biological membranes- the general name of functionally active surface structures that limit cells (cellular or plasma membranes) and intracellular organelles (membranes of mitochondria, nuclei, lysosomes, endoplasmic reticulum, etc.). They contain lipids, proteins, heterogeneous molecules (glycoproteins, glycolipids) and, depending on the function performed, numerous minor components: coenzymes, nucleic acids, antioxidants, carotenoids, inorganic ions, etc.

The coordinated functioning of membrane systems - receptors, enzymes, transport mechanisms - helps maintain cell homeostasis and at the same time quickly respond to changes in the external environment.

To main functions of biological membranes can be attributed:

separation of the cell from the environment and the formation of intracellular compartments (compartments);

control and regulation of the transport of a huge variety of substances through membranes;

participation in providing intercellular interactions, transmission of signals inside the cell;

conversion of the energy of food organic substances into the energy of chemical bonds of ATP molecules.

The molecular organization of the plasma (cell) membrane in all cells is approximately the same: it consists of two layers of lipid molecules with many specific proteins included in it. Some membrane proteins have enzymatic activity, while others bind nutrients from the environment and ensure their transport into the cell through membranes. Membrane proteins are distinguished by the nature of their association with membrane structures. Some proteins, called external or peripheral , loosely bound to the surface of the membrane, others, called internal or integrated , are immersed inside the membrane. Peripheral proteins are easily extracted, while integral proteins can only be isolated using detergents or organic solvents. On fig. 4 shows the structure of the plasma membrane.

The outer, or plasma, membranes of many cells, as well as the membranes of intracellular organelles, such as mitochondria, chloroplasts, were isolated in a free form and their molecular composition was studied. All membranes contain polar lipids in an amount ranging from 20 to 80% of its mass, depending on the type of membranes, the rest is mainly accounted for by proteins. So, in the plasma membranes of animal cells, the amount of proteins and lipids, as a rule, is approximately the same; the inner mitochondrial membrane contains about 80% proteins and only 20% lipids, while the myelin membranes of brain cells, on the contrary, contain about 80% lipids and only 20% proteins.

Rice. 4. Structure of the plasma membrane

The lipid part of the membranes is a mixture of various kinds of polar lipids. Polar lipids, which include phosphoglycerolipids, sphingolipids, glycolipids, are not stored in fat cells, but are incorporated into cell membranes, and in strictly defined ratios.

All polar lipids in membranes are constantly renewed during metabolism; under normal conditions, a dynamic stationary state is established in the cell, in which the rate of lipid synthesis is equal to the rate of their decay.

The membranes of animal cells contain mainly phosphoglycerolipids and, to a lesser extent, sphingolipids; triacylglycerols are found only in trace amounts. Some membranes of animal cells, especially the outer plasma membrane, contain significant amounts of cholesterol and its esters (Fig. 5).

Fig.5. Membrane lipids

Currently, the generally accepted model for the structure of membranes is the fluid mosaic model proposed in 1972 by S. Singer and J. Nicholson.

According to her, proteins can be likened to icebergs floating in a lipid sea. As mentioned above, there are 2 types of membrane proteins: integral and peripheral. Integral proteins penetrate the membrane through, they are amphipathic molecules. Peripheral proteins do not penetrate the membrane and are less strongly associated with it. The main continuous part of the membrane, that is, its matrix, is the polar lipid bilayer. At normal cell temperature, the matrix is ​​in a liquid state, which is ensured by a certain ratio between saturated and unsaturated fatty acids in the hydrophobic tails of polar lipids.

The fluid mosaic model also suggests that on the surface of integral proteins located in the membrane there are R-groups of amino acid residues (mainly hydrophobic groups, due to which proteins seem to “dissolve” in the central hydrophobic part of the bilayer). At the same time, on the surface of peripheral, or external proteins, there are mainly hydrophilic R-groups, which are attracted to the hydrophilic charged polar heads of lipids due to electrostatic forces. Integral proteins, and these include enzymes and transport proteins, are active only if they are located inside the hydrophobic part of the bilayer, where they acquire the spatial configuration necessary for the manifestation of activity (Fig. 6). It should be emphasized once again that no covalent bonds are formed between the molecules in the bilayer, nor between the proteins and lipids of the bilayer.

