The program of the elective course in biology "mysteries of the living cell". Physiological properties of the cell membrane Lab cell membrane
Objective: show that the cell membrane has selective permeability. Demonstrate the role of the membrane in the process of phagocytosis and pinocytosis.
Equipment: microscopes, coverslips and slides, scalpels, dissecting needles, cups for water and solutions, filter paper, pipettes, ink. Culture of ciliates, amoebas, elodea leaf. NaCl or KCl solutions, CaCl or MgCl solutions, 2% albumin solution, 10% NaCl solution, distilled water.
Working process:
1. Place ciliates in a weak solution of NaCl or KCl. Prepare a microscope slide. Wrinkling of the cells can be seen, indicating permeability of the cell wall. In this case, water leaves the cell environment. Transfer the cells to a drop of distilled water or draw the solution from under the coverslip with filter paper and replace it with distilled water. Watch the cells swell as water enters them.
Place the ciliate in a low concentration CaCl or MgCl solution (same as the previous solution). The ciliates continue to live, no deformations are observed. Ca and Mg ions reduce the permeability of the cell membrane, in contrast to Na and K ions. There is no movement of water through the membrane.
2. Place the amoeba in a drop of 2% albumin (chicken egg white). Prepare a microscope slide. After some time, bubbles, protrusions, tubules begin to form on the surface of the amoeba. It seems that the surface of the amoeba is "boiling". This is accompanied by intense fluid movement near the membrane surface. Fluid bubbles are surrounded by protrusions of the cytoplasm. which are then closed. Pinocytic vesicles sometimes appear suddenly, which indicates the rapid capture of a drop of liquid along with a substance soluble in it.
Place the amoeba in the sugar solution. Pinocytosis is absent. Pinocytosis is caused only by substances that lower the surface tension of the cell membrane, such as amino acids, some salts. In a drop of liquid in which amoebas are located, enter a little finely ground carcass. Prepare the preparation for the microscope. After some time, the amoebas begin to slowly move towards the grains of the carcass, releasing pseudopodia. Carcass grains are attached to the surface of pseudopodia, then slowly surrounded by them and after a while are immersed in the cytoplasm. Under a microscope, observe the phenomenon of phagocytosis in an amoeba.
3. In the cytoplasm of Elodea cells, many round-oval green bodies are visible - these are chloroplasts. Examine the cells near the central vein of the leaf. They can detect the movement of the cytoplasm and plastids along the walls. If the movement is hardly noticeable, heat the preparation under an electric lamp.
4. Sketch everything you saw on the slides. Discuss in groups the processes you have seen, try to explain them.
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Different substances pass through the membrane at different rates, so we say that membranes are selectively permeable. In this case, the rate of passage of substances varies depending on the physiological state of the cell or organelle.
Due to selective permeability, they regulate the transport of substances between the external environment and the cell, between organelles and the cytoplasm, etc.
By regulating the flow of substances into the cell and their excretion, the membranes thereby regulate the speed and direction of biochemical reactions, which form the basis of the body's metabolism. The very selective permeability of membranes depends on the metabolism in the cell.
Membranes regulate metabolism in another way - by changing the activity of enzymes. Some enzymes are active only when they are attached to the membrane, while others, on the contrary, do not show activity in this state and begin to act only after the membrane releases them to "freedom". A change in membrane permeability can facilitate the contact of the enzyme with the substrate, after which a chemical reaction begins, which was initially impossible.
Membrane enzymes work well only when they are in contact with lipids. In the presence of lipids, the shape of molecules can change membrane proteins– enzymes, so that their active centers become available to the substrate. In addition, the localization of the enzyme on the membrane determines the place of this reaction in the cell.
Another important aspect of the enzymatic activity of membranes is the coordination of chemical reactions taking place in cells. When several enzymes catalyze a chain of reactions in which the product of the first reaction serves as a substrate for another, and so on, these enzymes are located on the membrane in a certain sequence, forming a multienzyme system. There are many such systems in the membrane, for example, a chain of respiratory enzymes. In this case, the enzymes are arranged in strict sequence with a minimum distance between them.
cell compartmentalization- a necessary condition for life and one of the main functions of membranes. First, membranes increase inner surface cells on which enzymes are localized and pass chemical reactions. Secondly, different compartments differ in chemical composition. Further, since the compartments have different chemical composition different biochemical reactions take place in them, then with the help of membranes, the physical separation of metabolic processes, often of the opposite direction, is carried out. For example, protein synthesis occurs in ribosomes, and decay occurs in lysosomes. Each of these processes is regulated independently of one another. Let us give another example: the synthesis of fatty acids and their oxidation. The first process occurs in the cytoplasm, the second - in the mitochondria.
However, metabolic systems are not completely isolated from one another. The membranes dividing the cell into compartments have specialized mechanisms that transport substrates, reaction products, as well as cofactors and compounds that have a regulatory effect from one to another. Thus, the rate of individual metabolic processes that occur within compartments is partially regulated transport systems membranes.
Regulation of the rate of metabolic processes can occur due to the movement of regulated substances from one compartment to another.
Different compartments have different concentrations organic matter, ions, different chemical composition. For example, in vacuoles there is always a supply of amino acids, organic acids, sugars, ions. This leads to chemical heterogeneity in the cell. The uneven concentration of ions on both sides of the membrane leads to the appearance of a difference in electrical potentials. Thus, the plasma membrane carries a negative charge, while the tonoplast carries a positive charge. Different concentrations and chemical composition cause different viscosities in different parts cytoplasm.
Possessing selective permeability, passing the necessary substances into the cell, the membranes perform another function - they regulate homeostasis. Homeostasis called the property of a cell (organelle, organ, organism, ecosystem) to maintain the constancy of its internal environment.
Why should the internal environment of a cell remain constant? Membrane proteins and enzyme proteins are globular. The globular native structure of protein molecules depends on weak bonds, which are easily destroyed even with a small change in the internal environment of the cell. Thus, the cell must maintain homeostasis so that the native structure of proteins does not change. If the tertiary or quaternary structure of the protein changes, then the enzyme will lose or change its activity and the strict correspondence between the structure of the enzyme and the substrate will be violated in order for the reaction to proceed.
The structure of the protein molecule determines its placement in the membrane, and thus its properties and functions. A change in the conformation of protein molecules can change the amount of hydrophobic and hydrophilic radicals on its surface. This leads to a change in the arrangement of protein globules in the membrane. The latter will affect its selective ability and other properties, which, in turn, will cause a violation of heterogeneity, the disappearance of enzymes, and can lead to cell death.
Membranes are involved in cell adaptation to changing environmental conditions, which will be discussed below.
Most of the membranes, in addition to general functions, such as regulation of metabolism, compartmentalization, also perform special ones. For example, the membranes of mitochondria and chloroplasts are directly involved in the synthesis of ATP. Life is a continuous work, for the performance of which it is necessary to expend energy all the time.
Thus, the synthesis of ATP is necessary constantly, it is associated with a strictly defined structure of organelle membranes (chloroplasts, mitochondria). Violation of this structure leads to a decrease in ATP synthesis, which means death.
The labile structure of membranes allows them to perform various functions: barrier, transport osmotic, electrical, structural, energy, biosynthetic, secretory, receptor-regulatory, and some others.
