Cell membrane process. PVC membranes: essence and purpose
In 1972, the theory was put forward that a partially permeable membrane surrounds the cell and performs a number of vital tasks, and the structure and function of cell membranes are significant issues regarding the proper functioning of all cells in the body. became widespread in the 17th century, along with the invention of the microscope. It became known that plant and animal tissues consist of cells, but due to the low resolution of the device, it was impossible to see any barriers around the animal cell. In the 20th century, the chemical nature of the membrane was studied in more detail, and it was found that it is based on lipids.
Structure and functions of cell membranes
The cell membrane surrounds the cytoplasm of living cells, physically separating intracellular components from the external environment. Fungi, bacteria and plants also have cell walls that provide protection and prevent the passage of large molecules. Cell membranes also play a role in the formation of the cytoskeleton and the attachment of other vital particles to the extracellular matrix. This is necessary in order to hold them together, forming the tissues and organs of the body. Features of the structure of the cell membrane include permeability. The main function is protection. The membrane consists of a phospholipid layer with embedded proteins. This part is involved in processes such as cell adhesion, ionic conductance and signaling systems and serves as an attachment surface for several extracellular structures, including the wall, glycocalyx and internal cytoskeleton. The membrane also maintains cell potential by acting as a selective filter. It is selectively permeable to ions and organic molecules and controls the movement of particles.
Biological mechanisms involving the cell membrane
1. Passive diffusion: Some substances (small molecules, ions), such as carbon dioxide (CO2) and oxygen (O2), can penetrate the plasma membrane by diffusion. The shell acts as a barrier for certain molecules and ions, they can concentrate on either side.
2. Transmembrane channel and transporter protein: Nutrients such as glucose or amino acids must enter the cell, and some metabolic products must leave the cell.
3. Endocytosis is the process by which molecules are taken up. A slight deformation (invagination) is created in the plasma membrane in which the substance to be transported is ingested. This requires energy and is thus a form of active transport.
4. Exocytosis: Occurs in various cells to remove undigested remains of substances brought by endocytosis to secrete substances such as hormones and enzymes and transport the substance completely across the cell barrier.
Molecular structure
The cell membrane is a biological membrane consisting primarily of phospholipids and separating the contents of the entire cell from the external environment. The formation process occurs spontaneously under normal conditions. To understand this process and correctly describe the structure and functions of cell membranes, as well as properties, it is necessary to evaluate the nature of phospholipid structures, which are characterized by structural polarization. When phospholipids in the aqueous environment of the cytoplasm reach a critical concentration, they combine into micelles, which are more stable in the aqueous environment.
Membrane properties
- Stability. This means that once formed, membrane disintegration is unlikely.
- Strength. The lipid shell is reliable enough to prevent the passage of a polar substance; both solutes (ions, glucose, amino acids) and much larger molecules (proteins) cannot pass through the formed boundary.
- Dynamic character. This is perhaps the most important property when considering the structure of the cell. The cell membrane can undergo various deformations, can fold and bend without being destroyed. Under special circumstances, for example, during vesicle fusion or budding, it can be disrupted, but only temporarily. At room temperature, its lipid components are in constant, chaotic movement, forming a stable fluid boundary.
Liquid mosaic model
Speaking about the structure and functions of cell membranes, it is important to note that in the modern concept, the membrane as a liquid mosaic model was considered in 1972 by scientists Singer and Nicholson. Their theory reflects three main features of the membrane structure. Integrals promote a mosaic pattern for the membrane, and they are capable of lateral in-plane movement due to the variable nature of lipid organization. Transmembrane proteins are also potentially mobile. An important feature of the membrane structure is its asymmetry. What is the structure of a cell? Cell membrane, nucleus, proteins and so on. The cell is the basic unit of life, and all organisms are composed of one or many cells, each of which has a natural barrier separating it from its environment. This outer boundary of the cell is also called the plasma membrane. It is made up of four different types of molecules: phospholipids, cholesterol, proteins and carbohydrates. The fluid mosaic model describes the structure of the cell membrane as follows: flexible and elastic, with a consistency similar to vegetable oil, so that all individual molecules simply float in a liquid medium, and they are all capable of moving laterally within this membrane. A mosaic is something that contains many different pieces. In the plasma membrane it is represented by phospholipids, cholesterol molecules, proteins and carbohydrates.
