What are the main functions of the cell membrane. Cell (plasma) membrane, its main functions

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 coating of the membrane 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 is a dense cellulose layer with pores through which communication is carried out between neighboring cells through cytoplasmic bridges.

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

At bacteria – mureina.

Properties of biological membranes

1. Ability to self-assemble after destructive impacts. This property is determined by the physicochemical characteristics of phospholipid molecules, which in an aqueous solution come together so that the hydrophilic ends of the molecules turn outward, and the hydrophobic ends inward. Proteins can be incorporated into ready-made phospholipid layers. The ability to self-assemble is essential at the cellular level.

2. Semi-permeability(selectivity in the transmission of ions and molecules). Ensures the maintenance of the 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 oscillatory movements of lipid and protein molecules. This provides a high rate of enzymatic and other chemical processes in the membranes.

4. Fragments of membranes do not have free ends, as they are closed in bubbles.

Functions of the outer cell membrane (plasmalemma)

The main functions of the plasmalemma are as follows: 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 the external environment, and intracellular membranes divide the cytoplasm into separate reactionary 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 has a protein or glycoprotein nature. The main function of the receptor formations of the plasmalemma is the recognition of external signals, due to which the cells are correctly oriented and form tissues in the process of differentiation. The activity of various regulatory systems, as well as the formation of an immune response, is associated with the receptor function.

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

Transport function of membranes. The membrane provides selective penetration into the cell and from the cell into the environment of various chemicals. The transport of substances is necessary to maintain the appropriate pH in the cell, the proper ionic concentration, 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. It determines the removal of toxic waste from the cell, the secretion of various useful substances and the creation of ionic gradients necessary for nervous and muscle activity. 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 drugs.

There are two main ways in which substances enter the cell and out of the cell into the external environment;

passive transport,

active transport.

Passive transport goes along the gradient of chemical or electrochemical concentration 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 an 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 the selective channels. These channels are formed by some transmembrane proteins that form end-to-end transport pathways that are open permanently or only for a short time. Through the selective channels, various molecules penetrate, 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 freely diffuse, and the smaller the molecule, the faster it is transported. The rather high rate of water diffusion through the lipid bilayer is precisely due to the small size of its molecules and the absence of a charge.

With facilitated diffusion proteins are involved in the transport of substances - carriers that work on the principle of "ping-pong". In this case, the protein exists in two conformational states: in the “pong” state, the binding sites of the transported substance are open on the outside of the bilayer, and in the “ping” state, the same sites open on the other side. This process is reversible. From which side the binding site of a substance will be open at a given time 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 in comparison with simple diffusion.

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

Because they provide ion transport, they are called ionophores.

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

with the help of 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 the transport of ions, this process is called an ion pump.

There are four main ion transport systems in the animal cell. Three of them provide transfer through biological membranes. Na + and K +, Ca +, H +, and the fourth - the transfer of protons during the operation 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 less sodium ions in the cell than in the environment, and more potassium.

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

This transport of sodium-potassium ions is ensured by the ATP-dependent enzyme, which is 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.

ATP hydrolysis is required to power this pump. This process is provided by the same enzyme sodium-potassium-dependent ATP-ase. At the same time, 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 driven by energy stored in ionic gradients rather than by direct hydrolysis of ATP. All of them work as cotransport systems (facilitating the transport of low molecular weight compounds). For example, the active transport of certain sugars and amino acids into animal cells is determined by the sodium ion gradient, and the higher the sodium ion gradient, the greater the rate of glucose absorption. Conversely, if the concentration of sodium in the intercellular space decreases markedly, glucose transport stops. In this case, sodium must join the sodium - dependent glucose carrier protein, which has two binding sites: one for glucose, the other for sodium. Sodium ions penetrating into the cell contribute to the introduction of the carrier protein into the cell along with glucose. Sodium ions that have entered the cell along with glucose are pumped back out by the sodium-potassium-dependent ATPase, which, by maintaining the 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 in the membrane package, which is called endocytosis. The latter is formally divided into phagocytosis and pinocytosis. The capture of solid particles 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 (vesicles);

separation of the endocytic vesicle from the membrane with the expenditure of energy - phagosome formation and restoration of membrane integrity;

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

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

In the animal kingdom endocytosis is a characteristic way of feeding many unicellular organisms (for example, in amoebas), and among multicellular organisms this type of digestion of food particles is found in endodermal cells in coelenterates. As for mammals and humans, they have a reticulo-histio-endothelial system of cells with the ability to endocytosis. Examples are 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 way 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.

