What are the main functions of the cell membrane. Main functions of membranes
Short description:
Sazonov V.F. 1_1 The structure of the cell membrane [Electronic resource] // Kinesiologist, 2009-2018: [website]. Date of update: 06.02.2018..__.201_). _The structure and functioning of the cell membrane is described (synonyms: plasmalemma, plasmolemma, biomembrane, cell membrane, outer cell membrane, cell membrane, cytoplasmic membrane). This initial information is necessary both for cytology and for understanding the processes of nervous activity: nervous excitation, inhibition, the work of synapses and sensory receptors.
cell membrane (plasma a lemma or plasma about lemma)
Concept definition
The cell membrane (synonyms: plasmalemma, plasmolemma, cytoplasmic membrane, biomembrane) is a triple lipoprotein (i.e. "fat-protein") membrane that separates the cell from the environment and carries out a controlled exchange and communication between the cell and its environment.
The main thing in this definition is not that the membrane separates the cell from the environment, but just that it connects cell with the environment. The membrane is active structure of the cell, it is constantly working.
A biological membrane is an ultrathin bimolecular film of phospholipids encrusted with proteins and polysaccharides. This cellular structure underlies the barrier, mechanical and matrix properties of a living organism (Antonov VF, 1996).
Figurative representation of the membrane
To me, the cell membrane appears as a lattice fence with many doors in it, which surrounds a certain territory. Any small living creatures can freely move back and forth through this fence. But larger visitors can only enter through the doors, and even then not all. Different visitors have keys only to their own doors, and they cannot pass through other people's doors. So, through this fence there are constantly flows of visitors back and forth, because the main function of the membrane-fence is twofold: to separate the territory from the surrounding space and at the same time connect it with the surrounding space. For this, there are many holes and doors in the fence - !
Membrane properties
1. Permeability.
2. Semi-permeability (partial permeability).
3. Selective (synonym: selective) permeability.
4. Active permeability (synonym: active transport).
5. Controlled permeability.
As you can see, the main property of the membrane is its permeability with respect to various substances.
6. Phagocytosis and pinocytosis.
7. Exocytosis.
8. The presence of electrical and chemical potentials, more precisely, the potential difference between the inner and outer sides of the membrane. Figuratively, one can say that "the membrane turns the cell into an "electric battery" by controlling ion flows". Details: .
9. Changes in electrical and chemical potential.
10. Irritability. Special molecular receptors located on the membrane can connect with signal (control) substances, as a result of which the state of the membrane and the entire cell can change. Molecular receptors trigger biochemical reactions in response to the combination of ligands (control substances) with them. It is important to note that the signaling substance acts on the receptor from the outside, while the changes continue inside the cell. It turns out that the membrane transmitted information from the environment to the internal environment of the cell.
11. Catalytic enzymatic activity. Enzymes can be embedded in the membrane or associated with its surface (both inside and outside the cell), and there they carry out their enzymatic activity.
12. Changing the shape of the surface and its area. This allows the membrane to form outgrowths outward or, conversely, invaginations into the cell.
13. The ability to form contacts with other cell membranes.
14. Adhesion - the ability to stick to solid surfaces.
Brief list of membrane properties
- Permeability.
- Endocytosis, exocytosis, transcytosis.
- Potentials.
- Irritability.
- enzymatic activity.
- Contacts.
- Adhesion.
Membrane functions
1. Incomplete isolation of internal content from external environment.
2. The main thing in the work of the cell membrane is exchange various substances between the cell and the extracellular environment. This is due to such property of the membrane as permeability. In addition, the membrane regulates this exchange by regulating its permeability.
3. Another important function of the membrane is creating a difference in chemical and electrical potentials between its inner and outer sides. Due to this, inside the cell has a negative electrical potential -.
4. Through the membrane is also carried out information exchange between the cell and its environment. Special molecular receptors located on the membrane can bind to control substances (hormones, mediators, modulators) and trigger biochemical reactions in the cell, leading to various changes in the cell or in its structures.
Video:The structure of the cell membrane
Video lecture:Details about the structure of the membrane and transport
Membrane structure
The cell membrane has a universal three-layer structure. Its median fat layer is continuous, and the upper and lower protein layers cover it in the form of a mosaic of individual protein areas. The fat layer is the basis that ensures the isolation of the cell from the environment, isolating it from the environment. By itself, it passes water-soluble substances very poorly, but easily passes fat-soluble ones. Therefore, the permeability of the membrane for water-soluble substances (for example, ions) has to be provided with special protein structures - and.
Below are microphotographs of real cell membranes of contacting cells, obtained using an electron microscope, as well as a schematic drawing showing the three-layered membrane and the mosaic nature of its protein layers. To enlarge an image, click on it.
Separate image of the inner lipid (fatty) layer of the cell membrane, permeated with integral embedded proteins. The upper and lower protein layers are removed so as not to interfere with the consideration of the lipid bilayer
Figure above: An incomplete schematic representation of the cell membrane (cell wall) from Wikipedia.
Note that the outer and inner protein layers have been removed from the membrane here so that we can better see the central fatty double lipid layer. In a real cell membrane, large protein "islands" float above and below along the fatty film (small balls in the figure), and the membrane turns out to be thicker, three-layered: protein-fat-protein . So it's actually like a sandwich of two protein "slices of bread" with a thick layer of "butter" in the middle, ie. has a three-layer structure, not a two-layer one.
In this figure, small blue and white balls correspond to the hydrophilic (wettable) "heads" of the lipids, and the "strings" attached to them correspond to the hydrophobic (non-wettable) "tails". Of the proteins, only integral end-to-end membrane proteins (red globules and yellow helices) are shown. Yellow oval dots inside the membrane are cholesterol molecules Yellow-green chains of beads on the outside of the membrane are oligosaccharide chains that form the glycocalyx. Glycocalyx is like a carbohydrate ("sugar") "fluff" on the membrane, formed by long carbohydrate-protein molecules protruding from it.
Living is a small "protein-fat bag" filled with semi-liquid jelly-like contents, which is penetrated by films and tubes.
The walls of this sac are formed by a double fatty (lipid) film, covered inside and out with proteins - the cell membrane. Therefore, the membrane is said to have three-layer structure : proteins-fats-proteins. Inside the cell there are also many similar fatty membranes that divide its internal space into compartments. Cellular organelles are surrounded by the same membranes: nucleus, mitochondria, chloroplasts. So the membrane is a universal molecular structure inherent in all cells and all living organisms.