Recently, more and more evidence has been accumulating indicating that some membranes are formed by the physical transfer of membrane material from one cellular component to another. There is evidence that allows us to consider ES as the source of those building blocks that are ultimately included in the plasmalemma. Perhaps this occurs as a result of the lacing of the vesicles from the Golgi cisterns. In all likelihood, two types of membranes are restructured into the Golgi apparatus: membranes characteristic of ES, into membranes characteristic of the plasmalemma.
In conclusion, we point out the main properties of membranes:
1. Membranes are complex structures. They are composed of structural proteins and lipids, but may also include highly specific molecules of enzymes, pigments, and cofactors.
2. Due to the chemical variability of the protein and lipid molecules that make up the membrane, and depending on their functions, different membranes can have different structures.
3. The structure of the membranes provides a high degree of ordering which specific molecules can form complex functional units.
4. Enzymatic reactions and other processes in membranes can lead to spatially directed, or vector, reactions; membranes are asymmetric
Plasmolysis (from the Greek plásma - fashioned, decorated and lýsis - decomposition, decay), separation of the protoplast from the membrane when the cell is immersed in a hypertonic solution.
Plasmolysis is characteristic mainly for plant cells that have a strong cellulose membrane. Animal cells shrink when transferred to a hypertonic solution. Depending on the viscosity of the protoplasm, on the difference between the osmotic pressure of the cell and the external solution, and consequently on the rate and degree of water loss by the protoplasm, there are convex, concave, convulsive and cap plasmolysis. Sometimes plasmolyzed cells remain alive; when such cells are immersed in water or a hypotonic solution, deplasmolysis occurs.
For a comparative assessment of plasmolysis in tissues, there are two methods:
Border plasmolysis method
- Plasmometric method.
The first method, developed by Hugo De Vries (1884), consists in immersing tissues in solutions with various concentrations of KNO3, sucrose, or other osmotically active substance and establishing the concentration at which 50% of the cells are plasmolyzed. With the plasmometric method, after plasmolysis, the relative volume of the cell and protoplast is measured, and the osmotic pressure of the cell is calculated from the concentration of the solution (according to the corresponding formulas).
Deplasmolysis (from de ... and plasmolysis) - the return of the protoplast of plant cells from the state of plasmolysis to its original state, characterized by normal turgor.
Deplasmolysis occurs when plasmolyzed cells (that is, cells that have undergone plasmolysis) are transferred to water or hypotonic solutions.
Turgor (Late Latin turgor - swelling, filling, from Latin turgere - to be swollen, filled), the stressed state of the cell membrane, depending on the osmotic pressure of the intracellular fluid (P internal), the osmotic pressure of the external solution (P external) and the elasticity of the cell membrane ( UO). Usually, the UV of animal cells (excluding some coelenterates) is low, they lack high T. and retain their integrity only in isotonic solutions or those that differ little from isotonic ones (the difference between P internal and P external is less than 0.5-1.0 am). In living plant cells, the inner P is always greater than the outer P, but the rupture of the cell membrane does not occur due to the presence of a cellulose cell wall. The difference between P internal and P external in plants (for example, in plants of halophytes, fungi) reaches 50-100 am, but even in this case, the margin of safety of a plant cell is 60-70%. In most plants, the relative elongation of the cell membrane due to T. does not exceed 5–10%, and the turgor pressure lies in the range of 5–10 am. Thanks to T., plant tissues have elasticity and structural strength. All processes of autolysis, wilting and aging are accompanied by a drop in T.
Water(hydrogen oxide) - a binary inorganic compound, chemical formula H 2 O. The water molecule consists of two hydrogen atoms and one oxygen, which are interconnected covalent bond. Under normal conditions, it is a transparent liquid, colorless (in a small volume), odor and taste. In the solid state it is called ice (ice crystals can form snow or frost), and in the gaseous state it is called water vapor. Water can also exist in the form of liquid crystals (on hydrophilic surfaces). About 71% of the Earth's surface is covered with water (oceans, seas, lakes, rivers, ice) - 361.13 million km2. On Earth, approximately 96.5% of water is in the oceans, 1.7% of the world's reserves are groundwater, another 1.7% in the glaciers and ice caps of Antarctica and Greenland, a small part in rivers, lakes and swamps, and 0.001% in clouds (formed from particles of ice and liquid water suspended in the air). Most of the earth's water is salty and unsuitable for Agriculture and drink. Dolyapresnaya is about 2.5%, and 98.8% of this water is in glaciers and groundwater. Less than 0.3% of all fresh water found in rivers, lakes and the atmosphere, and an even smaller amount (0.003%) is found in living organisms.
It is a good highly polar solvent. AT natural conditions always contains dissolved substances (salts, gases).
Water is of key importance in creating and sustaining life on Earth, in chemical structure living organisms, in the formation of climate and weather. Is an essential substance for all living beings on planet earth.
The first feature: water is the only substance on Earth (except for mercury),
for which the dependency specific heat temperature has
minimum. Due to the fact that the specific heat of water has
minimum about 37°C, normal human body temperature,
consisting of two thirds of water, is in the temperature range
36°-38°C (internal organs have a higher temperature than
external).
The second feature: the heat capacity of water is anomalous
high. To heat a certain amount of it by one degree,
more energy must be expended than when heating other liquids, -
at least twice as compared to simple substances. From this
the unique ability of water to retain heat follows. overwhelming
most other substances do not have this property. This
the exceptional feature of water contributes to the fact that a person
normal body temperature is maintained at the same level and hot
during the day and cool at night.
So the water plays
the leading role in the processes of regulation of human heat exchange and
allows him to maintain a comfortable state with a minimum
energy costs. At normal body temperature, a person is
in the most favorable energy state.
Temperature
other warm-blooded mammals (32-39°C) also correlates well with
temperature of the minimum specific heat capacity of water.
Third
feature: water has a high specific heat of fusion, that is
water is very difficult to freeze, and ice is very difficult to melt. As a result, the climate
on Earth as a whole is quite stable and soft.
All three features of the thermal properties of water allow a person to optimally
way to exist in a favorable environment.
performs the transport function of "delivering" nutrients to tissues and
organs during root and leaf nutrition, metabolic processes and synthesis,
- thermoregulating, preventing tissue overheating and denaturation
(destruction) of proteins, including enzymes and hormones,
- is the main component of plant organisms (80-90%
plants are made up of water), which creates turgor - tissue elasticity,
- as a source of a battery - hydrogen (H), necessary in the processes
photosynthesis of primary sugars
Plant cells only at the earliest stage of development are completely filled with protoplasm. Very soon, cavities begin to appear in the protoplasm, vacuoles - reservoirs with cell sap. The formation of vacuoles is due to the presence in the protoplasm of substances that strongly attract water. As the cell grows and ages, individual vacuoles merge into one continuous cavity, and the protoplasm is reduced to a thin layer lining the cell walls. Only strands and filaments of protoplasm cross the vacuole that has grown to cover the entire cell.
Cellular sap, located in vacuoles, has a complex chemical composition. It contains dissolved mineral salts, organic acids (oxalic, malic, citric, tartaric) and their salts, sugars, nitrogenous substances, alkaloids, glucosides, tannins, etc.
Coloring substances are often found in cell sap - pigments (anthocyanin, less often anthochlor). The color of anthocyanin changes depending on the reaction of the environment. When acidic, it is red or purple, when alkaline, it is blue.
