What molecules does the cell membrane consist of? Biological mechanisms involving the cell membrane

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

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

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

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

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

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

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

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

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

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

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

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

Cell membrane

Image of a cell membrane. The small blue and white balls correspond to the hydrophobic “heads” of the phospholipids, and the lines attached to them correspond to the hydrophilic “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 outside membranes - chains of oligosaccharides that form the glycocalyx

A biological membrane also includes various proteins: integral (penetrating the membrane through), semi-integral (immersed at one end in the outer or inner lipid layer), surface (located on the outer or adjacent to the inner sides of the membrane). 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

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 various substances, creating ion gradients, maintaining the optimal concentration of ions in the cell that are necessary for the functioning of cellular enzymes.
Particles that for any reason are unable 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 large sizes), but necessary for the cell, can penetrate the membrane through special carrier proteins (transporters) and channel proteins or by endocytosis.
In passive transport, substances cross the lipid bilayer without expending energy along a concentration gradient 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). Greater role in ensuring mechanical function have cell walls, and in animals - 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, ensuring the conduction of nerve impulses) also bind to special receptor proteins of 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 countless configuration of the side chains, it is possible to make its own special 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. Important part membranes are made up of proteins that permeate 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 related friend on the other hand, areas 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 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 character, that is, they do not require energy expenditure; the last two - active processes related to energy consumption.

The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane right 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. Then membrane potential is being restored. Potassium channels are always open, allowing potassium ions to slowly enter the cell.

Literature

Antonov V.F., Smirnova E.N., Shevchenko E.V. Lipid membranes phase transitions. - M.: Science, 1994.
Gennis R. Biomembranes. Molecular structure and functions: translation from English. = Biomembranes. Molecular structure and function (by Robert B. Gennis). - 1st edition. - M.: Mir, 1997. - ISBN 5-03-002419-0
Ivanov V. G., Berestovsky T. N. Lipid bilayer of biological membranes. - M.: Nauka, 1982.
Rubin A. B. Biophysics, textbook in 2 vols. - 3rd edition, corrected and expanded. - M.: Moscow University Publishing House, 2004. -

Membranes are extremely viscous and at the same time plastic structures that surround all living cells. Functions cell membranes:

1. The plasma membrane is a barrier that maintains the different composition of the extra- and intracellular environment.

2.Membranes form specialized compartments inside the cell, i.e. numerous organelles - mitochondria, lysosomes, Golgi complex, endoplasmic reticulum, nuclear membranes.

3. Enzymes involved in energy conversion in processes such as oxidative phosphorylation and photosynthesis are localized in the membranes.

Structure and composition of membranes

The basis of the membrane is a double lipid layer, the formation of which involves phospholipids and glycolipids. The lipid bilayer is formed by two rows of lipids, the hydrophobic radicals of which are hidden inward, and the hydrophilic groups face outward and are in contact with the aqueous environment. Protein molecules are, as it were, “dissolved” in the lipid bilayer.

Structure of membrane lipids

Membrane lipids are amphiphilic molecules, because the molecule has both a hydrophilic region (polar heads) and a hydrophobic region, represented by hydrocarbon radicals of fatty acids, which spontaneously form a bilayer. Membranes contain three main types of lipids - phospholipids, glycolipids and cholesterol.

The lipid composition is different. The content of a particular lipid is apparently determined by the variety of functions performed by these lipids in membranes.

Phospholipids. All phospholipids can be divided into two groups - glycerophospholipids and sphingophospholipids. Glycerophospholipids are classified as phosphatidic acid derivatives. The most common glycerophospholipids are phosphatidylcholines and phosphatidylethanolamines. Sphingophospholipids are based on the amino alcohol sphingosine.

Glycolipids. In glycolipids, the hydrophobic part is represented by the alcohol ceramide, and the hydrophilic part is represented by a carbohydrate residue. Depending on the length and structure of the carbohydrate part, cerebrosides and gangliosides are distinguished. The polar “heads” of glycolipids are located on the outer surface of plasma membranes.

