How do substances enter the cell. Does the cream penetrate the skin
6.3. Entry mechanisms nutrients in a cage
The main obstacle to the transport of substances into the cell is the cytoplasmic membrane (CPM), which has selective permeability. The CPM regulates not only the entry of substances into the cell, but also the exit from it of water, various metabolic products and ions, which ensures the normal functioning of the cell.
There are several mechanisms for the transport of nutrients into the cell: simple diffusion, facilitated diffusion, and active transport.
simple diffusion - the penetration of molecules of a substance into a cell without the help of any carriers. The driving force of this process is the concentration gradient of the substance, i.e., the differences in its concentration on both sides of the CPM - in the external environment and in the cell. Molecules of water, some gases (molecular oxygen, nitrogen, hydrogen), some ions, the concentration of which in the external environment is higher than in the cell, move through the CPM by passive diffusion. Passive transfer proceeds until the concentration of substances on both sides of the cytoplasmic membrane is equalized. The incoming water presses the cytoplasm and CPM against the cell wall and an internal pressure is created in the cell on the cell wall, called turgor. Simple diffusion occurs without the expenditure of energy. The speed of such a process is insignificant.
The vast majority of substances can penetrate inside the cell only with the participation of carriers - specific proteins called permeases and localized on the cytoplasmic membrane. Permeases capture solute molecules and carry them to the inner surface of the cell. With the help of carrier proteins, solutes are transported by facilitated diffusion and active transport.
Facilitated diffusion occurs along a concentration gradient with the help of carrier proteins. Like passive diffusion, it proceeds without energy consumption. Its rate depends on the concentration of substances in the solution. It is assumed that the release of metabolic products from the cell is also carried out by facilitated diffusion. Through facilitated diffusion, monosaccharides and amino acids enter the cell.
active transport - solutes are transported regardless of the concentration gradient. This type of transport of substances requires energy (ATP). With active transport, the rate of entry of substances into the cell reaches a maximum even at a low concentration in the nutrient medium. Most substances penetrate into the cell of microorganisms as a result of active transport.
Prokaryotes and eukaryotes differ in their transport mechanisms. In prokaryotes, the selective intake of nutrients is carried out mainly by active transport, and in eukaryotes, by facilitated diffusion, and less often by active transport. The release of products from the cell is most often carried out by facilitated diffusion.
6.4. Nutritional needs and nutritional types of microorganisms
A variety of substances that microorganisms need and consume for the synthesis of basic organic substances of the cell, growth, reproduction and for energy are called nutrients and environment containing nutrients is called nutrient medium.
Nutrient requirements of microorganisms are diverse, but regardless of the needs, the nutrient medium must contain all necessary elements, which are present in the cells of microorganisms, and the ratio of organogenic elements should approximately correspond to this ratio in the cell.
The sources of hydrogen and oxygen are water, molecular hydrogen and oxygen, as well as chemicals containing these elements. Sources of macronutrients are mineral salts(potassium phosphate, magnesium sulfate, iron chloride, etc.).
Sources of carbon and nitrogen can be both organic and inorganic compounds.
According to the accepted classification of microorganisms ontype of food they are divided into groups depending on the carbon source, energy source and electron source (the nature of the oxidized substrate).
Depending on the carbon source microorganisms are divided into:
* autotrophs(self-feeding), which use carbon from inorganic compounds (carbon dioxide and carbonates);
* heterotrophs(feed at the expense of others) - use carbon from organic compounds.
Depending on the energy source distinguish:
* phototrophs - microorganisms that use the energy of sunlight as an energy source;
* chemotrophs - energy material for these microorganisms are a variety of organic and inorganic substances.
Depending on the electron source (nature of the oxidized
substrate microorganisms are divided into:
* lithotrophs - oxidize inorganic substances and thereby obtain energy;
* oraganotrophs - They obtain energy by oxidizing organic matter.
Among microorganisms, the most common microorganisms that have the following types of nutrition:
Photolithoautotrophy - a type of nutrition characteristic of microbes that use the energy of light and the energy of oxidation of inorganic compounds to synthesize cell substances from carbon dioxide.
Photoorganoheterotrophy - this type of nutrition of microorganisms, when, in addition to light energy, the energy of oxidation of organic compounds is used to obtain the energy necessary for the synthesis of cell substances from carbon dioxide.
Chemolitoautotrophy - type of nutrition in which microorganisms obtain energy from the oxidation of inorganic compounds, and inorganic compounds are the source of carbon.
photoautotrophs → photolithoautotrophs
photoorganoautotrophs
phototrophs photoheterotrophs → photolithoheterotrophs
photoorganoheterotrophs
microorganisms
Chemoorganoheterotrophy - type of nutrition of microorganisms that obtain energy and carbon from organic compounds. Microorganisms found in food products have exactly this type of nutrition.
