The cell membrane is an ultrathin film on the surface of a cell or cellular organelle, consisting of a bimolecular layer of lipids with embedded proteins and polysaccharides.
Membrane functions:
- · Barrier - provides regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides that are dangerous to the cell. Selective permeability means that the permeability of a membrane to different atoms or molecules depends on their size, electrical charge and chemical properties. Selective permeability ensures that the cell and cellular compartments are separated from the environment and supplied with the necessary substances.
- · Transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery of nutrients, removal of metabolic end products, secretion of various substances, creation of ion gradients, maintenance of optimal pH and ion concentrations in the cell, which are necessary for the functioning of cellular enzymes. Particles that for any reason are unable to cross the phospholipid bilayer (for example, due to hydrophilic properties, since the membrane inside is hydrophobic and does not allow hydrophilic substances to pass through, or due to their large size), but necessary for the cell, can penetrate the membrane through special carrier proteins (transporters) and channel proteins or by endocytosis. In passive transport, substances cross the lipid bilayer without expending energy along a concentration gradient by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance pass through the membrane. This molecule may have a channel that allows only one type of substance to pass through. Active transport requires energy as it occurs against a concentration gradient. There are special pump proteins on the membrane, including ATPase, which actively pumps potassium ions (K +) into the cell and pumps sodium ions (Na +) out of it.
- · matrix - ensures a certain relative position and orientation of membrane proteins, their optimal interaction.
- · mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play a major role in ensuring mechanical function, and in animals, the intercellular substance.
- · energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate;
- · receptor - some proteins located in the membrane are receptors (molecules with the help of which the cell perceives certain signals). For example, hormones circulating in the blood act only on target cells that have receptors corresponding to these hormones. Neurotransmitters (chemicals that ensure the conduction of nerve impulses) also bind to special receptor proteins in target cells.
- · enzymatic - membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.
- · implementation of generation and conduction of biopotentials. With the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K + ion inside the cell is much higher than outside, and the concentration of Na + is much lower, which is very important, since this ensures the maintenance of the potential difference on the membrane and the generation of a nerve impulse.
- · cell marking - there are antigens on the membrane that act as markers - “labels” that allow the cell to be identified. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of “antennas”. Because of the myriad configurations of side chains, it is possible to make a specific marker for each cell type. With the help of markers, cells can recognize other cells and act in concert with them, for example, in the formation of organs and tissues. This also allows the immune system to recognize foreign antigens.
Some protein molecules diffuse freely in the plane of the lipid layer; in the normal state, parts of protein molecules emerging on different sides of the cell membrane do not change their position.
The special morphology of cell membranes determines their electrical characteristics, among which the most important are capacitance and conductivity.
Capacitive properties are mainly determined by the phospholipid bilayer, which is impermeable to hydrated ions and at the same time thin enough (about 5 nm) to allow efficient charge separation and storage, and electrostatic interaction of cations and anions. In addition, the capacitive properties of cell membranes are one of the reasons that determine the time characteristics of electrical processes occurring on cell membranes.
Conductivity (g) is the reciprocal of electrical resistance and is equal to the ratio of the total transmembrane current for a given ion to the value that determined its transmembrane potential difference.
Various substances can diffuse through the phospholipid bilayer, and the degree of permeability (P), i.e., the ability of the cell membrane to pass these substances, depends on the difference in concentrations of the diffusing substance on both sides of the membrane, its solubility in lipids and the properties of the cell membrane. The rate of diffusion for charged ions under constant field conditions in a membrane is determined by the mobility of ions, the thickness of the membrane, and the distribution of ions in the membrane. For nonelectrolytes, the permeability of the membrane does not affect its conductivity, since nonelectrolytes do not carry charges, i.e., they cannot carry electric current.
The conductivity of a membrane is a measure of its ionic permeability. An increase in conductivity indicates an increase in the number of ions passing through the membrane.
An important property of biological membranes is fluidity. All cell membranes are mobile fluid structures: most of their constituent lipid and protein molecules are capable of moving quite quickly in the plane of the membrane
Cell membrane (also cytolemma, plasmalemma, or plasma membrane) is an elastic molecular structure consisting of proteins and lipids. Separates the contents of any cell from the external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments - compartments or organelles, in which certain environmental conditions are maintained.
If the cell has one (usually plant cells do), it covers the cell membrane.
