What is a membrane briefly. biological membrane. Consider the main functions of the cell membrane

The concept is used in various spheres of life and sciences. And in each of them it has a different meaning. But, one way or another, the use of this term is connected with the meaning of the word itself. Translated from Latin, "membrane" is a membrane.

Various interpretations of the concept

In technology and engineering, this concept is used when talking about a thin film or plate fixed along the contour, as in microphones or pressure gauges.

In biology, a membrane is understood as an elastic molecular structure that is present in every cell and performs the function of protection from environmental influences. It ensures the integrity of the cell and participates in metabolic processes with the outside world.

reverse osmosis membrane

One recent invention is the reverse osmosis module, which is used to purify water. This design is a pipe having a bottom and a lid. And inside this pipe is just the reverse osmosis membrane, the presence of which ensures the production of ultrapure water, freed from various bacteriological contaminants and biological deposits. The fluid purification mechanism is based on minimizing dead spaces where bacteria can accumulate.

These modules are widely used in medicine, and to be more precise, they supply devices for hemodialysis with ultrapure water.

Membranes of hydraulic accumulators and expansion tanks. Their replacement

Hydraulic accumulators and expansion tanks are devices that are used to compensate (volume) inside heating devices.

What is a membrane in this case? This element is the main component of devices of this type. It affects the performance and reliability of the entire system. The shape of the membrane may vary. It is diaphragm, ball and balloon. If the tank has a large volume, then a metal fitting is inserted into the back of the element, in which there is a hole for bleeding air. Depending on the scope of use of the device, the material for the manufacture of the membrane is selected. For example, in the expansion tanks of the heating system, the main criterion is the level of heat resistance and durability. In the case of cold water supply, the choice of membrane material is guided by the criterion of dynamic elasticity.

Unfortunately, there is no material that could be called universal. Therefore, its correct choice is one of the most important conditions for long-term operation of the device and its efficient operation. Most often, the plates are made from natural rubber, synthetic butyl or ethylene propylene rubber.

The membrane is replaced by disconnecting the accumulator or expansion tank from the system. First, the screws that hold the flange and body together are removed. In some devices there is also a mount in the nipple area. After removing it, the membrane can be easily removed. By performing the reverse action, you need to put a new membrane.

Polymer membranes

The term "polymer membrane" is used in several cases. Firstly, it is used, speaking about one of the most modern and advanced roofing materials in terms of practicality. This type of membrane is produced by using an extrusion method, which ensures that there are no voids in the composition of the finished material. The advantages of a polymer product include absolute water resistance, vapor permeability, light weight, strength, low flammability, environmental safety.

The term "polymer membrane" is often used when it comes to the reverse osmosis plates already mentioned above, as well as other types of membranes made from organic polymers. These are micro- and ultrafiltration products, membranes used in nanofiltration. The advantage of polymeric membranes in this context lies in high manufacturability and great possibilities for controlling the properties and structure of the material. In this case, small chemical and technological variations in the manufacturing process are used.

Cell membrane. Cells are the units of all living things

It has long been known that the basic structural unit of a living organism is a cell. It is a differentiated section of the cytoplasm, which is surrounded by a cell membrane. In the process of evolution, as the limits of functionality expanded, it acquired plasticity and subtlety, because the most important processes in the body occur precisely in the cells.

The cell membrane is the boundary of the cell, which is a natural barrier between its internal contents and the environment. The main characteristic feature of the membrane is semi-permeability, which ensures the penetration of moisture and nutrients into the cell and the removal of decay products from it. The cell membrane is the main structural component of the organization of the cell.

Historical facts related to the discovery and study of the cell membrane

In 1925, Grendel and Gorder successfully set up an experiment to identify the "shadows" of erythrocytes. It was they who first discovered the lipid bilayer in the course of experiments. The successors of their work Danielli, Dawson, Robertson, Nicholson in different years worked on the creation of a fluid-mosaic model of the membrane structure. Singscher finally managed to do this in 1972.

Basic Functions of the Cell Membrane

Separation of the internal contents of the cell from the components of the external environment.
Contributing to maintaining the constancy of the chemical composition inside the cell.
Regulation of the balance of metabolism.
Ensuring communication between cells.
Signal function.
protective function.

