The cell membrane has a rather complex structure, which can be considered in 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 of it. They mainly perform a transport function. In semi-integral proteins, one end is located in the thickness of the membrane, and the second goes 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.

The structure of the biomembrane. The cell-bounding membranes and membrane organelles of eukaryotic cells have a common chemical composition and structure. They include lipids, proteins and carbohydrates. Membrane lipids are mainly represented by phospholipids and cholesterol. Most membrane proteins are complex proteins, such as glycoproteins. Carbohydrates do not occur independently in the membrane; they are associated with proteins and lipids. The thickness of the membranes is 7-10 nm.

According to the currently generally accepted liquid mosaic model of membrane structure, lipids form a double layer, or lipid bilayer, in which the hydrophilic “heads” of lipid molecules face outward, and the hydrophobic “tails” are hidden inside the membrane (Fig. 2.24). These “tails,” due to their hydrophobicity, ensure the separation of the aqueous phases of the internal environment of the cell and its environment. Proteins are associated with lipids through various types of interactions. Some proteins are located on the surface of the membrane. Such proteins are called peripheral, or superficial. Other proteins are partially or completely immersed in the membrane - these are integral, or submerged proteins. Membrane proteins perform structural, transport, catalytic, receptor and other functions.

Membranes are not like crystals; their components are constantly in motion, as a result of which gaps appear between lipid molecules - pores through which various substances can enter or leave the cell.

Biological membranes differ in their location in the cell, chemical composition and functions. The main types of membranes are plasma and internal.

Plasma membrane(Fig. 2.24) contains about 45% lipids (including glycolipids), 50% proteins and 5% carbohydrates. Chains of carbohydrates, which are part of complex proteins-glycoproteins and complex lipids-glycolipids, protrude above the surface of the membrane. Plasmalemma glycoproteins are extremely specific. For example, they are used for mutual recognition of cells, including sperm and egg.

On the surface of animal cells, carbohydrate chains form a thin surface layer - glycocalyx. It is detected in almost all animal cells, but its degree of expression varies (10-50 µm). The glycocalyx provides direct communication between the cell and the external environment, where extracellular digestion occurs; Receptors are located in the glycocalyx. In addition to the plasmalemma, the cells of bacteria, plants and fungi are also surrounded by cell membranes.

Internal membranes eukaryotic cells delimit different parts of the cell, forming peculiar “compartments” - compartments, which promotes the separation of various metabolic and energy processes. They may vary according to chemical composition and the functions performed, but their general structural plan remains the same.

Membrane functions:

1. Limiting. The idea is that they separate the internal space of the cell from the external environment. The membrane is semi-permeable, that is, only those substances that the cell needs can freely pass through it, and there are mechanisms for transporting the necessary substances.

2. Receptor. Primarily associated with the perception of signals environment and transfer of this information inside the cell. Special receptor proteins are responsible for this function. Membrane proteins are also responsible for cellular recognition according to the “friend or foe” principle, as well as for the formation of intercellular connections, the most studied of which are the synapses of nerve cells.

3. Catalytic. Numerous enzyme complexes are located on the membranes, as a result of which intensive synthetic processes occur on them.

4. Energy transforming. Associated with the formation of energy, its storage in the form of ATP and consumption.

5. Compartmentalization. Membranes also delimit the space inside the cell, thereby separating the starting materials of the reaction and the enzymes that can carry out the corresponding reactions.

6. Formation of intercellular contacts. Despite the fact that the thickness of the membrane is so small that it cannot be distinguished with the naked eye, it, on the one hand, serves as a fairly reliable barrier for ions and molecules, especially water-soluble ones, and on the other, ensures their transport into and out of the cell.

Membrane transport. Due to the fact that cells, as elementary biological systems, are open systems, to ensure metabolism and energy, maintain homeostasis, growth, irritability and other processes, the transfer of substances through the membrane - membrane transport is required (Fig. 2.25). Currently, the transport of substances across the cell membrane is divided into active, passive, endo- and exocytosis.

