Outer membrane. Cell membrane. Structure of the cell membrane

The vast majority of organisms living on Earth consists of cells that are largely similar in their chemical composition, structure and vital functions. Metabolism and energy conversion occur in every cell. Cell division underlies the processes of growth and reproduction of organisms. Thus, the cell is a unit of structure, development and reproduction of organisms.

A cell can only exist as an integral system, indivisible into parts. Cell integrity is ensured by biological membranes. A cell is an element of a system of a higher rank - an organism. Cell parts and organelles, consisting of complex molecules, represent integral systems of a lower rank.

The cell is an open system connected with the environment by the exchange of substances and energy. It is a functional system in which each molecule performs specific functions. The cell has stability, the ability to self-regulate and self-reproduce.

The cell is a self-governing system. The control genetic system of a cell is represented by complex macromolecules - nucleic acids (DNA and RNA).

In 1838-1839 German biologists M. Schleiden and T. Schwann summarized knowledge about the cell and formulated the main position of the cell theory, the essence of which is that all organisms, both plant and animal, consist of cells.

In 1859, R. Virchow described the process of cell division and formulated one of the most important provisions of cell theory: “Every cell comes from another cell.” New cells are formed as a result of division of the mother cell, and not from non-cellular substance, as was previously thought.

The discovery of mammalian eggs by the Russian scientist K. Baer in 1826 led to the conclusion that the cell underlies the development of multicellular organisms.

Modern cell theory includes the following provisions:

1) cell - the unit of structure and development of all organisms;

2) cells of organisms from different kingdoms of living nature are similar in structure, chemical composition, metabolism, and basic manifestations of life activity;

3) new cells are formed as a result of division of the mother cell;

4) in a multicellular organism, cells form tissues;

5) organs are made up of tissues.

With the introduction of modern biological, physical and chemical research methods into biology, it has become possible to study the structure and functioning of various components of the cell. One of the methods for studying cells is microscopy. A modern light microscope magnifies objects 3000 times and allows you to see the largest cell organelles, observe the movement of the cytoplasm, and cell division.

Invented in the 40s. XX century An electron microscope gives magnification of tens and hundreds of thousands of times. An electron microscope uses a stream of electrons instead of light, and electromagnetic fields instead of lenses. Therefore, an electron microscope produces clear images at much higher magnifications. Using such a microscope, it was possible to study the structure of cell organelles.

The structure and composition of cell organelles is studied using the method centrifugation. Chopped tissues with destroyed cell membranes are placed in test tubes and rotated in a centrifuge at high speed. The method is based on the fact that different cellular organoids have different mass and density. More dense organelles are deposited in a test tube at low centrifugation speeds, less dense ones - at high speeds. These layers are studied separately.

Widely used cell and tissue culture method, which consists in the fact that from one or several cells on a special nutrient medium one can obtain a group of the same type of animal or plant cells and even grow a whole plant. Using this method, you can get an answer to the question of how various tissues and organs of the body are formed from one cell.

The basic principles of cell theory were first formulated by M. Schleiden and T. Schwann. A cell is a unit of structure, vital activity, reproduction and development of all living organisms. To study cells, methods of microscopy, centrifugation, cell and tissue culture, etc. are used.

The cells of fungi, plants and animals have much in common not only in chemical composition, but also in structure. When examining a cell under a microscope, various structures are visible in it - organoids. Each organelle performs specific functions. There are three main parts in a cell: the plasma membrane, the nucleus and the cytoplasm (Figure 1).

Plasma membrane separates the cell and its contents from the environment. In Figure 2 you see: the membrane is formed by two layers of lipids, and protein molecules penetrate the thickness of the membrane.

Main function of the plasma membrane transport. It ensures the flow of nutrients into the cell and the removal of metabolic products from it.

An important property of the membrane is selective permeability, or semi-permeability, allows the cell to interact with the environment: only certain substances enter and are removed from it. Small molecules of water and some other substances penetrate the cell by diffusion, partly through pores in the membrane.

Sugars, organic acids, and salts are dissolved in the cytoplasm, the cell sap of the vacuoles of a plant cell. Moreover, their concentration in the cell is much higher than in the environment. The higher the concentration of these substances in the cell, the more water it absorbs. It is known that water is constantly consumed by the cell, due to which the concentration of cell sap increases and water again enters the cell.

The entry of larger molecules (glucose, amino acids) into the cell is ensured by membrane transport proteins, which, combining with the molecules of transported substances, transport them across the membrane. This process involves enzymes that break down ATP.

