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Why is the membrane called the universal structural unit of the cell? Cell membrane: its structure and function. Functions of the outer membrane of the cell

Short description:

Sazonov V.F. 1_1 The structure of the cell membrane [Electronic resource] // Kinesiologist, 2009-2018: [site]. Updated date: 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 O lemma)

Definition of the concept

The cell membrane (synonyms: plasmalemma, plasmolemma, cytoplasmic membrane, biomembrane) is a triple lipoprotein (ie "fat-protein") membrane that separates the cell from the environment and carries out controlled exchange and communication between the cell and its environment.

The main thing in this definition is not that the membrane separates the cell from the environment, but precisely that it connects cage with the environment. The membrane is active the structure of the cell, it is constantly working.

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

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 creature 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 to 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 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 we can say that "the membrane turns the cell into an" electric battery "by controlling ionic flows"... Details: .

9. Changes in electrical and chemical potential.

10. Irritability. Special molecular receptors located on the membrane can bind with signaling (control) substances, as a result of which the state of the membrane and the entire cell can change. Molecular receptors trigger biochemical reactions in response to the combination of ligands (control substances) with them. It is important to note that the signaling substance acts on the receptor from the outside, and the changes continue inside the cell. It turns out that the membrane transmitted information from the environment to the internal environment of the cell.

11. Catalytic enzymatic activity. Enzymes can be embedded in the membrane or connected to 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, invagination into the cell.

13. Ability to form contacts with other cell membranes.

14. Adhesion is the ability to adhere to solid surfaces.

A short list of membrane properties

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

Membrane functions

1. Incomplete isolation of the 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 intercellular environment. This is due to such a 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, the inside of 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 controlling substances (hormones, mediators, modulators) and trigger biochemical reactions in the cell, leading to various changes in the functioning of the cell or in its structures.

Video:Cell membrane structure

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

Membrane structure

The cell membrane has a versatile three-layer structure. Its middle fat layer is continuous, and the upper and lower protein layers cover it in the form of a mosaic of separate protein areas. The fatty layer is the basis that ensures the isolation of the cell from the environment, isolating it from the environment. By itself, it very poorly permeates water-soluble substances, but easily permits fat-soluble substances. Therefore, the permeability of the membrane for water-soluble substances (for example, ions) must be provided with special protein structures - and.

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

Separate image of the inner lipid (fat) layer of the cell membrane, permeated with integral embedded proteins. Top and bottom protein layers removed so as not to interfere with viewing the lipid bilayer

Picture above: Incomplete schematic representation of the cell membrane (cell wall) as shown on Wikipedia.

Note that the outer and inner protein layers have been removed from the membrane 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-layer: protein-fat-protein ... So it actually looks like a sandwich of two protein "slices of bread" with a thick layer of "butter" in the middle, i.e. has a three-layer structure, not a two-layer one.

In this figure, small blue and white globules correspond to hydrophilic (wettable) lipid "heads", and the "strings" attached to them correspond to hydrophobic (non-wettable) "tails". Of the proteins, only integral end-to-end membrane proteins (red globules and yellow helices) are shown. The yellow oval dots inside the membrane are cholesterol molecules. The yellow-green bead chains 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 sticking out of it.

Alive is a small "protein-fat pouch" filled with a semi-liquid jelly-like content, which is permeated with films and tubes.

The walls of this sac are formed by a double fatty (lipid) film covered with proteins from the inside and outside - the cell membrane. Therefore, the membrane is said to have three-layer structure : protein-fat-protein... There are also many similar fatty membranes inside the cell, which divide its internal space into compartments. Cell 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 is not a real, but an artificial model of a piece of a biological membrane: this is an instant snapshot of a fatty phospholipid bilayer (i.e., a double layer) in the process of its molecular dynamics modeling. The calculated cell of the model is shown - 96 PC molecules ( f osfatidil NS olina) and 2304 water molecules, a total of 20544 atoms.

On the right is a visual model of a single molecule of that very lipid, from which the membrane lipid bilayer is assembled. At the top, it has a hydrophilic (water-loving) head, and at the bottom, two hydrophobic (water-afraid) tails. This lipid has a simple name: 1-steroyl-2-docosahexaenoyl-Sn-glycero-3-phosphatidylcholine (18: 0/22: 6 (n-3) cis PC), but you don't need to memorize it unless you plan to bring your teacher to a swoon with the depth of your knowledge.

A more precise scientific definition of a cell can be given:

Is a limited by an active membrane, an ordered, structured heterogeneous system of biopolymers participating in a single set of metabolic, energy and information processes, and also carrying out the maintenance and reproduction of the entire system as a whole.

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

The cell membrane (plasmalemma, plasmolemma) is an active membrane that separates the cell from the environment and connects it with the environment. © Sazonov V.F., 2016.

From this definition of a membrane, it follows that it does not just restrict the cell, but is actively working linking it to its environment.