On the left - no longer a real, but an artificial model of a piece of a biological membrane: this is an instant snapshot of an adipose phospholipid bilayer (i.e. a double layer) in the process of its molecular dynamics modeling. The calculation cell of the model is shown - 96 PQ molecules ( f osphatidil X oline) and 2304 water molecules, total 20544 atoms.
On the right is a visual model of a single molecule of the same lipid, from which the membrane lipid bilayer is assembled. It has a hydrophilic (water-loving) head at the top, and two hydrophobic (water-fearing) tails at the bottom. This lipid has a simple name: 1-steroyl-2-docosahexaenoyl-Sn-glycero-3-phosphatidylcholine (18:0/22:6(n-3)cis PC), but you don't need to memorize it unless you plan to make your teacher swoon with the depth of your knowledge.
You can give a more precise scientific definition of a cell:
is an ordered, structured heterogeneous system of biopolymers limited by an active membrane, participating in a single set of metabolic, energy and information processes, and also maintaining and reproducing the entire system as a whole.
Inside the cell is also penetrated by membranes, and between the membranes there is not water, but a viscous gel / sol of variable density. Therefore, the interacting molecules in the cell do not float freely, as in a test tube with an aqueous solution, but mostly sit (immobilized) on the polymer structures of the cytoskeleton or intracellular membranes. And therefore, chemical reactions take place inside the cell almost like in a solid body, and not in a liquid. The outer membrane that surrounds the cell is also covered in enzymes and molecular receptors, making it a very active part of the cell.
The cell membrane (plasmalemma, plasmolemma) is an active shell that separates the cell from the environment and connects it with the environment. © Sazonov V.F., 2016.
From this definition of a membrane, it follows that it does not simply limit the cell, but actively working linking it to its environment.
The fat that makes up the membranes is special, so its molecules are usually called not just fat, but lipids, phospholipids, sphingolipids. The membrane film is double, i.e. it consists of two films stuck together. Therefore, textbooks write that the base of the cell membrane consists of two lipid layers (or " bilayer", i.e. double layer). For each individual lipid layer, one side can be wetted by water, and the other cannot. So, these films stick together with each other precisely by their non-wetting sides.
bacteria membrane
The shell of a prokaryotic cell of gram-negative bacteria consists of several layers, shown in the figure below.
Layers of the shell of gram-negative bacteria:
1. The inner three-layer cytoplasmic membrane, which is in contact with the cytoplasm.
2. Cell wall, which consists of murein.
3. The outer three-layer cytoplasmic membrane, which has the same system of lipids with protein complexes as the inner membrane.
Communication of gram-negative bacterial cells with the outside world through such a complex three-step structure does not give them an advantage in surviving in harsh conditions compared to gram-positive bacteria that have a less powerful shell. They just don't take it well high temperatures, increased acidity and pressure drops.
Video lecture:Plasma membrane. E.V. Cheval, Ph.D.
Video lecture:The membrane as a cell boundary. A. Ilyaskin
Importance of Membrane Ion Channels
It is easy to understand that only fat-soluble substances can enter the cell through the membrane fatty film. These are fats, alcohols, gases. For example, in erythrocytes, oxygen and carbon dioxide easily pass in and out directly through the membrane. But water and water-soluble substances (for example, ions) simply cannot pass through the membrane into any cell. This means that they need special holes. But if you just make a hole in the fatty film, then it will immediately tighten back. What to do? A solution was found in nature: it is necessary to make special protein transport structures and stretch them through the membrane. This is how the channels for the passage of fat-insoluble substances are obtained - the ion channels of the cell membrane.
So, in order to give its membrane additional properties of permeability for polar molecules (ions and water), the cell synthesizes special proteins in the cytoplasm, which are then integrated into the membrane. They are of two types: transporter proteins (for example, transport ATPases) and channel-forming proteins (channel formers). These proteins are embedded in the double fatty layer of the membrane and form transport structures in the form of transporters or in the form of ion channels. Various water-soluble substances can now pass through these transport structures, which otherwise cannot pass through the fatty membrane film.
In general, proteins embedded in the membrane are also called integral, precisely because they are, as it were, included in the composition of the membrane and penetrate it through and through. Other proteins, not integral, form, as it were, islands that "float" on the surface of the membrane: either along its outer surface or along its inner one. After all, everyone knows that fat is a good lubricant and it is easy to slide on it!
conclusions
1. In general, the membrane is three-layered:
1) the outer layer of protein "islands",
2) fatty two-layer "sea" (lipid bilayer), i.e. double lipid film
3) the inner layer of protein "islands".
But there is also a loose outer layer - the glycocalyx, which is formed by glycoproteins sticking out of the membrane. They are molecular receptors to which signaling controls bind.
2. Special protein structures are built into the membrane, ensuring its permeability to ions or other substances. We must not forget that in some places the sea of fat is permeated through with integral proteins. And it is integral proteins that form special transport structures cell membrane (see section 1_2 Membrane transport mechanisms). Through them, substances enter the cell, and are also removed from the cell to the outside.
3. Enzyme proteins can be located on any side of the membrane (outer and inner), as well as inside the membrane, which affect both the state of the membrane itself and the life of the entire cell.
So the cell membrane is an active variable structure that actively works in the interests of the whole cell and connects it with the outside world, and is not just a "protective shell". This is the most important thing to know about the cell membrane.
In medicine, membrane proteins are often used as “targets” for drugs. Receptors, ion channels, enzymes, transport systems act as such targets. AT recent times in addition to the membrane, genes hidden in the cell nucleus also become targets for drugs.
Video:Introduction to Cell Membrane Biophysics: Structure of Membrane 1 (Vladimirov Yu.A.)
Video:History, structure and functions of the cell membrane: Structure of membranes 2 (Vladimirov Yu.A.)
© 2010-2018 Sazonov V.F., © 2010-2016 kineziolog.bodhy.
All living organisms on Earth are made up of cells, and each cell is surrounded by a protective shell - a membrane. However, the functions of the membrane are not limited to protecting organelles and separating one cell from another. The cell membrane is a complex mechanism that is directly involved in reproduction, regeneration, nutrition, respiration, and many other important cell functions.
