Among The main functions of the cell membrane can be distinguished as barrier, transport, enzymatic and receptor. The cell (biological) membrane (aka 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 "capture" various chemical and physical signals.

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

In the evolution of life on Earth, a cell in general could form only after the appearance of a membrane that separated and stabilized the internal contents, preventing it 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 the transport.

Small molecules are able to pass through the plasmalemma without any "helpers", along the concentration gradient, i.e. from a region with a high concentration of a given substance to a region with a low concentration. This is the case, for example, for the gases involved in respiration. Oxygen and carbon dioxide diffuse through the cell membrane in the direction where their concentration is currently lower.

Since the membrane is mostly hydrophobic (due to the double lipid 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-penetrating) proteins often operate on the principle of opening and closing channels. When a molecule approaches such a protein, it connects to it, and the channel opens. This substance or another passes through the protein channel, after which its conformation changes, and the channel closes for this substance, but may open for the passage of another. The sodium-potassium pump works according to this principle, pumping potassium ions into the cell and pumping sodium ions out of it.

Enzymatic function of the cell membrane to a greater extent implemented on the membranes of cell organelles. Most of the proteins synthesized in the cell perform an enzymatic function. Sitting down on the membrane in a certain order, they organize a conveyor when the reaction product catalyzed by one enzyme protein passes to the next one. Such a "pipeline" stabilizes the 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 the same in all organisms and in different membrane cell structures), their chemical composition may still differ. There are more liquid and more solid, some have more certain proteins, others less. In addition, different sides (inner and outer) of the same membrane also differ.

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

If, for example, a hormone binds to 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 penetrating the membrane. At the next stage, various biochemical reactions are started or stopped in the cell, i.e., its metabolism changes, and the cellular response to the “irritant” begins.

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

The basic structural unit of a living organism is a cell, which is a differentiated section of the cytoplasm surrounded by a cell membrane. In view of the fact that the cell performs many important functions, such as reproduction, nutrition, movement, the shell must be plastic and dense.

History of the discovery and research of the cell membrane

In 1925, Grendel and Gorder made a successful experiment to identify the "shadows" of erythrocytes, or empty shells. Despite several gross mistakes made, scientists discovered the lipid bilayer. Their work was continued by Danielli, Dawson in 1935, Robertson in 1960. As a result of many years of work and the accumulation of arguments in 1972, Singer and Nicholson created a fluid mosaic model of the structure of the membrane. Further experiments and studies confirmed the works of scientists.

Meaning

What is a cell membrane? This word began to be used more than a hundred years ago, translated from Latin it means "film", "skin". So designate the border of the cell, which is a natural barrier between the internal contents and the external environment. The structure of the cell membrane suggests semi-permeability, due to which moisture and nutrients and decay products can freely pass through it. This shell can be called the main structural component of the organization of the cell.

Consider the main functions of the cell membrane

1. Separates the internal contents of the cell and the components of the external environment.

2. Helps maintain a constant chemical composition of the cell.

3. Regulates the correct metabolism.

4. Provides interconnection between cells.

5. Recognizes signals.

6. Protection function.

"Plasma Shell"

The outer cell membrane, also called the plasma membrane, is an ultramicroscopic film that is five to seven nanometers thick. It consists mainly of protein compounds, phospholide, water. The film is elastic, easily absorbs water, and also quickly restores its integrity after damage.

Differs in a universal structure. This membrane occupies a boundary position, participates in the process of selective permeability, excretion of decay products, synthesizes them. The relationship with the "neighbors" and the reliable protection of the internal contents from damage makes it an important component in such a matter as the structure of the cell. The cell membrane of animal organisms sometimes turns out to be covered with the thinnest layer - glycocalyx, which includes proteins and polysaccharides. Plant cells outside the membrane are protected by a cell wall that acts as a support and maintains shape. The main component of its composition is fiber (cellulose) - a polysaccharide that is insoluble in water.

Thus, the outer cell membrane performs the function of repair, protection and interaction with other cells.