Fig.6. Membrane proteins

Membrane proteins can move freely in the lateral plane. Peripheral proteins literally float on the surface of the bilayer "sea", while integral proteins, like icebergs, are almost completely submerged in the hydrocarbon layer.

Most of the membranes are asymmetric, that is, they have unequal sides. This asymmetry is manifested in the following:

Firstly, the fact that the inner and outer sides of the plasma membranes of bacterial and animal cells differ in the composition of polar lipids. For example, the inner lipid layer of human erythrocyte membranes contains mainly phosphatidylethanolamine and phosphatidylserine, while the outer lipid layer contains phosphatidylcholine and sphingomyelin.

· secondly, some transport systems in membranes act only in one direction. For example, erythrocyte membranes have a transport system (“pump”) that pumps Na + ions from the cell to the environment, and K + ions into the cell due to the energy released during ATP hydrolysis.

Thirdly, the outer surface of the plasma membrane contains a very large number of oligosaccharide groups, which are the heads of glycolipids and oligosaccharide side chains of glycoproteins, while there are practically no oligosaccharide groups on the inner surface of the plasma membrane.

The asymmetry of biological membranes is preserved due to the fact that the transfer of individual phospholipid molecules from one side of the lipid bilayer to the other is very difficult for energy reasons. The polar lipid molecule is able to move freely on its side of the bilayer, but is limited in its ability to jump to the other side.

Lipid mobility depends on the relative content and type of unsaturated fatty acids present. The hydrocarbon nature of fatty acid chains gives the membrane properties of fluidity, mobility. In the presence of cis-unsaturated fatty acids, the cohesive forces between chains are weaker than in the case of saturated fatty acids alone, and lipids retain high mobility even at low temperatures.

On the outer side of the membranes there are specific recognition sites, the function of which is to recognize certain molecular signals. For example, it is through the membrane that some bacteria perceive slight changes in nutrient concentration, which stimulates their movement towards the food source; this phenomenon is called chemotaxis.

The membranes of various cells and intracellular organelles have a certain specificity due to their structure, chemical composition and functions. The following main groups of membranes in eukaryotic organisms are distinguished:

plasma membrane (outer cell membrane, plasmalemma),

the nuclear membrane

The endoplasmic reticulum

membranes of the Golgi apparatus, mitochondria, chloroplasts, myelin sheaths,

excitable membranes.

In prokaryotic organisms, in addition to the plasma membrane, there are intracytoplasmic membrane formations; in heterotrophic prokaryotes, they are called mesosomes. The latter are formed by invagination into the outer cell membrane and in some cases remain in contact with it.

erythrocyte membrane consists of proteins (50%), lipids (40%) and carbohydrates (10%). The main part of carbohydrates (93%) is associated with proteins, the rest - with lipids. In the membrane, lipids are arranged asymmetrically in contrast to the symmetrical arrangement in micelles. For example, cephalin is found predominantly in the inner layer of lipids. This asymmetry is maintained, apparently, due to the transverse movement of phospholipids in the membrane, carried out with the help of membrane proteins and due to the energy of metabolism. In the inner layer of the erythrocyte membrane are mainly sphingomyelin, phosphatidylethanolamine, phosphatidylserine, in the outer layer - phosphatidylcholine. The erythrocyte membrane contains an integral glycoprotein glycophorin, consisting of 131 amino acid residues and penetrating the membrane, and the so-called band 3 protein, consisting of 900 amino acid residues. The carbohydrate components of glycophorin perform a receptor function for influenza viruses, phytohemagglutinins, and a number of hormones. Another integral protein containing few carbohydrates and penetrating the membrane was also found in the erythrocyte membrane. He is called tunnel protein(component a), as it is assumed that it forms a channel for anions. The peripheral protein associated with the inner side of the erythrocyte membrane is spectrin.