Cholesterol (CS). CS is present in all membranes of animal cells. Its molecule consists of a rigid hydrophobic core and a flexible hydrocarbon chain. The single hydroxyl group at the 3-position is the “polar head”. For an animal cell, the average molar ratio of cholesterol/phospholipids is 0.3-0.4, but in the plasma membrane this ratio is much higher (0.8-0.9). The presence of cholesterol in membranes reduces the mobility of fatty acids, reduces the lateral diffusion of lipids and therefore can affect the functions of membrane proteins.

Membrane properties:

1. Selective permeability. The closed bilayer provides one of the main properties of the membrane: it is impermeable to most water-soluble molecules, since they do not dissolve in its hydrophobic core. Gases such as oxygen, CO 2 and nitrogen have the ability to easily penetrate into cells due to the small size of their molecules and weak interaction with solvents. Molecules of a lipid nature, such as steroid hormones, also easily penetrate the bilayer.

2. Liquidity. Membranes are characterized by liquidity (fluidity), the ability of lipids and proteins to move. Two types of phospholipid movements are possible: somersault (in scientific literature called “flip-flop”) and lateral diffusion. In the first case, phospholipid molecules opposing each other in the bimolecular layer turn over (or somersault) towards each other and change places in the membrane, i.e. the outside becomes the inside and vice versa. Such jumps are associated with energy consumption. More often, rotations around the axis (rotation) and lateral diffusion are observed - movement within the layer parallel to the surface of the membrane. The speed of movement of molecules depends on the microviscosity of the membranes, which, in turn, is determined by the relative content of saturated and unsaturated fatty acids in the lipid composition. Microviscosity is lower if unsaturated fatty acids predominate in the lipid composition, and higher if the content of saturated fatty acids is high.

3. Membrane asymmetry. The surfaces of the same membrane differ in the composition of lipids, proteins and carbohydrates (transverse asymmetry). For example, phosphatidylcholines predominate in the outer layer, and phosphatidylethanolamines and phosphatidylserines predominate in the inner layer. The carbohydrate components of glycoproteins and glycolipids come to the outer surface, forming a continuous structure called the glycocalyx. There are no carbohydrates on the inner surface. Proteins - hormone receptors are located on the outer surface of the plasma membrane, and the enzymes they regulate - adenylate cyclase, phospholipase C - on the inner surface, etc.

Membrane proteins

Membrane phospholipids act as a solvent for membrane proteins, creating a microenvironment in which the latter can function. Proteins account for 30 to 70% of the mass of membranes. The number of different proteins in the membrane varies from 6-8 in the sarcoplasmic reticulum to more than 100 in the plasma membrane. These are enzymes, transport proteins, structural proteins, antigens, including antigens of the major histocompatibility system, receptors for various molecules.

Based on their localization in the membrane, proteins are divided into integral (partially or completely immersed in the membrane) and peripheral (located on its surface). Some integral proteins cross the membrane once (glycophorin), others cross the membrane many times. For example, the retinal photoreceptor and β 2 -adrenergic receptor cross the bilayer 7 times.

Peripheral proteins and domains of integral proteins, located on the outer surface of all membranes, are almost always glycosylated. Oligosaccharide residues protect the protein from proteolysis and are also involved in ligand recognition or adhesion.

Cytoplasm- an obligatory part of the cell, enclosed between the plasma membrane and the nucleus; is divided into hyaloplasm (the main substance of the cytoplasm), organelles (permanent components of the cytoplasm) and inclusions (temporary components of the cytoplasm). Chemical composition cytoplasm: the basis is water (60-90% of the total mass of the cytoplasm), various organic and inorganic compounds. The cytoplasm has an alkaline reaction. Feature cytoplasm of a eukaryotic cell - constant movement ( cyclosis). It is detected primarily by the movement of cell organelles, such as chloroplasts. If the movement of the cytoplasm stops, the cell dies, since only being in constant movement, it can perform its functions.