Figure 1. Generalized diagram of the structure of a eukaryotic cell.
(to enlarge the image, click on the picture)

Figure 2. Structure of the plasma membrane.
1 - piercing proteins, 2 - submerged proteins, 3 - external proteins

Figure 3. Diagram of pinocytosis and phagocytosis.

Even larger molecules of proteins and polysaccharides enter the cell by phagocytosis (from the Greek. phagos- devouring and kitos- vessel, cell), and drops of liquid - by pinocytosis (from the Greek. pinot- I drink and kitos) (Figure 3).

Animal cells, unlike plant cells, are surrounded by a soft and flexible “coat” formed mainly by polysaccharide molecules, which, joining some membrane proteins and lipids, surround the cell from the outside. The composition of polysaccharides is specific to different tissues, due to which cells “recognize” each other and connect with each other.

Plant cells do not have such a “coat”. They have a pore-ridden plasma membrane above them. cell membrane, consisting predominantly of cellulose. Through the pores, threads of cytoplasm stretch from cell to cell, connecting the cells to each other. This is how communication between cells is achieved and the integrity of the body is achieved.

The cell membrane in plants plays the role of a strong skeleton and protects the cell from damage.

Most bacteria and all fungi have a cell membrane, only its chemical composition is different. In fungi it consists of a chitin-like substance.

The cells of fungi, plants and animals have a similar structure. A cell has three main parts: the nucleus, the cytoplasm, and the plasma membrane. The plasma membrane is composed of lipids and proteins. It ensures the entry of substances into the cell and their release from the cell. In the cells of plants, fungi and most bacteria there is a cell membrane above the plasma membrane. It performs a protective function and plays the role of a skeleton. In plants, the cell wall consists of cellulose, and in fungi, it is made of a chitin-like substance. Animal cells are covered with polysaccharides that provide contacts between cells of the same tissue.

Do you know that the main part of the cell is cytoplasm. It consists of water, amino acids, proteins, carbohydrates, ATP, and ions of inorganic substances. The cytoplasm contains the nucleus and organelles of the cell. In it, substances move from one part of the cell to another. Cytoplasm ensures the interaction of all organelles. Chemical reactions take place here.

The entire cytoplasm is permeated with thin protein microtubules that form cell cytoskeleton, thanks to which it maintains a constant shape. The cell cytoskeleton is flexible, since microtubules are able to change their position, move from one end and shorten from the other. Various substances enter the cell. What happens to them in the cage?

In lysosomes - small round membrane vesicles (see Fig. 1) molecules of complex organic substances are broken down into simpler molecules with the help of hydrolytic enzymes. For example, proteins are broken down into amino acids, polysaccharides into monosaccharides, fats into glycyrin and fatty acids. For this function, lysosomes are often called the “digestive stations” of the cell.

If the membrane of lysosomes is destroyed, the enzymes contained in them can digest the cell itself. Therefore, lysosomes are sometimes called “cell killing weapons.”

The enzymatic oxidation of small molecules of amino acids, monosaccharides, fatty acids and alcohols formed in lysosomes to carbon dioxide and water begins in the cytoplasm and ends in other organelles - mitochondria. Mitochondria are rod-shaped, thread-like or spherical organelles, delimited from the cytoplasm by two membranes (Fig. 4). The outer membrane is smooth, and the inner one forms folds - cristas, which increase its surface. The inner membrane contains enzymes that participate in the oxidation of organic substances to carbon dioxide and water. This releases energy that is stored by the cell in ATP molecules. Therefore, mitochondria are called the “power stations” of the cell.

In the cell, organic substances are not only oxidized, but also synthesized. The synthesis of lipids and carbohydrates is carried out on the endoplasmic reticulum - EPS (Fig. 5), and proteins - on ribosomes. What is EPS? This is a system of tubules and cisterns, the walls of which are formed by a membrane. They permeate the entire cytoplasm. Substances move through the ER channels to different parts of the cell.

There is smooth and rough EPS. On the surface of the smooth ER, carbohydrates and lipids are synthesized with the participation of enzymes. The roughness of the ER is given by the small round bodies located on it - ribosomes(see Fig. 1), which are involved in protein synthesis.

The synthesis of organic substances also occurs in plastids, which are found only in plant cells.

Rice. 4. Scheme of the structure of mitochondria.
1.- outer membrane; 2.- inner membrane; 3.- folds of the inner membrane - cristae.

Rice. 5. Scheme of the structure of rough EPS.