Anthocyanin stains beet roots, red cabbage leaves, purple, red and blue flower petals. The second soluble pigment anthochlor is also sometimes found in the petals and colors them yellow.
The usefulness of many cultivated plants depends on the composition of cell sap. The sugar content of sugar beets, the sweet taste of watermelon and fruits are determined by cell sap. living cell plants is an osmotic system, where various substances are directed through membranes from a higher concentration to a lower one until they equalize.
When the cell is in water or in a very weak salt solution (like a soil solution), water enters the cell sap, as a result of which the vacuole increases in volume, stretches the protoplasm and presses it tightly against the shell. The shell also stretches somewhat and is, as they say, in a state of turgor (tension). With a high content of sugar in the cells (fruits of cherries, sweet cherries, grapes) and abundant moisture (frequent rains), the turgor can be so large that the cells burst.
The reverse phenomenon is observed in plasmolysis. If a living plant cell is placed in a hypertonic solution of sugar or salt (stronger than cell sap), then water will come out of the cell to the outside, since the osmotic (attractive) force of such a solution is greater than the osmotic force of cell sap.
The osmotic pressure is especially high in plants growing in deserts and salt marshes. In many cases it reaches 50 and even 100 atm. atm). By quantitative indicators Based on concentration, the osmotic pressure in some plants is many times the pressure of steam in the most powerful locomotives. In reality, cells have to experience only the difference in osmotic pressures between cell sap and soil solutions, the concentration of which is high in desert soils and solonchaks.
The process of entry of substances into the cell is called endocytosis. Distinguish between pinocytosis and phagocytosis.
Phagocytosis (Greek fago - to devour) - the absorption of solid organic substances by the cell. Once near the cell, the solid particle is surrounded by outgrowths of the membrane, or an invagination of the membrane is formed under it. As a result, the particle is enclosed in a membrane vesicle inside the cell. This vesicle is called a phagosome. The term "phagocytosis" was proposed by I. I. Mechnikov in 1882. Phagocytosis is characteristic of protozoa, coelenterates, leukocytes, as well as cells of the capillaries of the bone marrow, spleen, liver, and adrenal glands.
The second way substances enter the cell is called pinocytosis (Greek pinot - I drink) - this is the process of absorption by the cell of small drops of liquid with macromolecular substances dissolved in it. It is carried out by capturing these drops by outgrowths of the cytoplasm. Captured drops are immersed in the cytoplasm and absorbed there. The phenomenon of pinocytosis is characteristic of animal cells and unicellular protozoa.
Another way substances enter the cell is osmosis - the passage of water through the selectively permeable cell membrane. Water moves from a less concentrated solution to a more concentrated one. Substances can also pass through the membrane by diffusion - this is how substances that can dissolve in lipids (simple and esters, fatty acids, etc.). By diffusion along the concentration gradient, some ions go through special channels of the membrane (for example, the potassium ion leaves the cell).
In addition, the transport of substances across the membrane is carried out by the sodium-potassium pump: it moves sodium ions out of the cell and potassium ions into the cell against a concentration gradient with the expenditure of ATP energy.
Phagocytosis, pinocytosis, and the sodium-potassium pump are examples of active transport, while osmosis and diffusion are examples of passive transport.
WATER BALANCE OF PLANTS
The ratio between the amount of water that plants receive and the amount of water they use in the same period of time.
Water balance and wilting. One of the most dynamic processes in a plant is water metabolism, which is in close correlation with other plant life processes. Water balance is the intake and expenditure of water by a plant. With moderate transpiration and sufficient water supply to the plant, a favorable water balance is created. On a clear sunny day, this balance is disturbed and a water deficit occurs in the plant, which is usually 5-10%. Such a deficiency is considered quite normal and does not cause much harm to the plant.
With intensive transpiration or drying up of the soil, when the supply of water to the plant stops, there is a significant loss of water by plant cells, which is not replenished by its absorption from the soil, resulting in a water deficit, often observed in the hottest hours of the day in plants.
With water deficiency, the leaves lose turgor, wither, hang.
Some plants that have a large number of mechanical tissues in their organs, such as immortelle (genus Helichrysum), do not change their appearance with water deficiency, with a significant loss of water and even death.
Observations have shown that usually at dawn the internal gradient in the plant and soil is almost equalized, the water potentials of the plant and soil are balanced. In the morning, when the leaves begin to transpire, the water potential becomes somewhat lower than at dawn, but the flow of water into the plant begins; when the necessary gradient of water potentials is created from the leaves to the root-soil interface.
Withering is temporary and long-term.
Date added: 2015-02-02 | Views: 1725 | Copyright infringement
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SECTION 2
LABORATORY CLASSES
Lab #1
Comparison of membrane permeability of living and dead cells
Exercise: identify differences in the permeability of membranes of living and dead cells and draw a conclusion about the causes of these differences.
Materials and equipment: test tubes, test tube stand, scalpel, spirit lamp or gas burner, 30% acetic acid solution, beet root.
Operating procedure
1. After removing the integumentary tissues, the beet root is cut into cubes (the side of the cube is 5 mm) and thoroughly washed with water to remove the pigment that has come out of the damaged cells.
2. One piece of beets is dipped into three test tubes. In the first and second pour 5 ml of water, in the third - 5 ml of a 30% solution of acetic acid. The first test tube is left for control. The contents of the second boil for 2-3 minutes.
3. The vacuoles of beet root cells contain betacyanin, a pigment that gives color to the root tissue. The tonoplasts of living cells are impermeable to the molecules of this pigment. After cell death, the tonoplast loses its semipermeability property, becomes permeable, pigment molecules leave the cells and color the water.
In the second and third tubes, where the cells were killed by boiling or acid, the water is stained, while in the first tube it remains unstained.
4. Write down the results of observations.
Lab #2
Turgor, plasmolysis and deplasmolysis
Exercise: to study under a microscope the phenomena of turgor, plasmolysis and deplasmolysis in the cells of the blue onion epidermis.
Materials and equipment: microscopes, dissecting accessories, alcohol lamps, blue onions, table beet roots, 30% sugar solution, 5-8% potassium nitrate solution.
Operating procedure
1. Make a planar cut of the epidermis of a blue onion, put it on a glass slide in a drop of water.
2. Close the drop with a cover glass and observe the cells in the state of turgor in a microscope.
3. Take a drop of 30% sugar solution and place it next to the coverslip.
4. Touching the opposite end of the coverslip with filter paper, replace the water in the preparation with a sugar solution.
5. Again observe under the microscope. If plasmolysis is not yet noticeable, repeat the replacement of water with a sugar solution.
Under a microscope, plasmolysis in living cells of the epidermis will be clearly visible.
6. Carry out the experiment in the reverse order, i.e. return the water again and observe the phenomenon of deplasmolysis.
7. Draw cells in a state of turgor, plasmolysis and deplasmolysis.
8. To prove that plasmolysis and deplasmolysis occur only in living cells, conduct such an experiment in parallel. One of the sections of the onion epidermis, placed in a drop of water, hold over the flame of an alcohol lamp to kill the cells. Then apply a sugar solution and see if plasmolysis occurs.
The described experience allows you to get acquainted not only with the processes of turgor, plasmolysis and deplasmolysis, but also with the process of substances entering the cell (in this case, sugar molecules from solution).