Plasma shell

What is the membrane, which is called the plasma sheath? This is the outer one, which in its structure is an ultramicroscopic film with a thickness of 5-7 nanometers. It consists of protein compounds, phospholipids, water. The film, being very elastic, absorbs moisture well, and also has the ability to quickly restore its integrity.

The plasma membrane is characterized by a universal structure. Its boundary position causes participation in the process of selective permeability during the removal of decay products from the cell. Interacting with neighboring elements and reliably protecting the contents from damage, the outer membrane is one of the most important components of the cell structure.

The thinnest layer that sometimes covers the cell membrane of living organisms is called the glycocalyx. It is made up of proteins and polysaccharides. And in plant cells, the membrane is protected from above by a special wall, which also performs a supporting function and maintains its shape. It mainly consists of fiber, an insoluble polysaccharide.

Thus, it can be concluded that the main functions of the outer cell membrane are repair, protection, and interaction with neighboring cells.

Structural features

What is a membrane? This is a mobile shell, the width of which is 6-10 nanometers. The basis of its structure is the lipid bilayer and proteins. Carbohydrates are also present in the membrane, but they account for only 10% of the mass of the membranes. But they are necessarily contained in glycolipids or glycoproteins.

If we talk about the ratio of the amount of proteins and lipids, then it can vary greatly. It all depends on the type of fabric. For example, myelin contains about 20% protein, while mitochondria contain about 80%. The composition of the membrane directly affects its density. The higher the protein content, the higher the shell density.

Diversity of lipid functions

Each lipid is inherently a phospholipid, resulting from the interaction of glycerol and sphingosine. Membrane proteins are densely packed around the lipid scaffold, but their layer is not continuous. Some of them are immersed in the lipid layer, while others, as it were, penetrate it. This is due to the presence of areas permeable to water.

It is obvious that the composition of lipids in different membranes is not random, but a clear explanation for this phenomenon has not yet been found. Any given shell can contain up to one hundred different types of lipid molecules. Let us consider the factors that possibly influence the determination of the lipid composition of the membrane molecule.

First, a mixture of lipids must necessarily have the ability to form a stable bilayer in which proteins can function.
Secondly, lipids should contribute to the stabilization of severely deformed membranes, the establishment of contact between membranes, or the binding of certain proteins.
Thirdly, lipids are bioregulators.
Fourth, some lipids are active participants in biosynthesis reactions.

Cell membrane proteins

Proteins perform several functions. Some play the role of enzymes, while others transport various kinds of substances from the environment into the cell and back.

The structure and functions of the membrane are arranged in such a way that they penetrate it through and through, providing a close connection. But peripheral proteins are not closely associated with the membrane. Their function is to maintain the structure of the shell, receive and convert signals from the external environment, and also serve as catalysts for various reactions.

The composition of the membrane is represented primarily by a bimolecular layer. Its continuity ensures the barrier and mechanical properties of the cell. In the process of vital activity, a violation of the structure of the bilayer can occur, which leads to the formation of structural defects through hydrophilic pores. Following this, all functions of the cell membrane can be disrupted.

Shell Properties

The features of the cell membrane are due to its fluidity, due to which it does not have a rigid structure. The lipids that make up its composition can move freely. You can observe the asymmetry of the cell membrane. This is the reason for the difference in the compositions of the protein and lipid layers.

The polarity of the cell membrane has been proven, that is, its outer side has a positive charge, and the inner side has a negative one. It should also be noted that the shell has selective insight. It passes inside, in addition to water, only certain groups of molecules and ions of dissolved substances.

Features of the structure of the cell membrane in plant and animal organisms

The outer membrane and the endoplasmic reticulum of the cell are closely connected. Often the surface of the shell is also covered with various protrusions, folds, microvilli. the cells of animal organisms are covered on the outside with a glycoprotein layer that performs receptor and signal functions. In plant cells, outside this shell is another one, thick and clearly visible under a microscope. The fiber that it is made of is involved in the formation of a support at the origin, for example, wood.

Animal cells also have external structures located outside the membrane. They perform an exclusively protective function. An example is chitin, which is found in the integumentary tissue of insects.

In addition to the cellular, there is an intracellular, or inner membrane. It divides the cell into specialized closed compartments called organelles. They must constantly maintain a certain environment.

Based on the foregoing, we can conclude that the cell membrane, the characteristics of which prove its importance in the functioning of the whole organism, has a complex composition and structure, depending on many internal and external factors. Damage to this film can lead to cell death.