The term "cell membrane" has been used for about a hundred years. The word "membrane" in translation from Latin means "film". But in the case of a cell membrane, it would be more correct to speak of a combination of two films interconnected in a certain way, moreover, different sides of these films have different properties.
The cell membrane (cytolemma, plasmalemma) is a three-layer lipoprotein (fat-protein) shell that separates each cell from neighboring cells and the environment, and carries out a controlled exchange between cells and the environment.
Of decisive importance in this definition is not that the cell membrane separates one cell from another, but that it ensures its interaction with other cells and the environment. The membrane is a very active, constantly working structure of the cell, on which many functions are assigned by nature. From our article, you will learn everything about the composition, structure, properties and functions of the cell membrane, as well as the danger posed to human health by disturbances in the functioning of cell membranes.
History of cell membrane research
In 1925, two German scientists, Gorter and Grendel, were able to conduct a complex experiment on human red blood cells, erythrocytes. Using osmotic shock, the researchers obtained the so-called "shadows" - empty shells of red blood cells, then put them in one pile and measured the surface area. The next step was to calculate the amount of lipids in the cell membrane. With the help of acetone, the scientists isolated lipids from the "shadows" and determined that they were just enough for a double continuous layer.
However, during the experiment, two gross errors were made:
The use of acetone does not allow all lipids to be isolated from the membranes;
The surface area of the "shadows" was calculated by dry weight, which is also incorrect.
Since the first error gave a minus in the calculations, and the second - a plus, overall result turned out to be surprisingly accurate, and German scientists brought in scientific world The most important discovery is the lipid bilayer of the cell membrane.
In 1935, another pair of researchers, Danielly and Dawson, after long experiments on bilipid films, came to the conclusion that proteins are present in cell membranes. There was no other way to explain why these films have such a high surface tension. Scientists have presented to the attention of the public a schematic model of a cell membrane, similar to a sandwich, where the role of slices of bread is played by homogeneous lipid-protein layers, and between them instead of oil is emptiness.
In 1950, using the first electron microscope, the Danielly-Dawson theory was partially confirmed - two layers consisting of lipid and protein heads were clearly visible on micrographs of the cell membrane, and between them there was a transparent space filled only with tails of lipids and proteins.
In 1960, guided by these data, the American microbiologist J. Robertson developed a theory about the three-layer structure of cell membranes, which for a long time considered to be the only correct one. However, as science developed, more and more doubts were born about the homogeneity of these layers. From the point of view of thermodynamics, such a structure is extremely unfavorable - it would be very difficult for cells to transport substances in and out through the entire “sandwich”. In addition, it has been proven that the cell membranes of different tissues have different thickness and method of attachment, which is due to different functions of organs.
In 1972, microbiologists S.D. Singer and G.L. Nicholson was able to explain all the inconsistencies of Robertson's theory with the help of a new, fluid-mosaic model of the cell membrane. Scientists have found that the membrane is heterogeneous, asymmetric, filled with fluid, and its cells are in constant motion. And the proteins that make up it have a different structure and purpose, in addition, they are located differently relative to the bilipid layer of the membrane.
Cell membranes contain three types of proteins:
Peripheral - attached to the surface of the film;
semi-integral- partially penetrate the bilipid layer;
Integral - completely penetrate the membrane.
Peripheral proteins are associated with the heads of membrane lipids through electrostatic interaction, and they never form a continuous layer, as was previously believed. And semi-integral and integral proteins serve to transport oxygen and nutrients into the cell, as well as to remove decay products from it and more for several important features, which you will learn about later.
The cell membrane performs the following functions:
Barrier - the permeability of the membrane for different types of molecules is not the same. To bypass the cell membrane, the molecule must have a certain size, chemical properties and electric charge. Harmful or inappropriate molecules, due to the barrier function of the cell membrane, simply cannot enter the cell. For example, with the help of the peroxide reaction, the membrane protects the cytoplasm from peroxides that are dangerous for it;
Transport - a passive, active, regulated and selective exchange passes through the membrane. Passive metabolism is suitable for fat-soluble substances and gases consisting of very small molecules. Such substances penetrate into and out of the cell without energy expenditure, freely, by diffusion. The active transport function of the cell membrane is activated when necessary, but difficult to transport substances need to be carried into or out of the cell. For example, those with a large molecular size, or unable to cross the bilipid layer due to hydrophobicity. Then protein pumps begin to work, including ATPase, which is responsible for the absorption of potassium ions into the cell and the ejection of sodium ions from it. Regulated transport is essential for secretion and fermentation functions, such as when cells produce and secrete hormones or gastric juice. All these substances leave the cells through special channels and in a given volume. And the selective transport function is associated with the very integral proteins that penetrate the membrane and serve as a channel for the entry and exit of strictly defined types of molecules;
Matrix - the cell membrane determines and fixes the location of organelles relative to each other (nucleus, mitochondria, chloroplasts) and regulates the interaction between them;
Mechanical - provides a restriction of one cell from another, and, at the same time time is correct the connection of cells into a homogeneous tissue and the resistance of organs to deformation;
Protective - both in plants and in animals, the cell membrane serves as the basis for building a protective frame. An example would be hardwood, dense peel, prickly thorns. In the animal world, there are also many examples of the protective function of cell membranes - turtle shell, chitinous shell, hooves and horns;
Energy - the processes of photosynthesis and cellular respiration would be impossible without the participation of cell membrane proteins, because it is with the help of protein channels that cells exchange energy;
Receptor - proteins embedded in the cell membrane may have another important function. They serve as receptors through which the cell receives a signal from hormones and neurotransmitters. And this, in turn, is necessary for the conduction of nerve impulses and the normal course of hormonal processes;
Enzymatic - another important function inherent in some proteins of cell membranes. For example, in the intestinal epithelium, digestive enzymes are synthesized with the help of such proteins;
Biopotential- the concentration of potassium ions inside the cell is much higher than outside, and the concentration of sodium ions, on the contrary, is greater outside than inside. This explains the potential difference: the charge is negative inside the cell, positive outside, which contributes to the movement of substances into the cell and out in any of the three types of metabolism - phagocytosis, pinocytosis and exocytosis;
Marking - on the surface of cell membranes there are so-called "labels" - antigens consisting of glycoproteins (proteins with branched oligosaccharide side chains attached to them). Since side chains can have a huge variety of configurations, each type of cell receives its own unique label that allows other cells in the body to recognize them “by sight” and respond to them correctly. That is why, for example, human immune cells, macrophages, easily recognize a foreigner that has entered the body (infection, virus) and try to destroy it. The same thing happens with diseased, mutated and old cells - the label on their cell membrane changes and the body gets rid of them.