Hyaloplasma ( cytosol) is a colorless, slimy, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it ensures the interconnection of the nucleus and all organelles. Depending on the predominance of the liquid part or large molecules in the hyaloplasm, two forms of hyaloplasm are distinguished: sol- more liquid hyaloplasm and gel- thicker hyaloplasm. Mutual transitions are possible between them: the gel turns into a sol and vice versa.

Functions of the cytoplasm:

combining all cell components into a single system,
environment for the passage of many biochemical and physiological processes,
environment for the existence and functioning of organelles.

Cell membranes

Cell membranes limit eukaryotic cells. In each cell membrane, at least two layers can be distinguished. The inner layer is adjacent to the cytoplasm and is represented by plasma membrane(synonyms - plasmalemma, cell membrane, cytoplasmic membrane), over which the outer layer is formed. IN animal cell it's thin and it's called glycocalyx(formed by glycoproteins, glycolipids, lipoproteins), in plant cell- thick, called cell wall(formed by cellulose).

All biological membranes have common structural features and properties. It is currently generally accepted fluid mosaic model of membrane structure. The basis of the membrane is a lipid bilayer formed mainly by phospholipids. Phospholipids are triglycerides in which one fatty acid residue is replaced by a phosphoric acid; The section of the molecule containing the phosphoric acid residue is called the hydrophilic head, the sections containing the fatty acid residues are called the hydrophobic tails. In the membrane, phospholipids are arranged in a strictly ordered manner: the hydrophobic tails of the molecules face each other, and the hydrophilic heads face outward, towards the water.

In addition to lipids, the membrane contains proteins (on average ≈ 60%). They determine most of the specific functions of the membrane (transport of certain molecules, catalysis of reactions, receiving and converting signals from the environment, etc.). There are: 1) peripheral proteins(located on the outside or inner surface lipid bilayer), 2) semi-integral proteins(immersed in the lipid bilayer to varying depths), 3) integral, or transmembrane, proteins(pierce the membrane through, contacting both the external and internal environment of the cell). Integral proteins are in some cases called channel-forming or channel proteins, since they can be considered as hydrophilic channels through which polar molecules pass into the cell (the lipid component of the membrane would not let them through).

A - hydrophilic phospholipid head; B - hydrophobic phospholipid tails; 1 - hydrophobic regions of proteins E and F; 2 — hydrophilic regions of protein F; 3 - branched oligosaccharide chain attached to a lipid in a glycolipid molecule (glycolipids are less common than glycoproteins); 4 - branched oligosaccharide chain attached to a protein in a glycoprotein molecule; 5 - hydrophilic channel (functions as a pore through which ions and some polar molecules can pass).

The membrane may contain carbohydrates (up to 10%). The carbohydrate component of membranes is represented by oligosaccharide or polysaccharide chains associated with protein molecules (glycoproteins) or lipids (glycolipids). Carbohydrates are mainly located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form a supra-membrane complex, the glycocalyx, which is several tens of nanometers thick. It contains many cell receptors, and with its help cell adhesion occurs.

Molecules of proteins, carbohydrates and lipids are mobile, capable of moving in the plane of the membrane. The thickness of the plasma membrane is approximately 7.5 nm.

Functions of membranes

Membranes perform the following functions:

separation of cellular contents from the external environment,
regulation of metabolism between the cell and the environment,
dividing the cell into compartments (“compartments”),
place of localization of “enzymatic conveyors”,
ensuring communication between cells in the tissues of multicellular organisms (adhesion),
signal recognition.

The most important membrane property— selective permeability, i.e. membranes are highly permeable to some substances or molecules and poorly permeable (or completely impermeable) to others. This property underlies the regulatory function of membranes, ensuring the exchange of substances between the cell and the external environment. The process of substances passing through the cell membrane is called transport of substances. There are: 1) passive transport- the process of passing substances without energy consumption; 2) active transport- the process of passage of substances that occurs with the expenditure of energy.

