What is the membrane responsible for? What is the function of the cell membrane - its properties and functions

The study of the structure of organisms, as well as plants, animals and humans, is the branch of biology called cytology. Scientists have found that the contents of the cell, which is inside it, is quite complex. It is surrounded by the so-called surface apparatus, which includes the outer cell membrane, supra-membrane structures: glycocalyx and microfilaments, pelicule and microtubules that form its submembrane complex.

In this article, we will study the structure and functions of the outer cell membrane, which is part of the surface apparatus of various types of cells.

What are the functions of the outer cell membrane?

As described earlier, the outer membrane is part of the surface apparatus of each cell, which successfully separates its internal contents and protects cell organelles from adverse environmental conditions. Another function is to ensure the exchange of substances between the cell contents and the tissue fluid, therefore, the outer cell membrane transports molecules and ions entering the cytoplasm, and also helps to remove toxins and excess toxic substances from the cell.

The structure of the cell membrane

Membranes, or plasmalemmas, of different types of cells are very different from each other. Mainly, the chemical structure, as well as the relative content of lipids, glycoproteins, proteins in them and, accordingly, the nature of the receptors in them. External which is determined primarily by the individual composition of glycoproteins, takes part in the recognition of environmental stimuli and in the reactions of the cell itself to their actions. Some types of viruses can interact with proteins and glycolipids of cell membranes, as a result of which they penetrate into the cell. Herpes and influenza viruses can use to build their protective shell.

And viruses and bacteria, the so-called bacteriophages, attach to the cell membrane and dissolve it at the point of contact with the help of a special enzyme. Then a molecule of viral DNA passes into the hole formed.

Features of the structure of the plasma membrane of eukaryotes

Recall that the outer cell membrane performs the function of transport, that is, the transfer of substances into and out of it into the external environment. To carry out such a process, a special structure is required. Indeed, the plasmalemma is a constant, universal system of the surface apparatus for all. This is a thin (2-10 Nm), but fairly dense multilayer film that covers the entire cell. Its structure was studied in 1972 by such scientists as D. Singer and G. Nicholson, they also created a fluid-mosaic model of the cell membrane.

The main chemical compounds that form it are ordered molecules of proteins and certain phospholipids, which are interspersed in a liquid lipid environment and resemble a mosaic. Thus, the cell membrane consists of two layers of lipids, the nonpolar hydrophobic "tails" of which are inside the membrane, and the polar hydrophilic heads face the cytoplasm of the cell and the interstitial fluid.

The lipid layer is penetrated by large protein molecules that form hydrophilic pores. It is through them that aqueous solutions of glucose and mineral salts are transported. Some protein molecules are located both on the outer and inner surfaces of the plasmalemma. Thus, on the outer cell membrane in the cells of all organisms with nuclei, there are carbohydrate molecules bound by covalent bonds with glycolipids and glycoproteins. The content of carbohydrates in cell membranes ranges from 2 to 10%.

The structure of the plasmalemma of prokaryotic organisms

The outer cell membrane in prokaryotes performs similar functions to the plasma membranes of cells of nuclear organisms, namely: the perception and transmission of information coming from the external environment, the transport of ions and solutions into and out of the cell, and the protection of the cytoplasm from foreign reagents from the outside. It can form mesosomes - structures that arise when the plasmalemma protrudes into the cell. They may contain enzymes involved in the metabolic reactions of prokaryotes, for example, in DNA replication, protein synthesis.

Mesosomes also contain redox enzymes, while photosynthetics contain bacteriochlorophyll (in bacteria) and phycobilin (in cyanobacteria).

The role of outer membranes in intercellular contacts

Continuing to answer the question of what functions the outer cell membrane performs, let us dwell on its role in plant cells. In plant cells, pores are formed in the walls of the outer cell membrane, passing into the cellulose layer. Through them, the exit of the cytoplasm of the cell to the outside is possible; such thin channels are called plasmodesmata.