Beyond carbon essential element the nutrient medium is nitrogen. Autotrophs usually use nitrogen from mineral compounds, and heterotrophs use ammonium salts in addition to inorganic nitrogen compounds. organic acids, amino acids, peptones and other compounds. Some heterotrophs assimilate atmospheric nitrogen (nitrogen fixers).
There are microorganisms that themselves are not capable of synthesizing one or another organic substance (for example, amino acids, vitamins). Such microorganisms are called auxotrophic for this substance . Substances that are added to accelerate growth and metabolic processes called growth substances.
Questions for self-examination
1. What ways of feeding living beings do you know?
2. What is "extracellular digestion"?
3. What are the mechanisms by which nutrients enter the cell?
4. What is the difference between simple diffusion and facilitated diffusion?
5. AT What is the essential difference between passive and facilitated diffusion from active transport?
6. What is the role of permeases in the transport of solutes into the cell?
7. What is the mechanism of entry of water and gases into the cell?
8. How do they get into the cell simple sugars and amino acids?
9. How do prokaryotes and eukaryotes differ in the mechanisms of transport of substances?
10. What are "organogenic elements"?
11. What are macronutrients?
12 . What are the nutrient requirements of microorganisms?
13 . How are microorganisms classified depending on the source of carbon and energy?
14. What are "chemoorganoheterotrophs"?
16 . What types of food do you know?
17 . What are nitrogen-fixing microorganisms?
18. What are "auxotrophic microorganisms"?
Literature
Churbanova I.N. Microbiology. - M.: Higher school, 1987.
Mudretsova-Wiss K.A. Microbiology. - M.: Economics, 1985. - 255 p.
Mishustin E.N., Emtsev V.T. Microbiology. - M.: Agropromizdat, 1987, 350s.
Verbina N.M., Kaptereva Yu.V. Microbiology of food production.- M.: Agropromizdat, 1988.- 256 p.
"Introduction to general biology and ecology. Grade 9. A.A. Kamensky (gdz)
Characteristics of the cell. cell membrane
Question 1. What are the functions of the outer membrane of the cell?
The outer cell membrane consists of a double lipid layer and protein molecules, some of which are located on the surface, and some penetrate both layers of lipids through and through. Functions of the plasma membrane:
1. Delimiting. Plasma membranes form closed systems without interruption anywhere, i.e. they have no risers, so they separate the inside from the environment. For example, the cell membrane protects the contents of the cytoplasm from physical and chemical damage.
2. Transport - one of essential functions related to the ability of the membrane to pass into or out of the cell various substances, this is necessary to maintain the constancy of its composition, i.e. homeostasis (Greek homos - similar and stasis - state).
3. Contact. In the composition of tissues and organs, complex special structures are formed between cells - intercellular contacts.
4. The plasma membrane of many cells can form special structures (microvilli, cilia, flagella).
5. A difference in electrical potentials is created on the plasma membrane. For example, the glycoproteins of mammalian erythrocytes create a negative charge on their surface, which prevents them from agglutinating (sticking together).
6. Receptor. It is provided by molecules of integral proteins that have polysaccharide ends on the outside. The membranes have big number receptors are special proteins whose role is to transmit signals from the outside to the inside of the cell. Glycoproteins are involved in the recognition individual factors environment and in the response of cells to these factors. For example, the egg and sperm recognize each other by glycoproteins that fit together as separate elements of an integral structure (stereochemical connection like a "key to the lock") - this is the stage that precedes fertilization.
7. The plasma membrane can participate in synthesis and catalysis. The membrane is the basis for precise placement of enzymes. Hydrolytic enzymes can precipitate in the glycocalyx layer, which cleave various biopolymers and organic molecules, carrying out membrane or extracellular cleavage. This is how extracellular cleavage proceeds in heterotrophic bacteria and fungi. In mammals, for example, in the intestinal epithelium, in the zone of the brush border of the suction epithelium, a large number of various enzymes (amylase, lipase, various proteinases, exohydrolases, etc.), i.e. parietal digestion is carried out.
Question 2. In what ways can various substances enter the cell?
Substances can penetrate the outer cell membrane in several ways. First, through the thinnest channels formed by protein molecules, ions of small substances, such as sodium, potassium, and calcium ions, can pass into the cell. This so-called passive transport proceeds without energy expenditure by diffusion, osmosis and facilitated diffusion. Secondly, substances can enter the cell by phagocytosis or pinocytosis. Large molecules of biopolymers enter through the membrane due to phagocytosis, a phenomenon first described by I.I. Mechnikov. The process of capture and absorption of liquid droplets occurs by pinocytosis. By phagocytosis and pinocytosis, food particles usually enter the cell.
Question 3. How is pinocytosis different from phagocytosis?
Phagocytosis (Greek phagos - to devour, cytos - receptacle) is the capture and absorption of large particles (sometimes whole cells and their particles) by a cell. In this case, the plasma membrane forms outgrowths, surrounds the particles and moves them into the cell in the form of vacuoles. This process is associated with the cost of the membrane and ATP energy.