The fat of which the membranes are composed is special, therefore its molecules are usually called not just fat, but "Lipids", "phospholipids", "sphingolipids"... The membrane film is double, that is, it consists of two films adhered to each other. Therefore, in textbooks they write that the basis of the cell membrane consists of two lipid layers (or of " bilayer", ie a double layer). For each separate lipid layer, one side can be wetted with water, and the other cannot. So, these films stick to each other precisely with their non-wetting sides.

Membrane of bacteria

The prokaryotic cell membrane of gram-negative bacteria consists of several layers, shown in the figure below.
Coating layers of gram-negative bacteria:
1. Internal three-layer cytoplasmic membrane, which is in contact with the cytoplasm.
2. The cell wall, which is composed of murein.
3. 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-stage structure does not give them an advantage in survival in harsh conditions compared to gram-positive bacteria, which have a less powerful membrane. They just as poorly tolerate high temperatures, acidity and pressure drops.

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

Video lecture:Membrane as a cell border. A. Ilyaskin

The importance of membrane ion channels

It is easy to understand that only fat-soluble substances can enter the cell through the fatty membrane. 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 be pulled back. What to do? A way out in nature was found: it is necessary to make special protein transport structures and stretch them through the membrane. This is how channels for the passage of fat-insoluble substances are obtained - ion channels of the cell membrane.

So, to give its membrane additional properties of permeability to polar molecules (ions and water), the cell synthesizes special proteins in the cytoplasm, which are then incorporated into the membrane. They are of two types: transporter proteins (for example, transport ATPases) and channel forming proteins (channel makers). These proteins are incorporated into the double fat 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 cannot otherwise pass through the fatty membrane film.

In general, proteins built into the membrane are also called integral, precisely because they seem to be included in the composition of the membrane and penetrate it through and through. Other proteins, not integral, form, as it were, islands that “float” along the membrane surface: either along its outer surface or along its inner surface. 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-layer:

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 with which signaling agents bind.

2. Special protein structures are built into the membrane, ensuring its permeability for ions or other substances. Do not forget that in some places the sea of ​​fat is permeated with integral proteins through and through. And it is the integral proteins that form special transport structures cell membrane (see section 1_2 Membrane transport mechanisms). Through them, substances enter the cell, and are also removed from the cell to the outside.

3. On either side of the membrane (outer and inner), as well as inside the membrane, enzyme proteins can be located, which affect both the state of the membrane itself and the life of the entire cell.

So the cell membrane is an active changeable structure that actively works in the interests of the entire cell and connects it with the outside world, and is not just a "protective shell". This is the most important thing to know about the cell membrane.

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

Video:Introduction to the biophysics of the cell membrane: The structure of membranes 1 (Vladimirov Yu.A.)

Video:History, structure and function of the cell membrane: Membrane structure 2 (Vladimirov Yu.A.)

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

A cell membrane is an ultrathin film on the surface of a cell or cell organelle, consisting of a bimolecular lipid layer with embedded proteins and polysaccharides.

Membrane functions:

  • · Barrier - provides a regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides that are harmful to the cell. Selective permeability means that the membrane's permeability to various atoms or molecules depends on their size, electrical charge, and chemical properties. Selective permeability ensures the separation of the cell and cell compartments from the environment and supply them with the necessary substances.
  • · Transport - substances are transported through the membrane into and out of the cell. Transport through membranes provides: delivery of nutrients, removal of end metabolic products, secretion of various substances, creation of ionic gradients, maintenance of optimal pH and ion concentration in the cell, which 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 because of their large size), but necessary for the cell, can penetrate the membrane through special carrier proteins (transporters) and channel proteins or by endocytosis. With passive transport, substances cross the lipid bilayer without energy consumption along the concentration gradient by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance to pass through the membrane. This molecule can have a channel that allows only one type of substance to pass through. Active transport requires energy consumption, since it occurs against the concentration gradient. There are special pump proteins on the membrane, including ATPase, which actively pumps potassium ions (K +) into the cell and pumps out sodium ions (Na +) from it.
  • Matrix - provides a certain mutual arrangement 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 an important 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 are also involved;
  • Receptor - some proteins in the membrane are receptors (molecules through 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 that 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 carrying out 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 labeling - there are antigens on the membrane that act as markers - "labels" that allow you to identify the cell. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of "antennas". Due to the myriad of side chain configurations, 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, during the formation of organs and tissues. This also allows the immune system to recognize foreign antigens.

Some protein molecules freely diffuse in the plane of the lipid layer; in the normal state, parts of protein molecules emerging on opposite sides of the cell membrane do not change their position.

The specific morphology of cell membranes determines their electrical characteristics, among which the most important are capacitance and conductivity.

Capacitive properties are mainly determined by the phospholipid bilayer, which is impermeable to hydrated ions and at the same time thin enough (about 5 nm) to ensure efficient separation and accumulation of charges, and electrostatic interaction of cations and anions. In addition, the capacitive properties of cell membranes are one of the reasons that determine the temporal characteristics of electrical processes occurring on cell membranes.

Conductivity (g) is the reciprocal of electrical resistance and is equal to the ratio of the total transmembrane current for a given ion to the value that caused its transmembrane potential difference.