Myelin membranes , surrounding axons of neurons, are multilayered, they contain a large amount of lipids (about 80%, half of them are phospholipids). The proteins of these membranes are important for the fixation of membrane salts lying one above the other.

chloroplast membranes. Chloroplasts are covered with a two-layer membrane. The outer membrane bears some resemblance to that of mitochondria. In addition to this surface membrane, chloroplasts have an internal membrane system - lamellae. Lamellae form or flattened vesicles - thylakoids, which, located one above the other, are collected in packs (grana) or form a membrane system of the stroma (stromal lamellae). Lamella gran and stroma on the outer side of the thylakoid membrane are concentrated hydrophilic groups, galacto- and sulfolipids. The phytolic part of the chlorophyll molecule is immersed in the globule and is in contact with the hydrophobic groups of proteins and lipids. The porphyrin nuclei of chlorophyll are mainly localized between the adjoining membranes of the thylakoids of the gran.

Inner (cytoplasmic) membrane of bacteria similar in structure to the inner membranes of chloroplasts and mitochondria. It contains enzymes of the respiratory chain, active transport; enzymes involved in the formation of membrane components. The predominant component of bacterial membranes are proteins: the protein/lipid ratio (by weight) is 3:1. The outer membrane of gram-negative bacteria, compared with the cytoplasmic one, contains a smaller amount of various phospholipids and proteins. Both membranes differ in lipid composition. The outer membrane contains proteins that form pores for the penetration of many low molecular weight substances. A characteristic component of the outer membrane is also a specific lipopolysaccharide. A number of outer membrane proteins serve as receptors for phages.

Virus membrane. Among viruses, membrane structures are characteristic of those containing a nucleocapsid, which consists of a protein and a nucleic acid. This "core" of viruses is surrounded by a membrane (envelope). It also consists of a bilayer of lipids with glycoproteins included in it, located mainly on the surface of the membrane. In a number of viruses (microviruses), 70-80% of all proteins enter the membranes, the remaining proteins are contained in the nucleocapsid.

Thus, cell membranes are very complex structures; their constituent molecular complexes form an ordered two-dimensional mosaic, which gives the membrane surface biological specificity.

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 a 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 membranes.

Structure and composition of membranes

The basis of the membrane is a lipid bilayer, in the formation of which phospholipids and glycolipids participate. The lipid bilayer is formed by two rows of lipids, the hydrophobic radicals of which are hidden inside, and the hydrophilic groups are turned outward and are in contact with the aqueous medium. Protein molecules seem to be “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, spontaneously forming a bilayer. There are three main types of lipids in membranes: phospholipids, glycolipids, and cholesterol.

The lipid composition is different. The content of one or another lipid, apparently, is 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 derivatives of phosphatidic acid. 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 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. 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 only 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 the cell due to the small size of the molecules and weak interaction with solvents. Also, molecules of a lipid nature, for example, steroid hormones, easily penetrate through the bilayer.

2. Liquidity. The membranes are characterized by fluidity (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 the expenditure of energy. More often, rotations around the axis (rotation) and lateral diffusion are observed - movement within the layer parallel to the membrane surface. The speed of movement of molecules depends on the microviscosity of membranes, which, in turn, is determined by the relative content of saturated and unsaturated fatty acids in the composition of lipids. Microviscosity is lower if unsaturated fatty acids predominate in the composition of lipids, and higher if the content of saturated fatty acids is high.

3. Asymmetry of membranes. 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 pouch 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 regulated by them - adenylate cyclase, phospholipase C - on the inside, 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 main histocompatibility system, receptors for various molecules.

By 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), while others cross the membrane many times. For example, the retinal photoreceptor and β 2 -adrenergic receptor crosses 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 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.

9.5.1. One of the main functions of membranes is participation in the transport of substances. This process is provided by three main mechanisms: simple diffusion, facilitated diffusion and active transport (Figure 9.10). Remember the most important features of these mechanisms and examples of the transported substances in each case.

Figure 9.10. Mechanisms of transport of molecules across the membrane

simple diffusion- transfer of substances through the membrane without the participation of special mechanisms. Transport occurs along a concentration gradient without energy consumption. Small biomolecules - H2O, CO2, O2, urea, hydrophobic low molecular weight substances are transported by simple diffusion. The rate of simple diffusion is proportional to the concentration gradient.

Facilitated diffusion- the transfer of substances across the membrane using protein channels or special carrier proteins. It is carried out along the concentration gradient without energy consumption. Monosaccharides, amino acids, nucleotides, glycerol, some ions are transported. Saturation kinetics is characteristic - at a certain (saturating) concentration of the transferred substance, all carrier molecules take part in the transfer and the transport speed reaches a limiting value.

active transport- also requires the participation of special carrier proteins, but the transfer occurs against a concentration gradient and therefore requires energy. With the help of this mechanism, Na+, K+, Ca2+, Mg2+ ions are transported through the cell membrane, and protons through the mitochondrial membrane. The active transport of substances is characterized by saturation kinetics.