Cellular exchange occurs across membranes, and can be carried out through three main types of reactions:
Phagocytosis is a cellular process in which phagocytic cells embedded in the membrane capture and digest solid particles of nutrients. In the human body, phagocytosis is carried out by membranes of two types of cells: granulocytes (granular leukocytes) and macrophages (immune killer cells);
Pinocytosis is the process of capturing liquid molecules that come into contact with it by the surface of the cell membrane. For nutrition by the type of pinocytosis, the cell grows thin fluffy outgrowths in the form of antennae on its membrane, which, as it were, surround a drop of liquid, and a bubble is obtained. First, this vesicle protrudes above the surface of the membrane, and then it is “swallowed” - it hides inside the cell, and its walls merge with the inner surface of the cell membrane. Pinocytosis occurs in almost all living cells;
Exocytosis is the reverse process, in which vesicles with a secretory functional fluid (enzyme, hormone) are formed inside the cell, and it must somehow be removed from the cell into the environment. To do this, the bubble first merges with the inner surface of the cell membrane, then bulges outward, bursts, expels the contents and again merges with the surface of the membrane, this time from the outside. Exocytosis takes place, for example, in the cells of the intestinal epithelium and the adrenal cortex.
Cell membranes contain three classes of lipids:
Phospholipids;
Glycolipids;
Cholesterol.
Phospholipids (a combination of fats and phosphorus) and glycolipids (a combination of fats and carbohydrates), in turn, consist of a hydrophilic head, from which two long hydrophobic tails extend. But cholesterol sometimes occupies the space between these two tails and does not allow them to bend, which makes the membranes of some cells rigid. In addition, cholesterol molecules streamline the structure of cell membranes and prevent the transition of polar molecules from one cell to another.
But the most important component, as can be seen from the previous section on the functions of cell membranes, are proteins. Their composition, purpose and location are very diverse, but there is something in common that unites them all: annular lipids are always located around the proteins of cell membranes. These are special fats that are clearly structured, stable, have more saturated fatty acids in their composition, and are released from membranes along with "sponsored" proteins. This is a kind of personal protective shell for proteins, without which they simply would not work.
The structure of the cell membrane is three-layered. A relatively homogeneous liquid bilipid layer lies in the middle, and proteins cover it on both sides with a kind of mosaic, partially penetrating into the thickness. That is, it would be wrong to think that the outer protein layers of cell membranes are continuous. Proteins, in addition to their complex functions, are needed in the membrane in order to pass inside the cells and transport out of them those substances that are not able to penetrate the fat layer. For example, potassium and sodium ions. For them, special protein structures are provided - ion channels, which we will discuss in more detail below.
If you look at the cell membrane through a microscope, you can see a layer of lipids formed by the smallest spherical molecules, along which, like the sea, large protein cells float. different shapes. Exactly the same membranes divide the internal space of each cell into compartments in which the nucleus, chloroplasts and mitochondria are comfortably located. If there were no separate “rooms” inside the cell, the organelles would stick together and would not be able to perform their functions correctly.
A cell is a set of organelles structured and delimited by membranes, which is involved in a complex of energy, metabolic, informational and reproductive processes that ensure the vital activity of the organism.
As can be seen from this definition, the membrane is the most important functional component of any cell. Its significance is as great as that of the nucleus, mitochondria and other cell organelles. And the unique properties of the membrane are due to its structure: it consists of two films stuck together in a special way. Molecules of phospholipids in the membrane are located with hydrophilic heads outward, and hydrophobic tails inward. Therefore, one side of the film is wetted by water, while the other is not. So, these films are connected to each other with non-wettable sides inward, forming a bilipid layer surrounded by protein molecules. This is the very “sandwich” structure of the cell membrane.
Ion channels of cell membranes
Let us consider in more detail the principle of operation of ion channels. What are they needed for? The fact is that only fat-soluble substances can freely penetrate through the lipid membrane - these are gases, alcohols and fats themselves. So, for example, in red blood cells there is a constant exchange of oxygen and carbon dioxide, and for this our body does not have to resort to any additional tricks. But what about when it becomes necessary to transport through the cell membrane aqueous solutions such as sodium and potassium salts?
It would be impossible to pave the way for such substances in the bilipid layer, since the holes would immediately tighten and stick together back, such is the structure of any adipose tissue. But nature, as always, found a way out of the situation and created special protein transport structures.
There are two types of conductive proteins:
Transporters are semi-integral protein pumps;
Channeloformers are integral proteins.
Proteins of the first type are partially immersed in the bilipid layer of the cell membrane, and look out with their heads, and in the presence of the desired substance, they begin to behave like a pump: they attract a molecule and suck it into the cell. And proteins of the second type, integral, have an elongated shape and are located perpendicular to the bilipid layer of the cell membrane, penetrating it through and through. Through them, as through tunnels, substances that are unable to pass through fat move into and out of the cell. It is through ion channels that potassium ions penetrate into the cell and accumulate in it, while sodium ions, on the contrary, are brought out. There is a difference in electrical potentials, so necessary for correct operation all the cells in our body.
The most important conclusions about the structure and functions of cell membranes
Theory always looks interesting and promising if it can be usefully applied in practice. The discovery of the structure and functions of the cell membranes of the human body allowed scientists to make a real breakthrough in science in general, and in medicine in particular. It is no coincidence that we have dwelled on ion channels in such detail, because it is here that lies the answer to one of the most important questions of our time: why do people increasingly get sick with oncology?
Cancer claims about 17 million lives worldwide every year and is the fourth leading cause of all deaths. According to WHO, the incidence of cancer is steadily increasing, and by the end of 2020 it could reach 25 million per year.
What explains the real epidemic of cancer, and what does the function of cell membranes have to do with it? You will say: the reason is in a bad environmental situation, malnutrition, bad habits and heavy heredity. And, of course, you will be right, but if we talk about the problem in more detail, then the reason is the acidification of the human body. The negative factors listed above lead to disruption of the cell membranes, inhibit breathing and nutrition.
