1 Describe the structure and functions of membrane organisms. Cell membrane: definition, functions of membranes, physical properties. Steps in the process of pinocytosis

The cell membrane is an ultrathin film on the surface of a cell or cell organelle, consisting of a bimolecular layer of lipids 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 dangerous for 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 optimal pH in the cell and the concentration of ions that are necessary for the functioning of cellular enzymes. Particles that for some reason are unable to cross the phospholipid bilayer (for example, due to hydrophilic properties, since the membrane is hydrophobic inside 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 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 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 located 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 the 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.
Marking of the cell - there are antigens on the membrane that act as markers - "tags" 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.

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

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

The capacitance 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 allow 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 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), i.e., the ability of the cell membrane to pass these substances, depends on the difference in concentrations 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 permeability of the membrane does not affect its conductivity, since non-electrolytes do not carry charges, that is, they cannot carry electric current.

The conductivity of a membrane is a measure of its ion 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 the lipid and protein molecules that make up them are able to move quite quickly in the plane of the membrane

In 1972, the theory was put forward that a partially permeable membrane surrounds the cell and performs a number of vital tasks, and the structure and function of cell membranes are significant issues regarding the proper functioning of all cells in the body. became widespread in the 17th century, along with the invention of the microscope. It became known that plant and animal tissues are composed of cells, but due to the low resolution of the device, it was impossible to see any barriers around the animal cell. In the 20th century, the chemical nature of the membrane was studied in more detail, it was found that lipids are its basis.

The structure and functions of cell membranes

The cell membrane surrounds the cytoplasm of living cells, physically separating intracellular components from the external environment. Fungi, bacteria and plants also have cell walls that provide protection and prevent the passage of large molecules. Cell membranes also play a role in the development of the cytoskeleton and the attachment of other vital particles to the extracellular matrix. This is necessary in order to hold them together, forming the tissues and organs of the body. Structural features of the cell membrane include permeability. The main function is protection. The membrane consists of a phospholipid layer with embedded proteins. This part is involved in processes such as cell adhesion, ionic conduction, and signaling systems and serves as an attachment surface for several extracellular structures, including the wall, glycocalyx, and internal cytoskeleton. The membrane also maintains the potential of the cell by acting as a selective filter. It is selectively permeable to ions and organic molecules and controls the movement of particles.

Biological mechanisms involving the cell membrane

1. Passive diffusion: some substances (small molecules, ions), such as carbon dioxide (CO2) and oxygen (O2), can diffuse through the plasma membrane. The shell acts as a barrier to certain molecules and ions that can be concentrated on either side.

2. Transmembrane protein channels and transporters: Nutrients such as glucose or amino acids must enter the cell, and some metabolic products must leave it.

3. Endocytosis is the process by which molecules are taken up. A slight deformation (invagination) is created in the plasma membrane, in which the substance to be transported is swallowed. It requires energy and is thus a form of active transport.

4. Exocytosis: occurs in various cells to remove undigested residues of substances brought by endocytosis, to secrete substances such as hormones and enzymes, and transport the substance completely through the cell barrier.

molecular structure

The cell membrane is a biological membrane, consisting mainly of phospholipids and separating the contents of the entire cell from the external environment. The formation process occurs spontaneously under normal conditions. To understand this process and correctly describe the structure and functions of cell membranes, as well as properties, it is necessary to assess the nature of phospholipid structures, which are characterized by structural polarization. When phospholipids in the aquatic environment of the cytoplasm reach a critical concentration, they combine into micelles, which are more stable in the aquatic environment.

Membrane properties

Stability. This means that after the formation of the membrane is unlikely to disintegrate.
Strength. The lipid membrane is sufficiently reliable to prevent the passage of a polar substance; both dissolved substances (ions, glucose, amino acids) and much larger molecules (proteins) cannot pass through the formed boundary.
dynamic nature. This is perhaps the most important property when considering the structure of the cell. The cell membrane can be subjected to various deformations, it can fold and bend without collapsing. Under special circumstances, such as the fusion of vesicles or budding, it can be broken, but only temporarily. At room temperature, its lipid components are in constant, chaotic motion, forming a stable fluid boundary.

Liquid mosaic model

Speaking about the structure and functions of cell membranes, it is important to note that in the modern view, the membrane as a liquid mosaic model was considered in 1972 by scientists Singer and Nicholson. Their theory reflects three main features of the membrane structure. The integrals provide a mosaic template for the membrane, and they are capable of lateral in-plane movement due to the variable nature of lipid organization. Transmembrane proteins are also potentially mobile. An important feature of the membrane structure is its asymmetry. What is the structure of a cell? Cell membrane, nucleus, proteins and so on. The cell is the basic unit of life, and all organisms are made up of one or more cells, each with a natural barrier separating it from its environment. This outer border of the cell is also called the plasma membrane. It is made up of four different types of molecules: phospholipids, cholesterol, proteins and carbohydrates. The liquid mosaic model describes the structure of the cell membrane as follows: flexible and elastic, with a consistency similar to vegetable oil, so that all individual molecules simply float in the liquid medium, and they are all able to move sideways within this shell. A mosaic is something that contains many different details. In the plasma membrane, it is represented by phospholipids, cholesterol molecules, proteins and carbohydrates.