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 - the lipid layer, or bilayer. Each molecule consists of a hydrophilic head and a hydrophobic tail, and in biological membranes, lipids are located with heads outward, tails inward.

Numerous protein molecules are immersed in the bilipid layer. Some of them are 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 passes the necessary substances into the cell and removes metabolic products, and also provides communication between cells.

The barrier, delimiting function of the membrane provides a double layer of lipids. It does not allow the contents of the cell to spread, mix 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 due to the proteins immersed in it. With the help of receptor proteins, it can perceive various irritations on its surface. Transport proteins form the thinnest channels through which potassium, calcium, and other ions of small diameter pass into and out of the cell. Proteins - provide vital processes in itself.

Large food particles that are not able to pass through thin membrane channels enter the cell by phagocytosis or pinocytosis. The common 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 in this place. Then the particle, surrounded by a membrane, enters the cell, a digestive one is formed, and digestive enzymes penetrate into the formed vesicle.

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

In the case of pinocytosis, the invagination of the membrane does not capture solid particles, but droplets of liquid with substances dissolved in it. This mechanism is one of the main pathways for the penetration of substances into the cell.

Plant cells covered over the membrane with a solid layer of the cell wall are not capable of phagocytosis.

The reverse process of endocytosis is exocytosis. Synthesized substances (for example, hormones) are packed into membrane vesicles, approach, are embedded in it, and the contents of the vesicle are ejected from the cell. Thus, the cell can also get rid of unnecessary metabolic products.

Cytoplasm- an obligatory part of the cell, enclosed between the plasma membrane and the nucleus; It is subdivided into hyaloplasm (the main substance of the cytoplasm), organelles (permanent components of the cytoplasm) and inclusions (temporary components of the cytoplasm). The chemical composition of the cytoplasm: the basis is water (60-90% of the total mass of the cytoplasm), various organic and inorganic compounds. The cytoplasm is alkaline. A characteristic feature of the cytoplasm of a eukaryotic cell is 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 motion can it perform its functions.

Hyaloplasm ( cytosol) is a colorless, slimy, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it provides 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- denser hyaloplasm. Mutual transitions are possible between them: the gel turns into a sol and vice versa.

Functions of the cytoplasm:

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

Cell walls

Cell walls 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 an animal cell, it is thin and is called glycocalyx(formed by glycoproteins, glycolipids, lipoproteins), in a plant cell - thick, called cell wall(formed by cellulose).

All biological membranes have common structural features and properties. Currently generally accepted fluid mosaic model of the 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 residue; the section of the molecule in which the residue of phosphoric acid is located is called the hydrophilic head, the sections in which fatty acid residues are located are called 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 outwards, 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.). Distinguish: 1) peripheral proteins(located on the outer or inner surface of the lipid bilayer), 2) semi-integral proteins(immersed in the lipid bilayer to different depths), 3) integral or transmembrane proteins(permeate the membrane through and through, while in contact with both the external and internal environment of the cell). Integral proteins in some cases are called channel-forming, or channel, 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 head of the phospholipid; C, hydrophobic tails of the phospholipid; 1 - hydrophobic regions of proteins E and F; 2, hydrophilic regions of protein F; 3 - a 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). Basically, carbohydrates are located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form an epimembrane complex, the glycocalyx, several tens of nanometers thick. Many cell receptors are located in it, with its help cell adhesion occurs.

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

Membrane functions

The membranes perform the following functions:

1. separation of cellular contents from the external environment,
2. regulation of metabolism between the cell and the environment,
3. division of the cell into compartments ("compartments"),
4. location of "enzymatic conveyors",
5. providing communication between cells in the tissues of multicellular organisms (adhesion),
6. 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, which ensures the exchange of substances between the cell and the external environment. The process by which substances pass through the cell membrane is called transport of substances. Distinguish: 1) passive transport- the process of passing substances, going without energy; 2) active transport- the process of passing substances, going with the cost of energy.

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

The following types of passive transport can be distinguished: 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, water is the solvent).

Necessity active transport occurs when it is necessary to ensure the transfer of molecules through the membrane against the 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.

Work Na + /K + -pump. For normal functioning, the cell must maintain a certain ratio of K + and Na + ions in the cytoplasm and in the 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 freely diffuse through the membrane pores. The Na+/K+ pump counteracts the equalization of these ion concentrations 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, so that it can attach both K + and Na + . The cycle of operation of Na + /K + -pump can be divided into the following phases: 1) attachment of Na + from the inside of the membrane, 2) phosphorylation of the pump protein, 3) release of Na + in the extracellular space, 4) attachment of K + from the outside of the membrane , 5) dephosphorylation of the pump protein, 6) release of K + in the intracellular space. The sodium-potassium pump consumes almost a third of all the energy necessary for the life of the cell. During one cycle of operation, the pump pumps out 3Na + from the cell and pumps in 2K +.