Phospholipids
Phospholipids constitute the main structure of the cell membrane. These molecules have two different ends: a head and a tail. The head end contains a phosphate group and is hydrophilic. This means that it is attracted to water molecules. The tail is made up of hydrogen and carbon atoms called fatty acid chains. These chains are hydrophobic; they do not like to mix with water molecules. This process is similar to what happens when you pour vegetable oil into water, that is, it does not dissolve in it. The structural features of the cell membrane are associated with the so-called lipid bilayer, which consists of phospholipids. Hydrophilic phosphate heads are always located where there is water in the form of intracellular and extracellular fluid. The hydrophobic tails of phospholipids in the membrane are organized in such a way that they keep them away from water.
Cholesterol, proteins and carbohydrates
When people hear the word cholesterol, they usually think it's bad. However, cholesterol is actually a very important component of cell membranes. Its molecules consist of four hydrogen rings and carbon atoms. They are hydrophobic and occur among the hydrophobic tails in the lipid bilayer. Their importance lies in maintaining consistency, they strengthen the membranes, preventing crossing. Cholesterol molecules also keep the phospholipid tails from coming into contact and hardening. This ensures fluidity and flexibility. Membrane proteins function as enzymes to speed up chemical reactions, act as receptors for specific molecules, or transport substances across the cell membrane.
Carbohydrates, or saccharides, are found only on the extracellular side of the cell membrane. Together they form the glycocalyx. It provides cushioning and protection to the plasma membrane. Based on the structure and type of carbohydrates in the glycocalyx, the body can recognize cells and determine whether they should be there or not.
Membrane proteins
The structure of a cell membrane cannot be imagined without such an important component as protein. Despite this, they can be significantly smaller in size than another important component - lipids. There are three types of major membrane proteins.
- Integral. They completely cover the bilayer, cytoplasm and extracellular environment. They perform transport and signaling functions.
- Peripheral. Proteins are attached to the membrane by electrostatic or hydrogen bonds at their cytoplasmic or extracellular surfaces. They are involved mainly as a means of attachment for integral proteins.
- Transmembrane. They perform enzymatic and signaling functions, and also modulate the basic structure of the lipid bilayer of the membrane.
Functions of biological membranes
The hydrophobic effect, which regulates the behavior of hydrocarbons in water, controls the structures formed by membrane lipids and membrane proteins. Many membrane properties are conferred by the carrier lipid bilayers, which form the basic structure for all biological membranes. Integral membrane proteins are partially hidden in the lipid bilayer. Transmembrane proteins have a specialized organization of amino acids in their primary sequence.
Peripheral membrane proteins are very similar to soluble proteins, but they are also membrane bound. Specialized cell membranes have specialized cell functions. How do the structure and functions of cell membranes affect the body? The functionality of the entire organism depends on how biological membranes are structured. From intracellular organelles, extracellular and intercellular interactions of membranes, structures necessary for the organization and performance of biological functions are created. Many structural and functional features are common to bacteria and enveloped viruses. All biological membranes are built on a lipid bilayer, which results in a number of common characteristics. Membrane proteins have many specific functions.
- Controlling. Plasma membranes of cells determine the boundaries of interaction between the cell and the environment.
- Transport. The intracellular membranes of cells are divided into several functional units with different internal compositions, each of which is supported by the necessary transport function in combination with permeability control.
- Signal transduction. Membrane fusion provides a mechanism for intracellular vesicular signaling and preventing various types of viruses from freely entering the cell.