Where there should be a plus, a minus is formed, and the cell cannot function normally. But cancer cells do not need either oxygen or an alkaline environment - they are able to use an anaerobic type of nutrition. Therefore, in conditions of oxygen starvation and an off-scale pH level, healthy cells mutate, wanting to adapt to environment and become cancer cells. This is how a person gets cancer. To avoid this, you just need to drink enough clean water daily, and give up carcinogens in food. But, as a rule, people are well aware of harmful products and the need for high-quality water, and do nothing - they hope that trouble will bypass them.
Knowing the features of the structure and functions of the cell membranes of different cells, doctors can use this information to provide targeted, targeted therapeutic effects on the body. Many modern drugs, getting into our body, are looking for the right "target", which can be ion channels, enzymes, receptors and biomarkers of cell membranes. This method of treatment allows you to achieve better results with minimal side effects.
Antibiotics of the latest generation, when released into the blood, do not kill all the cells in a row, but look for exactly the cells of the pathogen, focusing on markers in its cell membranes. The newest anti-migraine drugs, triptans, narrow only the inflamed vessels of the brain, while almost no effect on the heart and peripheral circulatory system. And they recognize the necessary vessels precisely by the proteins of their cell membranes. There are many such examples, so we can say with confidence that knowledge about the structure and functions of cell membranes underlies the development of modern medical science and saves millions of lives every year.
Education: Moscow Medical Institute. I. M. Sechenov, specialty - "Medicine" in 1991, in 1993 "Occupational diseases", in 1996 "Therapy".
The basic structural unit of a living organism is a cell, which is a differentiated section of the cytoplasm surrounded by a cell membrane. In view of the fact that the cell performs many important functions, such as reproduction, nutrition, movement, the shell must be plastic and dense.
History of the discovery and research of the cell membrane
In 1925, Grendel and Gorder made a successful experiment to identify the "shadows" of erythrocytes, or empty shells. Despite several gross mistakes made, scientists discovered the lipid bilayer. Their work was continued by Danielli, Dawson in 1935, Robertson in 1960. As a result of many years of work and the accumulation of arguments in 1972, Singer and Nicholson created a fluid mosaic model of the structure of the membrane. 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". So designate the border of the cell, which is a natural barrier between the internal contents and the external environment. The structure of the cell membrane suggests semi-permeability, due to which moisture and nutrients and decay products can freely pass through it. This shell can be called the main structural component of the organization of the cell.
Consider the main functions of the cell membrane
1. Separates the internal contents of the cell and the components of the external environment.
2. Helps maintain a constant chemical composition of the cell.
3. Regulates the correct metabolism.
4. Provides interconnection between cells.
5. Recognizes signals.
6. Protection function.
"Plasma Shell"
The outer cell membrane, also called the plasma membrane, is an ultramicroscopic film that is five to seven nanometers thick. It consists mainly of protein compounds, phospholide, water. The film is elastic, easily absorbs water, and also quickly restores its integrity after damage.
Differs in a universal structure. This membrane occupies a boundary position, participates in the process of selective permeability, excretion of decay products, synthesizes them. The relationship with the "neighbors" and the reliable protection of the internal contents from damage makes it an important component in such a matter as the structure of the cell. The cell membrane of animal organisms sometimes turns out to be covered with the thinnest layer - glycocalyx, which includes proteins and polysaccharides. Plant cells outside the membrane are protected by a cell wall that acts as a 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 performs the function of repair, protection and interaction with other cells.
The structure of the cell membrane
The thickness of this movable shell varies from six to ten nanometers. The cell membrane of a cell has a special composition, the basis of which is the lipid bilayer. The hydrophobic tails, which are inert to water, are located on the inside, while the hydrophilic heads, which interact with water, are turned outward. Each lipid is a phospholipid, which is the result of the interaction of substances such as glycerol and sphingosine. The lipid scaffold is closely surrounded by proteins, which are located in a non-continuous layer. Some of them are immersed in the lipid layer, the rest pass through it. As a result, water-permeable areas are formed. The functions performed by these proteins are different. Some of them are enzymes, the rest are transport proteins that carry various substances from the external environment to the cytoplasm and vice versa.
The cell membrane is permeated through and closely connected with integral proteins, while 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. Due 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 a cell membrane can change. In this case, the nucleus may suffer from external influences.
Properties
The cell membrane of a cell has interesting features. Due to its fluidity, this shell is not a rigid structure, and the bulk of the proteins and lipids that make up its composition move freely on the plane of the membrane.
In general, the cell membrane is asymmetric, so the composition of the protein and lipid layers is different. Plasma membranes in animal cells have a glycoprotein layer on their outer side, which performs receptor and signal functions, and also plays an important role in the process of combining cells into tissue. The cell membrane is polar outside the charge is positive, and on the inside it 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. For potassium ions, a different ratio is characteristic: their number 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 performs a “pumping” role, leveling the concentration of substances: sodium ions are pumped out to the cell surface, and potassium ions are pumped inward. This feature part of the most important functions of the cell membrane.
This tendency of sodium and potassium ions to move inward from the surface plays a large 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 inflows 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 is the cell nourished through the cell membrane?
Many cells take in substances through processes such as phagocytosis and pinocytosis. With the first option of flexible outer membrane a small depression is created in which the captured particle is located. Then the diameter of the recess becomes larger until the surrounded particle enters the cell cytoplasm. Through phagocytosis, some protozoa, such as amoeba, as well as blood cells - leukocytes and phagocytes, are fed. Similarly, cells absorb fluid that contains the necessary useful material. This phenomenon is called pinocytosis.
The outer membrane is closely connected to the endoplasmic reticulum of the cell.
In many types of basic tissue components, protrusions, folds, and microvilli are located on the surface of the membrane. Plant cells on the outside of this shell are covered with another one, thick and clearly visible under a microscope. The fiber they are made of helps form the 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 the chitin contained in the integumentary cells of insects.
In addition to the cell 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 a cell membrane. Structure and function suggest significant expansion total area cell surface, improving 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 bonds are maintained at a sufficiently strong level, forming tissues. In this regard, we can conclude that one of the most important roles in the cell is played by the cell membrane. The structure and functions performed by it are radically different in different cells, depending on their purpose. Through these features, a variety of physiological activity of cell membranes and their roles in the existence of cells and tissues is achieved.
1 - polar head of the phospholipid molecule
2 - fatty acid tail of the phospholipid molecule
3 - integral protein
4 - peripheral protein
5 - semi-integral protein
6 - glycoprotein
7 - glycolipid
The outer cell membrane is inherent in all cells (animals and plants), has a thickness of about 7.5 (up to 10) nm and consists of lipid and protein molecules.