Various substances can diffuse through the phospholipid bilayer, and the degree of permeability (P), that is, the ability of the cell membrane to pass these substances, depends on the difference in the concentration of the diffusing substance on both sides of the membrane, its solubility in lipids and the properties of the cell membrane. The diffusion rate for charged ions in a constant field in the membrane is determined by the mobility of the ions, the thickness of the membrane, and the distribution of ions in the membrane. For non-electrolytes, the membrane permeability does not affect its conductivity, since non-electrolytes do not carry charges, i.e., they cannot carry an electric current.

The conductivity of a membrane is a measure of its ionic permeability. An increase in conductivity indicates an increase in the number of ions passing through the membrane.

An important property of biological membranes is fluidity. All cell membranes are mobile fluid structures: most of their constituent lipid and protein molecules are able to move fairly quickly in the membrane plane

The membrane is a superfine structure that forms the surfaces of organelles and the cell as a whole. All membranes have a similar structure and are linked into one system.

Chemical composition

Cell membranes are chemically homogeneous and consist of proteins and lipids of various groups:

  • phospholipids;
  • galactolipids;
  • sulfolipids.

They also include nucleic acids, polysaccharides and other substances.

Physical properties

At normal temperatures, the membranes are in a liquid crystal state and constantly fluctuate. Their viscosity is close to that of vegetable oil.

The membrane is recoverable, durable, elastic and porous. The thickness of the membranes is 7-14 nm.

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The membrane is impermeable for large molecules. Small molecules and ions can pass through the pores and the membrane itself under the influence of concentration differences on different sides of the membrane, as well as with the help of transport proteins.

Model

Typically, the structure of membranes is described using a fluid-mosaic model. The membrane has a framework - two rows of lipid molecules, tightly like bricks adjacent to each other.

Rice. 1. Biological membrane of the sandwich type.

On both sides, the surface of lipids is covered with proteins. The mosaic pattern is formed by protein molecules unevenly distributed on the membrane surface.

According to the degree of immersion in the bilipid layer, protein molecules are divided into three groups:

  • transmembrane;
  • submerged;
  • superficial.

Proteins provide the main property of the membrane - its selective permeability to various substances.

Membrane types

All cell membranes by localization can be divided into the following types:

  • outdoor;
  • nuclear;
  • membranes of organelles.

The outer cytoplasmic membrane, or plasmolemma, is the border of the cell. Connecting with the elements of the cytoskeleton, it maintains its shape and size.

Rice. 2. Cytoskeleton.

The nuclear membrane, or karyolemma, is the boundary of the nuclear content. It is built of two membranes, very similar to the outer one. The outer membrane of the nucleus is associated with membranes endoplasmic reticulum(EPS) and, through the pores, with an inner membrane.

The EPS membranes penetrate the entire cytoplasm, forming surfaces on which various substances are synthesized, including membrane proteins.

Organoid membranes

The majority of organelles have a membrane structure.

The walls are built from one membrane:

  • Golgi complex;
  • vacuoles;
  • lysosomes.

Plastids and mitochondria are built from two layers of membranes. Their outer membrane is smooth, while the inner membrane forms many folds.

The peculiarities of photosynthetic chloroplast membranes are embedded chlorophyll molecules.

Animal cells have a carbohydrate layer on the surface of the outer membrane called the glycocalyx.

Rice. 3. Glycocalyx.

The glycocalyx is most developed in the cells of the intestinal epithelium, where it creates conditions for digestion and protects the plasmolemma.

Table "Structure of the cell membrane"

What have we learned?

We examined the structure and function of the cell membrane. The membrane is a selective (selective) barrier of the cell, nucleus and organelles. The structure of the cell membrane is described by a liquid-mosaic model. According to this model, protein molecules are embedded in a double layer of viscous lipids.

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Cell- self-regulating structural and functional unit of tissues and organs. The cellular theory of the structure of organs and tissues was developed by Schleiden and Schwann in 1839. Later, using electron microscopy and ultracentrifugation, it was possible to elucidate the structure of all the main organelles of animal and plant cells (Fig. 1).

Rice. 1. Scheme of the structure of the cell of animal organisms

The main parts of the cell are the cytoplasm and the nucleus. Each cell is surrounded by a very thin membrane that limits its contents.

The cell membrane is called plasma membrane and is characterized by selective permeability. This property allows essential nutrients and chemical elements penetrate into the cell, and excess products leave it. The plasma membrane consists of two layers of lipid molecules with the inclusion of specific proteins. The main lipids of the membrane are phospholipids. They contain phosphorus, a polar head, and two non-polar tails of long-chain fatty acids. Membrane lipids include cholesterol and cholesterol esters. In accordance with the liquid-mosaic model of structure, membranes contain inclusions of protein and lipid molecules that can mix relative to the bilayer. Each type of membrane of any animal cell is characterized by its own relatively constant lipid composition.