Significance and conclusions
The structure of the outer cell membrane affects the entire body. It plays an important role in protecting the integrity by allowing only selected substances to penetrate. It is also a good base for the attachment of the cytoskeleton and cell wall, which helps in maintaining the shape of the cell. Lipids make up about 50% of the membrane mass of most cells, although this varies depending on the type of membrane. The structure of the outer cell membrane of mammals is more complex, containing four main phospholipids. An important property of lipid bilayers is that they behave as two-dimensional liquids in which individual molecules can freely rotate and move laterally. Such fluidity is an important property of membranes, which is determined depending on temperature and lipid composition. Due to its hydrocarbon ring structure, cholesterol plays a role in determining membrane fluidity. biological membranes for small molecules allows the cell to control and maintain its internal structure.
Considering the structure of the cell (cell membrane, nucleus, and so on), we can conclude that the body is a self-regulating system that, without outside help, cannot harm itself and will always look for ways to restore, protect and properly function each cell.
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 resulting vesicles, 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 remains 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.
The basic structural unit of a living organism is the cell, which is a differentiated section of the cytoplasm surrounded by a cell membrane. Due to the fact that the cell performs many important functions, such as reproduction, nutrition, movement, the membrane must be plastic and dense.
History of the discovery and research of the cell membrane
In 1925, Grendel and Gorder conducted a successful experiment to identify the “shadows” of red blood cells, or empty membranes. Despite several serious mistakes, scientists discovered the lipid bilayer. Their work was continued by Danielli, Dawson in 1935, and Robertson in 1960. As a result of many years of work and accumulation of arguments, in 1972 Singer and Nicholson created a fluid-mosaic model of the membrane structure. 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”. This is how the cell boundary is designated, which is a natural barrier between the internal contents and the external environment. The structure of the cell membrane implies semi-permeability, due to which moisture and nutrients and breakdown products can freely pass through it. This shell can be called the main structural component of the cell organization.
Let's consider the main functions of the cell membrane
1. Separates the internal contents of the cell and components of the external environment.
2. Helps maintain a constant chemical composition of the cell.
3. Regulates proper metabolism.
4. Provides communication between cells.
5. Recognizes signals.
6. Protection function.
"Plasma Shell"
The outer cell membrane, also called the plasma membrane, is an ultramicroscopic film whose thickness ranges from five to seven nanomillimeters. It consists mainly of protein compounds, phospholides, and water. The film is elastic, easily absorbs water, and quickly restores its integrity after damage.
It has a universal structure. This membrane occupies a border position, participates in the process of selective permeability, removal of decay products, and synthesizes them. The relationship with its “neighbors” and reliable protection of the internal contents from damage makes it an important component in such matters as the structure of the cell. The cell membrane of animal organisms is sometimes covered with a thin layer - the glycocalyx, which includes proteins and polysaccharides. Plant cells outside the membrane are protected by a cell wall, which serves as 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 has the function of repair, protection and interaction with other cells.
Structure of the cell membrane
The thickness of this movable shell varies from six to ten nanomillimeters. The cell membrane of a cell has a special composition, the basis of which is a lipid bilayer. Hydrophobic tails, inert to water, are located on the inside, while hydrophilic heads, interacting with water, face outward. Each lipid is a phospholipid, which is the result of the interaction of substances such as glycerol and sphingosine. The lipid framework is closely surrounded by proteins, which are arranged in a non-continuous layer. Some of them are immersed in the lipid layer, the rest pass through it. As a result, areas that are permeable to water are formed. The functions performed by these proteins are different. Some of them are enzymes, the rest are transport proteins that transfer various substances from the external environment to the cytoplasm and back.
The cell membrane is permeated through and closely connected by integral proteins, and 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. Thanks 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 the cell membrane can change. The core may suffer from external influences.
Properties
The cell membrane of a cell has interesting features. Due to its fluidity, this membrane is not a rigid structure, and the bulk of the proteins and lipids that make up it move freely on the plane of the membrane.
In general, the cell membrane is asymmetrical, so the composition of the protein and lipid layers differs. Plasma membranes in animal cells, on their outer side, have a glycoprotein layer that performs receptor and signaling functions, and also plays a large role in the process of combining cells into tissue. The cell membrane is polar, that is, the charge on the outside is positive and the charge on the inside 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. Potassium ions have a different ratio: their amount 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 plays a “pumping” role, leveling the concentration of substances: sodium ions are pumped to the surface of the cell, and potassium ions are pumped inside. This feature is one of the most important functions of the cell membrane.