The structure of the cell membrane

The thickness of this movable shell varies from six to ten nanometers. The cell membrane of a cell has a special composition, the basis of which is the lipid bilayer. The hydrophobic tails, which are inert to water, are located on the inside, while the hydrophilic heads, which interact with water, are turned outward. Each lipid is a phospholipid, which is the result of the interaction of substances such as glycerol and sphingosine. The lipid scaffold is closely surrounded by proteins, which are located in a non-continuous layer. Some of them are immersed in the lipid layer, the rest pass through it. As a result, water-permeable areas are formed. The functions performed by these proteins are different. Some of them are enzymes, the rest are transport proteins that carry various substances from the external environment to the cytoplasm and vice versa.

The cell membrane is permeated through and closely connected with integral proteins, while the connection with peripheral ones is less strong. These proteins perform an important function, which is to maintain the structure of the membrane, receive and convert signals from the environment, transport substances, and catalyze reactions that occur on membranes.

Compound

The basis of the cell membrane is a bimolecular layer. Due to its continuity, the cell has barrier and mechanical properties. At different stages of life, this bilayer can be disrupted. As a result, structural defects of through hydrophilic pores are formed. In this case, absolutely all functions of such a component as a cell membrane can change. In this case, the nucleus may suffer from external influences.

Properties

The cell membrane of a cell has interesting features. Due to its fluidity, this shell is not a rigid structure, and the bulk of the proteins and lipids that make up its composition move freely on the plane of the membrane.

In general, the cell membrane is asymmetric, so the composition of the protein and lipid layers is different. Plasma membranes in animal cells have a glycoprotein layer on their outer side, which performs receptor and signal functions, and also plays an important role in the process of combining cells into tissue. The cell membrane is polar, that is, the charge on the outside is positive, and on the inside it is negative. In addition to all of the above, the cell membrane has selective insight.

This means that in addition to water, only a certain group of molecules and ions of dissolved substances are allowed into the cell. The concentration of a substance such as sodium in most cells is much lower than in the external environment. For potassium ions, a different ratio is characteristic: their number in the cell is much higher than in the environment. In this regard, sodium ions tend to penetrate the cell membrane, and potassium ions tend to be released outside. Under these circumstances, the membrane activates a special system that performs a “pumping” role, leveling the concentration of substances: sodium ions are pumped out to the cell surface, and potassium ions are pumped inward. This feature is included in the most important functions of the cell membrane.

This tendency of sodium and potassium ions to move inward from the surface plays a large role in the transport of sugar and amino acids into the cell. In the process of actively removing sodium ions from the cell, the membrane creates conditions for new inflows of glucose and amino acids inside. On the contrary, in the process of transferring potassium ions into the cell, the number of "transporters" of decay products from inside the cell to the external environment is replenished.

How is the cell nourished through the cell membrane?

Many cells take in substances through processes such as phagocytosis and pinocytosis. In the first variant, a small recess is created by a flexible outer membrane, in which the captured particle is located. Then the diameter of the recess becomes larger until the surrounded particle enters the cell cytoplasm. Through phagocytosis, some protozoa, such as amoeba, as well as blood cells - leukocytes and phagocytes, are fed. Similarly, cells absorb fluid that contains the necessary nutrients. This phenomenon is called pinocytosis.

The outer membrane is closely connected to the endoplasmic reticulum of the cell.

In many types of basic tissue components, protrusions, folds, and microvilli are located on the surface of the membrane. Plant cells on the outside of this shell are covered with another one, thick and clearly visible under a microscope. The fiber they are made of helps form the support for plant tissues such as wood. Animal cells also have a number of external structures that sit on top of the cell membrane. They are exclusively protective in nature, an example of this is the chitin contained in the integumentary cells of insects.

In addition to the cell membrane, there is an intracellular membrane. Its function is to divide the cell into several specialized closed compartments - compartments or organelles, where a certain environment must be maintained.

