Structure and properties of biological cell membranes. Structure and functions of plasma membranes

It's no secret that all living beings on our planet are made up of cells, these countless "" organic matter. The cells, in turn, are surrounded by a special protective shell - a membrane, which plays a very important role in the life of the cell, and the functions of the cell membrane are not limited to just protecting the cell, but represent a complex mechanism involved in the reproduction, nutrition, and regeneration of the cell.

What is a cell membrane

The word “membrane” itself is translated from Latin as “film,” although a membrane is not just a kind of film in which a cell is wrapped, but a combination of two films connected to each other and having different properties. In fact, the cell membrane is a three-layer lipoprotein (fat-protein) membrane that separates each cell from neighboring cells and environment, and carrying out a controlled exchange between cells and the environment, this is the academic definition of what a cell membrane is.

The importance of the membrane is simply enormous, because it not only separates one cell from another, but also ensures the cell’s interaction with both other cells and the environment.

History of cell membrane research

An important contribution to the study of the cell membrane was made by two German scientists Gorter and Grendel back in 1925. It was then that they managed to conduct a complex biological experiment on red blood cells- red blood cells, during which scientists obtained the so-called “shadows”, empty shells of red blood cells, which were stacked in one stack and measured the surface area, and also calculated the amount of lipids in them. Based on the amount of lipids obtained, scientists came to the conclusion that they are precisely contained in the double layer of the cell membrane.

In 1935, another pair of cell membrane researchers, this time Americans Daniel and Dawson, after a series of long experiments, established the protein content in the cell membrane. There was no other way to explain why the membrane had such a high surface tension. Scientists have cleverly presented a model of a cell membrane in the form of a sandwich, in which the role of bread is played by homogeneous lipid-protein layers, and between them, instead of oil, there is emptiness.

In 1950, with the advent of electronics, the theory of Daniel and Dawson was confirmed by practical observations - in micrographs of the cell membrane, layers of lipid and protein heads and also the empty space between them were clearly visible.

In 1960, the American biologist J. Robertson developed a theory about the three-layer structure of cell membranes, which for a long time was considered the only true one, but with the further development of science, doubts began to arise about its infallibility. So, for example, from the point of view, it would be difficult and labor-intensive for cells to transport the necessary nutrients through the entire “sandwich”

And only in 1972, American biologists S. Singer and G. Nicholson were able to explain the inconsistencies in Robertson’s theory using a new fluid-mosaic model of the cell membrane. In particular, they found that the cell membrane is not homogeneous in its composition, moreover, it is asymmetrical and filled with liquid. In addition, cells are in constant motion. And the notorious proteins that are part of the cell membrane have different structures and functions.

Properties and functions of the cell membrane

Now let's look at what functions the cell membrane performs:

The barrier function of the cell membrane is the membrane as a real border guard, standing guard over the boundaries of the cell, delaying and not allowing harmful or simply inappropriate molecules to pass through.

Transport function of the cell membrane - the membrane is not only a border guard at the cell gate, but also a kind of customs checkpoint; useful substances are constantly exchanged with other cells and the environment through it.

Matrix function - it is the cell membrane that determines the location relative to each other and regulates the interaction between them.

Mechanical function - is responsible for limiting one cell from another and, at the same time, for correctly connecting cells to each other, for forming them into a homogeneous tissue.

The protective function of the cell membrane is the basis for building the cell's protective shield. In nature, an example of this function can be hard wood, a dense peel, a protective shell, all due to the protective function of the membrane.

Enzymatic function is another important function performed by certain proteins in the cell. For example, thanks to this function, the synthesis of digestive enzymes occurs in the intestinal epithelium.

Also, in addition to all this, cellular exchange occurs through the cell membrane, which can take place in three different reactions:

Phagocytosis is a cellular exchange in which membrane-embedded phagocyte cells capture and digest various nutrients.
Pinocytosis is the process of capture by the cell membrane of liquid molecules in contact with it. To do this, special tendrils are formed on the surface of the membrane, which seem to surround a drop of liquid, forming a bubble, which is subsequently “swallowed” by the membrane.
Exocytosis is reverse process when a cell releases a secretory functional fluid through a membrane to the surface.