Phospholipids

Phospholipids make up the basic structure of the cell membrane. These molecules have two distinct ends: a head and a tail. The head end contains a phosphate group and is hydrophilic. This means that it is attracted to water molecules. The tail is made up of hydrogen and carbon atoms called fatty acid chains. These chains are hydrophobic, they do not like to mix with water molecules. This process is similar to what happens when you pour vegetable oil into water, that is, it does not dissolve in it. The structural features of the cell membrane are associated with the so-called lipid bilayer, which consists of phospholipids. Hydrophilic phosphate heads are always located where there is water in the form of intracellular and extracellular fluid. The hydrophobic tails of phospholipids in the membrane are organized in such a way that they keep them away from water.

Cholesterol, proteins and carbohydrates

When people hear the word "cholesterol", people usually think it's bad. However, cholesterol is actually a very important component of cell membranes. Its molecules consist of four rings of hydrogen and carbon atoms. They are hydrophobic and occur among the hydrophobic tails in the lipid bilayer. Their importance lies in maintaining consistency, they strengthen the membranes, preventing crossover. Cholesterol molecules also keep the phospholipid tails from coming into contact and hardening. This guarantees fluidity and flexibility. Membrane proteins act as enzymes to speed up chemical reactions, act as receptors for specific molecules, or transport substances across the cell membrane.

Carbohydrates, or saccharides, are found only on the extracellular side of the cell membrane. Together they form the glycocalyx. It provides cushioning and protection to the plasma membrane. Based on the structure and type of carbohydrates in the glycocalyx, the body can recognize the cells and determine if they should be there or not.

Membrane proteins

The structure of the cell membrane cannot be imagined without such a significant component as protein. Despite this, they can be significantly inferior in size to another important component - lipids. There are three main types of membrane proteins.

Integral. They completely cover the bi-layer, cytoplasm and extracellular environment. They perform a transport and signaling function.
Peripheral. Proteins are attached to the membrane by electrostatic or hydrogen bonds at their cytoplasmic or extracellular surfaces. They are involved mainly as a means of attachment for integral proteins.
Transmembrane. They perform enzymatic and signaling functions, and also modulate the basic structure of the lipid bilayer of the membrane.

Functions of biological membranes

The hydrophobic effect, which regulates the behavior of hydrocarbons in water, controls structures formed by membrane lipids and membrane proteins. Many properties of membranes are conferred by carriers of lipid bilayers, which form the basic structure for all biological membranes. Integral membrane proteins are partially hidden in the lipid bilayer. Transmembrane proteins have a specialized organization of amino acids in their primary sequence.

Peripheral membrane proteins are very similar to soluble proteins, but they are also membrane bound. Specialized cell membranes have specialized cell functions. How do the structure and functions of cell membranes affect the body? The functionality of the whole organism depends on how biological membranes are arranged. From intracellular organelles, extracellular and intercellular interactions of membranes, the structures necessary for the organization and performance of biological functions are created. Many structural and functional features are shared between bacteria and enveloped viruses. All biological membranes are built on a lipid bilayer, which determines the presence of a number of common characteristics. Membrane proteins have many specific functions.

Controlling. Plasma membranes of cells determine the boundaries of the interaction of the cell with the environment.
Transport. The intracellular membranes of cells are divided into several functional blocks with different internal composition, each of which is supported by the necessary transport function in combination with control permeability.
signal transduction. Membrane fusion provides a mechanism for intracellular vesicular notification and preventing various kinds of viruses from freely entering the cell.

Significance and conclusions

The structure of the outer cell membrane affects the entire body. It plays an important role in protecting integrity by allowing only selected substances to penetrate. It is also a good base for anchoring the cytoskeleton and cell wall, which helps in maintaining the shape of the cell. Lipids make up about 50% of the membrane mass of most cells, although this varies depending on the type of membrane. The structure of the outer cell membrane of mammals is more complex, it contains four main phospholipids. An important property of lipid bilayers is that they behave like a two-dimensional fluid in which individual molecules can freely rotate and move laterally. Such fluidity is an important property of membranes, which is determined depending on temperature and lipid composition. Due to the hydrocarbon ring structure, cholesterol plays a role in determining the fluidity of membranes. biological membranes for small molecules allows the cell to control and maintain its internal structure.