Pinocytosis (Greek pino - drink) - the absorption of droplets of liquid with substances dissolved in it. It is carried out due to the formation of invaginations on the membrane and the formation of bubbles surrounded by the membrane, and moving them inside. This process is also associated with the cost of the membrane and ATP energy. The suction function of the intestinal epithelium is provided by pinocytosis.
Thus, during phagocytosis, the cell absorbs solid food particles, and during pinocytosis, liquid droplets. If the cell stops synthesizing ATP, then the processes of pino- and phagocytosis completely stop.
Question 4. Why plant cells no phagocytosis?
During phagocytosis, in the place where the food particle touches the outer membrane of the cell, an invagination is formed, and the particle enters the cell, surrounded by a membrane. A plant cell has a dense, non-plastic cellulose membrane on top of the cell membrane, which prevents phagocytosis.
1. What is the difference between the shells of animal and plant cells?
A plant cell, in addition to the cell membrane, is also covered with a cell wall of fiber, which gives it strength.
2. What is the fungus cell covered with?
Fungal cells, in addition to the cell membrane, are covered hard shell- a cell wall, which consists of 80-90% polysaccharides (in most it is chitin).
Questions
1. What are the functions of the outer membrane of the cell?
The cell membrane separates the internal content of the cell from the external environment. It protects the cytoplasm and nucleus from damage, provides communication between cells, selectively passes inside the cell necessary substances and removes metabolic products from the cell.
2. In what ways can various substances enter the cell?
Special proteins form the thinnest channels through which potassium, sodium, calcium ions and some other ions with a small diameter can pass into or out of the cell. However, larger particles cannot pass through the membrane channels. molecules nutrients- proteins, carbohydrates, lipids - enter the cell with the help of phagocytosis or pinocytosis.
3. How is pinocytosis different from phagocytosis?
Pinocytosis differs from phagocytosis only in that in this case, the invagination of the outer membrane captures not solid particles, but liquid droplets with substances dissolved in it.
4. Why do plant cells not have phagocytosis?
Since plant cells are covered with a dense layer of fiber over the outer cell membrane, they cannot capture substances by phagocytosis.
Tasks
1. Plan your paragraph.
1. General view about the structure of the cell.
2. Functions of the cell membrane.
3. The structure of the cell membrane.
4. Methods of transporting substances through the cell membrane.
2. After analyzing the text of the paragraph and figures 22 and 23, establish the relationship between the structure and functions of the cell membrane.
The basis of the plasmalemma is a layer of lipids, which has two rows of molecules. The dynamic properties of the membrane are due to the mobility of its molecular organization. Proteins and lipids are interconnected in the membrane inconstantly and form a mobile, flexible, temporarily linked into a single structure, capable of structural rearrangements. When this is easily changed, for example, the relative position of the components of the membrane. Due to this, the membranes can change their configuration, i.e. they have fluidity. This provides the possibility of phago- and pinocytosis.
Lipids are insoluble in water, so they create a barrier in the cell that prevents the movement of water and water-soluble substances from one compartment to another.
Protein molecules, however, make the membrane permeable to various substances through specialized structures called pores.
Apparently, some substances passively flow through the cell membrane under the action of a pressure difference, others are quite actively pumped into the cell through the membrane, and still others are drawn into the cell due to the invagination of the membrane.
Most of the cells live in an environment unsuitable for maintaining that extremely strict ratio of water, salts and organic substances, without which life is impossible. This entails the need for continuous and very careful regulation of the exchange of various substances that occurs between the outside world and the cytoplasm. The barrier that separates the internal contents of the cell from the environment is the cell membrane - the thinnest film, only ten millionths of a millimeter thick.
This membrane is permeable to many substances that flow in both directions (i.e. out of the cell and into the cell). Despite its negligible thickness, the membrane has a certain structure; this structure and chemical composition of the membrane, about which we still have a very vague idea, determine its selective and very uneven permeability. If the forces that ensure the passage of substances through the membrane are localized in the environment surrounding the cell, then one speaks of "passive transfer". If the energy expended on this is produced in the cell itself in the process of its metabolism, then one speaks of "active transfer". Such an interaction between the cell and its environment serves not only to ensure that the concentration in the cell of all the substances that make up its composition is always kept within certain limits, outside of which there can be no life; in some cells, for example, in nerve cells, this interaction is of paramount importance for the performance of the function that these cells carry in the body.
Many cells absorb the substances they need also by a kind of ingestion. This process is known as phagocytosis or pinocytosis (the words come from the Greek words for "eat" and "drink" respectively, and from the word for "cell"). With this method of absorption, the cell membrane forms pockets or invaginations that draw substances from outside into the cell; then these protrusions are laced off and a droplet of the external environment surrounded by a membrane in the form of a bubble or vacuole starts to float through the cytoplasm.