Thanks to them, the connection between neighboring plant cells is very strong. In human and animal cells, the sites of contact between adjacent cell membranes are called desmosomes. They are characteristic of endothelial and epithelial cells, and are also found in cardiomyocytes.

Auxiliary formations of the plasmalemma

To understand how plant cells differ from animals, it helps to study the structural features of their plasma membranes, which depend on what functions the outer cell membrane performs. Above it in animal cells is a layer of glycocalyx. It is formed by polysaccharide molecules associated with proteins and lipids of the outer cell membrane. Thanks to the glycocalyx, adhesion (sticking) occurs between cells, leading to the formation of tissues, therefore it takes part in the signaling function of the plasmalemma - the recognition of environmental stimuli.

How is the passive transport of certain substances across cell membranes

As mentioned earlier, the outer cell membrane is involved in the process of transporting substances between the cell and the external environment. There are two types of transport through the plasmalemma: passive (diffusion) and active transport. The first includes diffusion, facilitated diffusion and osmosis. The movement of substances along the concentration gradient depends primarily on the mass and size of the molecules passing through the cell membrane. For example, small non-polar molecules easily dissolve in the middle lipid layer of the plasmalemma, move through it and end up in the cytoplasm.

Large molecules of organic substances penetrate into the cytoplasm with the help of special carrier proteins. They are species-specific and, when combined with a particle or ion, passively transfer them through the membrane along the concentration gradient without expending energy (passive transport). This process underlies such property of the plasmalemma as selective permeability. In the process, the energy of ATP molecules is not used, and the cell saves it for other metabolic reactions.

Active transport of chemical compounds across the plasmalemma

Since the outer cell membrane ensures the transfer of molecules and ions from the external environment into the cell and back, it becomes possible to remove the products of dissimilation, which are toxins, to the outside, that is, to the intercellular fluid. occurs against a concentration gradient and requires the use of energy in the form of ATP molecules. It also involves carrier proteins called ATPases, which are also enzymes.

An example of such transport is the sodium-potassium pump (sodium ions pass from the cytoplasm to the external environment, and potassium ions are pumped into the cytoplasm). The epithelial cells of the intestine and kidneys are capable of it. Varieties of this method of transfer are the processes of pinocytosis and phagocytosis. Thus, having studied what functions the outer cell membrane performs, it can be established that heterotrophic protists, as well as cells of higher animal organisms, for example, leukocytes, are capable of pino- and phagocytosis.

Bioelectric processes in cell membranes

It has been established that there is a potential difference between the outer surface of the plasmalemma (it is positively charged) and the parietal layer of the cytoplasm, which is negatively charged. It was called the resting potential, and it is inherent in all living cells. And the nervous tissue has not only a resting potential, but is also capable of conducting weak biocurrents, which is called the process of excitation. The outer membranes of nerve cells-neurons, receiving irritation from receptors, begin to change charges: sodium ions massively enter the cell and the surface of the plasmalemma becomes electronegative. And the parietal layer of the cytoplasm, due to an excess of cations, receives a positive charge. This explains why the outer cell membrane of the neuron is recharged, which causes the conduction of nerve impulses that underlie the excitation process.

Outside, the cell is covered with a plasma membrane (or outer cell membrane) about 6-10 nm thick.

The cell membrane is a dense film of proteins and lipids (mainly phospholipids). Lipid molecules are arranged in an orderly manner - perpendicular to the surface, in two layers, so that their parts that interact intensively with water (hydrophilic) are directed outward, and the parts that are inert to water (hydrophobic) are directed inward.

Protein molecules are located in a non-continuous layer on the surface of the lipid framework on both sides. Some of them are immersed in the lipid layer, and some pass through it, forming areas permeable to water. These proteins perform various functions - some of them are enzymes, others are transport proteins involved in the transfer of certain substances from the environment to the cytoplasm and vice versa.