At passive transport substances move from an area of higher concentration to an area of lower, i.e. along the concentration gradient. In any solution there are solvent and solute molecules. The process of moving solute molecules is called diffusion, and the movement of solvent molecules is called osmosis. If the molecule is charged, then its transport is also affected by the electrical gradient. Therefore, people often talk about an electrochemical gradient, combining both gradients together. The speed of transport depends on the magnitude of the gradient.

You can select the following types passive transport: 1) simple diffusion— transport of substances directly through the lipid bilayer (oxygen, carbon dioxide); 2) diffusion through membrane channels— transport through channel-forming proteins (Na +, K +, Ca 2+, Cl -); 3) facilitated diffusion- transport of substances using special transport proteins, each of which is responsible for the movement of certain molecules or groups of related molecules (glucose, amino acids, nucleotides); 4) osmosis- transport of water molecules (in all biological systems The solvent is water.)

Necessity active transport occurs when it is necessary to ensure the transport of molecules across a membrane against an electrochemical gradient. This transport is carried out by special carrier proteins, the activity of which requires energy expenditure. The energy source is ATP molecules. Active transport includes: 1) Na + /K + pump (sodium-potassium pump), 2) endocytosis, 3) exocytosis.

Operation of Na + /K + pump. For normal functioning, the cell must maintain a certain ratio of K + and Na + ions in the cytoplasm and in external environment. The concentration of K + inside the cell should be significantly higher than outside it, and Na + - vice versa. It should be noted that Na + and K + can diffuse freely through the membrane pores. The Na + /K + pump counteracts the equalization of the concentrations of these ions and actively pumps Na + out of the cell and K + into the cell. The Na + /K + pump is a transmembrane protein capable of conformational changes, as a result of which it can attach both K + and Na +. The operating cycle of the Na + /K + pump can be divided into the following phases: 1) connection of Na + with inside membranes, 2) phosphorylation of the pump protein, 3) release of Na + in the extracellular space, 4) addition of K + to outside membranes, 5) dephosphorylation of the pump protein, 6) release of K + in the intracellular space. To work sodium potassium pump Almost a third of all the energy necessary for the life of the cell is spent. In one cycle of operation, the pump pumps out 3Na + from the cell and pumps in 2K +.

Endocytosis- the process of absorption of large particles and macromolecules by the cell. There are two types of endocytosis: 1) phagocytosis- capture and absorption of large particles (cells, parts of cells, macromolecules) and 2) pinocytosis— capture and absorption of liquid material (solution, colloidal solution, suspension). The phenomenon of phagocytosis was discovered by I.I. Mechnikov in 1882. During endocytosis, the plasma membrane forms an invagination, its edges merge, and structures delimited from the cytoplasm by a single membrane are laced into the cytoplasm. Many protozoa and some leukocytes are capable of phagocytosis. Pinocytosis is observed in intestinal epithelial cells and in the endothelium of blood capillaries.

Exocytosis- a process reverse to endocytosis: the removal of various substances from the cell. During exocytosis, the vesicle membrane merges with the outer cytoplasmic membrane, the contents of the vesicle are removed outside the cell, and its membrane is included in the outer cytoplasmic membrane. In this way, hormones are removed from the cells of the endocrine glands; in protozoa, undigested food remains are removed.

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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 nervous activity: nervous excitement, 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 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 cell structure underlies barrier, mechanical and matrix properties 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. Availability of electrical and chemical potentials, more precisely the differences potentials 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 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 electric 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 schematic drawing, showing the three-layer nature 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 its internal space into compartments. Surrounded by the same membranes cell organelles: nucleus, mitochondria, chloroplasts. So the membrane is universal molecular structure, characteristic of 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 right - visual model 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.

It is possible to give a more precise scientific definition cage:

- is limited active membrane, ordered, structured heterogeneous system biopolymers 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 aqueous solution, but mostly sit (immobilized) on 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. Outer membrane, surrounding the cell, is also covered with enzymes and molecular receptors, which makes 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.
The communication of gram-negative 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 that have a less powerful membrane. 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 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, 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 " containment"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, 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.)