Considering the structure of the cell (cell membrane, nucleus, and so on), we can conclude that the body is a self-regulating system that cannot harm itself without outside help and will always look for ways to restore, protect and properly function each cell.

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. Subsequently, 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 the necessary nutrients and chemical elements to penetrate into the cell, and excess products to leave it. The plasma membrane consists of two layers of lipid molecules with the inclusion of specific proteins in it. The main membrane lipids are phospholipids. They contain phosphorus, a polar head, and two non-polar long-chain fatty acid tails. Membrane lipids include cholesterol and cholesterol esters. In accordance with the fluid mosaic model of the 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 relatively constant lipid composition.

Membrane proteins are divided into two types according to their structure: 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 that allow water molecules and ions to move between cells; form ion channels specialized for the movement of certain types 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 gives 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.

Carbohydrates are also present in the membrane, which can covalently bind to lipids or proteins. There are three types of membrane carbohydrates: glycolipids (gangliosides), glycoproteins and proteoglycans. Most 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 outer side of the membrane there are receptor sites that bind various hormones. Other specific sections of the membrane can> t recognize and bind some proteins alien to these cells and various biologically active compounds.

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

Endoplasmic reticulum is a system of interconnected channels and cavities penetrating 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 informational and transport RNA, and protein synthesis is carried out in them. They synthesize proteins that enter the cisterns or the Golgi apparatus and are then released outside. Ribosomes that are free in the cytoplasm synthesize protein for use by the cell itself, and ribosomes associated with the endoplasmic reticulum produce protein that is excreted from the cell. Various functional proteins are synthesized in ribosomes: carrier proteins, enzymes, receptors, cytoskeletal proteins.

golgi apparatus formed by a system of tubules, cisterns and vesicles. It is associated with the endoplasmic reticulum, and the biologically active substances that have entered 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 merge with it, and the substances contained in the vesicles are removed from the cell in the process of 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 cleaved under the influence of peroxidases and catalase.

Mitochondria contain outer and inner membranes and are the "energy station" of the cell. Mitochondria are round or elongated structures with a double membrane. The inner membrane forms folds protruding into the mitochondria - cristae. ATP is synthesized in them, the substrates of the Krebs cycle are oxidized, and many biochemical reactions are carried out. 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, renewal and synthesis of new mitochondria takes place.

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 inside cells.

microtubules form the basis of the cytoskeleton and provide its strength. The cytoskeleton gives the cells a characteristic appearance and shape, serves as a site for attachment of 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 during cell division is carried out. They play the role of motor elements in the villi and flagella in eukaryotes.

Core is the main 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. Inside it is the nucleolus. 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, consisting of DNA, RNA, and a number of specific proteins.

Functions of the cell membrane

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

barrier function It 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 is 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 with the help of active transport.

Diffusion - the movement of a gas or solute along a concentration and electrochemical gradient. The diffusion rate depends on the permeability of the cell membrane, as well as the concentration gradient for uncharged particles, electric and concentration gradients for charged particles. simple diffusion occurs through the lipid bilayer or through channels. Charged particles move along an electrochemical gradient, while uncharged particles follow a chemical gradient. For example, oxygen, steroid hormones, urea, alcohol, etc. penetrate through the lipid layer of the membrane by simple diffusion. Various ions and particles move through the channels. Ion channels are formed by proteins and are divided into gated and uncontrolled channels. Depending on the selectivity, there are ion-selective ropes that allow only one ion to pass through, and channels that do not have selectivity. Channels have a mouth and a selective filter, and controlled channels have a gate mechanism.

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

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

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

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

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

At In endocytosis, cell membranes form invaginations into the cell, which, when laced, turn into vesicles. During exocytosis, vesicles with 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 functions of the cell membrane

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

At present, the fluid-mosaic model of the membrane, proposed by S. Singer and G. Nicholson in 1972, enjoys the greatest recognition. The basis of the membrane is a double layer of phospholipids (bilayer), the hydrophobic fragments of the molecule 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 in the hydrophobic zone. Some proteins penetrate through the membrane, and different hydrophilic groups of the same protein are found on both sides of the cell membrane. 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 limit 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. So, 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 fluid-mosaic model of the Singer-Nicolson membrane

Globular integral proteins embedded in a lipid bilayer are shown. 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 regions of the tails of the phospholipid molecule are not limited in their movement and are responsible for the fluidity of the membrane (Bretscher, 1985)

There are channels in the membrane 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 the receptors interact with substances. Channels can be opened or closed thanks to gates. Two types of gates are built into the membrane: activation(in the depth of the channel) and inactivation(on the surface of the channel). The gate can be in one of three states:

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

Another characteristic feature of membranes is the ability to selectively transfer inorganic ions, nutrients, and various metabolic products. There are 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 potentials 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 channel walls and mouth.