Endocytosis- the process of absorption by the cell of large particles and macromolecules. There are two types of endocytosis: 1) phagocytosis- capture and absorption of large particles (cells, cell parts, 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 the 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 the epithelial cells of the intestine, in the endothelium of blood capillaries.

Exocytosis- the reverse process of endocytosis: the removal of various substances from the cell. During exocytosis, the vesicle membrane fuses 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 excreted from the cells of the endocrine glands, and in protozoa, undigested food remains.

Go to lectures number 5"Cell Theory. Types of cellular organization»

Go to lectures number 7"Eukaryotic cell: structure and functions of organelles"

All living organisms, depending on the structure of the cell, are divided into three groups (see Fig. 1):

1. Prokaryotes (non-nuclear)

2. Eukaryotes (nuclear)

3. Viruses (non-cellular)

Rice. 1. Living organisms

In this lesson, we will begin to study the structure of the cells of eukaryotic organisms, which include plants, fungi and animals. Their cells are the largest and more complex compared to prokaryotic cells.

As you know, cells are capable of independent activity. They can exchange matter and energy with the environment, as well as grow and multiply, so the internal structure of the cell is very complex and primarily depends on the function that the cell performs in a multicellular organism.

The principles of construction of all cells are the same. In each eukaryotic cell, the following main parts can be distinguished (see Fig. 2):

1. The outer membrane that separates the contents of the cell from the external environment.

2. Cytoplasm with organelles.

Rice. 2. The main parts of a eukaryotic cell

The term "membrane" was proposed about a hundred years ago to denote the boundaries of the cell, but with the development of electron microscopy, it became clear that the cell membrane is part of the structural elements of the cell.

In 1959, J. D. Robertson formulated the elementary membrane hypothesis, according to which the cell membranes of animals and plants are built according to the same type.

In 1972, it was proposed by Singer and Nicholson, which is currently generally accepted. According to this model, the basis of any membrane is a double layer of phospholipids.

In phospholipids (compounds containing a phosphate group), the molecules consist of a polar head and two non-polar tails (see Figure 3).

Rice. 3. Phospholipid

In the phospholipid bilayer, hydrophobic fatty acid residues face inward, while hydrophilic heads, including a phosphoric acid residue, face outward (see Fig. 4).

Rice. 4. Phospholipid bilayer

The phospholipid bilayer is presented as a dynamic structure, lipids can move, changing their position.

The double layer of lipids provides the barrier function of the membrane, preventing the contents of the cell from spreading, and prevents the entry of toxic substances into the cell.

The presence of a boundary membrane between the cell and the environment was known long before the advent of the electron microscope. Physical chemists denied the existence of the plasma membrane and believed that there was an interface between the living colloidal contents and the environment, but Pfeffer (a German botanist and plant physiologist) in 1890 confirmed its existence.

At the beginning of the last century, Overton (a British physiologist and biologist) discovered that the rate of penetration of many substances into erythrocytes is directly proportional to their lipid solubility. In this regard, the scientist suggested that the membrane contains a large amount of lipids and substances, dissolving in it, pass through it and find themselves on the other side of the membrane.

In 1925, Gorter and Grendel (American biologists) isolated lipids from the cell membrane of erythrocytes. The resulting lipids were distributed over the surface of water with a thickness of one molecule. It turned out that the surface area occupied by the lipid layer is twice the area of ​​the erythrocyte itself. Therefore, these scientists concluded that the cell membrane consists of not one, but two layers of lipids.

Dawson and Danielli (English biologists) in 1935 suggested that in cell membranes the bimolecular lipid layer is enclosed between two layers of protein molecules (see Fig. 5).

Rice. 5. Membrane model proposed by Dawson and Danielli

With the advent of the electron microscope, it became possible to get acquainted with the structure of the membrane, and then it was found that the membranes of animal and plant cells look like a three-layer structure (see Fig. 6).

Rice. 6. Cell membrane under a microscope

In 1959, the biologist J. D. Robertson, combining the data available at that time, put forward a hypothesis about the structure of the "elementary membrane", in which he postulated a structure common to all biological membranes.