Structurally, membrane proteins are divided into two types: integral and peripheral. Peripheral proteins can be removed from the membrane without destroying it. There are four types of membrane proteins: transport proteins, enzymes, receptors, and structural proteins. Some membrane proteins have enzymatic activity, while others bind certain substances and facilitate their transfer into the cell. Proteins provide several pathways for the movement of substances across membranes: they form large pores, consisting of several protein subunits, which allow water molecules and ions to move between cells; form ion channels specialized for the movement of certain species of ions across the membrane under certain conditions. Structural proteins are associated with the inner lipid layer and provide the cytoskeleton of the cell. The cytoskeleton imparts mechanical strength to the cell membrane. In various membranes, proteins account for 20 to 80% of the mass. Membrane proteins can move freely in the lateral plane.

The membrane also contains carbohydrates, which can covalently bind to lipids or proteins. There are three types of membrane carbohydrates: glycolipids (gangliosides), glycoproteins, and proteoglycans. Most of the membrane lipids are in a liquid state and have a certain fluidity, i.e. the ability to move from one area to another. On the outside of the membrane there are receptor sites that bind various hormones. Other specific membrane regions can recognize and bind some proteins foreign to these cells and a variety of biologically active compounds.

The inner space of the cell is filled with cytoplasm, in which most of the reactions of cell metabolism catalyzed by enzymes take place. The cytoplasm consists of two layers: an internal one, called endoplasm, and a peripheral one - ectoplasm, which has a high viscosity and is devoid of granules. All components of a cell or organelle are located in the cytoplasm. The most important of the cell organelles are endoplasmic reticulum, ribosomes, mitochondria, Golgi apparatus, lysosomes, microfilaments and microtubules, peroxisomes.

Endoplasmic reticulum is a system of interconnected channels and cavities that permeate the entire cytoplasm. It provides transport of substances from the environment and inside cells. The endoplasmic reticulum also serves as a depot for intracellular Ca 2+ ions and serves as the main site for lipid synthesis in the cell.

Ribosomes - microscopic spherical particles with a diameter of 10-25 nm. Ribosomes are freely located in the cytoplasm or attached to the outer surface of the membranes of the endoplasmic reticulum and the nuclear membrane. They interact with messenger and transport RNA, and proteins are synthesized in them. They synthesize proteins that enter the cisterns or the Golgi apparatus, and then are released outside. Ribosomes, freely located in the cytoplasm, synthesize protein for use by the cell itself, and ribosomes associated with the endoplasmic reticulum produce protein that is removed from the cell. Various functional proteins are synthesized in ribosomes: carrier proteins, enzymes, receptors, cytoskeleton proteins.

Golgi apparatus formed by a system of tubules, cisterns and vesicles. It is associated with the endoplasmic reticulum, and biologically active substances received here are stored in a compacted form in secretory vesicles. The latter are constantly separated from the Golgi apparatus, transported to the cell membrane and merged with it, and the substances contained in the vesicles are removed from the cell during exocytosis.

Lysosomes - particles surrounded by a membrane with a size of 0.25-0.8 microns. They contain numerous enzymes involved in the breakdown of proteins, polysaccharides, fats, nucleic acids, bacteria and cells.

Peroxisomes formed from a smooth endoplasmic reticulum, resemble lysosomes and contain enzymes that catalyze the decomposition of hydrogen peroxide, which is broken down under the influence of peroxidases and catalase.

Mitochondria contain outer and inner membranes and are the "power station" of the cell. Mitochondria are round or elongated structures with a double membrane. The inner membrane forms folds protruding into the mitochondria - cristae. They synthesize ATP, oxidize substrates of the Krebs cycle and carry out many biochemical reactions. ATP molecules formed in mitochondria diffuse into all parts of the cell. Mitochondria contain a small amount of DNA, RNA, ribosomes, and with their participation, the renewal and synthesis of new mitochondria occurs.

Microfilaments are thin protein filaments, consisting of myosin and actin, and form the contractile apparatus of the cell. Microfilaments are involved in the formation of folds or protrusions of the cell membrane, as well as in the movement of various structures within cells.

Microtubules form the basis of the cytoskeleton and ensure its strength. The cytoskeleton gives cells a characteristic appearance and shape, serves as an attachment point for intracellular organelles and various bodies. In nerve cells, bundles of microtubules are involved in the transport of substances from the cell body to the ends of axons. With their participation, the functioning of the mitotic spindle is carried out during cell division. They play the role of motor elements in the villi and flagella in eukaryotes.

Core is the basic structure of the cell, is involved in the transmission of hereditary traits and in the synthesis of proteins. The nucleus is surrounded by a nuclear membrane containing many nuclear pores through which various substances are exchanged between the nucleus and the cytoplasm. There is a nucleolus inside it. The important role of the nucleolus in the synthesis of ribosomal RNA and histone proteins has been established. The rest of the nucleus contains chromatin, which consists of DNA, RNA, and a number of specific proteins.

Cell membrane functions

Cell membranes play an important role in the regulation of intracellular and intercellular metabolism. They are selectively permeable. Their specific structure makes it possible to provide barrier, transport and regulatory functions.

Barrier function manifests itself in limiting the penetration of compounds dissolved in water through the membrane. The membrane is impermeable to large protein molecules and organic anions.