Thus, the structure and functions of the membrane depend on the field of science or industry in which this concept is applied. In any case, this element is a shell or partition, which is flexible and fixed at the edges.

The word membrane has a number of meanings, but in a general sense, the term means a thin flexible septum, membrane or plate that can perform various functions. In this article we will tell you what a membrane is from the point of view of biology and technology.

Membrane in biology

Membrane (or cell membrane) is an elastic molecular structure whose role is to protect the cell from the environment. The cell membrane ensures its integrity, and is also responsible for the exchange processes between the environment and the cell.

The cell membrane consists of proteins and lipids and has a thickness of about 7 nm. Each "brick" of the membrane is responsible for a specific function of a given cellular organ. Lipids in the membrane are represented by three types - phospholipids, glycolipids and cholesterol.

Phospholipids and glycolipids form hydrophobic and hydrophilic sections (hydrophobic sections are directed inside the cell, and hydrophilic sections are directed outward), which regulate the exchange of water and similar molecules between the cell and the environment. Cholesterol stiffens the membrane.

The proteins that make up the membrane can perform many functions, for example, there are transporter proteins that help the necessary substances get into the cell.

Membrane in engineering

The safety membrane is part of the membrane safety device, the task of which is to ensure the necessary discharge of the gas-vapor mixture at a certain pressure. Such devices are used as fuses for process equipment, pipelines, etc.

In the presence of dangerous overloads, the membrane breaks, providing the necessary "discharge", while maintaining the integrity of an expensive and complex technical system.

Look for more interesting concepts in the section.

cell membrane

Image of a cell membrane. Small blue and white balls correspond to the hydrophilic "heads" of lipids, and the lines attached to them correspond to the hydrophobic "tails". The figure shows only integral membrane proteins (red globules and yellow helices). Yellow oval dots inside the membrane - cholesterol molecules Yellow-green chains of beads on the outside of the membrane - oligosaccharide chains that form the glycocalyx

The biological membrane also includes various proteins: integral (penetrating the membrane through), semi-integral (immersed at one end into the outer or inner lipid layer), surface (located on the outer or adjacent to the inner sides of the membrane). Some proteins are the points of contact of the cell membrane with the cytoskeleton inside the cell, and the cell wall (if any) outside. Some of the integral proteins function as ion channels, various transporters, and receptors.

Functions of biomembranes

barrier - provides a regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides dangerous to the cell. Selective permeability means that the permeability of a membrane to various atoms or molecules depends on their size, electrical charge, and chemical properties. Selective permeability ensures the separation of the cell and cellular compartments from the environment and supply them with the necessary substances.

transport - through the membrane there is a transport of substances into the cell and out of the cell. Transport through membranes provides: the delivery of nutrients, the removal of end products of metabolism, the secretion of various substances, the creation of ionic gradients, the maintenance of the appropriate pH and ionic concentration in the cell, which are necessary for the operation of cellular enzymes.

Particles that for some reason are not able 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.

In passive transport, substances cross the lipid bilayer without energy expenditure, 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 may have a channel that allows only one type of substance to pass through.

Active transport requires energy, as it occurs against a concentration gradient. There are special pump proteins on the membrane, including ATPase, which actively pumps potassium ions (K +) into the cell and pumps sodium ions (Na +) out of it.

matrix - provides a certain relative position and orientation of membrane proteins, their optimal interaction;

mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play an important role in providing mechanical function, and in animals - intercellular substance.

energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate;

receptor - some proteins sitting in the membrane are receptors (molecules with which the cell perceives certain signals).

For example, hormones circulating in the blood only act on target cells that have receptors corresponding to those hormones. Neurotransmitters (chemicals that conduct nerve impulses) also bind to specific receptor proteins on target cells.

enzymatic - membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.

implementation of generation and conduction of biopotentials.

With the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K + ion inside the cell is much higher than outside, and the concentration of Na + is much lower, which is very important, since this maintains the potential difference across the membrane and generates a nerve impulse.

cell marking - there are antigens on the membrane that act as markers - "labels" that allow the cell to be identified. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of "antennas". 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, when forming organs and tissues. It also allows the immune system to recognize foreign antigens.

