Biological membranes.
The term “membrane” (Latin membrana - skin, film) began to be used more than 100 years ago to designate a cell boundary that serves, on the one hand, as a barrier between the contents of the cell and the external environment, and on the other, as a semi-permeable partition through which water can pass. and some substances. However, the functions of the membrane are not limited to this, since biological membranes form the basis of the structural organization of the cell.
Membrane structure. According to this model, the main membrane is a lipid bilayer in which the hydrophobic tails of the molecules face inward and the hydrophilic heads face outward. Lipids are represented by phospholipids - derivatives of glycerol or sphingosine. Proteins are associated with the lipid layer. Integral (transmembrane) proteins penetrate the membrane through and are firmly associated with it; peripheral ones do not penetrate and are less firmly connected to the membrane. Functions of membrane proteins: maintaining membrane structure, receiving and converting signals from the environment. environment, transport of certain substances, catalysis of reactions occurring on membranes. The membrane thickness ranges from 6 to 10 nm.
Membrane properties:
1. Fluidity. The membrane is not a rigid structure; most of its constituent proteins and lipids can move in the plane of the membrane.
2. Asymmetry. The composition of the outer and inner layers of both proteins and lipids is different. In addition, the plasma membranes of animal cells have a layer of glycoproteins on the outside (glycocalyx, which performs signaling and receptor functions, and is also important for uniting cells into tissues)
3. Polarity. The outer side of the membrane carries a positive charge, while the inner side carries a negative charge.
4. Selective permeability. The membranes of living cells, in addition to water, allow only certain molecules and ions of dissolved substances to pass through. (The use of the term “semi-permeability” in relation to cell membranes is not entirely correct, since this concept implies that the membrane allows only solvent molecules to pass through, while retaining all molecules and ions of dissolved substances.)
The outer cell membrane (plasmalemma) is an ultramicroscopic film 7.5 nm thick, consisting of proteins, phospholipids and water. An elastic film that is well wetted by water and quickly restores its integrity after damage. It has a universal structure, typical of all biological membranes. The border position of this membrane, its participation in the processes of selective permeability, pinocytosis, phagocytosis, excretion of excretory products and synthesis, in interaction with neighboring cells and protection of the cell from damage makes its role extremely important. Animal cells outside the membrane are sometimes covered with a thin layer consisting of polysaccharides and proteins - the glycocalyx. In plant cells, outside the cell membrane there is a strong cell wall that creates external support and maintains the shape of the cell. It consists of fiber (cellulose), a water-insoluble polysaccharide.
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, are well studied. , H+, Mg 2+ isolated or conjugated (Na+ and K+; H+ and K+). The molecular mechanism of active transport is not fully understood.
Among The main functions of the cell membrane can be distinguished: barrier, transport, enzymatic and receptor. The cellular (biological) membrane (also known as plasmalemma, plasma or cytoplasmic membrane) protects the contents of the cell or its organelles from the environment, provides selective permeability for substances, enzymes are located on it, as well as molecules that can “catch” various chemical and physical signals.
This functionality is ensured by the special structure of the cell membrane.
In the evolution of life on Earth, a cell could generally form only after the appearance of a membrane, which separated and stabilized the internal contents and prevented them from disintegrating.
In terms of maintaining homeostasis (self-regulation of the relative constancy of the internal environment) the barrier function of the cell membrane is closely related to transport.
Small molecules are able to pass through the plasmalemma without any “helpers”, along a concentration gradient, i.e., from an area with a high concentration of a given substance to an area with a low concentration. This is the case, for example, for gases involved in respiration. Oxygen and carbon dioxide diffuse through the cell membrane in the direction where their concentration is currently lower.
Since the membrane is mostly hydrophobic (due to the lipid double layer), polar (hydrophilic) molecules, even small ones, often cannot penetrate through it. Therefore, a number of membrane proteins act as carriers of such molecules, binding to them and transporting them through the plasmalemma.
Integral (membrane-permeating) proteins often operate on the principle of opening and closing channels. When any molecule approaches such a protein, it binds to it and the channel opens. This substance or another passes through the protein channel, after which its conformation changes, and the channel closes to this substance, but can open to allow the passage of another. The sodium-potassium pump works on this principle, pumping potassium ions into the cell and pumping sodium ions out of it.
Enzymatic function of the cell membrane to a greater extent realized on the membranes of cell organelles. Most proteins synthesized in the cell perform an enzymatic function. “Sitting” on the membrane in a certain order, they organize a conveyor when the reaction product catalyzed by one enzyme protein moves on to the next. This “conveyor” is stabilized by surface proteins of the plasmalemma.
Despite the universality of the structure of all biological membranes (they are built according to a single principle, they are almost identical in all organisms and in different membrane cell structures), their chemical composition can still differ. There are more liquid and more solid ones, some have more of certain proteins, others have less. In addition, different sides (inner and outer) of the same membrane also differ.
The membrane that surrounds the cell (cytoplasmic) on the outside has many carbohydrate chains attached to lipids or proteins (resulting in the formation of glycolipids and glycoproteins). Many of these carbohydrates serve receptor function, being susceptible to certain hormones, detecting changes in physical and chemical indicators in the environment.
If, for example, a hormone connects with its cellular receptor, then the carbohydrate part of the receptor molecule changes its structure, followed by a change in the structure of the associated protein part that penetrates the membrane. At the next stage, various biochemical reactions are started or suspended in the cell, i.e. its metabolism changes, and a cellular response to the “stimulus” begins.
In addition to the listed four functions of the cell membrane, others are also distinguished: matrix, energy, marking, formation of intercellular contacts, etc. However, they can be considered as “subfunctions” of those already discussed.