Rice. 6. Diagram of the structure of a chloroplast.
1.- outer membrane; 2.- inner membrane; 3.- internal contents of the chloroplast; 4.- folds of the inner membrane, collected in “stacks” and forming grana.

In colorless plastids - leucoplasts(from Greek leukos- white and plastos- created) starch accumulates. Potato tubers are very rich in leucoplasts. Yellow, orange, and red colors are given to fruits and flowers. chromoplasts(from Greek chromium- color and plastos). They synthesize pigments involved in photosynthesis - carotenoids. In plant life, it is especially important chloroplasts(from Greek chloros- greenish and plastos) - green plastids. In Figure 6 you see that chloroplasts are covered with two membranes: an outer and an inner. The inner membrane forms folds; between the folds there are bubbles arranged in stacks - grains. Granas contain chlorophyll molecules, which are involved in photosynthesis. Each chloroplast has about 50 grains arranged in a checkerboard pattern. This arrangement ensures maximum illumination of each face.

In the cytoplasm, proteins, lipids, and carbohydrates can accumulate in the form of grains, crystals, and droplets. These inclusion- reserve nutrients that are consumed by the cell as needed.

In plant cells, some of the reserve nutrients, as well as breakdown products, accumulate in the cell sap of vacuoles (see Fig. 1). They can account for up to 90% of the volume of a plant cell. Animal cells have temporary vacuoles that occupy no more than 5% of their volume.

Rice. 7. Scheme of the structure of the Golgi complex.

In Figure 7 you see a system of cavities surrounded by a membrane. This Golgi complex, which performs various functions in the cell: participates in the accumulation and transportation of substances, their removal from the cell, the formation of lysosomes and the cell membrane. For example, cellulose molecules enter the cavity of the Golgi complex, which, using vesicles, move to the cell surface and are included in the cell membrane.

Most cells reproduce by division. Participating in this process cell center. It consists of two centrioles surrounded by dense cytoplasm (see Fig. 1). At the beginning of division, the centrioles move towards the poles of the cell. Protein threads emanate from them, which connect to the chromosomes and ensure their uniform distribution between the two daughter cells.

All cell organelles are closely interconnected. For example, protein molecules are synthesized in ribosomes, they are transported through ER channels to different parts of the cell, and proteins are destroyed in lysosomes. Newly synthesized molecules are used to build cell structures or accumulate in the cytoplasm and vacuoles as reserve nutrients.

The cell is filled with cytoplasm. The cytoplasm contains the nucleus and various organelles: lysosomes, mitochondria, plastids, vacuoles, ER, cell center, Golgi complex. They differ in their structure and functions. All organelles of the cytoplasm interact with each other, ensuring the normal functioning of the cell.

Table 1. CELL STRUCTURE

The most important role in the life activity and division of cells of fungi, plants and animals belongs to the nucleus and the chromosomes located in it. Most cells of these organisms have a single nucleus, but there are also multinucleated cells, such as muscle cells. The nucleus is located in the cytoplasm and has a round or oval shape. It is covered with a shell consisting of two membranes. The nuclear envelope has pores through which the exchange of substances occurs between the nucleus and the cytoplasm. The nucleus is filled with nuclear juice, in which nucleoli and chromosomes are located.

Nucleoli- these are “workshops for the production” of ribosomes, which are formed from ribosomal RNA produced in the nucleus and proteins synthesized in the cytoplasm.

The main function of the nucleus - storage and transmission of hereditary information - is associated with chromosomes. Each type of organism has its own set of chromosomes: a certain number, shape and size.

All cells of the body, except the sex cells, are called somatic(from Greek soma- body). Cells of an organism of the same species contain the same set of chromosomes. For example, in humans, each cell of the body contains 46 chromosomes, in the fruit fly Drosophila - 8 chromosomes.

Somatic cells, as a rule, have a double set of chromosomes. It is called diploid and is denoted by 2 n. So, a person has 23 pairs of chromosomes, that is, 2 n= 46. Sex cells contain half as many chromosomes. Is it single, or haploid, kit. Person has 1 n = 23.

All chromosomes in somatic cells, unlike chromosomes in germ cells, are paired. The chromosomes that make up one pair are identical to each other. Paired chromosomes are called homologous. Chromosomes that belong to different pairs and differ in shape and size are called non-homologous(Fig. 8).

In some species the number of chromosomes may be the same. For example, red clover and peas have 2 n= 14. However, their chromosomes differ in shape, size, and nucleotide composition of DNA molecules.

Rice. 8. Set of chromosomes in Drosophila cells.