When studying the phenomena of plasmolysis and deplasmolysis in table beet root cells, the procedure is the same, but instead of a sugar solution, it is better to use a 5% solution of potassium nitrate.
Lab #3
Determination of transpiration by weight
Exercise: determine the amount of water evaporated by a plant over a certain period of time using the weight method.
Materials and equipment: scales, weights, scissors, dishes, stand, live plants.
Operating procedure
1. Attach the U-tube to the stand and pour water into it. Cut off one leaf from the plant (or a small branch with two leaves) and use a cotton plug to strengthen it in one knee (the cotton plug should not touch the water, otherwise the water will evaporate through it). Close the other knee with a rubber or plastic stopper (if there is no such tube, you can take a simple test tube and pour vegetable oil on the surface of the water so that there is no evaporation).
2. Weigh the instrument and at the same time the small mold filled with water. Place the device and crystallizer on the window.
3. After 1-2 hours, re-weigh. The mass decreases in both cases, as the water evaporates.
Lab #4
Observation of the movement of stomata
Exercise: observe stomatal movements, explain the cause of stomatal movements, draw stomata in water and in solutions of 5 and
20%-
th glycerin.
Objective: observe stomatal movements in water and in a solution of glycerol.
Materials and equipment: glycerol solutions (5 and 20%), 1M sucrose solution, microscopes, slides and coverslips, dissecting needles, filter paper, bottles, leaves of any plants.
Operating procedure
1. Prepare several sections of the lower leaf epidermis and place them for 2 hours in a 5% glycerol solution. Glycerin penetrates into the vacuoles of the guard cells, lowers their water potential and, consequently, increases their ability to absorb water. Sections are placed on a glass slide in the same solution, the state of the cells is noted and they are sketched.
2. Replace the glycerin with water by pulling it out from under the glass with filter paper. In this case, the opening of the stomatal fissures is observed. Draw the preparation.
3. Replace water with a strong osmotic agent - 20% glycerol solution or 1M sucrose solution. Observe the closing of the stomata.
4. Draw conclusions.
Lab #5
Photosynthesis products
Exercise: study the process of formation of primary starch in leaves.
Materials and equipment: alcohol lamps, water baths, scissors, electric stoves, incandescent lamps of 200-300 W, dishes, living plants (pumpkin, beans, pelargonium, primrose, etc.), ethyl alcohol, a solution of iodine in potassium iodide.
Operating procedure
1. Using a starch test, prove that starch is formed during photosynthesis.
A well-watered plant should be placed for 2-3 days in a dark place. During this time, there will be an outflow of assimilates from the leaves. New starch cannot form in the dark.
To get the contrast from the photosynthesis process, part of the leaf must be darkened. To do this, you can use a photo negative or two identical opaque screens, attaching them from above and below. Drawings on the screen (cutouts) can be very different.
An incandescent lamp of 200-300 W is placed at a distance of 0.5 m from the sheet. After an hour or two, the sheet must be processed, as indicated above. It is more convenient to do this on a flat plate. At the same time, a sheet that remained dark all the time is processed.
Parts exposed to illumination are colored blue, while the rest are yellow.
In summer, you can modify the experience - close a few leaves on the plant, putting on them bags of black opaque paper with appropriate cutouts; after two or three days, at the end sunny day, cut the leaves, boil them first in water, and then discolor with alcohol and treat with a solution of iodine in potassium iodide. The darkened areas of the leaves will be light, and the illuminated areas will turn black.
In some plants (for example, onions), the primary product of photosynthesis is not starch, but sugar, so the starch test is not applicable to them.
2. Record the results of observations.
Lab #6
Obtaining pigments from the leaves of an alcohol extract
and their separation
Exercise: obtain an alcohol extract of pigments, separate them and get acquainted with the basic properties of pigments.
Materials and equipment: scissors, mortars with pestles, racks with test tubes, dishes, spirit lamps, water baths, fresh or dry leaves (nettle, aspidistra, ivy or other plants), ethyl alcohol, gasoline, 20% NaOH (or KOH) solution, dry chalk , sand.
Operating procedure
1. Place dry leaves chopped with scissors into a clean mortar, add a little chalk to neutralize the acids of the cell sap. Thoroughly grind the mass with a pestle, adding ethyl alcohol (100 cm 3), then filter the solution.
The resulting extract of chlorophyll has fluorescence: in transmitted light it is green, in reflected light it is cherry red.
2. Separate the pigments using the Kraus method.
To do this, pour 2-3 cm 3 of extract into a test tube and add one and a half volume of gasoline and 2-3 drops of water; then you need to shake the test tube and wait until two layers become clearly visible - gasoline at the top, alcohol at the bottom. If separation does not occur, add more gasoline and shake the tube again.
In case of turbidity, add a little alcohol.
Since gasoline does not dissolve in alcohol, it ends up at the top. Green color the top layer indicates that chlorophyll has passed into gasoline. In addition to it, carotene also dissolves in gasoline. Below, in alcohol, xanthophyll remains. The bottom layer is yellow.
After settling the solution, two layers are formed. As a result of saponification of chlorophyll, alcohols are cleaved off and the sodium salt of chlorophyllin is formed, which, unlike chlorophyll, does not dissolve in gasoline.
For better saponification, a test tube with the addition of NaOH can be placed in a water bath with boiling water and, as soon as the solution boils, removed. After that, gasoline is poured. Carotene and xanthophyll (the color will be yellow) will pass into the gasoline layer (upper), and sodium salt of chlorophyll acid will pass into the alcohol layer.
Lab #7
Plant respiration detection
Exercise: prove that CO 2 is released during plant respiration, draw a device that helps to detect respiration by CO 2 release, make captions for the figure.
Materials and equipment: 2 glass jars with a capacity of 300-400 ml, 2 rubber test tubes with holes for funnels and tubes, 2 funnels, 2 curved "P" glass tubes 18-20 cm long and 4-5 mm in diameter, 2 test tubes, a beaker, Ba(OH) 2 solution, germinated seeds of wheat, sunflower, corn, peas, etc.
Operating procedure
1. 50-60 g of germinated seeds are poured into a glass jar, tightly closed with a cork into which a funnel and a curved glass tube are inserted and left for 1-1.5 hours. During this time, carbon dioxide will accumulate in the jar as a result of respiration of the seeds. It is heavier than air, so it is concentrated at the bottom of the can and does not enter the atmosphere through a funnel or tube.
2. At the same time, they take a control jar without seeds, also close it with a rubber stopper with a funnel and a glass tube and place it next to the first jar.
3. The free ends of the glass tubes are lowered into two test tubes with barite water. In both jars, through the funnels, they begin to gradually pour water. Water displaces air enriched with CO 2 from the jars, which enters the test tubes with a solution of Ba(OH) 2 . As a result, barite water becomes cloudy.
4. Compare the degree of turbidity Ba(OH) 2 in both test tubes.
Lab #8
Determination of respiration intensity in Conway cups
Exercise: to do the experiment and calculate the intensity of breathing of the objects under study, depending on the variants of the experiment.
Materials and equipment: Conway cups, vaseline, burettes, tripods, filter paper, scissors, scales, weights, reagents: 0.1n Ba(OH) 2 ; 0.1 N HCl, phenolphthalein, any seedlings and adult plants or their organs.
Operating procedure
1. Conway's cups are calibrated before the experiment; they must be of the same volume for the control and experimental options. Each variant of the experiment is set in triplicate.