Structure and composition of biomembranes

Membranes are composed of three classes of lipids: phospholipids, glycolipids, and cholesterol. Phospholipids and glycolipids (lipids with carbohydrates attached to them) consist of two long hydrophobic hydrocarbon "tails" that are associated with a charged hydrophilic "head". Cholesterol stiffens the membrane by occupying the free space between the hydrophobic lipid tails and preventing them from bending. Therefore, membranes with a low cholesterol content are more flexible, while those with a high cholesterol content are more rigid and brittle. Cholesterol also serves as a “stopper” that prevents the movement of polar molecules from and into the cell. An important part of the membrane is made up of proteins penetrating it and responsible for various properties of membranes. Their composition and orientation in different membranes differ.

Cell membranes are often asymmetric, that is, the layers differ in lipid composition, the transition of an individual molecule from one layer to another (the so-called flip flop) is difficult.

Membrane organelles

These are closed single or interconnected sections of the cytoplasm, separated from the hyaloplasm by membranes. Single-membrane organelles include endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes; to two-membrane - nucleus, mitochondria, plastids. Outside, the cell is limited by the so-called plasma membrane. The structure of the membranes of various organelles differs in the composition of lipids and membrane proteins.

Selective permeability

Cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, and the membranes themselves actively regulate this process to a certain extent - some substances pass through, while others do not. There are four main mechanisms for the entry of substances into the cell or out of the cell: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive, i.e. do not require energy costs; the last two are active processes associated with energy consumption.

The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane through and through, forming a kind of passage. The elements K, Na and Cl have their own channels. With respect to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open, and there is a sharp influx of sodium ions into the cell. This results in an imbalance in the membrane potential. After that, the membrane potential is restored. Potassium channels are always open, through which potassium ions slowly enter the cell.

Basic Functions of the Cell Membrane

One of the main properties of biological membranes is selective permeability (semipermeability)- some substances pass through them with difficulty, others easily and even towards a higher concentration. Thus, for most cells, the concentration of Na ions inside is much lower than in the environment. For K ions, the reverse ratio is characteristic: their concentration inside the cell is higher than outside. Therefore, Na ions always tend to enter the cell, and K ions - to go outside. The equalization of the concentrations of these ions is prevented by the presence in the membrane of a special system that plays the role of a pump that pumps Na ions out of the cell and simultaneously pumps K ions inside.

The desire of Na ions to move from outside to inside is used to transport sugars and amino acids into the cell. With the active removal of Na ions from the cell, conditions are created for the entry of glucose and amino acids into it.

In many cells, absorption of substances also occurs by phagocytosis and pinocytosis. At phagocytosis the flexible outer membrane forms a small depression where the captured particle enters. This recess increases, and, surrounded by a portion of the outer membrane, the particle is immersed in the cytoplasm of the cell. The phenomenon of phagocytosis is characteristic of amoeba and some other protozoa, as well as leukocytes (phagocytes). Similarly, the cells absorb liquids containing the substances necessary for the cell. This phenomenon has been called pinocytosis.

The outer membranes of various cells differ significantly both in the chemical composition of their proteins and lipids, and in their relative content. It is these features that determine the diversity in the physiological activity of the membranes of various cells and their role in the life of cells and tissues.

The endoplasmic reticulum of the cell is connected to the outer membrane. With the help of outer membranes, various types of intercellular contacts are carried out, i.e. communication between individual cells.

Many types of cells are characterized by the presence on their surface of a large number of protrusions, folds, microvilli. They contribute both to a significant increase in the surface area of cells and improve metabolism, as well as to stronger bonds of individual cells with each other.

On the outside of the cell membrane, plant cells have thick membranes that are clearly visible in an optical microscope, consisting of cellulose (cellulose). They create a strong support for plant tissues (wood).

Some cells of animal origin also have a number of external structures that are located on top of the cell membrane and have a protective character. An example is the chitin of the integumentary cells of insects.

Functions of the cell membrane (briefly)

Function	Description
protective barrier	Separates the internal organelles of the cell from the external environment
Regulatory	It regulates the exchange of substances between the internal contents of the cell and the external environment.
Delimiting (compartmentalization)	Separation of the internal space of the cell into independent blocks (compartments)
Energy	- Accumulation and transformation of energy; - light reactions of photosynthesis in chloroplasts; - Absorption and secretion.
Receptor (information)	Participates in the formation of excitation and its conduct.
Motor	Carries out the movement of the cell or its individual parts.

