The cell membrane who has it. Cell membrane functions. What is the function of the plasma membrane in the cell?

The cell membrane is called the plasmalemma or plasma membrane. The main functions of the cell membrane are to maintain the integrity of the cell and to maintain a relationship with the external environment.

Structure

Cell membranes are composed of lipoprotein (fat-protein) structures and are 10 nm thick. The membrane walls are formed by lipids of three classes:

• phospholipids - compounds of phosphorus and fats;
• glycolipids - compounds of lipids and carbohydrates;
• cholesterol (cholesterol) - fatty alcohol.

These substances form a liquid-mosaic structure, consisting of three layers. Phospholipids form two outer layers. They have a hydrophilic head, from which two hydrophobic tails extend. The ponytails are turned towards the inside of the structure, forming an inner layer. When cholesterol is incorporated into the tails of phospholipids, the membrane becomes stiff.

Rice. 1. The structure of the membrane.

Between phospholipids, glycolipids are inserted that perform a receptor function, and proteins of two types:

• peripheral (external, superficial) - are located on the lipid surface, without penetrating deep into the membrane;
• integral - embedded at different levels, can penetrate the entire membrane, only the inner or outer lipid layer;

All proteins differ in their structure and perform different functions. For example, globular protein compounds have a hydrophobic-hydrophilic structure and carry out a transport function.

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Rice. 2. Types of membrane proteins.

Plasmalemma is a fluid structure, because lipids are not related to each other, but are simply lined up in dense rows. Due to this property, the membrane can change its configuration, be mobile and elastic, and also transport substances.

Functions

What functions does the cell membrane perform:

• barrier - separates the contents of the cell from the external environment;
• transport - regulates metabolism;
• enzymatic - carries out enzymatic reactions;
• receptor - recognizes external stimuli.

The most important function is the transport of substances during metabolism. Liquid and solid substances constantly enter the cell from the external environment. Products of exchange come out. All substances pass through the cell membrane. Transport takes place in several ways, which are described in the table.

Rice. 3. Endocytosis and exocytosis.

Active transport of molecules of substances (sodium-potassium pump) is carried out with the help of protein structures built into the membrane, and requires energy consumption in the form of ATP.

What have we learned?

We examined the main functions of the membrane and the methods of transporting substances into the cell and back. The membrane is a lipoprotein structure consisting of three layers. The absence of strong bonds between lipids provides membrane plasticity and allows the transport of substances. Plasmalemma gives the cell a shape, protects it from external influences, and communicates with the environment.

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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. 2.9).

Membrane proteins are localized on the membrane surface or can be embedded at various 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. 2.9.

Depicted are globular integral proteins embedded in a lipid bilayer. Some proteins are ion channels, others (glycoproteins) contain oligosaccharide side chains that are 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. There are two types of gates 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 the chemical nature of the groups lining the walls of the channel and its mouth.

Currently, the best 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 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 ions, respectively. 2+, H +, Mg 2+ isolated or conjugated (Na + and K +; H + and K +). The molecular mechanism of active transport is not fully understood.

It has a thickness of 8-12 nm, so it is impossible to examine it through a light microscope. The structure of the membrane is studied using an electron microscope.

The plasma membrane is formed by two layers of lipids - the bilipid layer, or bilayer. Each molecule consists of a hydrophilic head and a hydrophobic tail, and in biological membranes, lipids are located with their heads outward and tails inward.

Numerous protein molecules are immersed in the bilipid layer. Some of them are located on the surface of the membrane (external or internal), others penetrate the membrane.

Plasma membrane functions

The membrane protects the contents of the cell from damage, maintains the shape of the cell, selectively passes the necessary substances into the cell and removes metabolic products, and also ensures the communication of cells with each other.

The barrier, delimiting function of the membrane is provided by a double layer of lipids. It prevents the contents of the cell from spreading, mixing with the environment or intercellular fluid, and prevents hazardous substances from entering the cell.

A number of the most important functions of the cytoplasmic membrane are carried out due to proteins immersed in it. With the help of receptor proteins, it can perceive various stimuli on its surface. Transport proteins form the thinnest channels through which potassium, calcium, and other ions of small diameter pass into and out of the cell. Proteins-enzymes provide vital processes in the cell itself.

Large food particles, unable to pass through thin membrane channels, enter the cell by phagocytosis or pinocytosis. The general name for these processes is endocytosis.

How endocytosis occurs - the penetration of large food particles into the cell

The food particle comes into contact with the outer membrane of the cell, and an invagination is formed in this place. Then a particle surrounded by a membrane enters the cell, a digestive vacuole is formed, and digestive enzymes penetrate into the formed vesicle.

Blood leukocytes that can capture and digest foreign bacteria are called phagocytes.

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

Plant cells covered over the membrane with a solid layer of the cell wall are incapable of phagocytosis.

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

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.