The cell membrane is a double layer (bilayer) of molecules of the lipid class, most of which are so-called complex lipids - phospholipids. Lipid molecules have a hydrophilic (“head”) and a hydrophobic (“tail”) part. When membranes are formed, the hydrophobic regions of the molecules turn inward, and the hydrophilic regions turn outward. The biological membrane also includes various proteins:
- integral (piercing the membrane through),
- semi-integral (immersed at one end in the outer or inner lipid layer),
- superficial (located on the outer or adjacent to the inner sides of the membrane).
Some proteins are the points of contact between the cell membrane and the cytoskeleton inside the cell and the cell wall outside.
Membrane functions:
- Barrier - provides regulated, selective, passive and active metabolism with the environment.
- Transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery of nutrients, removal of metabolic end products, secretion of various substances, creation of ion gradients, maintenance of optimal pH and ion concentrations in the cell, which are necessary for the functioning of cellular enzymes.
- Matrix - ensures a certain relative position and orientation of membrane proteins, their optimal interaction.
- Mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play a major role in ensuring mechanical function.
- Energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate.
Membranes consist of three classes of lipids:
- phospholipids,
- glycolipids,
- cholesterol
Phospholipids and glycolipids(lipids with carbohydrates attached) consist of two long hydrophobic hydrocarbon “tails” that are connected to a charged hydrophilic “head”.
Cholesterol imparts rigidity to the membrane by occupying the free space between the hydrophobic tails of lipids and preventing them from bending. Therefore, membranes with a low cholesterol content are more flexible, and those with a high cholesterol content are more rigid and fragile. Cholesterol also serves as a “stopper” that prevents the movement of polar molecules from the cell and into the cell.
An important part of the membrane is proteins, penetrating it and responsible for the various properties of membranes. Their composition and orientation differ in different membranes. Next to the proteins are annular lipids - they are more ordered, less mobile, contain more saturated fatty acids and are released from the membrane along with the protein. Without annular lipids, membrane proteins do not function.
Cell membranes are often asymmetrical, that is, the layers differ in lipid composition, the outer one contains mainly phosphatidylinositol, phosphatidylcholine, sphingomyelins and glycolipids, the inner one contains phosphatidylserine, phosphatidylethanolamine and phosphatidylinositol. The transition of an individual molecule from one layer to another (the so-called flip-flop) is difficult, but can occur spontaneously, approximately once every 6 months, or with the help of flippases proteins and scramblase of the plasma membrane. If phosphatidylserine appears in the outer layer, this is a signal for macrophages to destroy the cell.
Membrane organelles- these are closed single or interconnected sections of the cytoplasm, separated from the hyaloplasm by membranes. Single-membrane organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes; to double membranes - nucleus, mitochondria, plastids. The structure of the membranes of various organelles differs in the composition of lipids and membrane proteins.
Cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, and the membranes themselves, to a certain extent, actively regulate this process - some substances pass through, but others do not. There are four main mechanisms for the entry of substances into the cell or their removal from the cell to the outside: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive in nature, that is, they do not require energy; the last two are active processes associated with energy consumption.
The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane right through, forming a kind of passage. The elements K, Na and Cl have their own channels. Relative to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open and a sudden influx of sodium ions into the cell occurs. In this case, an imbalance of membrane potential occurs. After which the membrane potential is restored. Potassium channels are always open, allowing potassium ions to slowly enter the cell.
Cell- a self-regulating structural and functional unit of tissues and organs. The cellular theory of the structure of organs and tissues was developed by Schleiden and Schwann in 1839. Subsequently, with the help of electron microscopy and ultracentrifugation, it was possible to clarify the structure of all the main organelles of animal and plant cells (Fig. 1).
Rice. 1. Scheme of the structure of an animal cell
The main parts of a cell are the cytoplasm and the nucleus. Each cell is surrounded by a very thin membrane that limits its contents.
The cell membrane is called plasma membrane and is characterized by selective permeability. This property allows necessary nutrients and chemical elements to penetrate into the cell, and excess products to leave it. The plasma membrane consists of two layers of lipid molecules containing specific proteins. The main membrane lipids are phospholipids. They contain phosphorus, a polar head and two non-polar tails of long-chain fatty acids. Membrane lipids include cholesterol and cholesteryl esters. In accordance with the liquid mosaic model of structure, membranes contain inclusions of protein and lipid molecules that can mix relative to the bilayer. Each type of membrane of any animal cell has its own relatively constant lipid composition.