At present, the fluid-mosaic model of the construction of the cell membrane is widespread. According to this model, lipid molecules are arranged in two layers, with their water-repellent ends (hydrophobic - fat-soluble) facing each other, and water-soluble (hydrophilic) - to the periphery. Protein molecules are embedded in the lipid layer. Some of them are located on the outer or inner surface of the lipid part, others are partially immersed or penetrate the membrane through and through.
Membrane functions :
Protective, border, barrier;
Transport;
Receptor - is carried out at the expense of proteins - receptors, which have a selective ability for certain substances (hormones, antigens, etc.), enter into chemical interactions with them, conduct signals inside the cell;
Participate in the formation of intercellular contacts;
They provide the movement of some cells (amoeboid movement).
Animal cells have a thin layer of glycocalyx on top of the outer cell membrane. It is a complex of carbohydrates with lipids and carbohydrates with proteins. The glycocalyx is involved in intercellular interactions. The cytoplasmic membranes of most cell organelles have exactly the same structure.
In plant cells outside of the cytoplasmic membrane. the cell wall is made up of cellulose.
Transport of substances across the cytoplasmic membrane .
There are two main mechanisms for the entry of substances into the cell or out of the cell to the outside:
1. Passive transport.
2. Active transport.
Passive transport of substances occurs without the expenditure of energy. An example of such transport is diffusion and osmosis, in which the movement of molecules or ions is carried out from a region of high concentration to a region of lower concentration, for example, water molecules.
Active transport - in this type of transport, molecules or ions penetrate the membrane against a concentration gradient, which requires energy. An example of active transport is the sodium-potassium pump, which actively pumps sodium out of the cell and absorbs potassium ions from the external environment, transferring them into the cell. The pump is a special membrane protein that sets it in motion with ATP.
Active transport maintains a constant cell volume and membrane potential.
Substances can be transported by endocytosis and exocytosis.
Endocytosis - the penetration of substances into the cell, exocytosis - out of the cell.
During endocytosis, the plasma membrane forms an invagination or outgrowths, which then envelop the substance and, lacing off, turn into vesicles.
There are two types of endocytosis:
1) phagocytosis - the absorption of solid particles (phagocyte cells),
2) pinocytosis - the absorption of liquid material. Pinocytosis is characteristic of amoeboid protozoa.
By exocytosis, various substances are removed from the cells: undigested food residues are removed from the digestive vacuoles, their liquid secret is removed from the secretory cells.
Cytoplasm -(cytoplasm + nucleus form protoplasm). The cytoplasm consists of a watery ground substance (cytoplasmic matrix, hyaloplasm, cytosol) and various organelles and inclusions in it.
Inclusions– cell waste products. There are 3 groups of inclusions - trophic, secretory (gland cells) and special (pigment) values.
Organelles - These are permanent structures of the cytoplasm that perform certain functions in the cell.
Isolate organelles general meaning and special. Special ones are found in most cells, but are present in significant numbers only in cells that perform a specific function. These include microvilli of intestinal epithelial cells, cilia of the epithelium of the trachea and bronchi, flagella, myofibrils (providing muscle contraction, etc.).
Organelles of general importance include EPS, the Golgi complex, mitochondria, ribosomes, lysosomes, centrioles of the cell center, peroxisomes, microtubules, microfilaments. Plant cells contain plastids and vacuoles. Organelles of general importance can be divided into organelles having a membrane and non-membrane structure.
Organelles having a membrane structure are two-membrane and one-membrane. Two-membrane cells include mitochondria and plastids. To single-membrane - endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, vacuoles.
Membraneless organelles: ribosomes, cell center, microtubules, microfilaments.
Mitochondria – These are round or oval organelles. They consist of two membranes: internal and external. The inner membrane has outgrowths - cristae, which divide the mitochondria into compartments. The compartments are filled with a substance - a matrix. The matrix contains DNA, mRNA, tRNA, ribosomes, calcium and magnesium salts. This is where protein biosynthesis takes place. The main function of mitochondria is the synthesis of energy and its accumulation in ATP molecules. New mitochondria are formed in the cell as a result of the division of old ones.
plastids – organelles found predominantly in plant cells. They are of three types: chloroplasts containing a green pigment; chromoplasts (pigments of red, yellow, orange color); leucoplasts (colorless).
Chloroplasts, thanks to the green pigment chlorophyll, are able to synthesize organic matter from inorganic, using the energy of the sun.
Chromoplasts give bright colors to flowers and fruits.
Leucoplasts are able to accumulate reserve nutrients: starch, lipids, proteins, etc.
Endoplasmic reticulum ( EPS ) is a complex system of vacuoles and channels that are limited by membranes. There are smooth (agranular) and rough (granular) EPS. Smooth has no ribosomes on its membrane. It contains the synthesis of lipids, lipoproteins, the accumulation and removal of toxic substances from the cell. Granular EPS has ribosomes on membranes in which proteins are synthesized. Then the proteins enter the Golgi complex, and from there out.
Golgi complex (Golgi apparatus) is a stack of flattened membrane sacs - cisterns and a system of bubbles associated with them. The stack of cisterns is called a dictyosome.
Functions of the Golgi complex : protein modification, polysaccharide synthesis, substance transport, cell membrane formation, lysosome formation.
Lysosomes – are membrane-bound vesicles containing enzymes. They carry out intracellular cleavage of substances and are divided into primary and secondary. Primary lysosomes contain enzymes in an inactive form. After entering the organelles various substances enzymes are activated and the process of digestion begins - these are secondary lysosomes.
Peroxisomes have the appearance of bubbles bounded by a single membrane. They contain enzymes that break down hydrogen peroxide, which is toxic to cells.
Vacuoles – These are plant cell organelles that contain cell sap. Cell sap may contain spare nutrients, pigments, and waste products. Vacuoles are involved in the creation of turgor pressure, in the regulation of water-salt metabolism.
Ribosomes – organelles made up of large and small subunits. They can be located either on the ER or located freely in the cell, forming polysomes. They are composed of rRNA and protein and are produced in the nucleolus. Protein synthesis takes place in ribosomes.