2. A sample of plant material weighing 0.5-1.0 g is laid out in the outer circle of the Conway cup. 1 or 2 ml of 0.1 n Ba (OH) 2 is poured into the inner cylinder. a transparent contour of the thin section of the cup appeared) and put it in the dark for 20–40 minutes (to exclude photosynthesis in the green tissues of plants). During the exposure, accumulated in the volume of the Conway cup carbon dioxide reacts with barium hydroxide:
CO 2 + Ba (OH) 2 \u003d BaCO 3 + H 2 O.
An excess of Ba(OH) 2 is titrated with 0.1 N HCl for phenolphthalein until the pink color disappears.
3. Simultaneously with the experimental one put a control cup of Conway (without a sample). The same volume of a solution of 0.1 N Ba(OH) 2 is poured into it, closed with a ground-in lid and left next to the experimental cup. Barium hydroxide in this cup reacts with carbon dioxide, which was originally in its volume as part of the air. The excess barite is titrated.
4. By the difference in the volumes of the hydrochloric acid solution used to titrate the excess of Ba (OH) 2 in the control and experimental cups, calculate the respiratory rate (I. d.):
Mg CO 2 / (g ∙ h),
where V HC1k is the volume of 0.1 n HC1 used for titration of excess Ba(OH) 2 in the control cup; V HC1op is the volume of 0.1n HC1 used for titration of excess Ba(OH) 2 in the test dish; R- sample weight, g;
t - time, h; 2.2 - conversion factor of HC1 to CO 2 (1 ml of 0.1 n HC1 or Ba(OH) 2 is equivalent to 2.2 mg of CO 2).
Lab #9
The importance of various elements for plants
Exercise: to study the importance of various mineral elements for the growth of the aspergillus fungus.
Materials and equipment: scales, thermostat, cotton plugs, filters, five 100 cm 3 flasks, test tubes, pipette, two glasses, funnel, mineral salts, sucrose, organic acid(lemon), aspergillus fungus grown on slices of potato or bread for 3-4 days.
Operating procedure
1. Grow a mushroom on nutrient mixtures.
It has been established that aspergillus makes approximately the same demands on the conditions of mineral nutrition as higher plants. Of the mineral elements, the fungus does not need only calcium. Nutrient mixtures are prepared in flasks of 100 cm 3 and are made according to a certain scheme (Table 1).
The numbering of the flasks corresponds to the numbering of the variants of the experiment. Record the results of the experiment below.
Table 1
Scheme for the composition of nutrient mixtures
Substances | Concentration | Amount of substance (in ml) in flasks |
||||
No. 1 - complete mixture | No. 2 - without N | No. 3 - without P | No. 4 - without K | No. 5 - without minerals |
||
sucrose Lemon acid | ||||||
results Mycelium weight, g | ||||||
Citric acid is added to create an acidic environment that is favorable for Aspergillus but inhibits the development of other microorganisms.
2. Pour sterile water into a test tube or flask and place the mycelium of the fungus, taken with a sterile loop, into it, stir the contents by rotating between the fingers or palms.
Add the resulting suspension with a sterile pipette to all flasks.
Close the flasks with cotton plugs and place in a thermostat at a temperature of 30-35 °C. Follow up in a week.
The essence of the experiment lies in the fact that, by determining the mass of mycelium of a fungus grown on various nutrient mixtures, one can find out its need for individual elements.
3. Weigh, for which we take two clean glasses, one funnel and several identical paper filters. Weigh one beaker (No. 1) with funnel and filter and record the weight. Then put the funnel in another beaker (No. 2), transfer the mycelium of the fungus from the first flask onto the filter, rinse with water and, after the water groans, transfer the funnel back to beaker No. 1. Weigh again. It is clear that the result will be greater, since the mycelium of the fungus has been added.
Teaching aid... - Balashov: Nikolaev, 2007. - 48 p. ISBN 978-5-94035-300-3 educational-methodicalallowances methods are outlined... physiologyplants: studies. allowance/ ed. V. B. Ivanova. - Academy, 2001. - 144 p. Zanina, M. A. Physiologyplants: textbook-method. allowance ...
Training and metodology complex
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THEORY AND PRACTICE OF SCIENTIFIC SPEECH Special course for non-humanitarian specialties of universities Educational and methodological complex Balashov - 2008
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Educational and methodical complex (219)
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Objective: show that the cell membrane has selective permeability. Demonstrate the role of the membrane in the process of phagocytosis and pinocytosis.
Equipment: microscopes, coverslips and slides, scalpels, dissecting needles, cups for water and solutions, filter paper, pipettes, ink. Culture of ciliates, amoebas, elodea leaf. NaCl or KCl solutions, CaCl or MgCl solutions, 2% albumin solution, 10% NaCl solution, distilled water.
Working process:
Place ciliates in a weak solution of NaCl or KCl. Prepare a microscope slide. Wrinkling of the cells can be seen, indicating permeability of the cell wall. In this case, the water from the cell is released into the environment. Transfer the cells to a drop of distilled water or draw the solution from under the coverslip with filter paper and replace it with distilled water. Watch the cells swell as water enters them.
Place the infusoria in a low concentration CaCl or MgCl solution (same as the previous solution). The ciliates continue to live, no deformations are observed. Ca and Mg ions reduce the permeability of the cell membrane, in contrast to Na and K ions. There is no movement of water through the shell.
Place the amoeba in a drop of 2% albumin solution (chicken egg white). Prepare a microscope slide. After some time, bubbles, protrusions, tubules begin to form on the surface of the amoeba. It seems that the surface of the amoeba is "boiling". This is accompanied by intense fluid movement near the membrane surface. Fluid bubbles are surrounded by protrusions of the cytoplasm. which are then closed. Pinocytic vesicles sometimes appear suddenly, which indicates the rapid capture of a drop of liquid along with a substance soluble in it.
Place the amoeba in the sugar solution. Pinocytosis is absent. Pinocytosis is caused only by substances that lower the surface tension of the cell membrane, such as amino acids, some salts. In a drop of liquid in which amoebas are located, enter a little finely ground carcass. Prepare the preparation for the microscope. After some time, the amoebas begin to slowly move towards the grains of the carcass, releasing pseudopodia. Carcass grains are attached to the surface of pseudopodia, then slowly surrounded by them and after a while are immersed in the cytoplasm. Under a microscope, observe the phenomenon of phagocytosis in an amoeba.
In the cytoplasm of Elodea cells, many round-oval green bodies are visible - these are chloroplasts. Examine the cells near the central vein of the leaf. They can detect the movement of the cytoplasm and plastids along the walls. If the movement is hardly noticeable, heat the preparation under an electric lamp.
Sketch everything you saw on the slides. Discuss in groups the processes you have seen, try to explain them.
Laboratory work identification of aromorphosis and idioadaptation in plants and animals
Objective: show on specific examples the origin of large systematic groups by aromorphosis, get acquainted with examples of possible idioadaptation of organisms (degenerations), reveal the influence of human activity on the main directions of organic evolution
Equipment: herbariums of plants (moss, plantain, conifers, angiosperms), plants with thorns, pile (camel thorn, wild rose), drawings of the beak and legs of birds, animals with a protective (masking) color, stingray fish.