Regulatory function membrane consists in the regulation of intracellular metabolism in response to chemical, biological and mechanical influences. Various influences are perceived by special membrane receptors with a subsequent change in the activity of enzymes.

Transport function through biological membranes can be carried out passively (diffusion, filtration, osmosis) or using active transport.

Diffusion - movement of a gas or soluble substance along the concentration and electrochemical gradient... The diffusion rate depends on the permeability of the cell membrane, as well as the concentration gradient for uncharged particles, electrical and concentration gradients for charged particles. Simple diffusion occurs through the lipid bilayer or through channels. Charged particles move according to an electrochemical gradient, while uncharged ones follow a chemical gradient. For example, oxygen, steroid hormones, urea, alcohol, etc., penetrate by simple diffusion through the lipid layer of the membrane. Various ions and particles move through the channels. Ionic channels are formed by proteins and are subdivided into controlled and uncontrolled channels. Depending on the selectivity, a distinction is made between ion-selective ropes that allow only one ion to pass through and channels that do not have selectivity. The channels have a mouth and a selective filter, and the controlled channels also have a gate mechanism.

Facilitated diffusion - a process in which substances are transported across a membrane using special membrane carrier proteins. In this way, amino acids and monosaccharides enter the cell. This type of transport is very fast.

Osmosis - the movement of water through the membrane from a solution with a lower to a solution with a higher osmotic pressure.

Active transport - transport of substances against the concentration gradient using transport ATPases (ion pumps). This transfer takes place with the expenditure of energy.

The Na + / K + -, Ca 2+ - and H + -pumps have been studied to a greater extent. The pumps are located on cell membranes.

A kind of active transport are endocytosis and exocytosis. These mechanisms transport larger substances (proteins, polysaccharides, nucleic acids) that cannot be transported through the channels. This transport is more common in the epithelial cells of the intestine, renal tubules, and vascular endothelium.

At endocytosis, cell membranes form invaginations into the cell, which detach and turn into vesicles. During exocytosis, vesicles with their contents are transferred to the cell membrane and merge with it, and the contents of the vesicles are released into the extracellular environment.

The structure and function of the cell membrane

To understand the processes that ensure the existence of electrical potentials in living cells, first of all, it is necessary to understand the structure of the cell membrane and its properties.

Currently, the most recognized is the liquid-mosaic membrane model proposed by S. Singer and G. Nicholson in 1972.The membrane is based on a double layer of phospholipids (bilayer), the hydrophobic fragments of the molecules of which are immersed in the thickness of the membrane, and the polar hydrophilic groups are oriented outward, those. into the surrounding aquatic environment (Fig. 2).

Membrane proteins are localized on the membrane surface or can be embedded at different depths into the hydrophobic zone. Some proteins permeate the membrane and different hydrophilic groups of the same protein are found on both sides of the cell membrane. The proteins found in the plasma membrane play a very important role: they participate in the formation of ion channels, play the role of membrane pumps and carriers of various substances, and can also perform a receptor function.

The main functions of the cell membrane: barrier, transport, regulatory, catalytic.

The barrier function is to restrict the diffusion of water-soluble compounds through the membrane, which is necessary to protect cells from foreign, toxic substances and to maintain a relatively constant content of various substances inside the cells. Thus, the cell membrane can slow down the diffusion of various substances by 100,000-10,000,000 times.

Rice. 2. Three-dimensional scheme of the liquid-mosaic model of the Singer-Nicholson membrane

Depicted are globular integral proteins embedded in a lipid bilayer. Some proteins are ion channels, others (glycoproteins) contain oligosaccharide side chains involved in cell recognition of each other and in the intercellular tissue. Cholesterol molecules are closely adjacent to the phospholipid heads and fix the adjacent areas of the "tails". The inner parts of the tails of the phospholipid molecule are not limited in their movement and are responsible for the fluidity of the membrane (Bretscher, 1985)

The membrane contains channels through which ions penetrate. Channels are potential dependent and potential independent. Potential gated channels open when the potential difference changes, and potential-independent(hormone-regulated) open when receptors interact with substances. The channels can be opened or closed thanks to the gate. Two types of gates are built into the membrane: activation(deep in the channel) and inactivating(on the channel surface). The gate can be in one of three states:

  • open state (both types of gates are open);
  • closed state (activation gate is closed);
  • inactivation state (inactivation gate closed).

Another characteristic feature of membranes is the ability to carry out the selective transfer of inorganic ions, nutrients, and various metabolic products. Distinguish between systems of passive and active transfer (transport) of substances. Passive transport is carried out through ion channels with or without the help of carrier proteins, and its driving force is the difference in the electrochemical potential of ions between the intra- and extracellular space. The selectivity of ion channels is determined by its geometric parameters and chemical nature groups lining the walls of the canal and its mouth.

Currently, the most well-studied channels are those with selective permeability for Na +, K +, Ca 2+ ions, as well as for water (the so-called aquaporins). The diameter of the ion channels, according to various studies, is 0.5-0.7 nm. The throughput of the channels can vary, 10 7 - 10 8 ions per second can pass through one ion channel.