Despite all the similarity of this process with simple "swallowing", we still have no right to talk about the entry of substances into the cell, since this immediately entails the question of what the expression "inside" means. From our, so to speak, macroscopic, human point of view, we are inclined to frivolously assert that as soon as we swallowed a piece of food, this is how it got inside us. However, such a statement is not entirely correct. Interior digestive tract in the topological sense, it is the outer surface; true absorption of food begins only when it penetrates the cells of the intestinal wall. Therefore, the substance that has entered the cell as a result of pinocytosis or phagocytosis is still “outside”, since it is still surrounded by the membrane that has captured it. In order to really enter the cage and turn into an accessible metabolic processes component of the cytoplasm similar substances must in one way or another penetrate the membrane.
One of the forces that act on the entire cell membrane is due to the concentration gradient. This force arises due to the random movement of particles, seeking to be evenly distributed in space. If two solutions of the same composition but different concentrations come into contact, then diffusion of the solute begins from the region of higher concentration, and this diffusion continues until the concentration becomes the same everywhere. Concentration equalization occurs even if the two solutions are separated by a membrane, provided, of course, that the membrane is permeable to the solute. If the membrane is permeable to the solvent, but impermeable to the solute, then the concentration gradient appears before us in the form of the well-known phenomenon of osmosis: in this case, the solvent passes through the membrane, going from a region of lower concentration of a solute to a region of higher concentration. The concentration gradient and osmotic forces acting on both sides of the cell membrane are very significant, since the concentrations of many substances in the cell differ sharply from their concentrations in the external environment.
In passive transfer, the penetration of substances through the membrane is regulated by the selective permeability of the membrane. The permeability of the membrane for a given molecule depends on chemical composition and the properties of this molecule, as well as its size; at the same time, the membrane is able not only to block the path of certain substances, but also to pass through itself different substances at different speeds.
Depending on the nature of the environment to which they are adapted, cells different types have very different permeability. So, for example, the permeability of an ordinary amoeba and human erythrocytes for water differs by more than 100 times. In the table of permeability constants (expressed as the number of cubic microns of water passing through 1 square micron of the cell membrane in 1 minute under the influence of an osmotic pressure difference of 1 atmosphere), the value of 0.26 is listed against the amoeba, i.e. its permeability is very insignificant. The adaptive value of such low permeability is obvious: organisms living in fresh water, face the largest difference in concentration between the outdoor and indoor environments, and therefore they are forced to limit the flow of water into the interior in order to save the energy that would be required to pump this water back. Red blood cells do not need such a safety device, since they are usually surrounded by blood plasma - an environment that is in relative osmotic equilibrium with their internal environment. Once in the water, these cells immediately begin to swell and burst rather quickly, because their membrane is not elastic enough to withstand this sudden pressure of water.
If, as is usually the case in nature, the solute molecules are dissociated into ions that carry a certain electrical charge, then new forces come into play. It is well known that the membranes of many, and perhaps even all, cells have the ability to maintain a known potential difference between their outer and inner surface. As a result, a certain potential gradient arises, which, along with the concentration gradient, serves as the driving force for passive transfer through the cell membrane.
The third force involved in passive transport across a membrane is the transport of solutes along with the solvent (solvent pull). It comes into play only if the solution can actually flow through the membrane; in other words, if the membrane is porous. In this case, the movement of particles of the dissolved substance, diffusing in the direction of the flow, is accelerated, and the diffusion of particles in the opposite direction is slowed down. This pull-in effect usually does not play big role, however, in some special occasions its significance is quite large.
All three forces involved in passive transference can act separately or together. However, no matter what kind of force causes the movement - whether the concentration gradient, the potential gradient or the effect of retraction - the movement always occurs in a "downward" direction and the membrane serves as a passive barrier. At the same time, many important examples are known in cytology when none of these three forces can explain the transfer of substances through the membrane. In these cases, the movement occurs in an "upward" direction, i.e., against the forces that cause passive transfer, and therefore it must occur due to the energy released as a result of the metabolic processes taking place in the cell. In this active transport, the membrane is no longer just a passive barrier, but acts as a kind of dynamic organ.
Until recently, all the information that we had about the structure of the cell membrane was obtained exclusively as a result of studying its permeability and, therefore, was purely indirect. For example, it has been found that many substances that are soluble in lipids (fats) easily pass through the cell membrane. In this regard, the assumption arose that there is a layer of lipids in the cell membrane and that substances soluble in lipids pass through the membrane, dissolving on one side of it and releasing it on the other side. However, it turned out that water-soluble molecules also pass through the cell membrane. I had to assume that the structure of the membrane to some extent resembles a sieve, that is, that the membrane is equipped with pores or non-lipid areas, and possibly both at the same time; in addition, in order to explain the features of the passage of various ions, the presence of sections in the membrane that carry an electric charge was admitted. Finally, a protein component was also introduced into this hypothetical scheme of the membrane structure, since data appeared that, in particular, testify to the wettability of the membrane, which is incompatible with a purely fatty composition.