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

Active transport occurs with the expenditure of energy and is carried out by the so-called ion pumps. Ion pumps are molecular protein structures embedded in the membrane and carrying out the transfer of ions towards a higher electrochemical potential.

The operation of the pumps is carried out due to the energy of ATP hydrolysis. At present, Na + / K + - ATPase, Ca 2+ - ATPase, H + - ATPase, H + / K + - ATPase, Mg 2+ - ATPase, which ensure 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 has not been fully elucidated.

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

Membrane proteins are localized on the membrane surface or can be embedded at different depths in the hydrophobic zone. Some proteins penetrate through the membrane, and different hydrophilic groups of the same protein are found on both sides of the cell membrane. Proteins found in the plasma membrane play a very important role: they are involved 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.

Rice. 2.9.

There are channels in the membrane 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) are opened during the interaction of receptors with substances. Channels can be opened or closed thanks to gates. Two types of gates are built into the membrane: activation(in the depth of the channel) and inactivation(on the surface of the channel). The gate can be in one of three states:

open state (both types of gate are open);
closed state (activation gate closed);
inactivation state (inactivation gates are closed). Another characteristic feature of membranes is the ability to selectively transfer inorganic ions, nutrients, and various metabolic products. There are 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 potentials 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 channel walls and mouth.

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

Active transport occurs with the expenditure of energy and is carried out by the so-called ion pumps. Ion pumps are molecular protein structures embedded in the membrane and carrying out the transfer of ions towards a higher electrochemical potential.

The operation of the pumps is carried out due to the energy of ATP hydrolysis. Na + /K + - ATPase, Ca 2+ - ATPase, H + - ATPase, H + /K + - ATPase, Mg 2+ - ATPase, which ensure the movement of Na + , K + , Ca ions, respectively, are well studied. 2+, H +, Mg 2+ isolated or conjugated (Na + and K +; H + and K +). The molecular mechanism of active transport has not been fully elucidated.

cell membrane - molecular structure that is made up of lipids and proteins. Its main properties and functions:

separation of the contents of any cell from the external environment, ensuring its integrity;
management and adjustment of the exchange between the environment and the cell;
intracellular membranes divide the cell into special compartments: organelles or compartments.

The word "membrane" in Latin means "film". If we talk about the cell membrane, then this is a combination of two films that have different properties.

The biological membrane includes three types of proteins:

Peripheral - located on the surface of the film;
Integral - completely penetrate the membrane;
Semi-integral - at one end penetrate into the bilipid layer.

What are the functions of the cell membrane

1. Cell wall - a strong shell of the cell, which is located outside the cytoplasmic membrane. It performs protective, transport and structural functions. Present in many plants, bacteria, fungi and archaea.

2. Provides a barrier function, that is, selective, regulated, active and passive metabolism with the external environment.

3. Able to transmit and store information, and also takes part in the process of reproduction.

4. Performs a transport function that can transport substances through the membrane into and out of the cell.

5. The cell membrane has one-way conductivity. Due to this, water molecules can pass through the cell membrane without delay, and molecules of other substances penetrate selectively.

6. With the help of the cell membrane, water, oxygen and nutrients are obtained, and through it the products of cellular metabolism are removed.

7. Performs cell exchange across membranes, and can perform them through 3 main types of reactions: pinocytosis, phagocytosis, exocytosis.

8. The membrane provides the specificity of intercellular contacts.

9. There are numerous receptors in the membrane that are able to perceive chemical signals - mediators, hormones and many other biologically active substances. So she is able to change the metabolic activity of the cell.

10. The main properties and functions of the cell membrane:

matrix
Barrier
Transport
Energy
Mechanical
Enzymatic
Receptor
Protective
Marking
Biopotential

What is the function of the plasma membrane in the cell?

Delimits the contents of the cell;
Carries out the flow of substances into the cell;
Provides removal of a number of substances from the cell.

cell membrane structure

Cell membranes include lipids of 3 classes:

Glycolipids;
Phospholipids;
Cholesterol.

Basically, the cell membrane consists of proteins and lipids, and has a thickness of no more than 11 nm. From 40 to 90% of all lipids are phospholipids. It is also important to note glycolipids, which are one of the main components of the membrane.

The structure of the cell membrane is three-layered. A homogeneous liquid bilipid layer is located in the center, and proteins cover it from both sides (like a mosaic), partly penetrating into the thickness. Proteins are also necessary for the membrane to pass inside the cells and transport out of them special substances that cannot penetrate the fat layer. For example, sodium and potassium ions.