Active transport takes place with the expenditure of energy and is carried out by the so-called ion pumps. Ion pumps are molecular protein structures built into the membrane and transferring ions towards a higher electrochemical potential.

The pumps are powered by the energy of ATP hydrolysis. Currently, Na + / K + - ATPase, Ca 2+ - ATPase, H + - ATPase, H + / K + - ATPase, Mg 2+ - ATPase are well studied, which provide the movement of Na +, K +, Ca 2+ ions, respectively. , H +, Mg 2+ isolated or conjugated (Na + and K +; H + and K +). The molecular mechanism of active transport is not fully understood.

All living organisms on Earth are composed of cells, and each cell is surrounded by a protective shell - a membrane. However, the functions of the membrane are not limited to protecting organelles and separating one cell from another. The cell membrane is a complex mechanism that is directly involved in reproduction, regeneration, nutrition, respiration and many other important functions of the cell.

The term "cell membrane" has been around for nearly a century. The very word "membrane" translated from Latin means "film". But in the case of a cell membrane, it would be more correct to speak of a set of two films connected in a certain way, and, moreover, different sides of these films have different properties.

The cell membrane (cytolemma, plasmalemma) is a three-layer lipoprotein (fat-protein) membrane that separates each cell from neighboring cells and the environment, and carries out controlled exchange between cells and the environment.

Of decisive importance in this definition is not that the cell membrane separates one cell from another, but that it ensures its interaction with other cells and the environment. The membrane is a very active, constantly working structure of the cell, on which many functions are assigned by nature. From our article you will learn everything about the composition, structure, properties and functions of the cell membrane, as well as the danger that violations in the functioning of cell membranes pose to human health.

History of cell membrane research

In 1925, two German scientists, Gorter and Grendel, were able to conduct a complex experiment on red blood cells of human blood, erythrocytes. With the help of an osmotic shock, the researchers obtained the so-called "shadows" - empty shells of red blood cells, then put them in one pile and measured the surface area. The next step was to calculate the amount of lipids in the cell membrane. With the help of acetone, scientists isolated lipids from the "shadows" and determined that they were just enough for a double continuous layer.

However, during the experiment, two gross mistakes were made:

    The use of acetone does not allow the isolation of absolutely all lipids from the membranes;

    The surface area of ​​the "shadows" was calculated based on dry weight, which is also incorrect.

Since the first error gave a minus in the calculations, and the second - a plus, the overall result turned out to be surprisingly accurate, and German scientists brought the most important discovery to the scientific world - the lipid bilayer of the cell membrane.

In 1935, another pair of researchers, Danielle and Dawson, after long experiments on bilipid films, came to the conclusion about the presence of proteins in cell membranes. There was no other way to explain why these films have such a high surface tension. Scientists presented to the public a schematic model of a cell membrane, similar to a sandwich, where homogeneous lipid-protein layers play the role of slices of bread, and between them, instead of butter, there is a void.

In 1950, with the help of the first electron microscope, Danielle-Dawson's theory was partially confirmed - two layers consisting of lipid and protein heads were clearly visible on micrographs of the cell membrane, and between them a transparent space filled only with tails of lipids and proteins.

In 1960, guided by these data, the American microbiologist J. Robertson developed the theory of the three-layer structure of cell membranes, which for a long time was considered the only correct one. However, as science developed, more and more doubts arose about the homogeneity of these layers. From the point of view of thermodynamics, such a structure is extremely disadvantageous - it would be very difficult for cells to transport substances in and out through the entire "sandwich". In addition, it has been proven that the cell membranes of different tissues have different thicknesses and methods of attachment, which are due to different functions of organs.

In 1972, microbiologists S.D. Singer and G.L. Nicholson was able to explain all the inconsistencies in Robertson's theory with the help of a new, fluid-mosaic model of the cell membrane. Scientists have found that the membrane is heterogeneous, asymmetric, filled with liquid, and its cells are in constant motion. And the proteins that make up it have a different structure and purpose, in addition, they are located in different ways relative to the bilipid layer of the membrane.

The composition of cell membranes contains proteins of three types:

    Peripheral - attached to the surface of the film;

    Semi-integral- partially penetrate into the bilipid layer;

    Integral - completely penetrate the membrane.

Peripheral proteins are associated with the heads of membrane lipids through electrostatic interaction, and they never form a continuous layer, as was previously believed, while semi-integral and integral proteins serve to transport oxygen and nutrients inside the cell, as well as to remove decay products from it, and more. for several important functions, which you will learn about next.