Structure of the cell membrane

There are three classes of lipids in the cell membrane:

phospholipids (which are a combination of fats and phosphorus),
glycolipids (a combination of fats and carbohydrates),
cholesterol

Phospholipids and glycolipids, in turn, consist of a hydrophilic head, into which two long hydrophobic tails extend. Cholesterol occupies the space between these tails, preventing them from bending; all this, in some cases, makes the membrane of certain cells very rigid. In addition to all this, cholesterol molecules organize the structure of the cell membrane.

But be that as it may, important part The structure of the cell membrane is a protein, or rather different proteins that play different important roles. Despite the diversity of proteins contained in the membrane, there is something that unites them - annular lipids are located around all membrane proteins. Annular lipids are special structured fats that serve as a kind of protective shell for proteins, without which they simply would not work.

The structure of the cell membrane has three layers: the basis of the cell membrane is a homogeneous liquid bilipid layer. Proteins cover it on both sides like a mosaic. It is proteins, in addition to the functions described above, that also play the role of peculiar channels through which substances that are unable to penetrate through the liquid layer of the membrane pass through the membrane. These include, for example, potassium and sodium ions; for their penetration through the membrane, nature provides special ion channels in cell membranes. In other words, proteins ensure the permeability of cell membranes.

If we look at the cell membrane through a microscope, we will see a layer of lipids formed by small spherical molecules on which proteins swim as if on the sea. Now you know what substances make up the cell membrane.

Cell membrane video

And finally, an educational video about the cell membrane.

Outer cell membrane (plasmalemma, cytolemma, plasma membrane) of animal cells covered on the outside (i.e., on the side not in contact with the cytoplasm) with a layer of oligosaccharide chains covalently attached to membrane proteins (glycoproteins) and, to a lesser extent, to lipids (glycolipids). This carbohydrate membrane coating is called glycocalyx. The purpose of the glycocalyx is not yet very clear; there is an assumption that this structure takes part in the processes of intercellular recognition.

In plant cells On top of the outer cell membrane there is a dense cellulose layer with pores, through which communication between neighboring cells occurs through cytoplasmic bridges.

In cells mushrooms on top of the plasmalemma - a dense layer chitin.

U bacteria – mureina.

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 plasmalemma limits the contents of the cell, separating it from external environment, and intracellular membranes divide the cytoplasm into separate reaction compartments.

2. Receptor function. One of the most important functions of the plasmalemma is to ensure communication (connection) of the cell with the external environment through the receptor apparatus present in the membranes, which is of a protein or glycoprotein nature. The main function of the receptor formations of the plasmalemma is the recognition of external signals, thanks to which cells are correctly oriented and form tissues during the process of differentiation. The 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 that 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. Various molecules of the size and charge corresponding to the channels penetrate through selective 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. All of them work as cotransport 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 a characteristic method of nutrition for many unicellular organisms (for example, amoebas), and among multicellular 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.

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 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

barrier - ensures regulated, selective, passive and active metabolism with the 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 final metabolic products, secretion of various substances, creation of ion gradients, maintenance of optimal ion concentrations 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 their large size), 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). 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, 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 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 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 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. After which the membrane potential is 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 during 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. -

Cell membrane (also cytolemma, plasmalemma, or plasma membrane) is an elastic molecular structure consisting of proteins and lipids. Separates the contents of any cell from the external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments - compartments or organelles, in which certain environmental conditions are maintained.

If the cell has one (usually plant cells do), it covers the cell membrane.

The cell membrane is a double layer (bilayer) of molecules of the lipid class, most of which are so-called complex lipids - phospholipids. Lipid molecules have a hydrophilic (“head”) and a hydrophobic (“tail”) part. When membranes are formed, the hydrophobic regions of the molecules turn inward, and the hydrophilic regions turn outward. The biological membrane also includes various proteins:

integral (piercing the membrane through),
semi-integral (immersed at one end in the outer or inner lipid layer),
superficial (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 outside.

Membrane functions:

Barrier - provides regulated, selective, passive and active metabolism with the environment.
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 optimal pH and ion concentrations in the cell, which are necessary for the functioning of cellular enzymes.
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.
Energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate.

Membranes consist of three classes of lipids:

phospholipids,
glycolipids,
cholesterol

Phospholipids and glycolipids(lipids with carbohydrates attached) consist of two long hydrophobic hydrocarbon “tails” that are connected to a charged hydrophilic “head”.