This tendency of sodium and potassium ions to move inward from the surface plays a big 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 intakes 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 does cell nutrition occur through the cell membrane?
Many cells take up substances through processes such as phagocytosis and pinocytosis. In the first option, a flexible outer membrane creates a small depression in which the captured particle ends up. The diameter of the recess then becomes larger until the enclosed particle enters the cell cytoplasm. Through phagocytosis, some protozoa, such as amoebas, are fed, as well as blood cells - leukocytes and phagocytes. Similarly, cells absorb fluid, which contains the necessary nutrients. This phenomenon is called pinocytosis.
The outer membrane is closely connected to the endoplasmic reticulum of the cell.
Many types of main tissue components have protrusions, folds, and microvilli on the surface of the membrane. Plant cells on the outside of this shell are covered with another, thick and clearly visible under a microscope. The fiber they are made of helps form 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 chitin contained in the integumentary cells of insects.
In addition to the cellular 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 the cell membrane. The structure and functions suggest a significant expansion of the total surface area of the cell and an improvement in 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 connections are maintained at a fairly strong level, forming tissues. In this regard, we can conclude that the cell membrane plays one of the most important roles in the cell. The structure and functions performed by it differ radically in different cells, depending on their purpose. Through these features, a variety of physiological activities of cell membranes and their roles in the existence of cells and tissues is achieved.
The cell membrane has a rather complex structure, which can be viewed with an electron microscope. Roughly speaking, it consists of a double layer of lipids (fats), in which various peptides (proteins) are embedded in different places. The total thickness of the membrane is about 5-10 nm.
The general structure of the cell membrane is universal for the entire living world. However, animal membranes contain cholesterol inclusions, which determine their rigidity. The differences between the membranes of different kingdoms of organisms mainly concern the supra-membrane formations (layers). So in plants and fungi there is a cell wall above the membrane (on the outside). In plants it consists mainly of cellulose, and in fungi it consists mainly of chitin. In animals, the supra-membrane layer is called the glycocalyx.
Another name for the cell membrane cytoplasmic membrane or plasma membrane.
A deeper study of the structure of the cell membrane reveals many of its features related to the functions it performs.
The lipid bilayer is mainly composed of phospholipids. These are fats, one end of which contains a phosphoric acid residue that has hydrophilic properties (that is, it attracts water molecules). The second end of the phospholipid is chains of fatty acids that have hydrophobic properties (they do not form hydrogen bonds with water).
Phospholipid molecules in the cell membrane are arranged in two rows so that their hydrophobic “ends” are on the inside and their hydrophilic “heads” are on the outside. The result is a fairly strong structure that protects the contents of the cell from the external environment.
Protein inclusions in the cell membrane are distributed unevenly, in addition, they are mobile (since phospholipids in the bilayer have lateral mobility). Since the 70s of the XX century they began to talk about fluid-mosaic structure of the cell membrane.
Depending on how the protein is included in the membrane, three types of proteins are distinguished: integral, semi-integral and peripheral. Integral proteins pass through the entire thickness of the membrane, and their ends protrude on both sides 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.
Biological membranes- the general name for functionally active surface structures that bound 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 basic 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 ensuring intercellular interactions, transmitting signals into 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 (cellular) membrane is approximately the same in all cells: 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 transport them into the cell across membranes. Membrane proteins are distinguished by the nature of their connection with membrane structures. Some proteins called external or peripheral , are loosely bound to the surface of the membrane, others, called internal or integral , immersed inside the membrane. Peripheral proteins are easily extracted, while integral proteins can only be isolated using detergents or organic solvents. In Fig. Figure 4 shows the structure of the plasma membrane.