9.5.2. An example of a transport system that performs active ion transport is Na+,K+ -adenosine triphosphatase (Na+,K+ -ATPase or Na+,K+ -pump). This protein is located in the thickness of the plasma membrane and is able to catalyze the reaction of ATP hydrolysis. The energy released during the hydrolysis of 1 ATP molecule is used to transfer 3 Na + ions from the cell to the extracellular space and 2 K + ions in the opposite direction (Figure 9.11). As a result of the action of Na + , K + -ATPase, a concentration difference is created between the cytosol of the cell and the extracellular fluid. Since the transport of ions is non-equivalent, a difference in electrical potentials arises. Thus, an electrochemical potential arises, which is the sum of the energy of the difference in electric potentials Δφ and the energy of the difference in the concentrations of substances ΔС on both sides of the membrane.

Figure 9.11. Scheme of Na+, K+ -pump.

9.5.3. Transfer through membranes of particles and macromolecular compounds

Along with the transport of organic substances and ions carried out by carriers, there is a very special mechanism in the cell designed to absorb and remove macromolecular compounds from the cell by changing the shape of the biomembrane. Such a mechanism is called vesicular transport.

Figure 9.12. Types of vesicular transport: 1 - endocytosis; 2 - exocytosis.

During the transfer of macromolecules, sequential formation and fusion of vesicles (vesicles) surrounded by a membrane occur. According to the direction of transport and the nature of the transferred substances, the following types of vesicular transport are distinguished:

Endocytosis(Figure 9.12, 1) - the transfer of substances into the cell. Depending on the size of the resulting vesicles, there are:

a) pinocytosis - absorption of liquid and dissolved macromolecules (proteins, polysaccharides, nucleic acids) using small bubbles (150 nm in diameter);

b) phagocytosis — absorption of large particles, such as microorganisms or cell debris. In this case, large vesicles are formed, called phagosomes with a diameter of more than 250 nm.

Pinocytosis is characteristic of most eukaryotic cells, while large particles are absorbed by specialized cells - leukocytes and macrophages. At the first stage of endocytosis, substances or particles are adsorbed on the membrane surface; this process occurs without energy consumption. At the next stage, the membrane with the adsorbed substance deepens into the cytoplasm; the resulting local invaginations of the plasma membrane are laced from the cell surface, forming vesicles, which then migrate into the cell. This process is connected by a system of microfilaments and is energy dependent. The vesicles and phagosomes that enter the cell can merge with lysosomes. Enzymes contained in lysosomes break down substances contained in vesicles and phagosomes to low molecular weight products (amino acids, monosaccharides, nucleotides), which are transported to the cytosol, where they can be used by the cell.

Exocytosis(Figure 9.12, 2) - the transfer of particles and large compounds from the cell. This process, like endocytosis, proceeds with the absorption of energy. The main types of exocytosis are:

a) secretion - removal from the cell of water-soluble compounds that are used or affect other cells of the body. It can be carried out both by non-specialized cells and cells of the endocrine glands, the mucosa of the gastrointestinal tract, adapted for the secretion of the substances they produce (hormones, neurotransmitters, proenzymes), depending on the specific needs of the body.

Secreted proteins are synthesized on ribosomes associated with the membranes of the rough endoplasmic reticulum. These proteins are then transported to the Golgi apparatus, where they are modified, concentrated, sorted, and then packaged into vesicles, which are cleaved into the cytosol and subsequently fuse with the plasma membrane so that the contents of the vesicles are outside the cell.

Unlike macromolecules, small secreted particles, such as protons, are transported out of the cell using facilitated diffusion and active transport mechanisms.

b) excretion - removal from the cell of substances that cannot be used (for example, the removal of a reticular substance from reticulocytes during erythropoiesis, which is an aggregated remnant of organelles). The mechanism of excretion, apparently, consists in the fact that at first the excreted particles are in the cytoplasmic vesicle, which then merges with the plasma membrane.

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.