Passive transport- this is a type of transport that occurs without energy consumption from a higher concentration to a lower one. Lipid-soluble small non-polar molecules (0 2, C0 2) easily penetrate the cell by simple diffusion. Those insoluble in lipids, including charged small particles, are picked up by carrier proteins or pass through special channels (glucose, amino acids, K +, PO 4 3-). This type of passive transport is called facilitated diffusion. Water enters the cell through pores in the lipid phase, as well as through special channels lined with proteins. The transport of water through a membrane is called by osmosis(Fig. 2.26).

Osmosis is extremely important in the life of a cell, because if it is placed in a solution with a higher concentration of salts than in the cell solution, then water will begin to leave the cell and the volume of living contents will begin to decrease. In animal cells, the cell as a whole shrinks, and in plant cells, the cytoplasm lags behind the cell wall, which is called plasmolysis(Fig. 2.27).

When a cell is placed in a solution less concentrated than the cytoplasm, water transport occurs in the opposite direction - into the cell. However, there are limits to the extensibility of the cytoplasmic membrane, and an animal cell eventually ruptures, while a plant cell does not allow this to happen due to its strong cell wall. The phenomenon of filling the entire internal space of a cell with cellular contents is called deplasmolysis. The intracellular concentration of salts should be taken into account when preparing medications, especially for intravenous administration, as this can lead to damage to blood cells (for this, saline solution with a concentration of 0.9% sodium chloride is used). This is no less important when cultivating cells and tissues, as well as animal and plant organs.

Active transport proceeds with the expenditure of ATP energy from a lower concentration of a substance to a higher one. It is carried out using special pump proteins. Proteins pump K + , Na + , Ca 2+ and other ions through the membrane, which promotes the transport of essential organic substances, as well as the emergence of nerve impulses, etc.

Endocytosis- this is an active process of absorption of substances by the cell, in which the membrane forms invaginations and then forms membrane vesicles - phagosomes, in which the absorbed objects are contained. Then the primary lysosome fuses with the phagosome and forms secondary lysosome, or phagolysosome, or digestive vacuole. The contents of the vesicle are digested by lysosome enzymes, and the breakdown products are absorbed and assimilated by the cell. Undigested residues are removed from the cell by exocytosis. There are two main types of endocytosis: phagocytosis and pinocytosis.

Phagocytosis is the process of capture by the cell surface and absorption of solid particles by the cell, and pinocytosis- liquids. Phagocytosis occurs mainly in animal cells (unicellular animals, human leukocytes), it provides their nutrition, and often protection of the body (Fig. 2.28).

By pinocytosis, proteins, antigen-antibody complexes are absorbed during immune reactions, etc. However, many viruses also enter the cell by pinocytosis or phagocytosis. In plant and fungal cells, phagocytosis is practically impossible, as they are surrounded by durable cell membranes.

Exocytosis- a process reverse to endocytosis. In this way, undigested food remains are released from the digestive vacuoles, and substances necessary for the life of the cell and the body as a whole are removed. For example, the transmission of nerve impulses occurs due to the release of chemical messengers by the neuron sending the impulse - mediators, and in plant cells this is how auxiliary carbohydrates of the cell membrane are secreted.

Cell walls of plant cells, fungi and bacteria. Outside the membrane, the cell can secrete a strong framework - cell membrane, or cell wall.

In plants, the basis of the cell wall is cellulose, packed in bundles of 50-100 molecules. The spaces between them are filled with water and other carbohydrates. The plant cell membrane is permeated with channels - plasmodesmata(Fig. 2.29), through which the membranes of the endoplasmic reticulum pass.

Plasmodesmata carry out the transport of substances between cells. However, transport of substances, such as water, can also occur along the cell walls themselves. Over time, various substances, including tannins or fat-like substances, accumulate in the cell wall of plants, which leads to lignification or suberization of the cell wall itself, displacement of water and death of cellular contents. Between the cell walls of neighboring plant cells there are jelly-like spacers - middle plates that hold them together and cement the plant body as a whole. They are destroyed only during the process of fruit ripening and when the leaves fall.

The cell walls of fungal cells are formed chitin- a carbohydrate containing nitrogen. They are quite strong and are the external skeleton of the cell, but still, like in plants, they prevent phagocytosis.