Rice. 9. Chromosome structure.

To understand the role of chromosomes in the transmission of hereditary information, it is necessary to become familiar with their structure and chemical composition.

The chromosomes of a non-dividing cell look like long thin threads. Before cell division, each chromosome consists of two identical strands - chromatid, which are connected between the waists of the waist - (Fig. 9).

Chromosomes are made up of DNA and proteins. Because the nucleotide composition of DNA varies among species, the composition of chromosomes is unique to each species.

Every cell, except bacterial cells, has a nucleus in which nucleoli and chromosomes are located. Each species is characterized by a certain set of chromosomes: number, shape and size. In the somatic cells of most organisms the set of chromosomes is diploid, in the sex cells it is haploid. Paired chromosomes are called homologous. Chromosomes are made up of DNA and proteins. DNA molecules ensure the storage and transmission of hereditary information from cell to cell and from organism to organism.

Having worked through these topics, you should be able to:

1. Explain in what cases a light microscope (structure) or a transmission electron microscope should be used.
2. Describe the structure of the cell membrane and explain the relationship between the structure of the membrane and its ability to exchange substances between the cell and its environment.
3. Define the processes: diffusion, facilitated diffusion, active transport, endocytosis, exocytosis and osmosis. Indicate the differences between these processes.
4. Name the functions of the structures and indicate in which cells (plant, animal or prokaryotic) they are located: nucleus, nuclear membrane, nucleoplasm, chromosomes, plasma membrane, ribosome, mitochondrion, cell wall, chloroplast, vacuole, lysosome, smooth endoplasmic reticulum (agranular) and rough (granular), cell center, Golgi apparatus, cilium, flagellum, mesosoma, pili or fimbriae.
5. Name at least three signs by which a plant cell can be distinguished from an animal cell.
6. List the most important differences between prokaryotic and eukaryotic cells.

Ivanova T.V., Kalinova G.S., Myagkova A.N. "General Biology". Moscow, "Enlightenment", 2000

• Topic 1. "Plasma membrane." §1, §8 pp. 5;20
• Topic 2. "Cage." §8-10 pp. 20-30
• Topic 3. "Prokaryotic cell. Viruses." §11 pp. 31-34

The cell membrane is an ultrathin film on the surface of a cell or cellular organelle, consisting of a bimolecular layer of lipids with embedded proteins and polysaccharides.

Membrane functions:

• · Barrier - provides regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides that are dangerous to the cell. Selective permeability means that the permeability of a membrane to different atoms or molecules depends on their size, electrical charge and chemical properties. Selective permeability ensures that the cell and cellular compartments are separated from the environment and supplied with the necessary substances.
• · Transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery of nutrients, removal of metabolic end products, secretion of various substances, creation of ion gradients, maintenance of optimal pH and ion concentrations in the cell, which are necessary for the functioning of cellular enzymes. Particles that for any reason are unable to cross the phospholipid bilayer (for example, due to hydrophilic properties, since the membrane inside is hydrophobic and does not allow hydrophilic substances to pass through, or due to 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 expending energy along a concentration gradient by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance 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 - ensures 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 a major role in ensuring mechanical function, and in animals, the 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 the help of which the cell perceives certain signals). For example, hormones circulating in the blood act only on target cells that have receptors corresponding to these hormones. Neurotransmitters (chemicals that ensure the conduction of nerve impulses) also bind to special receptor proteins in 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 ensures the maintenance of the potential difference on the membrane and the generation of 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”. Because of the myriad configurations of side chains, 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, in the formation of organs and tissues. This 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 emerging on different 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.

Capacitive 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 charge separation and storage, and electrostatic interaction of cations and anions. In addition, the capacitive properties of cell membranes are one of the reasons that determine the time characteristics of electrical processes occurring on cell membranes.

Conductivity (g) is the reciprocal of electrical resistance and is equal to the ratio of the total transmembrane current for a given ion to the value that determined 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 rate of diffusion for charged ions under constant field conditions in a membrane is determined by the mobility of ions, the thickness of the membrane, and the distribution of ions in the membrane. For nonelectrolytes, the permeability of the membrane does not affect its conductivity, since nonelectrolytes do not carry charges, i.e., they cannot carry electric current.

The conductivity of a membrane is a measure of its ionic 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 their constituent lipid and protein molecules are capable of moving quite quickly in the plane of the membrane

Among The main functions of the cell membrane can be distinguished: barrier, transport, enzymatic and receptor. The cellular (biological) membrane (also known as plasmalemma, plasma or cytoplasmic membrane) protects the contents of the cell or its organelles from the environment, provides selective permeability for substances, enzymes are located on it, as well as molecules that can “catch” various chemical and physical signals.