STRUCTURALORGANIZATION AND FUNCTION OF BIOLOGICAL MEMBRANES

Biological membranes are an active molecular complex with highly selective properties that ensures the exchange of substances and energy with the environment. The membranes contain specific molecular pumps and channels that regulate the molecular and ionic composition of the intracellular environment. In addition to external cytoplasmic membrane (plasmolemma) eukaryotic cells also have internal membranes that limit various intracellular compartments(compartments), such as mitochondria, lysosomes, chloroplasts, etc. Membranes also regulate the exchange of information between cells and the environment (perception of external stimuli), etc. Membranes differ both in function and in structure. However, they all share the following basic properties:

■ membranes are a dense structure several molecules thick, 60-100 A, forming a continuous partition between individual cells and intracellular compartments;

■ Membranes are mainly composed of lipids and proteins. The membranes also contain carbohydrate ingredients, associated with lipids and proteins;

■ Membrane lipids are represented by relatively small molecules bearing hydrophilic and hydrophobic groups. In an aqueous medium, these molecules spontaneously form closed bimolecular layers, which serve as a barrier to the penetration of polar compounds;

■ most membrane functions are mediated by specific proteins that can act as pumps, channels, receptors, enzymes, etc.

There are three main types of lipids in membranes: phospholipids, glycolipids And cholesterol.

STRUCTURE OF MEMBRANES

membrane phospholipids. Among the lipid components of membranes, the leading role belongs to phospholipids- Substances derived from or trihydric alcohol glycerol (glycerophospholipids), or more complex alcohol sphingosine (sphingophospholipids). All major glycerophospholipids are derivatives phosphatidic acid, esterified with the hydroxyl group of alcohols such as series (serine phosphatides- cephalins), ethanolamine, choline (cholinphosphatides), cardiolipin (diphosphatidylglycerol) and inositol (phosphate-fatidylinositol).

Of the sphingophospholipids, the main one is sphingomyelin, which is based on sphingosine- amino alcohol with a long unsaturated hydrocarbon chain. Sphingomyelin also contains the nitrogenous base choline.

Regardless of structural diversity, each phospholipid molecule in the aquatic environment is an amphipathic molecule with a polar head and a nonpolar tail. The polar head is formed by residues of alcohol groups, nitrogenous bases and phosphoric acid. The tail part is due to the radicals of two fatty acids of a saturated and unsaturated series. Due to their amphipathic properties, phospholipids in an aqueous medium spontaneously form lipid bilayers, where the polar heads of the phospholipids are directed towards the soluble part of the cell with the formation of hydrogen bonds with water dipoles, and the nonpolar tails are inside the bilayer, being fastened together due to hydrophobic interactions. It is the bilayer structure of phospholipids that determines the semipermeable properties of membranes.

Examples include phosphatidylethanolamine and phosphatidylcholine. Both of them have polar heads NH4 (phosphatidylethanolamine) and N+ (phosphatidylcholine) in the upper part of the molecule, which are attached to two fatty acid residues through the residue of phosphoric acid and glycerol, of which one is saturated, the other is unsaturated (Fig. 1).

In 1972, S. J. Singer and G. Nicholson formulated the theory of membrane structure, according to which membranes have a fluid-mosaic structure. At normal cell temperature, the membrane bilayer is in a liquid state, which is provided by a certain ratio between saturated and unsaturated fatty acids in the hydrophobic tails of polar phospholipids. Fatty acids with unsaturated bonds are characterized by greater flexibility (unlike saturated fatty acids) and the ability to create bends, which prevents dense packing, makes it difficult to “freeze” membranes and thus affects their fluidity ().

The packing of hydrocarbons in a bilayer depends on temperature. At low temperatures, the bilayer is in the form of a gel and packed tightly, while at high temperatures (body temperature), the bilayer actually “melts” and becomes fluid, allowing lipid molecules to move around their axis, rotate, and change places. This, in turn, promotes the movement of other components in the membrane, in particular proteins.

membrane glycolipids. The next important component of membranes are glycolipids - lipids containing carbohydrates. Animal cell glycolipids, like sphingomyelin, are derivatives of the sphingosine alcohol linked to an acyl radical. The difference between these lipids is that in glycolipids one or more sugar residues are attached to the sphingosine residue, and in sphingomyelin - phosphorylcholine.