Thus, it is impossible to overestimate the role of such a component of the basic unit of a living organism as a cell membrane. The structure and functions imply a significant expansion of the total cell surface area, improvement of metabolic processes. This molecular structure consists of proteins and lipids. Separating the cell from the external environment, the membrane ensures its integrity. With its help, intercellular bonds are maintained at a sufficiently strong level, forming tissues. In this regard, we can conclude that one of the most important roles in the cell is played by the cell membrane. The structure and functions performed by it are radically different in different cells, depending on their purpose. Through these features, a variety of physiological activity of cell membranes and their roles in the existence of cells and tissues is achieved.

Short description:

Sazonov V.F. 1_1 The structure of the cell membrane [Electronic resource] // Kinesiologist, 2009-2018: [website]. Date of update: 06.02.2018..__.201_). _The structure and functioning of the cell membrane is described (synonyms: plasmalemma, plasmolemma, biomembrane, cell membrane, outer cell membrane, cell membrane, cytoplasmic membrane). This initial information is necessary both for cytology and for understanding the processes of nervous activity: nervous excitation, inhibition, the work of synapses and sensory receptors.

cell membrane (plasma a lemma or plasma about lemma)

Concept definition

The cell membrane (synonyms: plasmalemma, plasmolemma, cytoplasmic membrane, biomembrane) is a triple lipoprotein (i.e. "fat-protein") membrane that separates the cell from the environment and carries out a 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 just that it connects cell with the environment. The membrane is active structure of the cell, it is constantly working.

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

Figurative representation of the membrane

To me, the cell membrane appears as a lattice fence with many doors in it, which surrounds a certain territory. Any small living creatures can freely move back and forth through this fence. But larger visitors can only enter through the doors, and even then not all. Different visitors have keys only to their own doors, and they cannot pass 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. For this, 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 the membrane is its permeability with respect to various substances.

6. Phagocytosis and pinocytosis.

7. Exocytosis.

8. The presence of electrical and chemical potentials, more precisely, the potential difference between the inner and outer sides of the membrane. Figuratively, one can say that "the membrane turns the cell into an "electric battery" by controlling ion flows". Details: .

9. Changes in electrical and chemical potential.

10. Irritability. Special molecular receptors located on the membrane can connect with signal (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 combination of ligands (control substances) with them. It is important to note that the signaling substance acts on the receptor from the outside, while the changes continue inside the cell. It turns out that the membrane transmitted information from the environment to the internal environment of the cell.

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

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 solid surfaces.

Brief list of membrane properties

• Permeability.
• Endocytosis, exocytosis, transcytosis.
• Potentials.
• Irritability.
• enzymatic activity.
• Contacts.
• Adhesion.

Membrane functions

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

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

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

4. Through the membrane is also carried 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 cell or in its structures.

Video:The structure of the cell membrane

Video lecture:Details about the structure of the membrane and transport

Membrane structure

The cell membrane has a universal three-layer structure. Its median fat layer is continuous, and the upper and lower protein layers cover it in the form of a mosaic of individual 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 passes water-soluble substances very poorly, but easily passes fat-soluble ones. Therefore, the permeability of the membrane for water-soluble substances (for example, ions) has to be provided with special protein structures - and.

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

Separate image of the inner lipid (fatty) layer of the cell membrane, permeated with integral embedded proteins. The upper and lower protein layers are removed so as not to interfere with the consideration of the lipid bilayer

Figure above: An incomplete schematic representation of the cell membrane (cell wall) from Wikipedia.

Note that the outer and inner protein layers have been removed from the membrane here so that we can better see the central fatty double lipid layer. In a real cell membrane, large protein "islands" float above and below along 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 "slices of bread" with a thick layer of "butter" in the middle, ie. has a three-layer structure, not a two-layer one.

In this figure, 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. Yellow oval dots inside the membrane are cholesterol molecules Yellow-green chains of beads on the outside of the membrane are oligosaccharide chains that form the glycocalyx. Glycocalyx is like a carbohydrate ("sugar") "fluff" on the membrane, formed by long carbohydrate-protein molecules protruding from it.

Living is a small "protein-fat bag" filled with semi-liquid jelly-like contents, which is penetrated by films and tubes.