These observations and hypotheses are summarized in the cell membrane model proposed in 1940 by J. Danielli. According to this model, the membrane consists of a double layer of lipid molecules covered by two protein layers. Lipid molecules lie parallel to each other, but perpendicular to the plane of the membrane, with their uncharged ends facing each other, and the charged groups directed towards the membrane surface. At these charged ends, protein layers are adsorbed, consisting of protein chains, which form a tangle on the outer and inner surfaces of the membrane, thereby giving it a certain elasticity and resistance to mechanical damage and low surface tension. The length of lipid molecules is approximately 30 angstroms, and the thickness of the monomolecular protein layer is 10 angstroms; therefore, Danielli believed that the total thickness of the cell membrane is about 80 angstroms.
Results obtained with electron microscope, confirmed the correctness of the model created by Danielli. The "elementary membrane" studied from Robertson's electron micrographs matches Danielli's predictions in shape and size, and has been observed in many cells. various types. It can be distinguished two more dark stripes about 20 angstroms thick, which may well correspond to two protein layers of the model; these two strips are separated by a 35 angstrom lighter core corresponding to the lipid layer. The total membrane thickness of 75 angstroms is quite close to the value provided by the model.
Without violating the general symmetry of this model, it should be supplemented in order to take into account the differences in the chemical nature of the inner and outer surfaces of the membrane. This would make it possible to explain the existence of chemical gradients between the inner and outer surfaces of the membrane, revealed in some observations. In addition, we know that many cells are covered with a carbohydrate-containing mucoprotein membrane, the thickness of which varies in different cell types. Regardless of whether this layer has an effect on permeability, it can be assumed that it plays important role in pinocytosis.
In addition to these features of the structure of the membrane, so to speak in the "cross section", when studying the permeability, it turns out that its structure is also inhomogeneous in the other direction. It is known, for example, that cell membranes allow particles whose size does not exceed known limits to pass through, while retaining larger and larger particles, and this suggests the presence of pores in these membranes. So far, the existence of pores has not been confirmed by electron microscopic studies. This is not surprising, since it is assumed that these pores are very small and located very far from each other, so that their total area does not exceed one thousandth of the total surface of the membrane. If we call the membrane a sieve, then it should be added that there are very few holes in this sieve.
An even more important circumstance is that in order to explain the high selectivity that allows many cells to distinguish one substance from another, it is necessary to assume different chemical specificity of different parts of the membrane. It turned out, for example, that some enzymes are localized on the cell surface. Apparently, their function is to convert substances that are insoluble in the membrane into soluble derivatives that can pass through it. Many cases are known when a cell, permeable to one substance, does not let another substance close to the first one and similar to it in terms of molecular size and electrical properties.
So, we see that a thin cell membrane is a rather complex apparatus designed to actively interfere with the movement of substances entering the cell and released from it. Such an apparatus is indispensable for the process of active transfer, by means of which this transfer is mainly carried out. In order for this "upward" movement to occur, the cell must act against the forces of passive transference. However, despite the efforts of many scientists, it has not yet been possible to reveal the mechanism by which the energy released as a result of cellular metabolism is used to transport various substances through the cell membrane. It is possible that various mechanisms are involved in this energy transfer.
The problem of active ion transport attracts the liveliest interest. Biologists already 100 years ago knew the existence of a potential difference between the outer and inner surface of the membrane; Since about the same time, they have known that this potential difference has an effect on the transport and distribution of ions. However, only recently they began to understand that this potential difference itself arises and is maintained due to the active transport of ions.
The importance of this problem is evidenced by the fact that the cytoplasm of many cells contains much more potassium than sodium, and meanwhile they are forced to live in an environment that is characterized by just the opposite ratio between the content of these two ions. For example, blood plasma contains 20 times more sodium than potassium, while red blood cells contain 20 times more potassium than sodium. The erythrocyte membrane has a well-defined, albeit low, passive permeability for both sodium and potassium ions. If this permeability could freely manifest itself, then sodium ions would flow into the cell, and potassium ions would begin to flow out of it. Therefore, to maintain the existing ratio of ions, the cell has to continuously “pump out” sodium ions and accumulate potassium ions against a 50-fold concentration gradient.
Most of the models proposed to explain active transport are based on the assumption of the existence of some kind of carrier molecules. It is assumed that these still hypothetical carriers come into contact with ions located on one surface of the membrane, pass through the membrane in this form and again release ions on the other surface of the membrane. The movement of such compounds (carrier molecules that have attached ions to themselves), in contrast to the movement of the ions themselves, is believed to occur in a "descending" direction, i.e., in accordance with a chemical concentration gradient.