Basic Functions of the Cell Membrane

One of the main properties of biological membranes is selective permeability (semipermeability)- some substances pass through them with difficulty, others easily and even towards a higher concentration. Thus, for most cells, the concentration of Na ions inside is much lower than in the environment. For K ions, the reverse ratio is characteristic: their concentration inside the cell is higher than outside. Therefore, Na ions always tend to enter the cell, and K ions - to go outside. The equalization of the concentrations of these ions is prevented by the presence in the membrane of a special system that plays the role of a pump that pumps Na ions out of the cell and simultaneously pumps K ions inside.

The desire of Na ions to move from outside to inside is used to transport sugars and amino acids into the cell. With the active removal of Na ions from the cell, conditions are created for the entry of glucose and amino acids into it.

In many cells, absorption of substances also occurs by phagocytosis and pinocytosis. At phagocytosis the flexible outer membrane forms a small depression where the captured particle enters. This recess increases, and, surrounded by a portion of the outer membrane, the particle is immersed in the cytoplasm of the cell. The phenomenon of phagocytosis is characteristic of amoeba and some other protozoa, as well as leukocytes (phagocytes). Similarly, the cells absorb liquids containing the substances necessary for the cell. This phenomenon has been called pinocytosis.

The outer membranes of various cells differ significantly both in the chemical composition of their proteins and lipids, and in their relative content. It is these features that determine the diversity in the physiological activity of the membranes of various cells and their role in the life of cells and tissues.

The endoplasmic reticulum of the cell is connected to the outer membrane. With the help of outer membranes, various types of intercellular contacts are carried out, i.e. communication between individual cells.

Many types of cells are characterized by the presence on their surface of a large number of protrusions, folds, microvilli. They contribute both to a significant increase in the surface area of ​​cells and improve metabolism, as well as to stronger bonds of individual cells with each other.

On the outside of the cell membrane, plant cells have thick membranes that are clearly visible in an optical microscope, consisting of cellulose (cellulose). They create a strong support for plant tissues (wood).

Some cells of animal origin also have a number of external structures that are located on top of the cell membrane and have a protective character. An example is the chitin of the integumentary cells of insects.

Functions of the cell membrane (briefly)

biological membranes- complex supramolecular structures that surround all living cells and form closed, specialized compartments in them - organelles.

The membrane that bounds the cytoplasm of the cell from the outside is called the cytoplasmic or plasma membrane. The name of intracellular membranes usually comes from the name of the subcellular structures they limit or form.

Distinguish:

the nuclear

mitochondrial,

the lysosomal membrane

membranes of the Golgi complex

endoplasmic reticulum and others.

Membrane is a thin structure with a thickness of 7 nm.

According to its chemical composition, the membrane contains:

25% proteins,

25% phospholipids,

13% cholesterol,

4% lipids,

3% carbohydrates.

Structurally The basis of the membrane is a double layer of phospholipids.

A feature of phospholipid molecules is that they have hydrophilic and hydrophobic parts in their composition. The hydrophilic parts contain polar groups (phosphate groups in phospholipids and hydroxide groups in cholesterol). hydrophilic parts directed towards the surface. BUT hydrophobic (fatty tails) are directed towards the center of the membrane.

The molecule has two fatty tails, and these hydrocarbon chains can be found in two configurations. Stretched - trans configuration(cylinder 0.48 nm). The second type is the gosh-trans-gosh configuration. In this case, the two fat tails diverge and the area increases to 0.58 nm.

Lipid molecules under normal conditions have a liquid crystal form. And in this state they have mobility. Moreover, they can both move within their layer and turn over. When the temperature is lowered, the transition from the liquid state of the membrane to the jelly-like one occurs, and this reduces the mobility of the molecule.

When the lipid molecule moves, microstrips are formed, which are called kings, into which substances can be trapped. The lipid layer in the membrane is a barrier to water-soluble substances, but it allows fat-soluble substances to pass through..