The walls of this sac are formed by a double fatty (lipid) film, covered inside and out with proteins - the cell membrane. Therefore, the membrane is said to have three-layer structure : proteins-fats-proteins. Inside the cell there are also many similar fatty membranes that divide its internal space into compartments. Cellular organelles are surrounded by the same membranes: nucleus, mitochondria, chloroplasts. So the membrane is a universal molecular structure inherent in all cells and all living organisms.

On the left - no longer a real, but an artificial model of a piece of a biological membrane: this is an instant snapshot of an adipose phospholipid bilayer (i.e. a double layer) in the process of its molecular dynamics modeling. The calculation cell of the model is shown - 96 PQ molecules ( f osphatidil X oline) and 2304 water molecules, total 20544 atoms.

On the right is a visual model of a single molecule of the same lipid, from which the membrane lipid bilayer is assembled. It has a hydrophilic (water-loving) head at the top, and two hydrophobic (water-fearing) tails at the bottom. 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 memorize it unless you plan to make your teacher swoon with the depth of your knowledge.

You can give a more precise scientific definition of a cell:

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

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

The cell membrane (plasmalemma, plasmolemma) is an active shell 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 does not simply limit the cell, but actively working linking it to 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, i.e. it consists of two films stuck together. Therefore, textbooks write that the base of the cell membrane consists of two lipid layers (or " bilayer", i.e. double layer). For each individual lipid layer, one side can be wetted by water, and the other cannot. So, these films stick together with each other precisely by their non-wetting sides.

bacteria membrane

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

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

Video lecture:The membrane as a cell boundary. A. Ilyaskin

Importance of Membrane Ion Channels

It is easy to understand that only fat-soluble substances can enter the cell through the membrane fatty film. These are fats, alcohols, gases. For example, in erythrocytes, 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 need special holes. But if you just make a hole in the fatty film, then it will immediately tighten 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 how the channels for the passage of fat-insoluble substances are obtained - the ion channels of the cell membrane.

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

In general, proteins embedded in the membrane are also called integral, precisely because they are, as it were, included in the composition of the membrane and penetrate it through and through. Other proteins, not integral, form, as it were, islands that "float" on the surface of the membrane: either along its outer surface or along its inner one. After all, everyone knows that fat is a good lubricant and it is easy to slide on it!

conclusions

1. In general, the membrane is three-layered:

1) the outer layer of protein "islands",

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

3) the inner layer of protein "islands".

But there is also a loose outer layer - the glycocalyx, which is formed by glycoproteins sticking out of the membrane. They are molecular receptors to which signaling controls bind.

2. Special protein structures are built into the membrane, ensuring its permeability to ions or other substances. We must not forget that in some places the sea of ​​fat is permeated through with integral proteins. And it is 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. Enzyme proteins can be located on any side of the membrane (outer and inner), as well as inside the membrane, 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 whole cell and connects it with the outside world, and is not just a "protective shell". This is the most important thing to know about the cell membrane.

In medicine, membrane proteins are often used as “targets” for drugs. Receptors, ion channels, enzymes, transport systems act as such targets. Recently, in addition to the membrane, genes hidden in the cell nucleus have also become targets for drugs.

Video:Introduction to Cell Membrane Biophysics: Structure of Membrane 1 (Vladimirov Yu.A.)

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

© 2010-2018 Sazonov V.F., © 2010-2016 kineziolog.bodhy.

biological membranes.
The term "membrane" (lat. membrana - skin, film) began to be used more than 100 years ago to refer to the cell boundary, serving, on the one hand, as a barrier between the contents of the cell and the external environment, and on the other, as a semi-permeable partition through which water can pass and some substances. However, the functions of the membrane are not exhausted, since biological membranes form the basis of the structural organization of the cell.
The structure of the membrane. According to this model, the main membrane is a lipid bilayer, in which the hydrophobic tails of the molecules are turned inward and the hydrophilic heads are turned outward. Lipids are represented by phospholipids - derivatives of glycerol or sphingosine. Proteins are attached to the lipid layer. Integral (transmembrane) proteins penetrate the membrane through and are firmly associated with it; peripheral do not penetrate and are associated with the membrane less firmly. Functions of membrane proteins: maintaining the structure of membranes, receiving and converting signals from the environment. environment, transport of certain substances, catalysis of reactions occurring on membranes. the membrane thickness is from 6 to 10 nm.