Cholesterol imparts rigidity to the membrane 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 is proteins, penetrating it and responsible for the various properties of membranes. Their composition and orientation differ in different membranes. Next to the proteins are annular lipids - they are more ordered, less mobile, contain more saturated fatty acids and are released from the membrane along with the protein. Without annular lipids, membrane proteins do not function.

Cell membranes are often asymmetrical, that is, the layers differ in lipid composition, the outer one contains mainly phosphatidylinositol, phosphatidylcholine, sphingomyelins and glycolipids, the inner one contains phosphatidylserine, phosphatidylethanolamine and phosphatidylinositol. The transition of an individual molecule from one layer to another (the so-called flip-flop) is difficult, but can occur spontaneously, approximately once every 6 months, or with the help of flippases proteins and scramblase of the plasma membrane. If phosphatidylserine appears in the outer layer, this is a signal for macrophages to destroy the cell.

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 structure of the membranes of various organelles differs in the composition of lipids and membrane proteins.

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.

Cell membrane.
The cell membrane separates the contents of any cell from the external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments - compartments or organelles, in which certain environmental conditions are maintained.
Structure.
The cell membrane is a double layer (bilayer) of molecules of the class of lipids (fats), most of which are so-called complex lipids - phospholipids. Lipid molecules have a hydrophilic (“head”) and a hydrophobic (“tail”) part. When membranes are formed, the hydrophobic regions of the molecules turn inward, and the hydrophilic regions turn outward. Membranes are structures that are very similar in different organisms. The thickness of the membrane is 7-8 nm. (10−9 meters)
Hydrophilicity- the ability of a substance to be wetted by water.
Hydrophobicity- the inability of a substance to be wetted by water.
The biological membrane also includes various proteins:
- integral (piercing the membrane through)
- semi-integral (immersed at one end into the outer or inner lipid layer)
- superficial (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.
Cytoskeleton- a cellular framework inside a cell.
Functions.
1) Barrier- provides regulated, selective, passive and active metabolism with the environment.
2) Transport- transport of substances into and out of the cell occurs through the membrane. Matrix - ensures a certain relative position and orientation of membrane proteins, their optimal interaction.
3) Mechanical- ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). The intercellular substance plays a major role in ensuring mechanical function.
4) 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 (chemicals that ensure the conduction of nerve impulses) also bind to special receptor proteins in target cells.
Hormones- biologically active signaling chemicals.
5) Enzymatic- membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.
6) 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.
Nerve impulse – a wave of excitation transmitted along a nerve fiber.
7) 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.
Features of permeability.
Cell membranes are selectively permeable: they are slowly penetrated in different ways:
Glucose is the main source of energy.

Amino acids are the building blocks that make up all the proteins in the body.

Fatty acids – structural, energetic and other functions.

Glycerol – causes the body to retain water and reduces urine production.

Ions are enzymes for reactions.

Moreover, the membranes themselves, to a certain extent, actively regulate this process - some substances pass through, while 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:
Passive permeability mechanisms:
1) 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.
Diffusion- the process of mutual penetration of molecules of one substance between molecules of another.
Osmosis– the process of one-way diffusion through a semi-permeable membrane of solvent molecules towards a higher concentration of the solute.
The membrane surrounding a normal blood cell is permeable only to molecules of water, oxygen, some of the nutrients dissolved in the blood and cellular waste products
Active permeability mechanisms:
1) Active transport.
Active transport– transfer of a substance from an area of low concentration to an area of high concentration.
Active transport requires energy as it occurs from an area of low concentration to an area of high concentration. There are special pump proteins on the membrane that actively pump potassium ions (K+) into the cell and pump sodium ions (Na+) out of it, using ATP as energy.
ATP– a universal source of energy for all biochemical processes. .(more later)
2) Endocytosis.
Particles that for some reason are unable to cross the cell membrane, but are necessary for the cell, can penetrate the membrane by endocytosis.
Endocytosis– capture process external material cell.
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. After which the membrane potential is restored. Potassium channels are always open, allowing potassium ions to slowly enter the cell.
Membrane structure
Permeability
Active transport
Osmosis
Endocytosis