One such model, created by T. Shaw in 1954, makes it possible not only to explain the transfer of potassium and sodium ions through the membrane, but also to establish some connection between them. According to the Shaw model, potassium and sodium ions (K + and Na +) are transported across the membrane by fat-soluble carriers (X and Y) specific for ions. The resulting compounds (KX and NaY) are able to diffuse through the membrane, while the membrane is impermeable to free carriers. On the outer surface sodium transporter membranes are converted to potassium transporters, losing energy in the process. On the inner surface of the membrane, potassium carriers are again converted into sodium carriers due to the receipt of energy arising in the process of cell metabolism (the suppliers of this energy are, in all likelihood, energy-rich compounds in the molecule of which there are phosphate bonds).
Many of the assumptions made in this model are difficult to confirm experimentally, and it is by no means recognized by everyone. Nevertheless, we considered it necessary to mention it, since this model itself shows the entire complexity of the active transfer phenomenon.
Long before biologists deciphered the complex game physical strength, involved in the transfer of substances through the cell membrane, they already had to observe the cells, so to speak, "for food." AT late XIX century, Ilya Mechnikov saw for the first time how white blood cells(leukocytes) devoured bacteria, and gave them the name "phagocytes". In 1920, A. Schaeffer depicted how an amoeba catches its prey - a drawing that has become a classic. The process of pinocytosis, expressed less clearly, was first discovered by W. Lewis only in 1931. Studying the behavior of cells in tissue culture using time-lapse photography, he noticed membrane outgrowths on the cell periphery, which undulated so vigorously that from time to time they closed, like a compressed fist, capturing part of the medium as if in a bubble. To Lewis, all this seemed so similar to the process of drinking that he came up with an appropriate name for this phenomenon - “pinocytosis”.
Lewis' discovery at first did not attract attention, except for the work of S. Maet and W. Doyle, published in 1934, who reported a similar phenomenon observed by them in an amoeba. Pinocytosis remained a mere curiosity until, in the middle of this century, electron microscopy revealed that such ingestion was much more widespread.
In amoebae and in cells from tissue culture, pinocytosis can be observed under a conventional microscope. Due to the high resolution of the electron microscope, many other types of cells have also been found to form microscopic bubbles. From a physiological point of view, one of the most interesting examples of this kind are cells brush epithelium kidneys and intestines: vesicles that bring various substances into the cell are formed at the base of the brush border, to which this epithelium owes its name. The main feature of pinocytosis or phagocytosis is the same in all cells: some section of the cell membrane detaches from the cell surface and forms a vacuole or vesicle that breaks away from the periphery and migrates into the cell.
The size of the vesicles formed during pinocytosis varies widely. In amoebae and in cells taken from tissue culture, the average diameter of a newly detached pinocytic vacuole is 1-2 microns; the sizes of vacuoles, which we manage to detect using an electron microscope, vary from 0.1 to 0.01 microns. Quite often such vacuoles merge with each other and their sizes at the same time, naturally, increase. Because the most of cells contains a number of other vacuoles and granules, pinocytic vacuoles are soon lost from sight unless they are provided with some kind of "label". The vacuoles formed during phagocytosis are, of course, much larger and can accommodate whole bacterial cells, protozoan cells, and in the case of phagocytes, fragments of destroyed tissues.
On the basis of simple experiments with the amoeba, it can be seen that pyocytosis cannot be observed in any tissue at any time, since it is caused by the presence of certain certain substances in the environment. AT clean water pinocytosis does not occur in amoebas: in any case, it cannot be detected under a microscope. If sugar or some other carbohydrates are added to the water in which the amoebas are, then this will not lead to anything. When salts, proteins or certain amino acids are added, pinocytosis begins. S. Chapman-Andersen found that in amoeba each such induced pinocytosis can last about 30 minutes, regardless of the nature of the factor that caused it, and during this time up to 100 pinocytic channels are formed and the corresponding number of vacuoles is swallowed. Then pinocytosis stops and can resume only after 3-4 hours. According to Chapman Andersen, this is due to the fact that after 30 minutes of pinocytosis, all areas of the outer membrane capable of invagination are used.
In addition, Chapman-Andersen helped solve an old problem, namely, showed that phagocytosis and pinocytosis, from a physiological point of view, are the same process. In her experiment, amoebas were first given the opportunity to phagocytose as many ciliates edible for them as they could capture from an environment teeming with these microorganisms. Then they were transferred to a medium containing a factor that induces pinocytosis. It turned out that these amoeba are able to form only a few channels (less than 10% of the usual number). Conversely, amoebae that had exhausted their potential for pinocytosis did not phagocytize when transferred to a medium containing the organisms they normally use as food. Thus, the membrane surface appears to be the limiting factor in both cases.
S. Bennett in 1956 suggested that pinocytosis is caused by the adsorption of inductor molecules or ions on the surface of the cell membrane. This assumption was fully confirmed in the works of a number of researchers. It can hardly be doubted that in the amoeba adsorption occurs on a special membrane, which consists of mucus and envelops the entire amoeba. Since it is assumed that such a shell also exists in many other cells, it would be interesting to find out whether it performs a similar function in all cases.