Membrane properties:
1. Fluidity. The membrane is not a rigid structure; most of its proteins and lipids can move in the plane of the membranes.
2. Asymmetry. The composition of the outer and inner layers of both proteins and lipids is different. In addition, the plasma membranes of animal cells have a layer of glycoproteins on the outside (a glycocalyx that performs signal and receptor functions, and is also important for uniting cells into tissues)
3. Polarity. The outside of the membrane carries a positive charge, while the inside carries a negative charge.
4. Selective permeability. The membranes of living cells pass, in addition to water, only certain molecules and ions of dissolved substances. (The use of the term "semipermeability" in relation to cell membranes is not entirely correct, since this concept implies that the membrane passes only solvent molecules, while retaining all molecules and solute ions.)

The outer cell membrane (plasmalemma) is an ultramicroscopic film 7.5 nm thick, consisting of proteins, phospholipids and water. Elastic film, well wetted by water and quickly recovering integrity after damage. It has a universal structure, those typical of all biological membranes. The boundary position of this membrane, its participation in the processes of selective permeability, pinocytosis, phagocytosis, excretion of excretory products and synthesis, in conjunction with neighboring cells and protecting the cell from damage, makes its role extremely important. Animal cells outside the membrane are sometimes covered with a thin layer consisting of polysaccharides and proteins - the glycocalyx. Plant cells outside the cell membrane have a strong cell wall that creates an external support and maintains the shape of the cell. It consists of fiber (cellulose), a water-insoluble polysaccharide.

biological membranes- the general name of functionally active surface structures that limit cells (cellular or plasma membranes) and intracellular organelles (membranes of mitochondria, nuclei, lysosomes, endoplasmic reticulum, etc.). They contain lipids, proteins, heterogeneous molecules (glycoproteins, glycolipids) and, depending on the function performed, numerous minor components: coenzymes, nucleic acids, antioxidants, carotenoids, inorganic ions, etc.

The coordinated functioning of membrane systems - receptors, enzymes, transport mechanisms - helps maintain cell homeostasis and at the same time quickly respond to changes in the external environment.

To main functions of biological membranes can be attributed:

separation of the cell from the environment and the formation of intracellular compartments (compartments);

control and regulation of the transport of a huge variety of substances through membranes;

participation in providing intercellular interactions, transmission of signals inside the cell;

conversion of the energy of food organic substances into the energy of chemical bonds of ATP molecules.

The molecular organization of the plasma (cell) membrane in all cells is approximately the same: it consists of two layers of lipid molecules with many specific proteins included in it. Some membrane proteins have enzymatic activity, while others bind nutrients from the environment and ensure their transport into the cell through membranes. Membrane proteins are distinguished by the nature of their association with membrane structures. Some proteins, called external or peripheral , loosely bound to the surface of the membrane, others, called internal or integral , are immersed inside the membrane. Peripheral proteins are easily extracted, while integral proteins can only be isolated using detergents or organic solvents. On fig. 4 shows the structure of the plasma membrane.

The outer, or plasma, membranes of many cells, as well as the membranes of intracellular organelles, such as mitochondria, chloroplasts, were isolated in a free form and their molecular composition was studied. All membranes contain polar lipids in an amount ranging from 20 to 80% of its mass, depending on the type of membranes, the rest is mainly accounted for by proteins. So, in the plasma membranes of animal cells, the amount of proteins and lipids, as a rule, is approximately the same; the inner mitochondrial membrane contains about 80% proteins and only 20% lipids, while the myelin membranes of brain cells, on the contrary, contain about 80% lipids and only 20% proteins.