A closed lipid bilayer determines the main properties of membranes:

1) fluidity- depends on the ratio of saturated and unsaturated fatty acids in the composition of membrane lipids. Hydrophobic chains of saturated fatty acids are oriented parallel to each other and form a rigid crystalline structure (Figure 14.8, a). Unsaturated fatty acids, which have a bent hydrocarbon chain, break the compactness of the package and make the membrane more fluid (Figure 14.8, b). Cholesterol, embedding between fatty acids, condenses them and increases the rigidity of the membranes.

Figure 14.8. Influence of the fatty acid composition of phospholipids on the fluidity of the lipid bilayer.

2) lateral diffusion- free movement of molecules relative to each other in the plane of the membranes (Figure 14.9, a).

Figure 14.9. Types of movements of phospholipid molecules in the lipid bilayer.

3) limited capacity for transverse diffusion(the transition of molecules from the outer layer to the inner and vice versa, see Figure 14.9, b), which contributes to the preservation asymmetries– structural and functional differences between the outer and inner layers of the membrane.

4) impenetrability closed bilayer for most water-soluble molecules.

In addition to lipids, the membrane also contains protein molecules. Mostly glycoproteins.

Integral proteins pass through both layers. Other proteins are partially immersed in either the outer or inner layer. They are called peripheral proteins..

This membrane model is called liquid crystal model. Functionally, protein molecules perform structural, transport, enzymatic functions. In addition, they form ion channels with a diameter of 0.35 to 0.8 nm in diameter, through which ions can pass. Channels have their own specialization. Integral proteins are involved in active transport and facilitated diffusion.

Peripheral proteins on the inside of the membrane are characterized by an enzymatic function. On the inside - antigenic (antibodies) and receptor functions.

carbon chains can attach to protein molecules, and then form glycoproteins. Or to lipids, then they are called glycolipids.

Main Functions cell membranes will be:

1. barrier function(expressed in the fact that the membrane, using appropriate mechanisms, participates in the creation of concentration gradients, preventing free diffusion. At the same time, the membrane takes part in the mechanisms of electrogenesis. These include the mechanisms for creating a resting potential, the generation of an action potential, the mechanisms for the propagation of bioelectric impulses along a homogeneous and inhomogeneous excitable structures.)

2. Substance transfer.

Figure 14.10.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. By simple diffusion, small biomolecules are transported - H 2 O, CO 2, O 2, urea, hydrophobic low molecular weight substances. 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 the limit 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 +, Ca 2+, Mg 2+ ions are transported through the cell membrane, and protons through the mitochondrial membrane. The active transport of substances is characterized by saturation kinetics.

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 14.12.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 14.12, 1) - the transfer of substances into the cell. Depending on the size of the resulting vesicles, there are:

but) 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 14.12, 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:

but) 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 released particles are in the cytoplasmic vesicle, which then merges with the plasma membrane.

3. metabolic function(due to the presence of enzyme systems in them)

4. Membranes are involved in creation of electrical potentials at rest, and when excited - action currents.

5. Receptor function.

6. Immunological(associated with the presence of antigens and the production of antibodies).

7. Provide intercellular interaction and contact inhibition. (When homogeneous cells come into contact, inhibition of cell division occurs. This function is lost in cancer cells. In addition, cancer cells come into contact not only with their own cells, but also with other cells, infecting them.)

Receptors, their classification: by localization (membrane, nuclear), by the mechanism of development of processes (ionotropic and metaiotropic), by the speed of signal reception (fast, slow), by the type of receiving substances.

Receptors are final specialized formations designed to transform the energy of various types of stimuli into specific activity of the nervous system.

Classification:

by localization

membrane

nuclear

according to the process development mechanism

Ionotropic (they are membrane channels that open or close when bound to a ligand. The resulting ion currents cause changes in the transmembrane potential difference and, as a result, cell excitability, and also change intracellular concentrations of ions, which can secondarily lead to the activation of intracellular mediator systems One of the most fully studied ionotropic receptors is the n-cholinergic receptor.)