The bubble, which introduces the inducing substance into the cell, also introduces a certain amount of liquid medium into it. Chapman-Andersen and the author conducted a "double label" experiment to determine which of the two substances - inductor or liquid - belongs to the main role. We placed amoebae in a medium containing as an inducer a protein labeled with a radioactive isotope and sugar with another radioactive label, which made it possible to determine the amount of absorbed liquid. We proceeded from the fact that if the main consumed substance, as well as the substance inducing absorption, is protein, then the relative content of protein in vacuoles should be higher than in the medium. And so it turned out. However, the scale of this phenomenon far exceeded our expectations. Total protein absorbed within 30 minutes corresponded to approximately 25% of the total mass of the amoeba. This is a very impressive meal, which indicates that highest value for a cell during pinocytosis, they have substances adsorbed on the surface.
However, the food contained in the vacuole must still be considered outside the cell, since the case in which it is enclosed is part of the outer membrane. We must find out whether such communication with the external environment can provide raw materials for the metabolic apparatus of the cell, and if so, how. The simplest way to transfer substances from the vacuole to the cytoplasm would be the dissolution of the membrane under the action of cytoplasmic enzymes. However, electron microscopy data do not support this assumption: we have never been able to observe the disappearance of the membrane that forms the vacuole stalk.
Since the membrane is obviously preserved, the main task in the study of pinocytosis is the study of its permeability. There is no doubt that the pinocytic vesicle releases water into the cytoplasm; we are convinced of this by the noticeable wrinkling of the vacuoles. J. Marshall and the author have shown that wrinkling in amoebas is accompanied by a gradual increase in the concentration of the contents of the vacuole. It has been established by centrifugation that during the first few hours after pinocytosis, the density of vacuoles increases all the time compared to the density of the surrounding cytoplasm. Ultimately, these vacuoles turn into cytoplasmic granules, which resemble mitochondria in size and behavior during centrifugation.
It also turned out that the vacuole membrane is permeable not only to water, but also to such low molecular weight substances as glucose. Chapman-Andersen and the author, using radioactive glucose, found that the glucose absorbed in the process of pinocytosis quickly leaves the vacuoles and is evenly distributed throughout the cytoplasm. This glucose enters normal processes metabolism occurring in the cell, as if it had entered the cell in the usual way - as a result of diffusion from the cell surface; the product of its metabolism - radioactive carbon dioxide - soon appears among the excretory products of the amoeba. Chapman-Andersen and D. Prescott obtained the same results for some amino acids. Therefore, there is no doubt that with the help of pinocytosis, the cell can be “feeded” with substances that have small molecules. Experiments with "feeding" large molecules have not yet been carried out.
These results suggest that there is some change in membrane permeability. This change cannot be seen with an electron microscope; the membrane appears to be the same both before and after pinocytosis. However, there are reports that the mucus membrane lining the inside of the vacuole wall exfoliates and, together with the material adsorbed on it, remains in the center of the vacuole in the form of a small lump.
At the same time, another, probably very important, phenomenon occurs. Small secondary vacuoles form on the primary vacuole, which break away from it and migrate into the cytoplasm. We are not yet able to judge the role of this process for the distribution of the contents of the primary vacuole through the cytoplasm. Only one thing is clear: whatever permeability-related processes take place in the membranes of these microvacuoles, their flow is greatly facilitated due to such a huge increase in the area of the membrane surface inside the cell. It is possible that the secondary vacuoles are also involved in the creation of selective permeability, taking away some substances from the primary vacuole and leaving others in it.
The main difficulty that arises when trying to explain pinocytosis as one of the main physiological processes occurring in the cell is that it is completely devoid of specificity. True, in the activity of phagocytes sensitized by antibodies to the absorption of certain bacteria, a high specificity is manifested. A. Tyler believes that during fertilization, pinocytic ingestion of sperm by the egg occurs - a process that begins with the interaction of specific substances on the surfaces of the egg and sperm. However, generally speaking, the mechanical capture of adsorbed substances and liquids from the environment probably occurs without much choice. It is possible that as a result of this, useless or even harmful substances often enter the cell.
Probably, somewhere there is a mechanism with greater selectivity. It is easiest to assume that the choice, active or passive, occurs on the membranes that surround the vacuoles and vesicles that are in the cell. In this case, pinocytosis should be considered not as a process that excludes transfer through the membrane, but as a process that supplements such transfer. His the main task should consist in the creation of extensive internal surfaces, on which the activity of the forces associated with passive and active transfer could be even more effective than on the cell surface itself, and at the same time with a lower risk of loss of matter due to leakage.
All cells are separated from the environment by a plasma membrane. Cell membranes are not impenetrable barriers. Cells are able to regulate the amount and type of substances passing through the membranes, and often the direction of movement.
Transport across membranes is vital because it provides:
- appropriate pH value and ion concentration
- nutrient delivery
- disposal of toxic waste
- secretion of various nutrients
- creation of ionic gradients necessary for nerve and muscle activity.
