Cell membrane - this is the cell membrane that performs the following functions: separation of the contents of the cell and the external environment, selective transport of substances (exchange with the environment external to the cell), the site of some biochemical reactions, the union of cells into tissues and reception.
Cell membranes are divided into plasma (intracellular) and external. The main property of any membrane is semi-permeability, that is, the ability to pass only certain substances. This allows for selective exchange between the cell and the external environment or exchange between cell compartments.
Plasma membranes are lipoprotein structures. Lipids spontaneously form a bilayer (double layer), and membrane proteins “float” in it. The membranes contain several thousand different proteins: structural, transporters, enzymes, etc. Between the protein molecules there are pores through which hydrophilic substances pass (the lipid bilayer prevents their direct penetration into the cell). Glycosyl groups (monosaccharides and polysaccharides) are attached to some molecules on the surface of the membrane, which are involved in the process of cell recognition during tissue formation.
Membranes vary in thickness, usually ranging from 5 to 10 nm. The thickness is determined by the size of the amphiphilic lipid molecule and is 5.3 nm. A further increase in membrane thickness is due to the dimensions membrane proteins s complexes. Depending on the external conditions(cholesterol is the regulator) the structure of the bilayer can change so that it becomes more dense or liquid - the speed of movement of substances along the membranes depends on this.
Cell membranes include: plasma membrane, karyolemma, membranes of the endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria, inclusions, etc.
Lipids are insoluble in water (hydrophobicity), but soluble in organic solvents and fats (lipophilicity). The composition of lipids in different membranes is not the same. For example, the plasma membrane contains a lot of cholesterol. The most common lipids in the membrane are phospholipids (glycerophosphatides), sphingomyelins (sphingolipids), glycolipids and cholesterol.
Phospholipids, sphingomyelins, glycolipids consist of two functional various parts: hydrophobic nonpolar, which does not carry charges - “tails” consisting of fatty acids, and hydrophilic, containing charged polar “heads” - alcohol groups (for example, glycerol).
The hydrophobic part of the molecule usually consists of two fatty acids. One of the acids is saturated, and the second is unsaturated. This determines the ability of lipids to spontaneously form bilayer (bilipid) membrane structures. Membrane lipids perform the following functions: barrier, transport, protein microenvironment, electrical resistance membranes.
Membranes differ from each other in their set of protein molecules. Many membrane proteins consist of regions rich in polar (charge-bearing) amino acids and regions with nonpolar amino acids (glycine, alanine, valine, leucine). Such proteins in the lipid layers of membranes are located so that their non-polar sections are, as it were, immersed in the “fat” part of the membrane, where the hydrophobic sections of lipids are located. The polar (hydrophilic) part of these proteins interacts with the lipid heads and faces the aqueous phase.
Biological membranes have common properties:
membranes are closed systems that do not allow the contents of the cell and its compartments to mix. Violation of the integrity of the membrane can lead to cell death;
superficial (planar, lateral) mobility. In membranes there is a continuous movement of substances across the surface;
membrane asymmetry. The structure of the outer and surface layers is chemically, structurally and functionally heterogeneous.
Cell membrane
Image of a cell membrane. The small blue and white balls correspond to the hydrophobic “heads” of the phospholipids, and the lines attached to them correspond to the hydrophilic “tails”. The figure shows only integral membrane proteins (red globules and yellow helices). Yellow oval dots inside the membrane - cholesterol molecules Yellow-green chains of beads on the outside of the membrane - chains of oligosaccharides forming the glycocalyx
A biological membrane also includes various proteins: integral (penetrating the membrane through), semi-integral (immersed at one end into the outer or inner lipid layer), surface (located on the outer or adjacent to internal sides membranes). Some proteins are the points of contact between the cell membrane and the cytoskeleton inside the cell, and the cell wall (if there is one) outside. Some of the integral proteins function as ion channels, various transporters and receptors.
Functions
- barrier - ensures 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 the membrane to different atoms or molecules depends on their size, electrical charge and chemical properties. Selective permeability ensures separation of the cell and cellular compartments from environment and supplying them with the necessary substances.
- transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery nutrients, removal of metabolic end products, secretion various substances, creating ion gradients, maintaining the optimal concentration of ions in the cell that 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 large sizes), 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 ( chemical substances, ensuring the conduction of nerve impulses) also bind to special receptor proteins of 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.
Structure and composition of biomembranes
Membranes are composed of three classes of lipids: phospholipids, glycolipids and cholesterol. Phospholipids and glycolipids (lipids with carbohydrates attached) consist of two long hydrophobic hydrocarbon tails that are connected to a charged hydrophilic head. Cholesterol gives the membrane rigidity 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 consists of proteins that penetrate it and are responsible for the various properties of membranes. Their composition and orientation differ in different membranes.
Cell membranes are often asymmetrical, that is, the layers differ in lipid composition, the transition of an individual molecule from one layer to another (the so-called flip flop) is difficult.
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.
