Cell membrane cell division. Membranes: their structure and functioning

cell membrane - this is a cell membrane that performs the following functions: separation of the contents of the cell and the external environment, selective transport of substances (exchange with the external environment for the cell), the site of some biochemical reactions, the integration of cells into tissues and reception.

Cell membranes are divided into plasma (intracellular) and outer. The main property of any membrane is semi-permeability, that is, the ability to pass only certain substances. This allows selective exchange between the cell and the external environment, or exchange between compartments of the cell.

Plasma membranes are lipoprotein structures. Lipids spontaneously form a bilayer (double layer), and membrane proteins "swim" in it. There are several thousand different proteins in the membranes: structural, carriers, enzymes, etc. Between the protein molecules there are pores through which hydrophilic substances pass (the lipid bilayer prevents their direct penetration into the cell). Glycosyl groups (monosaccharides and polysaccharides) are attached to some molecules on the membrane surface, which are involved in the process of cell recognition during tissue formation.

The membranes differ in their thickness, usually between 5 and 10 nm. The thickness is determined by the size of the amphiphilic lipid molecule and is 5.3 nm. A further increase in the thickness of the membrane is due to the size of the membrane protein complexes. Depending on external conditions (cholesterol is the regulator), the structure of the bilayer can change so that it becomes more dense or liquid - the speed of movement of substances along the membranes depends on this.

Cell membranes include: plasmalemma, karyolemma, membranes of the endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria, inclusions, etc.

Lipids are insoluble in water (hydrophobicity), but readily soluble in organic solvents and fats (lipophilicity). The composition of lipids in different membranes is not the same. For example, the plasma membrane contains a lot of cholesterol. Of the lipids in the membrane, the most common are phospholipids (glycerophosphatides), sphingomyelins (sphingolipids), glycolipids, and cholesterol.

Phospholipids, sphingomyelins, glycolipids consist of two functionally different parts: hydrophobic non-polar, which does not carry charges - "tails" consisting of fatty acids, and hydrophilic, containing charged polar "heads" - alcohol groups (for example, glycerol).

The hydrophobic part of the molecule usually consists of two fatty acids. One of the acids is limiting, and the second is unsaturated. This determines the ability of lipids to spontaneously form two-layer (bilipid) membrane structures. Membrane lipids perform the following functions: barrier, transport, microenvironment of proteins, electrical resistance of the membrane.

Membranes differ from each other by a set of protein molecules. Many membrane proteins consist of regions rich in polar (charge-carrying) amino acids and regions with non-polar amino acids (glycine, alanine, valine, leucine). Such proteins in the lipid layers of membranes are located in such a way that their non-polar regions are, as it were, immersed in the "fat" part of the membrane, where the hydrophobic regions of lipids are located. The polar (hydrophilic) part of these proteins interacts with the lipid heads and is turned towards the aqueous phase.

Biological membranes have common properties:

membranes are closed systems that do not allow the contents of the cell and its compartments to mix. Violation of the integrity of the membrane can lead to cell death;

superficial (planar, lateral) mobility. In membranes, there is a continuous movement of substances over the surface;

membrane asymmetry. The structure of the outer and surface layers chemically, structurally and functionally heterogeneous.

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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 environment, transport of substances, catalyzing 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.

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, electric 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 across membranes provides: delivery nutrients, delete final products metabolism, the secretion of various substances, the creation of ionic gradients, the maintenance in the cell of the optimal and concentration of ions that are necessary for the operation 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 ( chemical substances, which ensure the conduction of nerve impulses) also bind to specific receptor proteins of target cells.
• enzymatic - membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.
• implementation of generation and 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.

see also

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 has enough complex structure which can be seen with an electron microscope. Roughly speaking, it consists of a double layer of lipids (fats), in which different peptides (proteins) are included in different places. The total thickness of the membrane is about 5-10 nm.

Overall plan structure of the cell membrane is universal for the entire living world. However, animal membranes contain inclusions of cholesterol, which determines its rigidity. The difference between the membranes of different kingdoms of organisms mainly concerns the supra-membrane formations (layers). So in plants and fungi above the membrane (on the outside) there is a cell wall. In plants, it consists mainly of cellulose, and in fungi - of the substance of chitin. In animals, the epimembrane layer is called the glycocalyx.

Another name for the cell membrane is cytoplasmic membrane or plasma membrane.

A deeper study of the structure of the cell membrane reveals many of its features associated with the functions performed.

The lipid bilayer is mainly composed of phospholipids. These are fats, one end of which contains a residue phosphoric acid, which has hydrophilic properties (i.e., attracts water molecules). The second end of the phospholipid is a chain of fatty acids that have hydrophobic properties (do not form hydrogen bonds with water).

Phospholipid molecules in the cell membrane line up in two rows so that their hydrophobic "ends" are inside, and the hydrophilic "heads" are outside. It turns out a fairly strong structure that protects the contents of the cell from the external environment.

Protein inclusions in the cell membrane are unevenly distributed, in addition, they are mobile (since phospholipids in the bilayer have lateral mobility). Since the 70s of the XX century, people began to talk about fluid-mosaic structure of the cell membrane.

Depending on how the protein is part of the membrane, there are three types of proteins: integral, semi-integral and peripheral. Integral proteins pass through the entire thickness of the membrane, and their ends stick out on both sides of it. They mainly perform a transport function. In semi-integral proteins, one end is located in the thickness of the membrane, and the second goes out (from the outside or inside) side. They perform enzymatic and receptor functions. Peripheral proteins are found on the outer or inner surface of the membrane.

The structural features of the cell membrane indicate that it is the main component of the surface complex of the cell, but not the only one. Its other components are the supra-membrane layer and the sub-membrane layer.

The glycocalyx (supramembrane layer of animals) is formed by oligosaccharides and polysaccharides, as well as peripheral proteins and protruding parts of integral proteins. The components of the glycocalyx perform a receptor function.

In addition to the glycocalyx, animal cells also have other supra-membrane formations: mucus, chitin, perilemma (similar to a membrane).

The supra-membrane formation in plants and fungi is the cell wall.

The submembrane layer of the cell is the surface cytoplasm (hyaloplasm) with the supporting-contractile system of the cell included in it, the fibrils of which interact with the proteins that make up the cell membrane. Various signals are transmitted through such compounds of molecules.

The membrane is a hyperfine structure that forms the surface of organelles and the cell as a whole. All membranes have a similar structure and are connected in one system.

Chemical composition

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

• phospholipids;
• galactolipids;
• sulfolipids.

They also include nucleic acids, polysaccharides and other substances.

Physical Properties

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

The membrane is recoverable, strong, elastic and has pores. The thickness of the membranes is 7 - 14 nm.

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

Model

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

Rice. 1. Sandwich-type biological membrane.

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

According to the degree of immersion in the bilipid layer protein molecules divide by three groups:

• transmembrane;
• submerged;
• superficial.

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

Membrane types

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

• outdoor;
• nuclear;
• organelle membranes.

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

Rice. 2. Cytoskeleton.

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

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

Organoid membranes

Most organelles have a membrane structure.

Walls are built from one membrane:

• Golgi complex;
• vacuoles;
• lysosomes.

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

Features of the photosynthetic membranes of chloroplasts are embedded chlorophyll molecules.

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

Rice. 3. Glycocalyx.

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

Table "Structure of the cell membrane"

What have we learned?

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

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