In bacteria, the cell wall contains carbohydrates with peptide fragments - murein, however, its content varies significantly among different groups of bacteria. Other polysaccharides can also be released outside the cell wall, forming a mucous capsule that protects bacteria from external influences.

The membrane determines the shape of the cell, serves as a mechanical support, performs a protective function, provides the osmotic properties of the cell, limiting the stretching of the living contents and preventing rupture of the cell, which increases due to the entry of water. In addition, water and substances dissolved in it overcome the cell wall before entering the cytoplasm or, conversely, when leaving it, while water is transported through the cell walls faster than through the cytoplasm.

9.5.1. One of the main functions of membranes is participation in the transfer of substances. This process is achieved through 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 substances transported 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. By simple diffusion, small biomolecules are transported - H2O, CO2, O2, urea, hydrophobic low-molecular substances. The rate of simple diffusion is proportional to the concentration gradient.

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

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

9.5.2. An example of a transport system that carries out active transport of ions is Na+,K+-adenosine triphosphatase (Na+,K+-ATPase or Na+,K+-pump). This protein is located deep in the plasma membrane and is capable of catalyzing 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 cell cytosol and the extracellular fluid. Since the transfer of ions is not equivalent, an electrical potential difference occurs. Thus, an electrochemical potential arises, which consists of the energy of the difference in electrical potentials Δφ and the energy of the difference in the concentrations of substances ΔC on both sides of the membrane.

Figure 9.11. Na+, K+ pump diagram.

9.5.3. Transport of particles and high molecular weight compounds across membranes

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 high-molecular compounds into the cell and remove high-molecular compounds from it by changing the shape of the biomembrane. This 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 membrane-surrounded vesicles (vesicles) occurs. Based on the direction of transport and the nature of the substances transported, the following types of vesicular transport are distinguished:

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

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 called phagosomes with a diameter of more than 250 nm are formed.

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 surface of the membrane; 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 detached 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 into low molecular weight products (amino acids, monosaccharides, nucleotides), which are transported into the cytosol, where they can be used by the cell.

Exocytosis(Figure 9.12, 2) - transfer of particles and large compounds from the cell. This process, like endocytosis, occurs 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 unspecialized cells and by cells of the endocrine glands, the mucous membrane 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 released 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 the mechanisms of facilitated diffusion and active transport.

b) excretion - removal from the cell of substances that cannot be used (for example, during erythropoiesis, removal from reticulocytes of the mesh substance, which is aggregated remnants of organelles). The mechanism of excretion appears to be that the excreted particles are initially trapped in a cytoplasmic vesicle, which then fuses with the plasma membrane.

Biological membranes.
The term “membrane” (Latin membrana - skin, film) began to be used more than 100 years ago to designate a cell boundary that serves, 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 limited to this, since biological membranes form the basis of the structural organization of the cell.
Membrane structure. According to this model, the main membrane is a lipid bilayer in which the hydrophobic tails of the molecules face inward and the hydrophilic heads face outward. Lipids are represented by phospholipids - derivatives of glycerol or sphingosine. Proteins are associated with the lipid layer. Integral (transmembrane) proteins penetrate the membrane through and are firmly associated with it; peripheral ones do not penetrate and are less firmly connected to the membrane. Functions of membrane proteins: maintaining membrane structure, 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 constituent proteins and lipids can move in the plane of the membrane.
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 (glycocalyx, which performs signaling and receptor functions, and is also important for uniting cells into tissues)
3. Polarity. The outer side of the membrane carries a positive charge, while the inner side carries a negative charge.
4. Selective permeability. The membranes of living cells, in addition to water, allow only certain molecules and ions of dissolved substances to pass through. (The use of the term “semi-permeability” in relation to cell membranes is not entirely correct, since this concept implies that the membrane allows only solvent molecules to pass through, while retaining all molecules and ions of dissolved substances.)

The outer cell membrane (plasmalemma) is an ultramicroscopic film 7.5 nm thick, consisting of proteins, phospholipids and water. An elastic film that is well wetted by water and quickly restores its integrity after damage. It has a universal structure, typical of all biological membranes. The borderline position of this membrane, its participation in the processes of selective permeability, pinocytosis, phagocytosis, excretion of excretory products and synthesis, in interaction with neighboring cells and protection of 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. In plant cells, outside the cell membrane there is a strong cell wall that creates external support and maintains the shape of the cell. It consists of fiber (cellulose), a water-insoluble polysaccharide.