Membrane proteins are divided into two types according to their structure: integral and peripheral. Peripheral proteins can be removed from the membrane without destroying it. There are four types of membrane proteins: transport proteins, enzymes, receptors and structural proteins. Some membrane proteins have enzymatic activity, others bind certain substances and facilitate their transport into the cell. Proteins provide several pathways for the movement of substances across membranes: they form large pores consisting of several protein subunits that allow water molecules and ions to move between cells; form ion channels specialized for the movement of certain types of ions across the membrane under certain conditions. Structural proteins are associated with the inner lipid layer and provide the cytoskeleton of the cell. The cytoskeleton provides mechanical strength to the cell membrane. In various membranes, proteins account for from 20 to 80% of the mass. Membrane proteins can move freely in the lateral plane.
The membrane also contains carbohydrates that can be covalently bound to lipids or proteins. There are three types of membrane carbohydrates: glycolipids (gangliosides), glycoproteins and proteoglycans. Most membrane lipids are in a liquid state and have a certain fluidity, i.e. the ability to move from one area to another. On the outer side of the membrane there are receptor sites that bind various hormones. Other specific areas of the membrane cannot recognize and bind certain proteins and various biologically active compounds that are foreign to these cells.
The internal space of the cell is filled with cytoplasm, in which most enzyme-catalyzed reactions of cellular metabolism take place. The cytoplasm consists of two layers: the internal one, called endoplasm, and the peripheral one, ectoplasm, which has a high viscosity and is devoid of granules. The cytoplasm contains all the components of a cell or organelle. The most important of the cell organelles are the endoplasmic reticulum, ribosomes, mitochondria, Golgi apparatus, lysosomes, microfilaments and microtubules, peroxisomes.
Endoplasmic reticulum is a system of interconnected channels and cavities that penetrate the entire cytoplasm. It ensures the transport of substances from the environment and inside cells. The endoplasmic reticulum also serves as a depot for intracellular Ca 2+ ions and serves as the main site of lipid synthesis in the cell.
Ribosomes - microscopic spherical particles with a diameter of 10-25 nm. Ribosomes are freely located in the cytoplasm or attached to the outer surface of the membranes of the endoplasmic reticulum and nuclear membrane. They interact with messenger and transport RNA, and protein synthesis occurs in them. They synthesize proteins that enter the cisternae or the Golgi apparatus and are then released outside. Ribosomes, freely located in the cytoplasm, synthesize protein for use by the cell itself, and ribosomes associated with the endoplasmic reticulum produce protein that is excreted from the cell. Ribosomes synthesize various functional proteins: carrier proteins, enzymes, receptors, cytoskeletal proteins.
Golgi apparatus formed by a system of tubules, cisterns and vesicles. It is associated with the endoplasmic reticulum, and the biologically active substances that enter here are stored in a compacted form in secretory vesicles. The latter are constantly separated from the Golgi apparatus, transported to the cell membrane and merge with it, and the substances contained in the vesicles are removed from the cell through the process of exocytosis.
Lysosomes - membrane-surrounded particles measuring 0.25-0.8 microns. They contain numerous enzymes involved in the breakdown of proteins, polysaccharides, fats, nucleic acids, bacteria and cells.
Peroxisomes formed from smooth endoplasmic reticulum, resemble lysosomes and contain enzymes that catalyze the decomposition of hydrogen peroxide, which is broken down under the influence of peroxidases and catalase.
Mitochondria contain outer and inner membranes and are the “energy station” of the cell. Mitochondria are round or elongated structures with a double membrane. The inner membrane forms folds protruding into the mitochondria - cristae. ATP synthesis occurs in them, oxidation of Krebs cycle substrates and many biochemical reactions occur. ATP molecules produced in mitochondria diffuse to all parts of the cell. Mitochondria contain a small amount of DNA, RNA, and ribosomes, and with their participation, the renewal and synthesis of new mitochondria occurs.
Microfilaments They are thin protein filaments consisting of myosin and actin and form the contractile apparatus of the cell. Microfilaments are involved in the formation of folds or protrusions of the cell membrane, as well as in the movement of various structures within cells.
Microtubules form the basis of the cytoskeleton and provide its strength. The cytoskeleton gives cells their characteristic appearance and shape and serves as a site for attachment of intracellular organelles and various bodies. In nerve cells, bundles of microtubules are involved in the transport of substances from the cell body to the ends of axons. With their participation, the mitotic spindle functions during cell division. They play the role of motor elements in villi and flagella in eukaryotes.