This functionality is ensured by the special structure of the cell membrane.

In the evolution of life on Earth, a cell could generally form only after the appearance of a membrane, which separated and stabilized the internal contents and prevented them from disintegrating.

In terms of maintaining homeostasis (self-regulation of the relative constancy of the internal environment) the barrier function of the cell membrane is closely related to transport.

Small molecules are able to pass through the plasmalemma without any “helpers”, along a concentration gradient, i.e., from an area with a high concentration of a given substance to an area with a low concentration. This is the case, for example, for gases involved in respiration. Oxygen and carbon dioxide diffuse through the cell membrane in the direction where their concentration is currently lower.

Since the membrane is mostly hydrophobic (due to the lipid double layer), polar (hydrophilic) molecules, even small ones, often cannot penetrate through it. Therefore, a number of membrane proteins act as carriers of such molecules, binding to them and transporting them through the plasmalemma.

Integral (membrane-permeating) proteins often operate on the principle of opening and closing channels. When any molecule approaches such a protein, it binds to it and the channel opens. This substance or another passes through the protein channel, after which its conformation changes, and the channel closes to this substance, but can open to allow the passage of another. The sodium-potassium pump works on this principle, pumping potassium ions into the cell and pumping sodium ions out of it.

Enzymatic function of the cell membrane to a greater extent realized on the membranes of cell organelles. Most proteins synthesized in the cell perform an enzymatic function. “Sitting” on the membrane in a certain order, they organize a conveyor when the reaction product catalyzed by one enzyme protein moves on to the next. This “conveyor” is stabilized by surface proteins of the plasmalemma.

Despite the universality of the structure of all biological membranes (they are built according to a single principle, they are almost identical in all organisms and in different membrane cell structures), their chemical composition can still differ. There are more liquid and more solid ones, some have more of certain proteins, others have less. In addition, different sides (inner and outer) of the same membrane also differ.

The membrane that surrounds the cell (cytoplasmic) on the outside has many carbohydrate chains attached to lipids or proteins (resulting in the formation of glycolipids and glycoproteins). Many of these carbohydrates serve receptor function, being susceptible to certain hormones, detecting changes in physical and chemical indicators in the environment.

If, for example, a hormone connects with its cellular receptor, then the carbohydrate part of the receptor molecule changes its structure, followed by a change in the structure of the associated protein part that penetrates the membrane. At the next stage, various biochemical reactions are started or suspended in the cell, i.e. its metabolism changes, and a cellular response to the “stimulus” begins.

In addition to the listed four functions of the cell membrane, others are also distinguished: matrix, energy, marking, formation of intercellular contacts, etc. However, they can be considered as “subfunctions” of those already considered.

Membranes are extremely viscous and at the same time plastic structures that surround all living cells. Functions cell membranes:

1. The plasma membrane is a barrier that maintains the different composition of the extra- and intracellular environment.

2.Membranes form specialized compartments inside the cell, i.e. numerous organelles - mitochondria, lysosomes, Golgi complex, endoplasmic reticulum, nuclear membranes.

3. Enzymes involved in energy conversion in processes such as oxidative phosphorylation and photosynthesis are localized in the membranes.

Structure and composition of membranes

The basis of the membrane is a double lipid layer, the formation of which involves phospholipids and glycolipids. The lipid bilayer is formed by two rows of lipids, the hydrophobic radicals of which are hidden inward, and the hydrophilic groups face outward and are in contact with the aqueous environment. Protein molecules are, as it were, “dissolved” in the lipid bilayer.

Structure of membrane lipids

Membrane lipids are amphiphilic molecules, because the molecule has both a hydrophilic region (polar heads) and a hydrophobic region, represented by hydrocarbon radicals of fatty acids, which spontaneously form a bilayer. Membranes contain three main types of lipids - phospholipids, glycolipids and cholesterol.

The lipid composition is different. The content of a particular lipid is apparently determined by the variety of functions performed by these lipids in membranes.

Phospholipids. All phospholipids can be divided into two groups - glycerophospholipids and sphingophospholipids. Glycerophospholipids are classified as phosphatidic acid derivatives. The most common glycerophospholipids are phosphatidylcholines and phosphatidylethanolamines. Sphingophospholipids are based on the amino alcohol sphingosine.