The cell membrane is the structure that covers the outside of the cell. It is also called cytolemma or plasmalemma.

This formation is built from a bilipid layer (bilayer) with proteins built into it. The carbohydrates that make up the plasmalemma are in a bound state.

The distribution of the main components of the plasmalemma is as follows: more than half of the chemical composition is proteins, a quarter is occupied by phospholipids, and a tenth is cholesterol.

Cell membrane and its types

The cell membrane is a thin film, the basis of which is made up of layers of lipoproteins and proteins.

According to localization, membrane organelles are distinguished, which have some features in plant and animal cells:

• mitochondria;
• core;
• endoplasmic reticulum;
• Golgi complex;
• lysosomes;
• chloroplasts (in plant cells).

There is also an inner and outer (plasmolemma) cell membrane.

Structure of the cell membrane

The cell membrane contains carbohydrates that cover it in the form of a glycocalyx. This is a supra-membrane structure that performs a barrier function. The proteins located here are in a free state. Unbound proteins participate in enzymatic reactions, providing extracellular breakdown of substances.

Proteins of the cytoplasmic membrane are represented by glycoproteins. Based on their chemical composition, proteins that are completely included in the lipid layer (along its entire length) are classified as integral proteins. Also peripheral, not reaching one of the surfaces of the plasmalemma.

The former function as receptors, binding to neurotransmitters, hormones and other substances. Insertion proteins are necessary for the construction of ion channels through which the transport of ions and hydrophilic substrates occurs. The latter are enzymes that catalyze intracellular reactions.

Basic properties of the plasma membrane

The lipid bilayer prevents the penetration of water. Lipids are hydrophobic compounds represented in the cell by phospholipids. The phosphate group faces outward and consists of two layers: the outer one, directed to the extracellular environment, and the inner one, delimiting the intracellular contents.

Water-soluble areas are called hydrophilic heads. The fatty acid sites are directed into the cell, in the form of hydrophobic tails. The hydrophobic part interacts with neighboring lipids, which ensures their attachment to each other. The double layer has selective permeability in different areas.

So, in the middle the membrane is impermeable to glucose and urea; hydrophobic substances pass through here freely: carbon dioxide, oxygen, alcohol. Cholesterol is important; the content of the latter determines the viscosity of the plasmalemma.

Functions of the outer cell membrane

The characteristics of the functions are briefly listed in the table:

Membrane function	Description
Barrier role	The plasmalemma performs a protective function, protecting the contents of the cell from the effects of foreign agents. Thanks to the special organization of proteins, lipids, and carbohydrates, the semipermeability of the plasmalemma is ensured.
Receptor function	Biologically active substances are activated through the cell membrane in the process of binding to receptors. Thus, immune reactions are mediated through the recognition of foreign agents by the cell receptor apparatus localized on the cell membrane.
Transport function	The presence of pores in the plasmalemma allows you to regulate the flow of substances into the cell. The transfer process occurs passively (without energy consumption) for compounds with low molecular weight. Active transport is associated with the expenditure of energy released during the breakdown of adenosine triphosphate (ATP). This method takes place for the transfer of organic compounds.
Participation in digestive processes	Substances are deposited on the cell membrane (sorption). Receptors bind to the substrate, moving it into the cell. A bubble is formed, lying freely inside the cell. Merging, such vesicles form lysosomes with hydrolytic enzymes.
Enzymatic function	Enzymes are essential components of intracellular digestion. Reactions requiring the participation of catalysts occur with the participation of enzymes.

What is the importance of the cell membrane

The cell membrane is involved in maintaining homeostasis due to the high selectivity of substances entering and exiting the cell (in biology this is called selective permeability).

Outgrowths of the plasmalemma divide the cell into compartments (compartments) responsible for performing certain functions. Specifically designed membranes corresponding to the fluid-mosaic pattern ensure the integrity of the cell.