Rice. 4. Structure of the plasma membrane

The lipid part of the membranes is a mixture of various kinds of polar lipids. Polar lipids, which include phosphoglycerolipids, sphingolipids, glycolipids, are not stored in fat cells, but are incorporated into cell membranes, and in strictly defined ratios.

All polar lipids in membranes are constantly renewed during metabolism; under normal conditions, a dynamic stationary state is established in the cell, in which the rate of lipid synthesis is equal to the rate of their decay.

The membranes of animal cells contain mainly phosphoglycerolipids and, to a lesser extent, sphingolipids; triacylglycerols are found only in trace amounts. Some membranes of animal cells, especially the outer plasma membrane, contain significant amounts of cholesterol and its esters (Fig. 5).

Fig.5. Membrane lipids

Currently, the generally accepted model for the structure of membranes is the fluid mosaic model proposed in 1972 by S. Singer and J. Nicholson.

According to her, proteins can be likened to icebergs floating in a lipid sea. As mentioned above, there are 2 types of membrane proteins: integral and peripheral. Integral proteins penetrate the membrane through, they are amphipathic molecules. Peripheral proteins do not penetrate the membrane and are less strongly associated with it. The main continuous part of the membrane, that is, its matrix, is the polar lipid bilayer. At normal cell temperature, the matrix is ​​in a liquid state, which is ensured by a certain ratio between saturated and unsaturated fatty acids in the hydrophobic tails of polar lipids.

The fluid mosaic model also suggests that on the surface of integral proteins located in the membrane there are R-groups of amino acid residues (mainly hydrophobic groups, due to which proteins seem to “dissolve” in the central hydrophobic part of the bilayer). At the same time, on the surface of peripheral, or external proteins, there are mainly hydrophilic R-groups, which are attracted to the hydrophilic charged polar heads of lipids due to electrostatic forces. Integral proteins, and these include enzymes and transport proteins, are active only if they are located inside the hydrophobic part of the bilayer, where they acquire the spatial configuration necessary for the manifestation of activity (Fig. 6). It should be emphasized once again that no covalent bonds are formed between the molecules in the bilayer, nor between the proteins and lipids of the bilayer.

Fig.6. Membrane proteins

Membrane proteins can move freely in the lateral plane. Peripheral proteins literally float on the surface of the bilayer "sea", while integral proteins, like icebergs, are almost completely submerged in the hydrocarbon layer.

Most of the membranes are asymmetric, that is, they have unequal sides. This asymmetry is manifested in the following:

Firstly, the fact that the inner and outer sides of the plasma membranes of bacterial and animal cells differ in the composition of polar lipids. For example, the inner lipid layer of human erythrocyte membranes contains mainly phosphatidylethanolamine and phosphatidylserine, while the outer lipid layer contains phosphatidylcholine and sphingomyelin.

· secondly, some transport systems in membranes act only in one direction. For example, erythrocyte membranes have a transport system (“pump”) that pumps Na + ions from the cell to the environment, and K + ions into the cell due to the energy released during ATP hydrolysis.

Thirdly, the outer surface of the plasma membrane contains a very large number of oligosaccharide groups, which are the heads of glycolipids and oligosaccharide side chains of glycoproteins, while there are practically no oligosaccharide groups on the inner surface of the plasma membrane.

The asymmetry of biological membranes is preserved due to the fact that the transfer of individual phospholipid molecules from one side of the lipid bilayer to the other is very difficult for energy reasons. The polar lipid molecule is able to move freely on its side of the bilayer, but is limited in its ability to jump to the other side.

Lipid mobility depends on the relative content and type of unsaturated fatty acids present. The hydrocarbon nature of fatty acid chains gives the membrane properties of fluidity, mobility. In the presence of cis-unsaturated fatty acids, the cohesive forces between chains are weaker than in the case of saturated fatty acids alone, and lipids retain high mobility even at low temperatures.

On the outer side of the membranes there are specific recognition sites, the function of which is to recognize certain molecular signals. For example, it is through the membrane that some bacteria perceive slight changes in nutrient concentration, which stimulates their movement towards the food source; this phenomenon is called chemotaxis.