Glycolipids. In glycolipids, the hydrophobic part is represented by the alcohol ceramide, and the hydrophilic part is represented by a carbohydrate residue. Depending on the length and structure of the carbohydrate part, cerebrosides and gangliosides are distinguished. The polar “heads” of glycolipids are located on the outer surface of plasma membranes.

Cholesterol (CS). CS is present in all membranes of animal cells. Its molecule consists of a rigid hydrophobic core and a flexible hydrocarbon chain. The single hydroxyl group at the 3-position is the “polar head”. For an animal cell, the average molar ratio of cholesterol/phospholipids is 0.3-0.4, but in the plasma membrane this ratio is much higher (0.8-0.9). The presence of cholesterol in membranes reduces the mobility of fatty acids, reduces the lateral diffusion of lipids and therefore can affect the functions of membrane proteins.

Membrane properties:

1. Selective permeability. The closed bilayer provides one of the main properties of the membrane: it is impermeable to most water-soluble molecules, since they do not dissolve in its hydrophobic core. Gases such as oxygen, CO 2 and nitrogen have the ability to easily penetrate into cells due to the small size of their molecules and weak interaction with solvents. Molecules of a lipid nature, such as steroid hormones, also easily penetrate the bilayer.

2. Liquidity. Membranes are characterized by liquidity (fluidity), the ability of lipids and proteins to move. Two types of phospholipid movements are possible: somersault (called “flip-flop” in the scientific literature) and lateral diffusion. In the first case, phospholipid molecules opposing each other in the bimolecular layer turn over (or somersault) towards each other and change places in the membrane, i.e. the outside becomes the inside and vice versa. Such jumps are associated with energy consumption. More often, rotations around the axis (rotation) and lateral diffusion are observed - movement within the layer parallel to the surface of the membrane. The speed of movement of molecules depends on the microviscosity of the membranes, which, in turn, is determined by the relative content of saturated and unsaturated fatty acids in the lipid composition. Microviscosity is lower if unsaturated fatty acids predominate in the lipid composition, and higher if the content of saturated fatty acids is high.

3. Membrane asymmetry. The surfaces of the same membrane differ in the composition of lipids, proteins and carbohydrates (transverse asymmetry). For example, phosphatidylcholines predominate in the outer layer, while phosphatidylethanolamines and phosphatidylserines predominate in the inner layer. The carbohydrate components of glycoproteins and glycolipids come to the outer surface, forming a continuous structure called the glycocalyx. There are no carbohydrates on the inner surface. Proteins - hormone receptors are located on the outer surface of the plasma membrane, and the enzymes they regulate - adenylate cyclase, phospholipase C - on the inner surface, etc.

Membrane proteins

Membrane phospholipids act as a solvent for membrane proteins, creating a microenvironment in which the latter can function. Proteins account for 30 to 70% of the mass of membranes. The number of different proteins in the membrane varies from 6-8 in the sarcoplasmic reticulum to more than 100 in the plasma membrane. These are enzymes, transport proteins, structural proteins, antigens, including antigens of the major histocompatibility system, receptors for various molecules.

Based on their localization in the membrane, proteins are divided into integral (partially or completely immersed in the membrane) and peripheral (located on its surface). Some integral proteins cross the membrane once (glycophorin), others cross the membrane many times. For example, the retinal photoreceptor and β 2 -adrenergic receptor cross the bilayer 7 times.

Peripheral proteins and domains of integral proteins, located on the outer surface of all membranes, are almost always glycosylated. Oligosaccharide residues protect the protein from proteolysis and are also involved in ligand recognition or adhesion.

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

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

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

A deeper study of the structure of the cell membrane reveals many of its features related to the functions it performs.

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

Phospholipid molecules in the cell membrane are arranged in two rows so that their hydrophobic “ends” are on the inside and their hydrophilic “heads” are on the outside. The result is a fairly strong structure that protects the contents of the cell from the external environment.

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

Depending on how the protein is included in the membrane, three types of proteins are distinguished: integral, semi-integral and peripheral. Integral proteins pass through the entire thickness of the membrane, and their ends protrude on both sides. They mainly perform a transport function. In semi-integral proteins, one end is located in the thickness of the membrane, and the second goes outside (from the outer or inner) side. 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 cell surface complex, but not the only one. Its other components are the supra-membrane layer and the sub-membrane layer.

The glycocalyx (the supra-membrane 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 (membrane-like).

The supra-membrane structure 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 proteins included in the cell membrane. Various signals are transmitted through such molecular connections.