Cell membrane structure. Membrane - what is it? Biological membrane: functions and structure

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

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

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

cell membrane

Image of a cell membrane. Small blue and white balls correspond to the hydrophobic "heads" of the phospholipids, and the lines attached to them correspond to the hydrophilic "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

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 the 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 optimal concentration of ions in the cell, which 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 these 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. 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 their removal from the cell to the outside: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive in nature, that is, they do not require energy; 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.

Literature

Antonov V. F., Smirnova E. N., Shevchenko E. V. Lipid membranes during phase transitions. - M .: Nauka, 1994.
Gennis R. Biomembranes. Molecular structure and functions: translation from English. = Biomembranes. Molecular structure and function (by Robert B. Gennis). - 1st edition. - M .: Mir, 1997. - ISBN 5-03-002419-0
Ivanov V. G., Berestovsky T. N. lipid bilayer of biological membranes. - M .: Nauka, 1982.
Rubin A. B. Biophysics, textbook in 2 vols. - 3rd edition, revised and expanded. - M .: Moscow University Press, 2004. - ISBN 5-211-06109-8
Bruce Alberts, et al.

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.

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 provide 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

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

History of the discovery and research of the cell membrane

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

Meaning

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

Consider the main functions of the cell membrane

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

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

3. Regulates the correct metabolism.

4. Provides interconnection between cells.

5. Recognizes signals.

6. Protection function.

"Plasma Shell"

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

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

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

The structure of the cell membrane

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

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

Compound

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

Properties

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

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

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

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

How is the cell nourished through the cell membrane?

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

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

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

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

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

The cell membrane (plasma membrane) is a thin, semi-permeable membrane that surrounds cells.

Function and role of the cell membrane

Its function is to protect the integrity of the interior by letting some essential substances into the cell and preventing others from entering.

It also serves as the basis for attachment to some organisms and to others. Thus, the plasma membrane also provides the shape of the cell. Another function of the membrane is to regulate cell growth through balance and.

In endocytosis, lipids and proteins are removed from the cell membrane as substances are absorbed. In exocytosis, vesicles containing lipids and proteins fuse with the cell membrane, increasing cell size. , and fungal cells have plasma membranes. Internal, for example, are also enclosed in protective membranes.

Cell membrane structure

The plasma membrane is mainly composed of a mixture of proteins and lipids. Depending on the location and role of the membrane in the body, lipids can make up 20 to 80 percent of the membrane, with the rest being proteins. While lipids help make the membrane flexible, proteins control and maintain the cell's chemistry and help transport molecules across the membrane.

Membrane lipids

Phospholipids are the main component of plasma membranes. They form a lipid bilayer in which the hydrophilic (water-attracted) "head" regions spontaneously organize to resist the aqueous cytosol and extracellular fluid, while the hydrophobic (water-repellent) "tail" regions face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only some molecules to diffuse across the membrane.

Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps keep cell membranes rigid by preventing phospholipids from being too tightly packed. Cholesterol is absent in plant cell membranes.

Glycolipids are located on the outer surface of cell membranes and are connected to them by a carbohydrate chain. They help the cell recognize other cells in the body.

Membrane proteins

The cell membrane contains two types of associated proteins. Peripheral membrane proteins are external and associated with it by interacting with other proteins. Integral membrane proteins are introduced into the membrane and most pass through it. Parts of these transmembrane proteins are located on both sides of it.

Plasma membrane proteins have a number of different functions. Structural proteins provide support and shape to cells. Membrane receptor proteins help cells communicate with their external environment through the use of hormones, neurotransmitters, and other signaling molecules. Transport proteins, such as globular proteins, carry molecules across cell membranes by facilitated diffusion. Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane, helping in the exchange and transport of molecules.