The outer, or plasma, membranes of many cells, as well as the membranes of intracellular organelles, for example, mitochondria, chloroplasts, were isolated in free form and their molecular composition was studied. All membranes contain polar lipids in quantities ranging from 20 to 80% of their mass, depending on the type of membrane; the rest is mainly proteins. Thus, 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 membrane is a mixture of various types of polar lipids. Polar lipids, which include phosphoglycerolipids, sphingolipids, and glycolipids, are not stored in fat cells, but are integrated into cell membranes, and in strictly defined proportions.
All polar lipids in membranes are constantly renewed during the metabolic process; 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 of membrane structure is the fluid mosaic model, proposed in 1972 by S. Singer and J. Nicholson.
According to it, 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 through the membrane; they are amphipathic molecules. Peripheral proteins do not penetrate the membrane and are less tightly bound to it. The main continuous part of the membrane, that is, its matrix, is the polar lipid bilayer. At normal cell temperatures, 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 liquid-mosaic model also assumes 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, which 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 covalent bonds are not formed either between the molecules in the bilayer or 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 immersed in the hydrocarbon layer.
For the most part, membranes are asymmetrical, that is, they have unequal sides. This asymmetry is manifested in the following:
· firstly, 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 red blood cell membranes contains mainly phosphatidylethanolamine and phosphatidylserine, and the outer layer contains phosphatidylcholine and sphingomyelin.
Secondly, some transport systems in membranes act only in one direction. For example, in the membranes of erythrocytes there is a transport system (“pump”) that pumps Na + ions from the cell into 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 glycolipid heads and oligosaccharide side chains of glycoproteins, while on the inner surface of the plasma membrane there are practically no oligosaccharide groups.
The asymmetry of biological membranes is maintained 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. A 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 the fatty acid chains imparts to the membrane properties of fluidity and mobility. In the presence of cis-unsaturated fatty acids, the cohesion forces between chains are weaker than in the case of saturated fatty acids alone, and lipids remain highly mobile even at low temperatures.
On the outside of the membranes there are specific recognition regions, the function of which is to recognize certain molecular signals. For example, it is through the membrane that some bacteria perceive slight changes in the concentration of a nutrient, 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),
· nuclear membrane,
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 of the outer cell membrane and in some cases retain contact with it.
Red blood cell membrane consists of proteins (50%), lipids (40%) and carbohydrates (10%). The bulk of carbohydrates (93%) are 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 lipid layer. This asymmetry is apparently maintained due to the transverse movement of phospholipids in the membrane, carried out with the help of membrane proteins and due to metabolic energy. The inner layer of the erythrocyte membrane contains mainly sphingomyelin, phosphatidylethanolamine, phosphatidylserine, and the outer layer contains phosphatidylcholine. The red blood cell 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 was found in the erythrocyte membrane, containing few carbohydrates and penetrating the membrane. He is called tunnel protein(component a), since it is believed to form a channel for anions. A peripheral protein associated with the inner side of the erythrocyte membrane is spectrin.
Myelin membranes , surrounding the 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 fixing membrane salts lying on top of each other.
Chloroplast membranes. Chloroplasts are covered with a two-layer membrane. The outer membrane has some similarities with that of mitochondria. In addition to this surface membrane, chloroplasts have an internal membrane system - lamellae. The lamellae form either flattened vesicles - thylakoids, which, located one above the other, are collected in packs (granas) or form a stromal membrane system (stromal lamellae). The lamellae of the grana and stroma on the outer side of the thylakoid membrane are concentrated hydrophilic groups, galacto- and sulfolipids. The phytol 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 contacting membranes of the grana thylakoids.
Inner (cytoplasmic) membrane of bacteria its structure is similar to the internal membranes of chloroplasts and mitochondria. Enzymes of the respiratory chain and active transport are localized in it; 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 to the cytoplasmic membrane, 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 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 protein and nucleic acid. This “core” of viruses is surrounded by a membrane (envelope). It also consists of a lipid bilayer with embedded glycoproteins located mainly on the surface of the membrane. In a number of viruses (microviruses), 70-80% of all proteins are contained in 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 imparts biological specificity to the membrane surface.