Glycolipids can be simple or complex. The simplest glycolipid cerebroside, containing only one sugar residue (glucose or galactose). In more complex glycolipids, the number of sugar residues can reach seven (gangliosides)

Glycolipids in membranes can play a protective, semiconductor, receptor-binding role. Among the molecules that can bind to glycolipids, there are also cell poisons such as cholera, tetanus toxin, etc.

Another representative of lipids in membranes is cholesterol. Its amount in membranes varies depending on the cell type. In plasma membranes, on average, there is approximately 1 molecule of cholesterol for each phospholipid molecule. Others (bacteria, for example) have no cholesterol at all. Cholesterol, like phospholipids, has polar and non-polar regions.

Inside the membranes, cholesterol intercalates between phospholipids and orients itself in the same direction as the phospholipid molecules themselves. Thus, the polar head of cholesterol is in the same plane as the polar heads of phospholipids (Fig. 2).

In membranes, cholesterol performs the following functions:

■ fix the first few nearest hydrocarbon groups that make up phospholipid fatty acids. This makes the lipid bilayer more resistant to deformation and limits the passage of small water-soluble molecules through them. In the absence of cholesterol (as, for example, in bacteria), the cell needs a shell;

■ prevents hydrocarbon crystallization and phase shifts in the membrane.

membrane proteins. IN while membrane lipids are responsible for creating a permeability barrier, membrane proteins mediate individual membrane functions, i.e. the transport of substances, the transfer of information, energy, etc. The ratio between lipids and proteins in different membranes can be different, for example, myelin, an insulator of nerve cells, contains only 18% proteins and 76% lipids, and the mitochondrial inner membrane, on the contrary, contains 76% proteins and only 24% lipids. Depending on the nature of localization in the membranes, integral (transmembrane), peripheral and "anchored" proteins are isolated.

Integral proteins penetrate the bilayer of the membrane through and through their biphilic properties are fixed in it. Proteins that cross the membrane only once are called singly penetrating proteins and several times repeatedly penetrating.

Peripheral proteins are localized on the surface of membranes and are held together only by electrostatic interactions and hydrogen bonds. Quite often, peripheral proteins attach to some regions of integral proteins (Fig. 3).

Oligosaccharides Glycoproteins Oligosaccharides

Rice. 3. Protein composition of membranes

"Anchored" proteins are fixed in membranes with the help of short tail lipophilic domains formed either at the expense of hydrophobic amino acid residues (cytochrome b5 ), or due to covalently bonded acyl radicals (enzyme alkaline phosphatase).

Regions of proteins that face the extracellular environment may be subjected to glycosylation.

transport proteins. Membrane proteins play a decisive role in the transport of substances across membranes, and integral proteins, which span both intracellular and intercellular space, are best suited to fulfill this role.

Proteins transport substances through membranes in various ways; they can act as protein pumps, channels, conveyors.

ATP - dependent pumps, represent ATPase, which promote the movement through membranes of ions or small molecules against their concentration gradient (or electrochemical potential) due to the energy of splitting APR. This mode of transport is known as active transport. Certain chemical reactions are associated with active transport, for example, thanks to such pumps in animal cells, low concentrations of Ca2 + are maintained inside the cell and a high content of Na + ions in the intercellular space lysosomes of cells, vacuoles of plant cells.

Protein channels provide fast (up to 108 molecules per second) simultaneous movement of water molecules and other molecules and ions in the direction of decreasing their concentration gradient (or electrochemical potential). Such movements of molecules are usually energetically favorable. Thus, the plasma membranes of all animal cells contain K + - specific protein channels that open and close at a certain time. Other protein channels are closed at this time and open only in response to special signals. Such channels play a particularly important role in nerve cells.

Transporter proteins promote the transport of various ions and molecules through the membrane; however, unlike channel proteins, transporter proteins bind one (or more) substrate molecules at the same time, which leads to a change in the conformation of the protein and, as a result, to the transport of these bound molecules across the membrane. Such transporters can carry about 102-104 molecules per second, which is much slower than the movement through protein channels.

3 types of transporter protein have been found.

Uniporters carry out transport through the membrane of animal cells of molecules of the same type in the direction of decreasing their concentration gradient, for example, glucose, amino acids.

Antiporters And simporters provide a coherent co-transport of some molecules or ions through the membrane against their concentration gradient with the movement of other molecules or ions in the process of their movement in the direction of decreasing their concentration gradient.