Cell Center – found in the cells of animals, fungi, lower plants and absent in higher plants. It consists of two centrioles and a radiant sphere. The centriole has the form of a hollow cylinder, the wall of which consists of 9 triplets of microtubules. When dividing, cells form threads of the mitotic spindle, which ensure the separation of chromatids in the anaphase of mitosis and homologous chromosomes during meiosis.
microtubules – tubular formations of various lengths. They are part of the centrioles, mitotic spindle, flagella, cilia, perform a supporting function, promote the movement of intracellular structures.
Microfilaments – filamentous thin formations located throughout the cytoplasm, but there are especially many of them under the cell membrane. Together with microtubules, they form the cytoskeleton of the cell, determine the flow of the cytoplasm, intracellular movements of vesicles, chloroplasts, and other organelles.
cell evolution
There are two stages in cell evolution:
1.Chemical.
2. Biological.
Chemical stage began about 4.5 billion years ago. Under the influence of ultraviolet radiation, radiation, lightning discharges (energy sources), at first simple chemical compounds- monomers, and then more complex - polymers and their complexes (carbohydrates, lipids, proteins, nucleic acids).
The biological stage of cell formation begins with the appearance of probionts - isolated complex systems capable of self-reproduction, self-regulation and natural selection. Probionts appeared 3-3.8 billion years ago. The first prokaryotic cells, bacteria, originated from probionts. Eukaryotic cells evolved from prokaryotes (1-1.4 billion years ago) in two ways:
1) By symbiosis of several prokaryotic cells - this is a symbiotic hypothesis;
2) By invagination of the cell membrane. The essence of the invagination hypothesis is that the prokaryotic cell contained several genomes attached to the cell membrane. Then invagination took place - invagination, detachment of the cell membrane, and these genomes turned into mitochondria, chloroplasts, and the nucleus.
Cell differentiation and specialization .
Differentiation is the formation of different types of cells and tissues during development multicellular organism. One of the hypotheses relates differentiation to gene expression during individual development. Expression is the process of turning certain genes into work, which creates conditions for directed synthesis of substances. Therefore, there is a development and specialization of tissues in one direction or another.
Similar information.
Membranes perform a large number of different functions:
membranes determine the shape of an organelle or cell;
barrier: control the exchange of soluble substances (for example, ions Na + , K + , Cl -) between the inner and outer compartment;
energy: ATP synthesis on the inner membranes of mitochondria and photosynthesis in chloroplast membranes; form a surface for flow chemical reactions(phosphorylation on mitochondrial membranes);
are a structure that provides recognition of chemical signals (hormone and neurotransmitter receptors are located on the membrane);
play a role in intercellular interaction and promote cell movement.
transport across the membrane. The membrane has a selective permeability for soluble substances, which is necessary for:
separation of the cell from the extracellular environment;
ensuring the penetration into the cell and retention in it of the necessary molecules (such as lipids, glucose and amino acids), as well as the removal of metabolic products (including unnecessary ones) from the cell;
maintaining a transmembrane ion gradient.
Intracellular organelles can also have a selectively permeable membrane. For example, in lysosomes, the membrane maintains a concentration of hydrogen ions (H +) 1000-10000 times greater than in the cytosol.
Transport across the membrane can be passive, lightweight or active.
Passive transport is the movement of molecules or ions along a concentration or electrochemical gradient. This can be simple diffusion, as in the case of gases (for example, O 2 and CO 2) or simple molecules (ethanol) penetrating the plasma membrane. In simple diffusion, small molecules dissolved in the extracellular fluid are successively dissolved in the membrane and then in the intracellular fluid. This process is nonspecific, while the rate of penetration through the membrane is determined by the degree of hydrophobicity of the molecule, that is, its fat solubility. The rate of diffusion through the lipid bilayer is directly proportional to hydrophobicity, as well as to the transmembrane concentration gradient or electrochemical gradient.
Facilitated diffusion is the rapid movement of molecules across a membrane by specific membrane proteins called permeases. This process is specific, it proceeds faster than simple diffusion, but it has a speed limit of transport.
Facilitated diffusion is usually characteristic of water-soluble substances. Most (if not all) membrane transporters are proteins. The specific mechanism of functioning of carriers during facilitated diffusion has not been studied enough. They can, for example, provide transfer by rotational movement in the membrane. Recently, information has appeared that carrier proteins, upon contact with the transported substance, change their conformation, as a result, peculiar “gates” or channels open in the membrane. These changes occur due to the energy released when the transported substance binds to the protein. Relay type transfer is also possible. In this case, the carrier itself remains immobile, and the ions migrate along it from one hydrophilic bond to another.
The antibiotic gramicidin can serve as a model for this type of carrier. In the lipid layer of the membrane, its long linear molecule takes the form of a spiral and forms a hydrophilic channel through which the K ion can migrate along the gradient.
Experimental evidence of the existence of natural channels in biological membranes has been obtained. Transport proteins are characterized by high specificity with respect to the substance transported through the membrane, resembling enzymes in many properties. They are highly sensitive to pH, competitively inhibited by compounds similar in structure to the transferred substance, and non-competitively - by agents that change specific functional groups of proteins.
Facilitated diffusion differs from the usual one not only in speed, but also in the ability to saturate. The increase in the rate of transfer of substances occurs in proportion to the growth of the concentration gradient only up to certain limits. The latter is determined by the "power" of the carrier.
Active transport is the movement of ions or molecules across a membrane against a concentration gradient due to the energy of ATP hydrolysis. There are three main types of active ion transport:
sodium-potassium pump - Na + /K + -adenosine triphosphatase (ATPase), carrying Na + out, and K + inside;
calcium (Ca 2+) pump - Ca 2+ -ATPase, which transports Ca 2+ from the cell or cytosol to the sarcoplasmic reticulum;
proton pump - H + -ATPase. The ion gradients created by active transport can be used to actively transport other molecules such as certain amino acids and sugars (secondary active transport).
Cotransport- this is the transport of an ion or molecule, coupled with the transfer of another ion. Symport- simultaneous transfer of both molecules in one direction; antiport- simultaneous transfer of both molecules in opposite directions. If the transport is not coupled with the transfer of another ion, this process is called uniport. Cotransport is possible both with facilitated diffusion and in the process of active transport.
Glucose can be transported by facilitated diffusion in a symport manner. Ions Cl - and HCO 3 - are transported through the membrane of erythrocytes by facilitated diffusion by a carrier called band 3, according to the type of antiport. In this case, Cl - and HCO 3 - are transferred in opposite directions, and the direction of transfer is determined by the prevailing concentration gradient.