The regulation of metabolism across membranes depends on the physical and chemical properties of the membranes and the ions or molecules passing through them.
Water is the main substance that enters and exits cells.
The movement of water both in living systems and in inanimate nature obeys the laws of volumetric flow and diffusion.
Diffusion is a familiar phenomenon. If a few drops of perfume are sprinkled in one corner of the room, the smell will gradually fill the whole room, even if the air in it is still. This is because matter moves from a region with more high concentration to an area with a lower one. In other words, diffusion is the spread of a substance as a result of the movement of their ions or molecules, which tend to equalize their concentration in the system.
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Signs of diffusion: each molecule moves independently of the others; these movements are chaotic. Diffusion is a slow process. But it can be accelerated as a result of plasma current, metabolic activity. Usually, substances are synthesized in one part of the cell and consumed in another. That. a concentration gradient is established, and substances can diffuse along the gradient from the place of formation to the place of consumption. Organic molecules are usually polar. Therefore, they cannot freely diffuse through the lipid barrier of cell membranes. However, carbon dioxide, oxygen and other lipid-soluble substances pass through the membranes freely. Water and some small ions pass in both directions. |
Cell membrane.
The cell is surrounded on all sides by a tight-fitting membrane that adapts to any change in its shape with apparent slight plasticity. This membrane is called the plasma membrane, or plasmalemma (Greek plasma - form; lemma - shell).
General characteristics of cell membranes:
- Different types of membranes differ in their thickness, but in most cases the thickness of the membranes is 5 - 10 nm; for example, the thickness of the plasma membrane is 7.5 nm.
- Membranes are lipoprotein structures (lipid + protein). To some lipid and protein molecules on external surfaces attached carbohydrate components (glycosyl groups). Typically, the proportion of carbohydrate in the membrane is from 2 to 10%.
- Lipids form a bilayer. This is because their molecules have polar heads and non-polar tails.
- Membrane proteins perform various functions Keywords: transport of substances, enzymatic activity, electron transfer, energy conversion, receptor activity.
- On the surfaces of glycoproteins are glycosyl groups - branched oligosaccharide chains resembling antennae. These glycosyl groups are associated with a recognition mechanism.
- The two sides of the membrane may differ from each other both in composition and in properties.
Functions of cell membranes:
- restriction of cellular contents from the environment
- regulation of metabolic processes at the border "cell - environment"
- transmission of hormonal and external signals that control cell growth and differentiation
- participation in the process of cell division.
Endocytosis and exocytosis.
Endocytosis and exocytosis are two active processes by which various materials are transported across the membrane, either into cells (endocytosis) or out of cells (exocytosis).
During endocytosis, the plasma membrane forms invaginations or outgrowths, which then, lacing off, turn into vesicles or vacuoles. There are two types of endocytosis:
1. Phagocytosis - the absorption of solid particles. Specialized cells that carry out phagocytosis are called phagocytes.
2. Pinocytosis - absorption of liquid material (solution, colloidal solution, suspension). Very small vesicles (micropinocytosis) often form.
Exocytosis is the reverse process of endocytosis. Hormones, polysaccharides, proteins, fat droplets and other cell products are excreted in this way. They are enclosed in membrane-bound vesicles and approach the plasmalemma. Both membranes fuse and the contents of the vesicle are released into the environment surrounding the cell.
Types of penetration of substances into the cell through membranes.
Molecules pass through membranes by three different processes: simple diffusion, facilitated diffusion, and active transport.
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Simple diffusion is an example of passive transport. Its direction is determined only by the difference in the concentrations of the substance on both sides of the membrane (concentration gradient). By simple diffusion, non-polar (hydrophobic) lipid-soluble substances and small uncharged molecules (for example, water) penetrate the cell.
Most of the substances needed by cells are transported through the membrane with the help of transport proteins (carrier proteins) immersed in it. All transport proteins appear to form a continuous protein passage across the membrane.
There are two main forms of carrier-assisted transport: facilitated diffusion and active transport.
Facilitated diffusion is due to a concentration gradient, and the molecules move along this gradient. However, if the molecule is charged, then its transport is affected by both the concentration gradient and the overall electrical gradient across the membrane (membrane potential).
Active transport is the movement of solutes against a concentration or electrochemical gradient using the energy of ATP. Energy is required because matter must move against its natural tendency to diffuse in the opposite direction.
Na-K pump.
One of the most important and best studied active transport systems in animal cells is the Na-K pump. Most animal cells maintain different concentration gradients of sodium and potassium ions on different sides of the plasma membrane: inside the cell, low concentration sodium ions and a high concentration of potassium ions. The energy needed to operate the Na-K pump is supplied by ATP molecules produced during respiration. The importance of this system for the whole organism is evidenced by the fact that in a resting animal more than a third of ATP is spent to ensure the operation of this pump.
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Na-K pump operation model.
BUT. The sodium ion in the cytoplasm combines with a transport protein molecule. |