Core is the main structure of the cell, participates in the transmission of hereditary characteristics and in the synthesis of proteins. The nucleus is surrounded by a nuclear membrane containing many nuclear pores through which various substances are exchanged between the nucleus and the cytoplasm. There is a nucleolus inside it. The important role of the nucleolus in the synthesis of ribosomal RNA and histone proteins has been established. The remaining parts of the nucleus contain chromatin, consisting of DNA, RNA and a number of specific proteins.
Functions of the cell membrane
Cell membranes play a crucial role in the regulation of intracellular and intercellular metabolism. They have selective permeability. Their specific structure allows them to provide barrier, transport and regulatory functions.
Barrier function manifests itself in limiting the penetration of compounds dissolved in water through the membrane. The membrane is impermeable to large protein molecules and organic anions.
Regulatory function membranes is to regulate intracellular metabolism in response to chemical, biological and mechanical influences. Various influences are perceived by special membrane receptors with a subsequent change in enzyme activity.
Transport function through biological membranes can be carried out passively (diffusion, filtration, osmosis) or using active transport.
Diffusion - movement of a gas or soluble substance along a concentration and electrochemical gradient. The rate of diffusion depends on the permeability of the cell membrane, as well as the concentration gradient for uncharged particles, and the electrical and concentration gradients for charged particles. Simple diffusion occurs through the lipid bilayer or through channels. Charged particles move according to an electrochemical gradient, and uncharged particles move according to a chemical gradient. For example, oxygen, steroid hormones, urea, alcohol, etc. penetrate through the lipid layer of the membrane by simple diffusion. Various ions and particles move through the channels. Ion channels are formed by proteins and are divided into gated and ungated channels. Depending on the selectivity, a distinction is made between ion-selective cables, which allow only one ion to pass through, and channels that do not have selectivity. The channels have an orifice and a selective filter, and the controlled channels have a gate mechanism.
Facilitated diffusion - a process in which substances are transported across a membrane using special membrane transport proteins. In this way, amino acids and monosaccharides penetrate into the cell. This type of transport happens very quickly.
Osmosis - movement of water through the membrane from a solution with a lower to a solution with a higher osmotic pressure.
Active transport - transport of substances against a concentration gradient using transport ATPases (ion pumps). This transfer occurs with the expenditure of energy.
Na + /K + -, Ca 2+ - and H + -pumps have been studied to a greater extent. The pumps are located on cell membranes.
A type of active transport is endocytosis And exocytosis. Using these mechanisms, larger substances (proteins, polysaccharides, nucleic acids) that cannot be transported through channels are transported. This transport is more common in intestinal epithelial cells, renal tubules, and vascular endothelium.
At In endocytosis, cell membranes form invaginations into the cell, which, when released, turn into vesicles. During exocytosis, vesicles with their contents are transferred to the cell membrane and merge with it, and the contents of the vesicles are released into the extracellular environment.
Structure and functions of the cell membrane
To understand the processes that ensure the existence of electrical potentials in living cells, you first need to understand the structure of the cell membrane and its properties.
Currently, the most widely accepted is the liquid mosaic model of the membrane, proposed by S. Singer and G. Nicholson in 1972. The membrane is based on a double layer of phospholipids (bilayer), the hydrophobic fragments of the molecule of which are immersed in the thickness of the membrane, and the polar hydrophilic groups are oriented outward, those. into the surrounding aquatic environment (Fig. 2).
Membrane proteins are localized on the surface of the membrane or can be embedded to varying depths in the hydrophobic zone. Some proteins span the membrane, and different hydrophilic groups of the same protein are found on both sides of the cell membrane. Proteins found in the plasma membrane play a very important role: they participate in the formation of ion channels, play the role of membrane pumps and transporters of various substances, and can also perform a receptor function.
The main functions of the cell membrane: barrier, transport, regulatory, catalytic.
The barrier function is to limit the diffusion of water-soluble compounds through the membrane, which is necessary to protect cells from foreign, toxic substances and maintain a relatively constant content of various substances inside the cells. Thus, the cell membrane can slow down the diffusion of various substances by 100,000-10,000,000 times.