Active transport of ions against a concentration gradient requires the energy released during the hydrolysis of ATP to ADP: ATP ADP + F (inorganic phosphate). Active transport, as well as facilitated diffusion, is characterized by: specificity, maximum rate limitation (i.e., the kinetic curve reaches a plateau), and the presence of inhibitors. An example is the primary active transport carried out by Na + /K + - ATPase. The functioning of this antiport fragment system requires the presence of Na + , K + and magnesium ions. It is present in almost all animal cells, and its concentration is especially high in excitable tissues (for example, in nerves and muscles) and in cells that are actively involved in the movement of Na + through the plasma membrane (for example, in the cortical layer of the kidneys and salivary glands) .
The ATPase enzyme itself is an oligomer consisting of 2 -subunits of 110 kDa each and 2 glycoprotein -subunits of 55 kDa each. During ATP hydrolysis, reversible phosphorylation of a certain aspartate residue on the -subunit occurs with the formation of -aspartamyl phosphate. Phosphorylation requires Na + and Mg 2+ but not K + , whereas dephosphorylation requires K + but not Na + or Mg 2+ . Two conformational states of the protein complex with different energy levels are described, which are usually denoted as E 1 and E 2, therefore ATPase is also called type E carrier 1 - E 2 . cardiac glycosides, for example digoxin and ouabain, inhibit the activity of ATPase. Due to its good solubility in water, ouabain is widely used in experimental studies to study the sodium pump.
The generally accepted idea of the work of Na + /K + - ATPase is as follows. Na and ATP ions are attached to the ATPase molecule in the presence of Mg 2+. The binding of Na ions triggers the hydrolysis of ATP, which results in the formation of ADP and the phosphorylated form of the enzyme. Phosphorylation induces the transition of the enzymatic protein to a new conformational state, and the Na-bearing site or sites turn out to be facing the external environment. Here, Na + is exchanged for K +, since the phosphorylated form of the enzyme is characterized by a high affinity for K ions. The reverse transition of the enzyme to the original conformation is initiated by hydrolytic cleavage of the phosphoryl group in the form of inorganic phosphate and is accompanied by the release of K + into the interior of the cell. The dephosphorylated active site of the enzyme is able to attach a new ATP molecule, and the cycle repeats.
The amounts of K and Na ions entering the cell as a result of the operation of the pump are not equal to each other. For three excreted Na ions, there are two introduced K ions with simultaneous hydrolysis of one ATP molecule. The opening and closing of the channel on opposite sides of the membrane and the alternating change in the efficiency of Na and K binding are provided by the energy of ATP hydrolysis. Transported ions - Na and K - cofactors of this enzymatic reaction. Theoretically, it is possible to imagine a wide variety of pumps operating on this principle, although at present only a few of them are known.
transport of glucose. Glucose transport can occur as both facilitated diffusion and active transport, in the first case it proceeds as a uniport, in the second - as a symport. Glucose can be transported into erythrocytes by facilitated diffusion. The Michaelis constant (Km) for glucose transport into erythrocytes is approximately 1.5 mmol/L (i.e., at this glucose concentration, about 50% of the available permease molecules will be bound to glucose molecules). Since the concentration of glucose in human blood is 4-6 mmol / l, its absorption by erythrocytes occurs almost at a maximum rate. The specificity of permease is already manifested in the fact that the L-isomer is almost not transported into erythrocytes, in contrast to D-galactose and D-mannose, but their higher concentrations are required to achieve half-saturation of the transport system. Once inside the cell, glucose undergoes phosphorylation and is no longer able to leave the cell. Permease for glucose is also called D-hexose permease. It is an integral membrane protein with molecular weight 45kD.
Glucose can also be transported by the Na + -dependent symport system found in the plasma membranes of a number of tissues, including the tubules of the kidneys and the intestinal epithelium. In this case, one glucose molecule is transported by facilitated diffusion against the concentration gradient, and one Na ion is transported along the concentration gradient. The entire system ultimately functions through the pumping function of Na + /K + - ATPase. Thus, symport is a secondary active transport system. Amino acids are transported in a similar way.
Ca 2+ -pump is an active transport system of the E 1 - E 2 type, consisting of an integral membrane protein, which, in the process of Ca 2+ transfer, is phosphorylated at an aspartate residue. During the hydrolysis of each ATP molecule, two Ca 2+ ions are transferred. In eukaryotic cells, Ca 2+ can bind to a calcium-binding protein called calmodulin, and the entire complex binds to the Ca 2+ pump. Ca 2+ -binding proteins also include troponin C and parvalbumin.
Ca ions, like Na ions, are actively removed from cells by Ca 2+ -ATPase. Especially a large number of calcium pump proteins contain the membranes of the endoplasmic reticulum. The chain of chemical reactions leading to ATP hydrolysis and Ca 2+ transfer can be written as the following equations:
2Ca n + ATP + E 1 Ca 2 - E - P + ADP
Ca 2 - E - P 2Ca ext + PO 4 3- + E 2
Where is San - Ca2 +, located outside;
Ca ext - Ca 2+ located inside;
E 1 and E 2 - different conformations of the carrier enzyme, the transition of which from one to another is associated with the use of ATP energy.
The system of active removal of H + from the cytoplasm is supported by two types of reactions: the activity of the electron transport chain (redox chain) and ATP hydrolysis. Both - both redox and hydrolytic H + pumps - are located in membranes capable of converting light or chemical energy into H + energy (that is, the plasma membranes of prokaryotes, the conjugating membranes of chloroplasts and mitochondria). As a result of the work of H + ATPase and / or redox chain, protons are translocated, and a proton-motive force (H +) appears on the membrane. The electrochemical gradient of hydrogen ions, as studies show, can be used for conjugated transport (secondary active transport) of a large number of metabolites - anions, amino acids, sugars, etc.
The activity of the plasma membrane is associated with the absorption of solid and liquid substances with a large molecular weight by the cell, - phagocytosis and pinocytosis(from Gerch. phagos- there is , pinos- drink, cytos- cell). The cell membrane forms pockets, or invaginations, that draw in substances from outside. Then such invaginations are laced off and surround a droplet of the external environment (pinocytosis) or solid particles (phagocytosis) with a membrane. Pinocytosis is observed in a wide variety of cells, especially in those organs where absorption processes occur.
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