Robertson's postulates on the structure of the "elementary membrane"

1. All membranes are about 7.5 nm thick.

2. In an electron microscope, they all appear to be three-layered.

3. The three-layer view of the membrane is the result of exactly the arrangement of proteins and polar lipids, which was provided for by the Dawson and Danielli model - the central lipid bilayer is enclosed between two layers of protein.

This hypothesis about the structure of the "elementary membrane" has undergone various changes, and in 1972 it was put forward by fluid mosaic model of the membrane(see Fig. 7), which is now generally accepted.

Rice. 7. Fluid mosaic model of the membrane

Protein molecules are immersed in the lipid bilayer of the membrane, they form a mobile mosaic. According to their location in the membrane and the way they interact with the lipid bilayer, proteins can be divided into:

- superficial (or peripheral) membrane proteins associated with the hydrophilic surface of the lipid bilayer;

- integral (membrane) proteins embedded in the hydrophobic region of the bilayer.

Integral proteins differ in the degree of their immersion in the hydrophobic region of the bilayer. They can be completely submerged integral) or partially submerged ( semi-integral), and can also penetrate the membrane through ( transmembrane).

Membrane proteins can be divided into two groups according to their functions:

- structural proteins. They are part of cell membranes and are involved in maintaining their structure.

- dynamic proteins. They are located on the membranes and participate in the processes taking place on it.

There are three classes of dynamic proteins.

1. Receptor. With the help of these proteins, the cell perceives various influences on its surface. That is, they specifically bind compounds such as hormones, neurotransmitters, toxins on the outside of the membrane, which serves as a signal to change various processes inside the cell or the membrane itself.

2. Transport. These proteins transport certain substances through the membrane, they also form channels through which various ions are transported into and out of the cell.

3. Enzymatic. These are enzyme proteins that are located in the membrane and are involved in various chemical processes.

Transport of substances across the membrane

Lipid bilayers are largely impermeable to many substances, so a large amount of energy is required to transport substances through the membrane, and the formation of various structures is also required.

There are two types of transport: passive and active.

Passive transport

Passive transport is the movement of molecules along a concentration gradient. That is, it is determined only by the difference in the concentration of the transferred substance on opposite sides of the membrane and is carried out without energy expenditure.

There are two types of passive transport:

- simple diffusion(see Fig. 8), which occurs without the participation of the membrane protein. The mechanism of simple diffusion is the transmembrane transfer of gases (oxygen and carbon dioxide), water and some simple organic ions. Simple diffusion is slow.

Rice. 8. Simple diffusion

- facilitated diffusion(see Fig. 9) differs from simple in that it takes place with the participation of carrier proteins. This process is specific and proceeds at a higher rate than simple diffusion.

Rice. 9. Facilitated diffusion

Two types of membrane transport proteins are known: carrier proteins (translocases) and channel-forming proteins. Transport proteins bind specific substances and carry them across the membrane along their concentration gradient, and, consequently, this process, as in simple diffusion, does not require the expenditure of ATP energy.

Food particles cannot pass through the membrane, they enter the cell by endocytosis (see Fig. 10). During endocytosis, the plasma membrane forms invaginations and outgrowths, captures a solid particle of food. A vacuole (or vesicle) is formed around the food bolus, which then detaches from the plasma membrane, and the solid particle in the vacuole is inside the cell.

Rice. 10. Endocytosis

There are two types of endocytosis.

1. Phagocytosis- absorption of solid particles. Specialized cells that perform phagocytosis are called phagocytes.

2. pinocytosis- absorption of liquid material (solution, colloidal solution, suspensions).

Exocytosis(see Fig. 11) - a process reverse to endocytosis. Substances synthesized in the cell, such as hormones, are packed into membrane vesicles that fit the cell membrane, are embedded in it, and the contents of the vesicle are ejected from the cell. In the same way, the cell can get rid of unnecessary metabolic products.

Rice. 11. Exocytosis

active transport

Unlike facilitated diffusion, active transport is the movement of substances against a concentration gradient. In this case, substances move from an area with a lower concentration to an area with a higher concentration. Since such movement occurs in the opposite direction to normal diffusion, the cell must expend energy in this process.

Among examples of active transport, the so-called sodium-potassium pump is best studied. This pump pumps sodium ions out of the cell and pumps potassium ions into the cell using the energy of ATP.

1. Structural (the cell membrane separates the cell from the environment).

2. Transport (substances are transported through the cell membrane, and the cell membrane is a highly selective filter).

3. Receptor (receptors located on the surface of the membrane perceive external influences, transmit this information into the cell, allowing it to quickly respond to environmental changes).