The cell membrane performs following functions:

    Barrier - the permeability of the membrane for different types of molecules is not the same.To bypass the cell membrane, the molecule must have a certain size, Chemical properties and electric charge. Harmful or unsuitable molecules, due to the barrier function of the cell membrane, simply cannot penetrate into the cell. For example, with the help of the peroxis reaction, the membrane protects the cytoplasm from peroxides that are dangerous to it;

    Transport - passive, active, regulated and selective exchange passes through the membrane. Passive metabolism is suitable for fat-soluble substances and gases composed of very small molecules. Such substances penetrate into and out of the cell without the expenditure of energy, freely, by the diffusion method. The active transport function of the cell membrane is activated when necessary but difficult to transport substances need to be transported into or out of the cell. For example, those with a large molecular size, or unable to cross the bilipid layer due to hydrophobicity. Then proteins-pumps begin to work, including ATPase, which is responsible for the absorption of potassium ions into the cell and the ejection of sodium ions from it. Regulated transport is necessary for the functions of secretion and fermentation, for example, when cells produce and secrete hormones or gastric juice. All these substances leave the cells through special channels and in a given volume. And the selective transport function is associated with the very integral proteins that permeate the membrane and serve as a channel for entry and exit of strictly defined types of molecules;

    Matrix - the cell membrane determines and fixes the arrangement of organelles relative to each other (nucleus, mitochondria, chloroplasts) and regulates the interaction between them;

    Mechanical - ensures the restriction of one cell from another, and, at the same time, - the correct connection of cells into a homogeneous tissue and the resistance of organs to deformation;

    Protective - in both plants and animals, the cell membrane serves as the basis for building a protective framework. An example is hard wood, dense skin, thorny thorns. In the animal kingdom, there are also many examples of the protective function of cell membranes - turtle shell, chitinous membrane, hooves and horns;

    Energy - the processes of photosynthesis and cellular respiration would be impossible without the participation of the proteins of the cell membrane, because it is with the help of protein channels that cells exchange energy;

    Receptor - proteins embedded in the cell membrane may have another important function. They serve as receptors through which the cell receives a signal from hormones and neurotransmitters. And this, in turn, is necessary for the conduction of nerve impulses and the normal course of hormonal processes;

    Enzymatic is another important function inherent in some proteins of cell membranes. For example, in the intestinal epithelium with the help of such proteins, digestive enzymes are synthesized;

    Biopotential- the concentration of potassium ions inside the cell is much higher than outside, and the concentration of sodium ions, on the contrary, is higher outside than inside. This explains the potential difference: inside the cell the charge is negative, outside it is positive, which promotes the movement of substances into the cell and outward in any of the three types of metabolism - phagocytosis, pinocytosis and exocytosis;

    Labeling - on the surface of cell membranes there are so-called "labels" - antigens consisting of glycoproteins (proteins with branched oligosaccharide side chains attached to them). Since the side chains can have a huge variety of configurations, each type of cell gets its own unique label, which allows other cells in the body to recognize them by sight and respond to them correctly. That is why, for example, human immune cells, macrophages, easily recognize a stranger who has entered the body (infection, virus) and try to destroy it. The same thing happens with sick, mutated and old cells - the label on their cell membrane changes and the body gets rid of them.

Cellular exchange occurs through membranes, and can be carried out using three main types of reactions:

    Phagocytosis is a cellular process in which phagocyte cells built into the membrane capture and digest solid particles of nutrients. In the human body, phagocytosis is carried out by the membranes of two types of cells: granulocytes (granular leukocytes) and macrophages (immune killer cells);

    Pinocytosis is the process of capture by the surface of the cell membrane of liquid molecules in contact with it. To feed by the type of pinocytosis, the cell grows on its membrane thin fluffy outgrowths in the form of tendrils, which, as it were, surround a droplet of liquid, and a bubble is obtained. First, this bubble protrudes above the surface of the membrane, and then "swallowed" - it hides inside the cell, and its walls merge with the inner surface of the cell membrane. Pinocytosis occurs in almost all living cells;

    Exocytosis is a reverse process in which bubbles with a secretory functional fluid (enzyme, hormone) are formed inside the cell, and it must be somehow removed from the cell into the environment. For this, the bubble first merges with the inner surface of the cell membrane, then protrudes outward, bursts, expels the contents and again merges with the membrane surface, this time from the outside. Exocytosis takes place, for example, in the cells of the intestinal epithelium and adrenal cortex.

Cell membranes contain lipids of three classes:

    Phospholipids;

    Glycolipids;

    Cholesterol.

Phospholipids (a combination of fats and phosphorus) and glycolipids (a combination of fats and carbohydrates), in turn, consist of a hydrophilic head, from which two long hydrophobic tails extend. But cholesterol sometimes takes up the space between these two tails and prevents them from bending, which makes the membranes of some cells rigid. In addition, cholesterol molecules order the structure of cell membranes and prevent the transition of polar molecules from one cell to another.

But the most important component, as you can see from the previous section on the functions of cell membranes, are proteins. Their composition, purpose and location are very diverse, but there is something in common that unites them all: annular lipids are always located around the proteins of cell membranes. These are special fats that are clearly structured, stable, contain more saturated fatty acids, and are released from membranes together with "sponsored" proteins. This is a kind of personal protective shell for proteins, without which they simply would not work.

The structure of the cell membrane is three-layered. In the middle lies a relatively homogeneous liquid bilipid layer, and proteins cover it on both sides like a mosaic, partially penetrating into the thickness. That is, it would be wrong to think that the outer protein layers of cell membranes are continuous. Proteins, in addition to their complex functions, are needed in the membrane in order to pass into the cells and transport out of them those substances that are not able to penetrate the fat layer. For example, potassium and sodium ions. For them, special protein structures are provided - ion channels, which we will discuss in more detail below.