Rice. 2. Three-dimensional diagram of the liquid-mosaic model of the Singer-Nicholson membrane
Depicted are globular integral proteins embedded in a lipid bilayer. Some proteins are ion channels, others (glycoproteins) contain oligosaccharide side chains that are involved in the recognition of cells among each other and in intercellular tissue. Cholesterol molecules are closely adjacent to the phospholipid heads and fix the adjacent sections of the “tails”. The internal sections of the tails of the phospholipid molecule are not limited in their movement and are responsible for the fluidity of the membrane (Bretscher, 1985)
The membrane contains channels through which ions penetrate. Channels can be voltage dependent or potential independent. Voltage-dependent channels open when the potential difference changes, and potential independent(hormone-regulated) open when receptors interact with substances. Channels can be opened or closed thanks to gates. Two types of gates are built into the membrane: activation(deep in the channel) and inactivation(on the channel surface). The gate can be in one of three states:
- open state (both types of gates are open);
- closed state (activation gate closed);
- inactivation state (inactivation gate closed).
Another characteristic feature of membranes is the ability to selectively transport inorganic ions, nutrients, and various metabolic products. There are systems of passive and active transfer (transport) of substances. Passive transport occurs through ion channels with or without the help of carrier proteins, and its driving force is the difference in the electrochemical potential of ions between the intra- and extracellular space. The selectivity of ion channels is determined by its geometric parameters and the chemical nature of the groups lining the walls of the channel and its mouth.
Currently, the most well studied channels are those that are selectively permeable to Na + , K + , Ca 2+ ions and also to water (the so-called aquaporins). The diameter of ion channels, according to various studies, is 0.5-0.7 nm. The channel capacity can vary; 10 7 - 10 8 ions per second can pass through one ion channel.
Active transport occurs with the expenditure of energy and is carried out by so-called ion pumps. Ion pumps are molecular protein structures embedded in a membrane that transport ions toward a higher electrochemical potential.
The pumps operate using the energy of ATP hydrolysis. Currently, Na+/K+ - ATPase, Ca 2+ - ATPase, H + - ATPase, H + /K + - ATPase, Mg 2+ - ATPase, which ensure the movement of Na +, K +, Ca 2+ ions, respectively, have been well studied , H+, Mg 2+ isolated or conjugated (Na+ and K+; H+ and K+). The molecular mechanism of active transport is not fully understood.
The membrane is an ultra-fine structure that forms the surfaces of organelles and the cell as a whole. All membranes have a similar structure and are connected into one system.
Chemical composition
Cell membranes are chemically homogeneous and consist of proteins and lipids of various groups:
- phospholipids;
- galactolipids;
- sulfolipids.
They also contain nucleic acids, polysaccharides and other substances.
Physical properties
At normal temperatures, the membranes are in a liquid crystalline state and constantly fluctuate. Their viscosity is close to that of vegetable oil.
The membrane is recoverable, durable, elastic and porous. Membrane thickness is 7 - 14 nm.
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The membrane is impermeable to large molecules. Small molecules and ions can pass through the pores and the membrane itself under the influence of concentration differences on different sides of the membrane, as well as with the help of transport proteins.
Model
Typically, the structure of membranes is described using a fluid mosaic model. The membrane has a framework - two rows of lipid molecules, tightly adjacent to each other, like bricks.
Rice. 1. Sandwich-type biological membrane.
On both sides the surface of lipids is covered with proteins. The mosaic pattern is formed by protein molecules unevenly distributed on the surface of the membrane.
According to the degree of immersion in the bilipid layer, protein molecules are divided into three groups:
- transmembrane;
- submerged;
- superficial.
Proteins provide the main property of the membrane - its selective permeability to various substances.
Membrane types
All cell membranes according to localization can be divided into the following types:
- external;
- nuclear;
- organelle membranes.
The outer cytoplasmic membrane, or plasmolemma, is the boundary of the cell. Connecting with the elements of the cytoskeleton, it maintains its shape and size.
Rice. 2. Cytoskeleton.
The nuclear membrane, or karyolemma, is the boundary of the nuclear contents. It is constructed of two membranes, very similar to the outer one. The outer membrane of the nucleus is connected to the membranes of the endoplasmic reticulum (ER) and, through pores, to the inner membrane.
ER membranes penetrate the entire cytoplasm, forming surfaces on which the synthesis of various substances, including membrane proteins, takes place.
Organelle membranes
Most organelles have a membrane structure.
The walls are built from one membrane:
- Golgi complex;
- vacuoles;
- lysosomes
Plastids and mitochondria are built from two layers of membranes. Their outer membrane is smooth, and the inner one forms many folds.
Features of photosynthetic membranes of chloroplasts are built-in chlorophyll molecules.
Animal cells have a carbohydrate layer on the surface of their outer membrane called the glycocalyx.