Working process:
Analyzing the main features of spore, gymnosperms and angiosperms, understand the aromorphoses of plants
Determine idioadaptation by plant thorn and glandular fibers
Analyze examples of idioadaptation: the structure of the beak and legs of birds living in different environmental conditions
To identify the causes of idioadaptation in the structure of the stingray fish
Introduction 2
1.Basic facts about the structure of the cell membrane 3
2. General representations about permeability 4
3. Transport of molecules across the membrane 4
3.1. Diffusion 5
3.2 Fick's Equation 6
3.3 Passive transport 7
3.3.1 Differences between facilitated and simple diffusion 8
4. Darcy's Law 8
5. Active transport 9
6. Structure and functions of ion channels 11
Conclusion 15
References 17
INTRODUCTION
Membrane transport - the transport of substances through the cell membrane into or out of the cell, carried out using various mechanisms - simple diffusion, facilitated diffusion and active transport.
The most important property of a biological membrane is its ability to pass various substances into and out of the cell. It has great importance for self-regulation and maintenance of a constant composition of the cell. This function of the cell membrane is performed due to selective permeability, i.e. the ability to pass some substances and not pass others. The easiest way to pass through the lipid bilayer is non-polar molecules with a small molecular weight (oxygen, nitrogen, benzene). Such small polar molecules as carbon dioxide, nitric oxide, water, and urea quickly penetrate through the lipid bilayer. Ethanol and glycerol, as well as steroids and thyroid hormones, pass through the lipid bilayer with a noticeable speed. For larger polar molecules (glucose, amino acids), as well as for ions, the lipid bilayer is practically impermeable, since its inner part is hydrophobic. So, for water, the permeability coefficient (cm/s) is about 10-2, for glycerol - 10-5, for glucose - 10-7, and for monovalent ions - less than 10-10.
The transport of large polar molecules and ions occurs due to channel proteins or carrier proteins. So, in cell membranes there are channels for sodium, potassium and chlorine ions, in the membranes of many cells - water channels aquaporins, as well as carrier proteins for glucose, different groups of amino acids and many ions. Active and passive transport.
Membranes form the structure of the cell and carry out its functions. Violation of the functions of cellular and intracellular membranes underlies irreversible cell damage and, as a result, the development of severe diseases of the cardiovascular, nervous, and endocrine systems.
1. Basic facts about the structure of the cell membrane.
Cell membranes include plasmolemma, karyolemma, mitochondrial membranes, EPS, Golgi apparatus, lysosomes, peroxisomes. A common feature of all cell membranes is that they are thin (6-10 nm) layers of lipoprotein nature (lipids in combination with proteins). Main chemical components cell membranes are lipids (40%) and proteins (60%); in addition, carbohydrates (5-10%) were found in many membranes.
The plasma membrane surrounds each cell, determines its size and maintains the differences between the cell's contents and the external environment. The membrane serves as a highly selective filter and is responsible for the active transport of substances, that is, the entry of nutrients into the cell and the removal of harmful waste products. Finally, the membrane is responsible for the perception of external signals, allowing the cell to respond to external changes. All biological membranes are ensembles of lipid and protein molecules held together by non-covalent interactions.
The basis of any molecular membrane is made up of lipid molecules that form a bilayer. Lipids include a large group of organic substances that have poor solubility in water (hydrophobicity) and good solubility in organic solvents and fats (lipophilicity). The composition of lipids in different membranes is not the same. For example, the plasma membrane, unlike the membranes of the endoplasmic reticulum and mitochondria, is enriched with cholesterol. Characteristic representatives of lipids found in cell membranes are phospholipids (glycerophosphatides), sphingomyelins, and cholesterol from steroid lipids.
A feature of lipids is the division of their molecules into two functionally different parts: hydrophobic non-polar, non-charge-carrying ("tails"), consisting of fatty acids, and hydrophilic, charged polar "heads". This determines the ability of lipids to spontaneously form two-layer (bilipid) membrane structures with a thickness of 5-7 nm.
The first experiments confirming this were carried out in 1925.
Bilayer formation is special property lipid molecules and is realized even outside the cell. The most important properties of the bilayer: the ability to self-assembly - fluidity - asymmetry.
2. General ideas about permeability.
Characteristics of membranes, vessel walls and epithelial cells, reflecting the ability to conduct chemical substances; distinguish between active (active transport of substances) and passive P. (phagocytosis
3. Transfer of molecules across the membrane.
Because the interior of the lipid layer is hydrophobic, it provides a virtually impenetrable barrier to most polar molecules. Due to the presence of this barrier, leakage of the contents of the cells is prevented, however, because of this, the cell was forced to create special mechanisms for the transport of water-soluble substances through the membrane. The transfer of small water-soluble molecules is carried out using special transport proteins. These are special transmembrane proteins, each of which is responsible for the transport of certain molecules or groups of related molecules.
In cells, there are also mechanisms for the transfer of macromolecules (proteins) and even large particles through the membrane. The process of absorption of macromolecules by the cell is called endocytosis. AT in general terms the mechanism of its course is as follows: local sections of the plasma membrane invaginate and close, forming an endocytic vesicle, then the absorbed particle usually enters the lysosomes and undergoes degradation.
3.1 Diffusion (Latin diffusio - distribution, spreading, scattering) - the process of transferring matter or energy from an area of high concentration to an area of low concentration (against the concentration gradient). The most famous example of diffusion is the mixing of gases or liquids (if you drop ink into water, the liquid will become uniformly colored after a while). Another example is related to a solid body: if one end of the rod is heated or electrically charged, heat spreads (or, respectively, electricity) from the hot (charged) part to the cold (uncharged) part. In the case of a metal rod, thermal diffusion develops rapidly, and the current flows almost instantly. If the rod is made of synthetic material, thermal diffusion is slow, and diffusion of electrically charged particles is very slow. Diffusion of molecules proceeds in general even more slowly. For example, if a piece of sugar is lowered to the bottom of a glass of water and the water is not stirred, it will take several weeks before the solution becomes homogeneous. Even slower is the diffusion of one solid into another. For example, if copper is covered with gold, then gold will diffuse into copper, but under normal conditions (room temperature and Atmosphere pressure) the gold-bearing layer will reach a thickness of several micrometers only after several thousand years.
All types of diffusion obey the same laws. The diffusion rate is proportional to the cross-sectional area of the sample, as well as the difference in concentrations, temperatures or charges (in the case of relatively small values of these parameters). Thus, heat will travel four times faster through a rod two centimeters in diameter than through a rod one centimeter in diameter. This heat will spread faster if the temperature difference per centimeter is 10°C instead of 5°C. The diffusion rate is also proportional to the parameter characterizing a particular material. In case of thermal diffusion this parameter is called thermal conductivity, in case of flow electric charges- electrical conductivity. The amount of a substance that diffuses during a certain time and the distance traveled by the diffusing substance are proportional square root diffusion time.
Diffusion is a process at the molecular level and is determined by the random nature of the movement of individual molecules. The diffusion rate is therefore proportional to the average velocity of the molecules. In the case of gases, the average speed of small molecules is greater, namely, it is inversely proportional to the square root of the mass of the molecule and increases with increasing temperature. Diffusion processes in solids at high temperatures often found practical use. For example, certain types of cathode ray tubes (CRTs) use metallic thorium diffused through metallic tungsten at 2000°C.
