ensured by the dependent ATPase, an enzyme localized in the membrane in such a way that it penetrates its entire thickness. Sodium and ATP enter this enzyme from the inside of the membrane, and potassium from the outside.

The transfer of sodium and potassium across the membrane occurs as a result of conformational changes that the sodium-potassium dependent ATPase undergoes, which is activated when the concentration of sodium inside the cell or potassium in the environment increases.

To supply energy to this pump, ATP hydrolysis is necessary. This process is ensured by the same enzyme, sodium-potassium dependent ATPase. Moreover, more than one third of the ATP consumed by an animal cell at rest is spent on the operation of the sodium-potassium pump.

Violation of the proper functioning of the sodium-potassium pump leads to various serious diseases.

The efficiency of this pump exceeds 50%, which is not achieved by the most advanced machines created by man.

Many active transport systems are powered by energy stored in ion gradients rather than by direct hydrolysis of ATP. They all work like transport systems(promoting the transport of low molecular weight compounds). For example, the active transport of some sugars and amino acids into animal cells is determined by a sodium ion gradient, and the higher the sodium ion gradient, the greater the rate of glucose absorption. And, conversely, if the sodium concentration in the intercellular space decreases markedly, glucose transport stops. In this case, sodium must join the sodium-dependent glucose transport protein, which has two binding sites: one for glucose, the other for sodium. Sodium ions penetrating the cell facilitate the introduction of the carrier protein into the cell along with glucose. Sodium ions that enter the cell along with glucose are pumped back by sodium-potassium dependent ATPase, which, by maintaining a sodium concentration gradient, indirectly controls glucose transport.

Transport of substances in membrane packaging. Large molecules of biopolymers practically cannot penetrate through the plasmalemma by any of the above-described mechanisms of transport of substances into the cell. They are captured by the cell and absorbed into membrane packaging, which is called endocytosis. The latter is formally divided into phagocytosis and pinocytosis. The uptake of particulate matter by the cell is phagocytosis, and liquid - pinocytosis. During endocytosis, the following stages are observed:

reception of the absorbed substance due to receptors in the cell membrane;

invagination of the membrane with the formation of a bubble (vesicle);

separation of the endocytic vesicle from the membrane with energy consumption – phagosome formation and restoration of membrane integrity;

Fusion of the phagosome with the lysosome and formation phagolysosomes (digestive vacuole) in which digestion of absorbed particles occurs;

removal of material undigested in the phagolysosome from the cell ( exocytosis).

In the animal world endocytosis is in a characteristic way nutrition of many unicellular organisms (for example, in amoebas), and among many cellular organisms, this type of digestion of food particles is found in the endodermal cells of coelenterates. As for mammals and humans, they have a reticulo-histio-endothelial system of cells with the ability to endocytosis. Examples include blood leukocytes and liver Kupffer cells. The latter line the so-called sinusoidal capillaries of the liver and capture various foreign particles suspended in the blood. Exocytosis- This is also a method of removing from the cell of a multicellular organism the substrate secreted by it, which is necessary for the function of other cells, tissues and organs.

The outside of the cell is covered with a 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.

The endoplasmic reticulum of the cell is connected to the outer membrane. With the help of outer membranes, various types of intercellular contacts are carried out, 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 between individual cells and 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 create a strong support for plant tissues (wood).

Some animal cells also have a number of external structures located on top of the cell membrane and have 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 conduction.
Motor	Carries out the movement of the cell or its individual parts.

It has a thickness of 8-12 nm, so it is impossible to examine it with a light microscope. The structure of the membrane is studied using an electron microscope.

The plasma membrane is formed by two layers of lipids - a bilipid layer, or bilayer. Each molecule consists of a hydrophilic head and a hydrophobic tail, and in biological membranes lipids are located with their heads outward and tails inward.

Numerous protein molecules are immersed in the bilipid layer. Some of them are located on the surface of the membrane (external or internal), others penetrate the membrane.

Functions of the plasma membrane

The membrane protects the contents of the cell from damage, maintains the shape of the cell, selectively allows necessary substances into the cell and removes metabolic products, and also ensures communication between cells.

The barrier, delimiting function of the membrane is provided by a double layer of lipids. It prevents the contents of the cell from spreading, mixing with the environment or intercellular fluid, and prevents the penetration of dangerous substances into the cell.

A number of the most important functions of the cytoplasmic membrane are carried out by proteins immersed in it. With the help of receptor proteins, it can perceive various irritations on its surface. Transport proteins form the finest channels through which potassium, calcium, and other ions of small diameter pass into and out of the cell. Proteins provide vital processes in the body itself.

Large food particles that are unable to pass through thin membrane channels enter the cell by phagocytosis or pinocytosis. The general name for these processes is endocytosis.

How does endocytosis occur - the penetration of large food particles into the cell?

The food particle comes into contact with the outer membrane of the cell, and an invagination forms at this point. Then the particle, surrounded by a membrane, enters the cell, a digestive vesicle is formed, and digestive enzymes penetrate into the resulting vesicle.

White blood cells that can capture and digest foreign bacteria are called phagocytes.

In the case of pinocytosis, the invagination of the membrane captures not solid particles, but droplets of liquid with substances dissolved in it. This mechanism is one of the main ways for substances to enter the cell.

Plant cells covered with a hard layer of cell wall on top of the membrane are not capable of phagocytosis.

The reverse process of endocytosis is exocytosis. Synthesized substances (for example, hormones) are packaged in membrane vesicles, approach the membrane, are built into it, and the contents of the vesicle are released from the cell. In this way, the cell can get rid of unnecessary metabolic products.

Short 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 of nervous activity: nervous excitation, inhibition, the functioning 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 the environment and carries 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, the cell membrane looks like 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 biochemical 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 the internal environment of the cell.

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. Another important function of the membrane is 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.

A separate image of the inner lipid (fat) layer of the cell membrane, permeated with integral embedded 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 it inner space to the 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 biological membrane: This is a snapshot of a fatty phospholipid bilayer (i.e. bilayer) during 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, energy 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.
Communication of gram-negative bacterial cells with 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 that 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 carbon dioxide easily pass in and out directly through the membrane. 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 just 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 your membrane additional properties 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 drugs. 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.)

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