Metabotropic (associated with systems of intracellular mediators. Changes in their conformation upon binding to a ligand leads to the launch of a cascade of biochemical reactions, and, ultimately, a change in the functional state of the cell.)

by signal reception speed

fast

slow

by type of absorbent

· Chemoreceptors- perceive the impact of dissolved or volatile chemicals.

· Osmoreceptors- perceive changes in the osmotic concentration of the liquid (as a rule, the internal environment).

· Mechanoreceptors- perceive mechanical stimuli (touch, pressure, stretching, vibrations of water or air, etc.)

· Photoreceptors- perceive visible and ultraviolet light

· thermoreceptors- perceive a decrease (cold) or an increase (thermal) temperature

· Baroreceptors- perceive changes in pressure

3. Ionotropic receptors, metaboprop receptors and their varieties. Systems of secondary mediators of action of metabotropic receptors (cAMP, cGMP, inositol-3-phosphate, diacylglycerol, Ca++ ions).

There are two types of receptors on the postsynaptic membrane - ionotropic and metabotropic.

Ionotropic
When ionotropic receptor the sensitive molecule contains not only an active site for mediator binding, but also an ion channel. The impact of the "primary mediator" (mediator) on the receptor leads to a rapid opening of the channel and the development of postsynaptic potential.
Metabotropic
When a mediator is attached and the metabotropic receptor is excited, intracellular metabolism changes, i.e. course of biochemical reactions

On the inside of the membrane, a number of other proteins are attached to such a receptor, performing enzymatic and partly transmitting ("intermediary") functions (Fig.). Intermediary proteins are referred to as G proteins. Under the influence of an excited receptor, the G-protein acts on the protein-enzyme, usually putting it into a "working" state. As a result, a chemical reaction is triggered: the precursor molecule turns into a signal molecule - a second messenger.

Rice. Scheme of the structure and functioning of the metabotropic receptor: 1 - mediator; 2 - receptor; 3 - ion channel; 4 - secondary intermediary; 5 - enzyme; 6 - G-protein; → - direction of signal transmission

Secondary intermediaries are small, fast-moving molecules or ions that transmit a signal inside the cell. In this they differ from the "primary mediators" - mediators and hormones that transmit information from cell to cell.

The best known second messenger is cAMP (cyclic adenosine monophosphoric acid), which is formed from ATP by the enzyme adenylate cyclase. It looks like cGMP (guanosine monophosphoric acid). Other important secondary messengers are inositol triphosphate and diacylglycerol, which are formed from the components of the cell membrane under the action of the enzyme phospholipase C. The role of Ca 2+, which enters the cell from the outside through ion channels or is released from special storage sites inside the cell (“depot” of calcium), is extremely important. Recently, much attention has been paid to the second messenger NO (nitric oxide), which is able to transmit a signal not only inside the cell, but also between cells, easily crossing the membrane, including from the postsynaptic neuron to the presynaptic one.

The final step in conducting a chemical signal is the action of a second messenger on the chemosensitive ion channel. This effect occurs either directly or through additional intermediates (enzymes). In any case, the ion channel opens and either EPSP or IPSP develops. The duration and amplitude of their first phase will be determined by the amount of the second messenger, which depends on the amount of the released mediator and the duration of its interaction with the receptor.

Thus, the mechanism of nerve stimulus transmission used by metabotropic receptors includes several successive steps. On each of them, regulation (weakening or strengthening) of the signal is possible, which makes the reaction of the postsynaptic cell more flexible and adapted to current conditions. However, this also slows down the process of information transfer.

cAMP system

Phospholipase C

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:

but) 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:

but) 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 released particles are in the cytoplasmic vesicle, which then merges with the plasma membrane.

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 provided 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 the 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.