3.2 Fick's equation
In most practical cases, the concentration C is used instead of the chemical potential. The direct replacement of µ by C becomes incorrect in the case of high concentrations, since the chemical potential is related to the concentration according to a logarithmic law. If we do not consider such cases, then the above formula can be replaced by the following:
which shows that the flux density of the substance J is proportional to the diffusion coefficient D and the concentration gradient. This equation expresses Fick's first law (Adolf Fick is a German physiologist who established the laws of diffusion in 1855). Fick's second law relates spatial and temporal changes in concentration (diffusion equation):
The diffusion coefficient D depends on the temperature. In a number of cases, in a wide temperature range, this dependence is the Arrhenius equation.
Diffusion processes are of great importance in nature:
Nutrition, respiration of animals and plants;
The penetration of oxygen from the blood into human tissues.
3.3 Passive transport
Passive transport is the movement of substances from places with great value electrochemical potential to places with its lower value.
In experiments with artificial lipid bilayers, it was found that the smaller the molecule and the less it forms hydrogen bonds, the faster it diffuses through the membrane. So, the smaller the molecule and the more fat-soluble (hydrophobic or non-polar) it is, the faster it will permeate the membrane. Diffusion of substances across the lipid bilayer is caused by a concentration gradient in the membrane. Molecules of lipid-insoluble substances and water-soluble hydrated ions (surrounded by water molecules) penetrate the membrane through lipid and protein pores. Small non-polar molecules are easily soluble and diffuse rapidly. Uncharged polar molecules at small sizes are also soluble and diffuse.
Importantly, water permeates the lipid bilayer very quickly despite being relatively insoluble in fats. This is due to the fact that its molecule is small and electrically neutral.
Osmosis is the predominant movement of water molecules through semi-permeable membranes (impermeable to solute and permeable to water) from places with a lower concentration of solute to places with greater concentration. Osmosis is essentially the simple diffusion of water from places of higher concentration to places of lower water concentration. Osmosis plays an important role in many biological phenomena. The phenomenon of osmosis causes hemolysis of erythrocytes in hypotonic solutions.
So, membranes can pass water and non-polar molecules through simple diffusion.
3.3.1 Differences between facilitated and simple diffusion:
1) the transfer of a substance with the participation of a carrier occurs much faster;
2) facilitated diffusion has the property of saturation: with an increase in concentration on one side of the membrane, the flux density of a substance increases only to a certain limit, when all carrier molecules are already occupied;
3) with facilitated diffusion, competition of transferred substances is observed in cases where different substances are transferred by the carrier; while some substances are better tolerated than others, and the addition of some substances makes it difficult to transport others; So among sugars, glucose is better tolerated than fructose, fructose is better than xylose, and xylose is better than arabinose, etc. etc.;
4) there are substances that block facilitated diffusion - they form a strong complex with carrier molecules, for example, phloridzin inhibits the transport of sugars through a biological membrane.
4. Darcy's law
Darcy's law (Henri Darcy, 1856) is the law of filtration of liquids and gases in a porous medium. Obtained experimentally. Expresses the dependence of the fluid filtration rate on the head gradient:
where: - filtration rate, K - filtration coefficient, - head gradient. Darcy's law is associated with several measurement systems. A medium with a permeability of 1 Darcy (D) allows 1 cm³/s of a liquid or gas with a viscosity of 1 cp (mPa s) to flow under a pressure gradient of 1 atm/cm acting over an area of 1 cm². 1 millidarcy (mD) is equal to 0.001 Darcy.
In the SI measurement system, 1 Darcy is equivalent to 9.869233×10−13m² or 0.9869233 µm². This conversion is usually approximated as 1 µm². Note that this is the reciprocal of 1.013250, the conversion factor from atmospheres to bars.
Transport through the lipid bilayer (simple diffusion) and transport with the participation of membrane proteins
5. Active transport
Other carrier proteins (sometimes called pump proteins) transport substances across the membrane at the expense of energy, which is usually supplied by ATP hydrolysis. This type of transport occurs against the concentration gradient of the transported substance and is called active transport.
Symport, antiport and uniport
Membrane transport of substances also differs in the direction of their movement and the amount of substances carried by this carrier:
1) Uniport - transport of one substance in one direction depending on the gradient
2) Symport - transport of two substances in one direction through one carrier.
3) Antiport - the movement of two substances in different directions through one carrier.
Uniport provides, for example, a voltage-dependent sodium channel, through which sodium ions move into the cell during the generation of an action potential.
Symport is carried out by a glucose transporter located on the outer (facing the intestinal lumen) side of the cells of the intestinal epithelium. This protein simultaneously captures a glucose molecule and a sodium ion and, changing its conformation, transfers both substances into the cell. In this case, the energy of the electrochemical gradient is used, which, in turn, is created due to the hydrolysis of ATP by sodium-potassium ATP-ase.
Antiport is carried out, for example, by sodium-potassium ATPase (or sodium-dependent ATPase). It transports potassium ions into the cell. and out of the cell - sodium ions.
Work of sodium-potassium ATPase as an example of antiport and active transport
Initially, this carrier attaches three Na + ions from the inside of the membrane. These ions change the conformation of the ATPase active site. After such activation, ATPase is able to hydrolyze one ATP molecule, and the phosphate ion is fixed on the surface of the carrier from the inside of the membrane.
The released energy is spent on changing the ATPase conformation, after which three Na + ions and an ion (phosphate) are on the outer side of the membrane. Here Na + ions are split off and replaced by two K + ions. Then the conformation of the carrier changes to the original one, and the K + ions are on the inner side of the membrane. Here, the K + ions are split off, and the carrier is again ready for operation.
More briefly, the actions of ATPase can be described as follows:
1) It “takes” three Na + ions from the inside of the cell, then splits the ATP molecule and attaches phosphate to itself
2) "Throws out" Na + ions and attaches two K + ions from the external environment.
3) Disconnects phosphate, throws two K + ions into the cell
As a result, a high concentration of Na + ions is created in the extracellular environment, and a high concentration of K + is created inside the cell. The work of Na +, K + - ATPase creates not only a difference in concentrations, but also a difference in charges (it works like an electrogenic pump). A positive charge is created on the outside of the membrane, and a negative charge on the inside.
6. Structure and functions of ion channels.
The excitable membrane model assumes a regulated transport of potassium and sodium ions across the membrane. However, the direct passage of the ion through the lipid bilayer is very difficult, so the ion flux density would be very low if the ion passed directly through the lipid phase of the membrane. This and a number of other considerations gave reason to believe that the membrane must contain some special structures - conducting ions.
Such structures were found and named ion channels. Similar channels have been isolated from various objects: the plasma membrane of cells, the postsynaptic membrane of muscle cells, and other objects. Ion channels formed by antibiotics are also known.
Main properties of ion channels:
1) selectivity;
2) independence of the operation of individual channels;
3) discrete character of conductivity;
4) dependence of channel parameters on membrane potential.
Let's consider them in order.
1. Selectivity is the ability of ion channels to selectively pass ions of any one type.
Even in the first experiments on the squid axon, it was found that sodium and potassium ions affect the membrane potential in different ways. Potassium ions change the resting potential, and sodium ions change the action potential.