Rice. 3. Glycocalyx.
The glycocalyx is most developed in the cells of the intestinal epithelium, where it creates conditions for digestion and protects the plasmalemma.
Table "Structure of the cell membrane"
What have we learned?
We looked at the structure and functions of the cell membrane. The membrane is a selective (selective) barrier of the cell, nucleus and organelles. The structure of the cell membrane is described by the fluid mosaic model. According to this model, protein molecules are built into the bilayer of viscous lipids.
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Cell- this is not only liquid, enzymes and other substances, but also highly organized structures called intracellular organelles. Organelles for a cell are no less important than its chemical components. Thus, in the absence of organelles such as mitochondria, the supply of energy extracted from nutrients will immediately decrease by 95%.
Most organelles in a cell are covered membranes consisting mainly of lipids and proteins. There are membranes of cells, endoplasmic reticulum, mitochondria, lysosomes, and Golgi apparatus.
Lipids are insoluble in water, so they create a barrier in the cell that prevents the movement of water and water-soluble substances from one compartment to another. Protein molecules, however, make the membrane permeable to various substances through specialized structures called pores. Many other membrane proteins are enzymes that catalyze numerous chemical reactions, which will be discussed in subsequent chapters.
Cell (or plasma) membrane is a thin, flexible and elastic structure with a thickness of only 7.5-10 nm. It consists mainly of proteins and lipids. The approximate ratio of its components is as follows: proteins - 55%, phospholipids - 25%, cholesterol - 13%, other lipids - 4%, carbohydrates - 3%.
Lipid layer of the cell membrane prevents water penetration. The basis of the membrane is a lipid bilayer - a thin lipid film consisting of two monolayers and completely covering the cell. Proteins are located throughout the membrane in the form of large globules.
Schematic representation of a cell membrane, reflecting its main elements- phospholipid bilayer and a large number of protein molecules protruding above the surface of the membrane.
Carbohydrate chains are attached to proteins on the outer surface
and to additional protein molecules inside the cell (not shown in the figure).
Lipid bilayer consists mainly of phospholipid molecules. One end of such a molecule is hydrophilic, i.e. soluble in water (a phosphate group is located on it), the other is hydrophobic, i.e. soluble only in fats (it contains a fatty acid).
Due to the fact that the hydrophobic part of the molecule phospholipid repels water, but is attracted to similar parts of the same molecules, phospholipids have a natural property of attaching to each other in the thickness of the membrane, as shown in Fig. 2-3. The hydrophilic part with the phosphate group forms two membrane surfaces: the outer one, which is in contact with the extracellular fluid, and the inner one, which is in contact with the intracellular fluid.
Middle of the lipid layer impermeable to ions and aqueous solutions of glucose and urea. Fat-soluble substances, including oxygen, carbon dioxide, and alcohol, on the contrary, easily penetrate this area of the membrane.
Molecules cholesterol, which is part of the membrane, also belongs to lipids by nature, since their steroid group is highly soluble in fats. These molecules seem to be dissolved in the lipid bilayer. Their main purpose is to regulate the permeability (or impermeability) of membranes for water-soluble components of body fluids. In addition, cholesterol is the main regulator of membrane viscosity.
Cell membrane proteins. In the figure, globular particles are visible in the lipid bilayer - these are membrane proteins, most of which are glycoproteins. There are two types of membrane proteins: (1) integral, which penetrate the membrane through; (2) peripheral, which protrude only above one of its surfaces, without reaching the other.
Many integral proteins form channels (or pores) through which water and water-soluble substances, especially ions, can diffuse into the intra- and extracellular fluid. Due to the selectivity of the channels, some substances diffuse better than others.
Other integral proteins function as carrier proteins, transporting substances for which the lipid bilayer is impermeable. Sometimes carrier proteins act in the direction opposite to diffusion; such transport is called active transport. Some integral proteins are enzymes.
Integral membrane proteins can also serve as receptors for water-soluble substances, including peptide hormones, since the membrane is impermeable to them. The interaction of a receptor protein with a specific ligand leads to conformational changes in the protein molecule, which, in turn, stimulates the enzymatic activity of the intracellular segment of the protein molecule or the transmission of a signal from the receptor into the cell using a second messenger. Thus, integral proteins embedded in the cell membrane involve it in the process of transmitting information about the external environment into the cell.
Molecules of peripheral membrane proteins often associated with integral proteins. Most peripheral proteins are enzymes or play the role of dispatcher of the transport of substances through membrane pores.