It's no secret that all living beings on our planet are made up of cells, these countless "" organic matter. The cells, in turn, are surrounded by a special protective shell - a membrane, which plays a very important role in the life of the cell, and the functions of the cell membrane are not limited to just protecting the cell, but represent a complex mechanism involved in the reproduction, nutrition, and regeneration of the cell.
What is a cell membrane
The word “membrane” itself is translated from Latin as “film,” although a membrane is not just a kind of film in which a cell is wrapped, but a combination of two films connected to each other and having different properties. In fact, the cell membrane is a three-layer lipoprotein (fat-protein) membrane that separates each cell from neighboring cells and the environment, and carries out controlled exchange between cells and the environment, this is the academic definition of what a cell membrane is.
The importance of the membrane is simply enormous, because it not only separates one cell from another, but also ensures the cell’s interaction with both other cells and the environment.
History of cell membrane research
An important contribution to the study of the cell membrane was made by two German scientists Gorter and Grendel back in 1925. It was then that they managed to conduct a complex biological experiment on red blood cells - erythrocytes, during which scientists obtained so-called “shadows”, empty shells of erythrocytes, which they stacked in one stack and measured the surface area, and also calculated the amount of lipids in them. Based on the amount of lipids obtained, scientists came to the conclusion that they are precisely contained in the double layer of the cell membrane.
In 1935, another pair of cell membrane researchers, this time Americans Daniel and Dawson, after a series of long experiments, established the protein content in the cell membrane. There was no other way to explain why the membrane had such a high surface tension. Scientists have cleverly presented a model of a cell membrane in the form of a sandwich, in which the role of bread is played by homogeneous lipid-protein layers, and between them, instead of oil, there is emptiness.
In 1950, with the advent of electronics, the theory of Daniel and Dawson was confirmed by practical observations - in micrographs of the cell membrane, layers of lipid and protein heads and also the empty space between them were clearly visible.
In 1960, the American biologist J. Robertson developed a theory about the three-layer structure of cell membranes, which for a long time was considered the only true one, but with the further development of science, doubts began to arise about its infallibility. So, for example, from the point of view, it would be difficult and labor-intensive for cells to transport the necessary nutrients through the entire “sandwich”
And only in 1972, American biologists S. Singer and G. Nicholson were able to explain the inconsistencies in Robertson’s theory using a new fluid-mosaic model of the cell membrane. In particular, they found that the cell membrane is not homogeneous in its composition, moreover, it is asymmetrical and filled with liquid. In addition, cells are in constant motion. And the notorious proteins that are part of the cell membrane have different structures and functions.
Properties and functions of the cell membrane
Now let's look at what functions the cell membrane performs:
The barrier function of the cell membrane is the membrane as a real border guard, standing guard over the boundaries of the cell, delaying and not allowing harmful or simply inappropriate molecules to pass through.
Transport function of the cell membrane - the membrane is not only a border guard at the cell gate, but also a kind of customs checkpoint; useful substances are constantly exchanged with other cells and the environment through it.
Matrix function - it is the cell membrane that determines the location relative to each other and regulates the interaction between them.
Mechanical function - is responsible for limiting one cell from another and, at the same time, for correctly connecting cells to each other, for forming them into a homogeneous tissue.
The protective function of the cell membrane is the basis for building the cell's protective shield. In nature, an example of this function can be hard wood, a dense peel, a protective shell, all due to the protective function of the membrane.
Enzymatic function is another important function performed by certain proteins in the cell. For example, thanks to this function, the synthesis of digestive enzymes occurs in the intestinal epithelium.
Also, in addition to all this, cellular exchange occurs through the cell membrane, which can take place in three different reactions:
- Phagocytosis is a cellular exchange in which membrane-embedded phagocyte cells capture and digest various nutrients.
- Pinocytosis is the process of capture by the cell membrane of liquid molecules in contact with it. To do this, special tendrils are formed on the surface of the membrane, which seem to surround a drop of liquid, forming a bubble, which is subsequently “swallowed” by the membrane.
- Exocytosis is a reverse process when a cell releases a secretory functional fluid to the surface through the membrane.
Structure of the cell membrane
There are three classes of lipids in the cell membrane:
- phospholipids (which are a combination of fats and phosphorus),
- glycolipids (a combination of fats and carbohydrates),
- cholesterol
Phospholipids and glycolipids, in turn, consist of a hydrophilic head, into which two long hydrophobic tails extend. Cholesterol occupies the space between these tails, preventing them from bending; all this, in some cases, makes the membrane of certain cells very rigid. In addition to all this, cholesterol molecules organize the structure of the cell membrane.
But be that as it may, the most important part of the structure of the cell membrane is protein, or rather different proteins that play different important roles. Despite the diversity of proteins contained in the membrane, there is something that unites them - annular lipids are located around all membrane proteins. Annular lipids are special structured fats that serve as a kind of protective shell for proteins, without which they simply would not work.
The structure of the cell membrane has three layers: the basis of the cell membrane is a homogeneous liquid bilipid layer. Proteins cover it on both sides like a mosaic. It is proteins, in addition to the functions described above, that also play the role of peculiar channels through which substances that are unable to penetrate through the liquid layer of the membrane pass through the membrane. These include, for example, potassium and sodium ions; for their penetration through the membrane, nature provides special ion channels in cell membranes. In other words, proteins ensure the permeability of cell membranes.
If we look at the cell membrane through a microscope, we will see a layer of lipids formed by small spherical molecules on which proteins swim as if on the sea. Now you know what substances make up the cell membrane.
Cell membrane video
And finally, an educational video about the cell membrane.