Measurements have shown that ion channels have absolute selectivity with respect to cations (cation-selective channels) or to anions (anion-selective channels). At the same time, various cations of various chemical elements, but the membrane conductivity for a minor ion, and hence the current through it, will be significantly lower, for example, for a sodium channel, the potassium current through it will be 20 times less. The ability of an ion channel to pass various ions is called relative selectivity and is characterized by a selectivity series - the ratio of channel conductivities for different ions taken at the same concentration.
2. Independence of individual channels. The passage of current through an individual ion channel is independent of whether current flows through other channels. For example, potassium channels can be turned on or off, but the current through the sodium channels does not change. The influence of channels on each other occurs indirectly: a change in the permeability of any channels (for example, sodium) changes the membrane potential, and it already affects the conductivities of other ion channels.
3. Discrete nature of the conduction of ion channels. Ion channels are a subunit complex of proteins that spans the membrane. In its center there is a tube through which ions can pass.
The number of ion channels per 1 μm of the membrane surface was determined using a radioactively labeled sodium channel blocker - tetrodotoxin. It is known that one TTX molecule binds to only one channel. Then the measurement of the radioactivity of the sample with famous area made it possible to show that there are about 500 sodium channels per 1 μm of the squid axon. This was first discovered in 1962 in studies of the conductivity of lipid bilayer membranes (BLMs) when microquantities of some substance that induced excitation were added to the solution surrounding the membrane. A constant voltage was applied to the BLM and the current was recorded. Recording the current in time had the form of jumps between two conducting states.
The results of experiments performed on various ion channels showed that the conductivity of the ion channel is discrete and it can be in two states: open or closed. Current surges are due to the simultaneous opening of 2 or 3 channels. Transitions between the states of the ion channel occur at random times and obey statistical patterns. It cannot be said that this ion channel will open exactly at this moment in time. One can only make a statement about the probability of opening a channel in a certain time interval.
Ion channels are described by characteristic lifetimes of the open and closed states.
4. Dependence of the channel parameters on the membrane potential. The ion channels of nerve fibers are sensitive to the membrane potential, for example, the sodium and potassium channels of the squid axon. This is manifested in the fact that after the beginning of membrane depolarization, the corresponding currents begin to change with one or another kinetics. In the language of "ion channels" this process occurs as follows. The ion-selective channel has a so-called
"sensor" - some element of its design, sensitive to the action electric field(see picture). When the membrane potential changes, the value of the force acting on it changes, as a result, this part of the ion channel moves and changes the probability of opening or closing the "gates" - a kind of shutters operating according to the "all or nothing" law.
Structure of the ion channel
The ion-selective channel consists of the following parts of the protein part immersed in the bilayer, which has a subunit structure; a selective filter formed by negatively charged oxygen atoms, which are rigidly located at a certain distance from each other and pass ions of only a certain diameter; gate part.
The “gates” of the ion channel are controlled by the membrane potential and can be either in the closed state (dashed line) or in the open state (solid line). The normal position of the sodium channel gate is closed. Under the action of an electric field, the probability of an open state increases, the gate opens and the flow of hydrated ions gets the opportunity to pass through the selective filter.
If the ion “fits” in diameter, then it sheds the hydration shell and jumps to the other side of the ion channel. If the ion is too large in diameter, such as tetraethylammonium, it is unable to pass through the filter and cannot cross the membrane. If, on the contrary, the ion is too small, then it has difficulties in the selective filter, this time due to the difficulty in shedding its hydration shell. For a “suitable” ion, the discharged water is replaced by bonds with oxygen atoms located in the filter; for an “inappropriate” ion, the steric correspondence is worse. Therefore, it is more difficult for it to pass through the filter and the conductivity of the channel is lower for it.
Ion channel blockers either cannot pass through it, getting stuck in the filter, or, if they are large molecules like TTX, they sterically match some channel entrance. Since blockers carry a positive charge, their charged part is drawn into the channel to the selective filter as an ordinary cation, and the macromolecule clogs it.
Thus, changes in the electrical properties of excitable biomembranes are carried out using ion channels. These are protein macromolecules penetrating the lipid bilayer, which can be in several discrete states. The properties of channels that are selective for potassium, sodium, and calcium ions can depend differently on the membrane potential, which determines the dynamics of the action potential in the membrane, as well as the differences in such potentials in the membranes of different cells.
Conclusion
Any molecule can pass through the lipid bilayer, but the rate of passive diffusion of substances, i.e. the transition of a substance from an area with a higher concentration to an area with a lower one can be very different. For some molecules, this takes such a long time that one can speak of their practical impermeability to the lipid bilayer of the membrane. The rate of diffusion of substances across a membrane depends mainly on the size of the molecules and their relative solubility in fats.
Small non-polar molecules such as O2, steroids, thyroid hormones, and fatty acids pass through the lipid membrane most easily by simple diffusion. Small polar uncharged molecules - CO2, NH3, H2O, ethanol, urea - also diffuse at a fairly high speed. Diffusion of glycerol is much slower, and glucose is practically unable to pass through the membrane on its own. For all charged molecules, regardless of size, the lipid membrane is impermeable.
The transport of such molecules is possible due to the presence in the membranes of either proteins that form channels (pores) filled with water in the lipid layer, through which substances of a certain size can pass by simple diffusion, or specific carrier proteins that, selectively interacting with certain ligands, facilitate their transfer through membrane (facilitated diffusion).
In addition to passive transport of substances, there are proteins in cells that actively pump certain substances dissolved in water against their gradient, i.e. from a lower concentration to a higher concentration. This process, called active transport, is always carried out with the help of carrier proteins and occurs with the expenditure of energy.
The outer part of the canal is relatively accessible for study, the study of the inner part presents significant difficulties. P. G. Kostyuk developed a method of intracellular dialysis, which makes it possible to study the function of the input and output structures of ion channels without the use of microelectrodes. It turned out that the part of the ion channel open to the extracellular space differs in its functional properties from the part of the channel facing the intracellular environment.
It is ion channels that provide two important properties of the membrane: selectivity and conductivity.
The selectivity, or selectivity, of the channel is provided by its special protein structure. Most of the channels are electrically controlled, i.e. their ability to conduct ions depends on the magnitude of the membrane potential. The channel is heterogeneous in its functional characteristics, especially for protein structures located at the entrance to the channel and at its exit (the so-called gate mechanisms).
Fick's equation
The “–” sign shows that the total density of the substance flux during diffusion is directed towards a decrease in density, D is the diffusion coefficient. The formula shows that the flux density of the substance J is proportional to the diffusion coefficient D and the concentration gradient. This equation expresses Fick's first law (Adolf Fick is a German physiologist who established the laws of diffusion in 1855).
The ion-selective channel consists of the following parts of the protein part immersed in the bilayer, which has a subunit structure; a selective filter formed by negatively charged oxygen atoms, which are rigidly located at a certain distance from each other and pass ions of only a certain diameter; gate part. It is ion channels that provide two important properties of the membrane: selectivity and conductivity. Calcium channels play an essential role in heart cells.
Bibliography
2. Yu. I. Afanasiev, N. A. Yurina, E. F. Kotovsky et al. Histology. M.
4. Filippovich Yu.B. Fundamentals of biochemistry. M., graduate School, 1985. Diffusion
5. Basniev K. S., Kochina N. I., Maksimov M. V. Underground hydromechanics. // M.: Nedra, 1993, p. 41-43
6. Gennis R. Biomembranes. Molecular structure and functions. M., Mir, 1997