In addition to those listed above, the membrane also performs a metabolic and energy-converting function.

metabolic function

Biological membranes directly or indirectly participate in the processes of metabolic transformations of substances in the cell, since most enzymes are associated with membranes.

The lipid environment of enzymes in the membrane creates certain conditions for their functioning, imposes restrictions on the activity of membrane proteins, and thus has a regulatory effect on metabolic processes.

Energy conversion function

The most important function of many biomembranes is the transformation of one form of energy into another.

Energy-converting membranes include internal membranes of mitochondria, thylakoids of chloroplasts (see Fig. 12).

Rice. 12. Mitochondria and chloroplast

Bibliography

1. Kamensky A.A., Kriksunov E.A., Pasechnik V.V. General biology 10-11 class Bustard, 2005.
2. Biology. Grade 10. General biology. Basic level / P.V. Izhevsky, O.A. Kornilova, T.E. Loshchilin and others - 2nd ed., revised. - Ventana-Graf, 2010. - 224 pages.
3. Belyaev D.K. Biology 10-11 class. General biology. A basic level of. - 11th ed., stereotype. - M.: Education, 2012. - 304 p.
4. Agafonova I.B., Zakharova E.T., Sivoglazov V.I. Biology 10-11 class. General biology. A basic level of. - 6th ed., add. - Bustard, 2010. - 384 p.

1. Ayzdorov.ru ().
2. Youtube.com().
3. Doctor-v.ru ().
4. Animals-world.ru ().

Homework

1. What is the structure of a cell membrane?
2. What are the properties of lipids to form membranes?
3. Due to what functions are proteins able to participate in the transport of substances across the membrane?
4. List the functions of the plasma membrane.
5. How does passive transport occur across the membrane?
6. How does active transport occur across the membrane?
7. What is the function of the sodium-potassium pump?
8. What is phagocytosis, pinocytosis?

The cell membrane is the structure that covers the outside of the cell. It is also called cytolemma or plasmolemma.

This formation is built from a bilipid layer (bilayer) with proteins embedded in it. The carbohydrates that make up the plasmalemma are in a bound state.

The distribution of the main components of the plasmalemma is as follows: more than half of the chemical composition falls on proteins, a quarter is occupied by phospholipids, and a tenth is cholesterol.

Cell membrane and its types

The cell membrane is a thin film, which is based on layers of lipoproteins and proteins.

By localization, membrane organelles are distinguished, which have some features in plant and animal cells:

• mitochondria;
• core;
• endoplasmic reticulum;
• Golgi complex;
• lysosomes;
• chloroplasts (in plant cells).

There is also an inner and outer (plasmolemma) cell membrane.

The structure of the cell membrane

The cell membrane contains carbohydrates that cover it in the form of a glycocalyx. This is a supra-membrane structure that performs a barrier function. The proteins located here are in a free state. Unbound proteins are involved in enzymatic reactions, providing extracellular breakdown of substances.

Proteins of the cytoplasmic membrane are represented by glycoproteins. According to the chemical composition, proteins are isolated that are completely included in the lipid layer (throughout) - integral proteins. Also peripheral, not reaching one of the surfaces of the plasmalemma.

The former function as receptors, binding to neurotransmitters, hormones, and other substances. Insertion proteins are necessary for the construction of ion channels through which ions and hydrophilic substrates are transported. The latter are enzymes that catalyze intracellular reactions.

Basic properties of the plasma membrane

The lipid bilayer prevents the penetration of water. Lipids are hydrophobic compounds present in the cell as phospholipids. The phosphate group is turned outward and consists of two layers: the outer one, directed to the extracellular environment, and the inner one, delimiting the intracellular contents.

Water-soluble areas are called hydrophilic heads. The fatty acid sites are directed inside the cell, in the form of hydrophobic tails. The hydrophobic part interacts with neighboring lipids, which ensures their attachment to each other. The double layer has selective permeability in different areas.

So, in the middle, the membrane is impermeable to glucose and urea, hydrophobic substances pass freely here: carbon dioxide, oxygen, alcohol. Cholesterol is important, the content of the latter determines the viscosity of the plasma membrane.

Functions of the outer membrane of the cell

The characteristics of the functions are briefly listed in the table:

What is the importance of the cell membrane

The cell membrane is involved in maintaining homeostasis due to the high selectivity of substances entering and leaving the cell (in biology this is called selective permeability).

Outgrowths of the plasmolemma divide the cell into compartments (compartments) responsible for performing certain functions. Specifically arranged membranes, corresponding to the fluid-mosaic scheme, ensure the integrity of the cell.