If you look at the cell membrane through a microscope, you can see a layer of lipids formed by the smallest spherical molecules, along which large protein cells float, like in the sea. different shapes... Exactly the same membranes divide the inner space of each cell into compartments in which the nucleus, chloroplasts and mitochondria are comfortably located. If there were no separate "rooms" inside the cell, the organelles would stick to each other and would not be able to perform their functions correctly.

A cell is a set of organelles structured and delimited by membranes, which participates in a complex of energy, metabolic, informational and reproductive processes that ensure the vital activity of the organism.

As you can see from this definition, the membrane is the most important functional component of any cell. Its significance is as great as the significance of the nucleus, mitochondria and other cellular organelles. And the unique properties of the membrane are due to its structure: it consists of two films, stuck together in a special way. Phospholipid molecules in the membrane are located with hydrophilic heads outward, and hydrophobic tails inward. Therefore, one side of the film is wetted with water, while the other is not. So, these films are connected to each other with non-wetted sides inward, forming a bilipid layer surrounded by protein molecules. This is the very "sandwich" structure of the cell membrane.

Ionic channels of cell membranes

Let us consider in more detail the principle of operation of ion channels. What are they needed for? The fact is that only fat-soluble substances can freely penetrate through the lipid membrane - these are gases, alcohols and fats themselves. For example, oxygen and carbon dioxide are constantly exchanged in red blood cells, and for this our body does not have to resort to any additional tricks. But what about when it becomes necessary to transport aqueous solutions such as sodium and potassium salts through the cell membrane?

It would be impossible to pave a path in the bilipid layer for such substances, since the holes would immediately tighten and stick back together, such is the structure of any adipose tissue. But nature, as always, found a way out of the situation and created special protein transport structures.

There are two types of conductive proteins:

    Conveyors - semi-integral protein pumps;

    Channel-formers are integral proteins.

Proteins of the first type are partially immersed in the bilipid layer of the cell membrane, and look out with their head, and in the presence of the necessary substance they begin to behave like a pump: they attract the molecule and suck it into the cell. And proteins of the second type, integral, have an elongated shape and are located perpendicular to the bilipid layer of the cell membrane, penetrating it through and through. Along them, as through tunnels, substances that are unable to pass through fat move into and out of the cell. It is through the ion channels that potassium ions penetrate into the cell and accumulate in it, while sodium ions, on the contrary, are removed outside. There is a difference in electrical potentials, which is so necessary for the proper functioning of all cells in our body.

The most important conclusions about the structure and function of cell membranes


A theory always looks interesting and promising if it can be put to good use in practice. The discovery of the structure and functions of the cell membranes of the human body allowed scientists to make a real breakthrough in science in general, and in medicine in particular. It is no coincidence that we dwelt in such detail on ion channels, because it is here that the answer to one of the most important questions of our time lies: why do people more and more often fall ill with oncology?

Cancer claims about 17 million lives worldwide every year and is the fourth most common cause of all deaths. According to the WHO, the incidence of cancer is steadily increasing, and by the end of 2020 may reach 25 million per year.

What explains this epidemic of cancer, and what does the function of cell membranes have to do with it? You will say: the reason is in a bad environmental situation, improper diet, bad habits and severe heredity. And, of course, you will be right, but if we talk about the problem in more detail, then the reason is the acidity of the human body. The above negative factors lead to disruption of cell membranes, inhibit respiration and nutrition.

Where there should be a plus, a minus is formed, and the cell cannot function normally. But cancer cells do not need either oxygen or an alkaline environment - they are able to use an anaerobic type of nutrition. Therefore, under conditions of oxygen starvation and an off-scale pH level, healthy cells mutate, wishing to adapt to environment, and become cancer cells. This is how a person gets sick with oncology. To avoid this, you just need to consume a sufficient amount of clean water daily, and give up carcinogens in food. But, as a rule, people are well aware of harmful products and the need for high-quality water, and do nothing - they hope that trouble will bypass them.

Knowing the features of the structure and functions of the cell membranes of different cells, doctors can use this information to provide targeted, targeted therapeutic effects on the body. Many modern medications getting into our body, they are looking for the right “target”, which can be ion channels, enzymes, receptors and biomarkers of cell membranes. This method of treatment allows you to achieve better results with minimal side effects.

When they enter the bloodstream, antibiotics of the last generation do not kill all cells in a row, but they are looking for the cells of the pathogen, focusing on markers in its cell membranes. The newest anti-migraine drugs, triptans, constrict only the inflamed blood vessels of the brain, with almost no effect on the heart and peripheral circulatory system. And they recognize the necessary vessels precisely by the proteins of their cell membranes. There are many such examples, so it is safe to say that knowledge about the structure and functions of cell membranes underlies the development of modern medical science, and saves millions of lives every year.


Education: Moscow Medical Institute. IM Sechenov, specialty - "General Medicine" in 1991, in 1993 "Occupational Diseases", in 1996 "Therapy".