Selective permeability
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 expenditure; the last two - active processes related to 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.
see also
Literature
- Antonov V.F., Smirnova E.N., Shevchenko E.V. Lipid membranes during phase transitions. - M.: Science, 1994.
- Gennis R. Biomembranes. Molecular structure and functions: translation from English. = Biomembranes. Molecular structure and function (by Robert B. Gennis). - 1st edition. - M.: Mir, 1997. - ISBN 5-03-002419-0
- Ivanov V. G., Berestovsky T. N. Lipid bilayer biological membranes. - M.: Nauka, 1982.
- Rubin A. B. Biophysics, textbook in 2 vols. - 3rd edition, corrected and expanded. - M.: Moscow University Publishing House, 2004. -
Cell membrane
Image of a cell membrane. The small blue and white balls correspond to the hydrophobic “heads” of the phospholipids, and the lines attached to them correspond to the hydrophilic “tails”. The figure shows only integral membrane proteins (red globules and yellow helices). Yellow oval dots inside the membrane - cholesterol molecules Yellow-green chains of beads on the outside of the membrane - chains of oligosaccharides forming the glycocalyx
A biological membrane also includes various proteins: integral (penetrating the membrane through), semi-integral (immersed at one end in the outer or inner lipid layer), surface (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 (if there is one) outside. Some of the integral proteins function as ion channels, various transporters and receptors.
Functions
- barrier - ensures 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 final metabolic products, secretion of various substances, creation of ion gradients, maintenance of optimal ion concentrations in the cell that 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 (chemical substances 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.
Structure and composition of biomembranes
Membranes are composed of three classes of lipids: phospholipids, glycolipids and cholesterol. Phospholipids and glycolipids (lipids with carbohydrates attached) consist of two long hydrophobic hydrocarbon tails that are connected to a charged hydrophilic head. Cholesterol gives the membrane rigidity 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 consists of proteins that penetrate it and are responsible for the various properties of membranes. Their composition and orientation differ in different membranes.
Cell membranes are often asymmetrical, that is, the layers differ in lipid composition, the transition of an individual molecule from one layer to another (the so-called flip flop) is difficult.
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.
Selective permeability
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 expenditure; 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.
see also
Literature
- Antonov V.F., Smirnova E.N., Shevchenko E.V. Lipid membranes during phase transitions. - M.: Science, 1994.
- Gennis R. Biomembranes. Molecular structure and functions: translation from English. = Biomembranes. Molecular structure and function (by Robert B. Gennis). - 1st edition. - M.: Mir, 1997. - ISBN 5-03-002419-0
- Ivanov V. G., Berestovsky T. N. Lipid bilayer of biological membranes. - M.: Nauka, 1982.
- Rubin A. B. Biophysics, textbook in 2 vols. - 3rd edition, corrected and expanded. - M.: Moscow University Publishing House, 2004. -
The cell membrane (plasma membrane) is a thin, semi-permeable membrane that surrounds cells.
Function and role of the cell membrane
Its function is to protect the integrity of the interior by allowing some necessary substances into the cage, and not allowing others to enter.
It also serves as the basis for attachment to some organisms and to others. Thus, the plasma membrane also provides the shape of the cell. Another function of the membrane is to regulate cell growth through balance and.
In endocytosis, lipids and proteins are removed from the cell membrane as substances are absorbed. During exocytosis, vesicles containing lipids and proteins fuse with the cell membrane, increasing cell size. , and fungal cells have plasma membranes. Internal ones, for example, are also enclosed in protective membranes.
Cell membrane structure
The plasma membrane is mainly composed of a mixture of proteins and lipids. Depending on the location and role of the membrane in the body, lipids can make up 20 to 80 percent of the membrane, with the remainder being proteins. While lipids help give the membrane flexibility, proteins control and maintain chemical composition cells and also help in the transport of molecules across the membrane.
Membrane lipids
Phospholipids are the main component of plasma membranes. They form a lipid bilayer in which the hydrophilic (water-attracted) head regions spontaneously organize to face the aqueous cytosol and extracellular fluid, while the hydrophobic (water-repelled) tail regions face away from the cytosol and extracellular fluid. The lipid bilayer is semipermeable, allowing only some molecules to diffuse across the membrane.
Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps maintain the rigidity of cell membranes by preventing phospholipids from becoming too dense. Cholesterol is absent in plant cell membranes.
Glycolipids are located with outer surface cell membranes and are connected to them by a carbohydrate chain. They help the cell recognize other cells in the body.
Membrane proteins
The cell membrane contains two types of associated proteins. Proteins of the peripheral membrane are external and are associated with it by interacting with other proteins. Integral membrane proteins are introduced into the membrane and most pass through. Parts of these transmembrane proteins are located on both sides of it.
Plasma membrane proteins have a number of various functions. Structural proteins provide support and shape to cells. Membrane receptor proteins help cells communicate with their external environment using hormones, neurotransmitters, and other signaling molecules. Transport proteins, such as globular proteins, transport molecules across cell membranes by facilitated diffusion. Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane, helping in the exchange and transport of molecules.
Organelle membranes
Some cellular organelles are also surrounded by protective membranes. Core,