ACTIVE TRANSPORTTHROUGH THE MEMBRANE

active transport- this is the transport of substances through membranes due to the consumption of cleavage energy APR. Active transport transports some ions and small molecules against their concentration gradient.

Proteins involved in active transport across membranes (protein pumps), conventionally divided into 4 classes: superfamily of proteins ABC, class proteins R.,F., And v. class proteins R.,F. and V transport only ions, and ABC- small molecules and ions.

Proteins (pumps) R. - class consist of 2 subunits - α and β; α - subunit contains ATP - binding site and is catalytic, and β - subunit - regulatory. Most proteins of this class are tetramers composed of 2 α and 2 β subunits. In the process of transport, at least one of the α - subunits is first phosphorylated (hence referred to as "P"), and it is through it that ion transport occurs.

P-class proteins include:

■ Na+/K+- ATPase - an enzyme localized in the plasma membrane and regulating the intracellular content of Na+ and K+ ions in animal cells;

■ Ca2+-ATRase - pumps pumping Ca2+ ions from the cytosol into the intercellular space against their concentration gradient to maintain a low level of calcium (10-2 M) in the cytoplasm of animal, yeast and plant cells. In addition to plasma Ca2+-ATPase muscle cells also contain another Ca2+-ATPase (muscle Ca2+ pump), which pumps calcium ions from the cytosol to the sarcoplasmic reticulum (SR), an intracellular calcium storage;

■ membrane proteins of gastric epithelial cells in mammals that promote the flow of hydrochloric acid into the stomach;

■ H+ pumps that transport hydrogen protons from the cell instead of the flow of K+ ions into the cell;

■ H+ - pumps that regulate the membrane electrical potential in the cells of plants, fungi, bacteria. These pumps do not contain a phosphoprotein part.

Ionic class pumps F And V structurally similar to each other, but much more complex than class P proteins. Pumps F and V consist of 3 transmembrane proteins and 5 different polypeptides that are oriented to the cytosolic part of the protein and form an intracytosolic domain. Some subunits of transmembrane proteins oriented to the outer part of biomembranes are structurally similar to intracytosolic domain polypeptides.

Class V pumps are mainly involved in maintaining a low pH in plant vacuoles and lysosomes and other acid vesicles in animal cells by expending cleavage energy. APR and pumping hydrogen protons across the membrane from the cytosol into the intercellular space against the proton electrochemical gradient. Class F pumps are found in bacterial plasma membranes, chloroplast and mitochondrial membranes. Unlike class V pumps, their function is mainly directed to the synthesis APR from BUTDR and inorganic phosphate due to the movement of hydrogen protons from the cytosolic intermembrane space towards a decrease in the electrochemical gradient.

The last class of ATP-dependent transport proteins is the superfamily ABC (APR-binding cassette). This class includes up to 100 different transport proteins, and they are found in the cells of all organisms. Each ABC protein is specific with respect to one particular substrate, or a group of substrates that are similar to each other, including ions, carbohydrates, peptides, polysaccharides, and even proteins.

All ABC - transport proteins are united by the presence of 4 main domains - two transmembrane domains (T), forming the so-called gate for the “passage” of molecules through the membrane, and two intracytosolic domains (A) involved in the binding APR. There may be one or two such ATP-binding sites in ABC-proteins, and they are often called ATPases, although they do not always show APR - hydrolyzing properties. In some cases, such transmembrane proteins can exhibit ATP-synthesizing properties, which plays a decisive role in the synthesis APR in mitochondrial membranes.

What is a membrane briefly. biological membrane. Consider the main functions of the cell membrane

Various interpretations of the concept

reverse osmosis membrane

Membranes of hydraulic accumulators and expansion tanks. Their replacement

Polymer membranes

Cell membrane. Cells are the units of all living things

Historical facts related to the discovery and study of the cell membrane

Basic Functions of the Cell Membrane

Plasma shell

Structural features

Diversity of lipid functions

Cell membrane proteins

Shell Properties

Features of the structure of the cell membrane in plant and animal organisms

Membrane in biology

Membrane in engineering

Functions of biomembranes

Structure and composition of biomembranes

Membrane organelles

Selective permeability

Links

see also

See what "Cell Membrane" is in other dictionaries:

Books

Basic Functions of the Cell Membrane

Functions of the cell membrane (briefly)