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 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 of them. 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 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, 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 creation of 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 adipose 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 ones. Therefore, the membrane permeability 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

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

Please 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, the small blue and white globules correspond to the hydrophilic (wettable) lipid "heads", and the "strings" attached to them correspond to the hydrophobic (non-wettable) "tails". Of the proteins, only integral end-to-end membrane proteins (red globules and yellow helices) are shown. The yellow oval dots inside the membrane are cholesterol molecules. The yellow-green 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 semi-liquid jelly-like contents, which are permeated with films and tubes.

The walls of this sac are formed by a double fat (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... Inside the cell, there are also many such fatty membranes that 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 X olina) and 2304 water molecules, a total of 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. At the top it has a hydrophilic (water-loving) head, and at the bottom there are two hydrophobic (water-afraid) tails. This lipid has a simple name: 1-steroyl-2-docosahexaenoyl-Sn-glycero-3-phosphatidylcholine (18: 0/22: 6 (n-3) cis PC), but you don't need to 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 not only restricts the cell, but is 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, 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", i.e. 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 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" over the surface of the membrane: 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, ion channels, enzymes, and 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 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.

Functions of the outer membrane of the cell

The characteristics of the functions are summarized in the table:

Purpose of diffusion membranes

The main purpose of superdiffusion roofing membranes is to provide protection against the penetration of internal and external moisture into the insulating layer. Sources of this moisture can be internal evaporation and precipitation. In addition, the diffusion membrane located in the roofing provides effective conditions for the removal of moisture already accumulated due to various reasons. The superdiffusion membrane can be confidently called one of the most important components of the heat-insulating circuit, since it indirectly contributes to the reduction of thermal energy losses. A thrifty owner of his own home, who knows a lot about savings, will never think about the need or lack thereof when deciding whether to buy and then install a diffusion membrane. Moreover, the cost of this material on the modern building material market can be confidently called purely symbolic.

Properties of biological membranes

1.
Self-assembly ability
after
destructive influences. This property
determined by physical and chemical
features of phospholipid molecules,
which are collected in aqueous solution
together so that the hydrophilic ends
molecules unfold outward, and
hydrophobic - inside. In ready-made
phospholipid layers can be embedded
proteins

Self-assembly ability has
essential at the cellular level

2. Semi-permeability
(selectivity in the transmission of ions
and molecules). Provides maintenance
constancy of ionic and molecular
composition in the cage.

3. Fluidity
membranes.
Membranes are not rigid structures,
they constantly fluctuate due to
rotational and oscillatory movements
molecules of lipids and proteins. This ensures
high flow rate of enzymatic
and other chemical processes in membranes.

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

What are superdiffusion membranes

A diffusion membrane is a special material with a two-, three- or even four-layer structure, the basis of which is a non-woven canvas. Diffusion membranes are used to protect the insulating layer from vapor penetration into its thickness. Also, diffusion membranes provide excellent protection against water and wind. When creating a roof that fully meets all modern requirements, every developer will certainly come across such a concept as a "roofing pie". In order for the roof to perform all the functions assigned to it during the entire service life, in addition to the main roofing, it is necessary to use some additional materials, which include superdiffusion membranes. Superdiffusion membranes can be used to create a roofing cake in any climatic zone of our country. The role of this additional layer is extremely important, as its presence allows to reduce the strength of adverse effects caused by extreme weather conditions, as well as to level out deficiencies and errors that have arisen during improper installation of the roof.

Cell membrane structure

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

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

The former function as receptors, binding with neurotransmitters, hormones, and other substances. Insert proteins are necessary for the construction of ion channels through which the transport of ions, hydrophilic substrates, is carried out. The latter are enzymes that catalyze intracellular reactions.

Benefits of Using Super Diffusion Membranes

The owner of a private house who decides to use superdiffusion membranes in the construction of a roofing cake, in comparison with homeowners using traditional technologies, will receive a number of undeniable advantages, among which the following are the main ones:

• The use of superdiffusion membranes allows one film to replace two, such as hydro and wind protection. The presence of a membrane allows the construction of a structure without a ventilation gap.
• Laying superdiffusion membranes is allowed directly on the surface of any coating, which makes it possible to lay thermal insulation with a thicker layer in comparison with traditional technologies. As a result, the homeowner receives enhanced thermal insulation.
• The use of superdiffusion membranes makes it possible to extend the service life of the insulation material and wooden roof structures. At the same time, wooden roof elements can be installed without preliminary treatment with special chemical compounds.
• The use of super diffusion membranes in the course of creating a roofing cake significantly reduces installation time and associated costs.

Basic properties of the plasma membrane

The lipid bilayer prevents water penetration. Lipids are hydrophobic compounds represented in the cell by phospholipids. The phosphate group faces outward and consists of two layers: the outer, directed into the extracellular environment, and the inner, delimiting the intracellular contents.

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

So, in the middle, the membrane is impermeable to glucose and urea, hydrophobic substances pass freely here: carbon dioxide, oxygen, alcohol

Cholesterol is important, the content of the latter determines the viscosity of the plasmolemma