The membranes of various cells and intracellular organelles have a certain specificity due to their structure, chemical composition and functions. The following main groups of membranes in eukaryotic organisms are distinguished:

plasma membrane (outer cell membrane, plasmalemma),

the nuclear membrane

The endoplasmic reticulum

membranes of the Golgi apparatus, mitochondria, chloroplasts, myelin sheaths,

excitable membranes.

In prokaryotic organisms, in addition to the plasma membrane, there are intracytoplasmic membrane formations; in heterotrophic prokaryotes, they are called mesosomes. The latter are formed by invagination into the outer cell membrane and in some cases remain in contact with it.

erythrocyte membrane consists of proteins (50%), lipids (40%) and carbohydrates (10%). The main part of carbohydrates (93%) is associated with proteins, the rest - with lipids. In the membrane, lipids are arranged asymmetrically in contrast to the symmetrical arrangement in micelles. For example, cephalin is found predominantly in the inner layer of lipids. This asymmetry is maintained, apparently, due to the transverse movement of phospholipids in the membrane, carried out with the help of membrane proteins and due to the energy of metabolism. In the inner layer of the erythrocyte membrane are mainly sphingomyelin, phosphatidylethanolamine, phosphatidylserine, in the outer layer - phosphatidylcholine. The erythrocyte membrane contains an integral glycoprotein glycophorin, consisting of 131 amino acid residues and penetrating the membrane, and the so-called band 3 protein, consisting of 900 amino acid residues. The carbohydrate components of glycophorin perform a receptor function for influenza viruses, phytohemagglutinins, and a number of hormones. Another integral protein containing few carbohydrates and penetrating the membrane was also found in the erythrocyte membrane. He is called tunnel protein(component a), as it is assumed that it forms a channel for anions. The peripheral protein associated with the inner side of the erythrocyte membrane is spectrin.

Myelin membranes , surrounding axons of neurons, are multilayered, they contain a large amount of lipids (about 80%, half of them are phospholipids). The proteins of these membranes are important for the fixation of membrane salts lying one above the other.

chloroplast membranes. Chloroplasts are covered with a two-layer membrane. The outer membrane bears some resemblance to that of mitochondria. In addition to this surface membrane, chloroplasts have an internal membrane system - lamellae. Lamellae form or flattened vesicles - thylakoids, which, located one above the other, are collected in packs (grana) or form a membrane system of the stroma (stromal lamellae). Lamella gran and stroma on the outer side of the thylakoid membrane are concentrated hydrophilic groups, galacto- and sulfolipids. The phytolic part of the chlorophyll molecule is immersed in the globule and is in contact with the hydrophobic groups of proteins and lipids. The porphyrin nuclei of chlorophyll are mainly localized between the adjoining membranes of the thylakoids of the gran.

Inner (cytoplasmic) membrane of bacteria similar in structure to the inner membranes of chloroplasts and mitochondria. It contains enzymes of the respiratory chain, active transport; enzymes involved in the formation of membrane components. The predominant component of bacterial membranes are proteins: the protein/lipid ratio (by weight) is 3:1. The outer membrane of gram-negative bacteria, compared with the cytoplasmic one, contains a smaller amount of various phospholipids and proteins. Both membranes differ in lipid composition. The outer membrane contains proteins that form pores for the penetration of many low molecular weight substances. A characteristic component of the outer membrane is also a specific lipopolysaccharide. A number of outer membrane proteins serve as receptors for phages.

Virus membrane. Among viruses, membrane structures are characteristic of those containing a nucleocapsid, which consists of a protein and a nucleic acid. This "core" of viruses is surrounded by a membrane (envelope). It also consists of a bilayer of lipids with glycoproteins included in it, located mainly on the surface of the membrane. In a number of viruses (microviruses), 70-80% of all proteins enter the membranes, the remaining proteins are contained in the nucleocapsid.

Thus, cell membranes are very complex structures; their constituent molecular complexes form an ordered two-dimensional mosaic, which gives the membrane surface biological specificity.