1. What is the structure of water?
Answer. The water molecule has an angular structure: its constituent nuclei form isosceles triangle, which has two hydrogens at its base and an oxygen atom at its apex. Internuclear O-N distances close to 0.1 nm, the distance between the nuclei of hydrogen atoms is 0.15 nm. Of the six electrons that make up the outer electron layer of the oxygen atom in the water molecule, two electron pairs form covalent O-N connections, and the remaining four electrons are two unshared electron pairs.
The water molecule is a small dipole containing positive and negative charges at the poles. Near the hydrogen nuclei there is a lack of electron density, and on the opposite side of the molecule, near the oxygen nucleus, there is an excess of electron density. It is this structure that determines the polarity of the water molecule.
2. What is the amount of water (in%) contained in different cells?
The amount of water varies in different tissues and organs. So, in a person in the gray matter of the brain, its content is 85%, and in the bone tissue - 22%. The highest water content in the body is observed in the embryonic period (95%) and gradually decreases with age.
The water content in various plant organs varies within fairly wide limits. It changes depending on the conditions. external environment, age and species of plants. Thus, the water content in lettuce leaves is 93-95%, corn - 75-77%. The amount of water is not the same in different organs of plants: sunflower leaves contain 80-83% of water, stems - 87-89%, roots - 73-75%. The water content, equal to 6-11%, is typical mainly for air-dry seeds, in which vital processes are inhibited. Water is contained in living cells, in the dead elements of the xylem and in the intercellular spaces. In the intercellular spaces, water is in a vapor state. Leaves are the main evaporating organs of a plant. In this regard, it is natural that the largest number water fills the intercellular spaces of the leaves. In a liquid state, water is found in various parts of the cell: cell membrane, vacuole, cytoplasm. Vacuoles are the most water-rich part of the cell, where its content reaches 98%. At the highest water content, the water content in the cytoplasm is 95%. The lowest water content is characteristic of cell membranes. Quantitative determination of water content in cell membranes is difficult; apparently, it ranges from 30 to 50%. Shapes of water in different parts plant cell are also different.
3. What is the role of water in living organisms?
Answer. Water is the predominant component of all living organisms. It has unique properties due to structural features: water molecules have the form of a dipole and hydrogen bonds form between them. The average water content in the cells of most living organisms is about 70%. Water in the cell is present in two forms: free (95% of all cell water) and bound (4-5% associated with proteins).
Water functions:
1. Water as a solvent. Many chemical reactions in the cell are ionic, therefore, they occur only in the aquatic environment. Substances that dissolve in water are called hydrophilic (alcohols, sugars, aldehydes, amino acids), insoluble - hydrophobic (fatty acids, cellulose).
2. Water as a reagent. Water is involved in many chemical reactions: polymerization reactions, hydrolysis, in the process of photosynthesis.
3. Transport function. Movement through the body along with water of substances dissolved in it to its various parts and the removal of unnecessary products from the body.
4. Water as a heat stabilizer and thermostat. This function is due to such properties of water as high heat capacity - it softens the effect on the body of significant temperature changes in the environment; high thermal conductivity - allows the body to maintain the same temperature throughout its volume; high heat of evaporation - used to cool the body during sweating in mammals and transpiration in plants.
5. Structural function. The cytoplasm of cells contains from 60 to 95% water, and it is she who gives the cells their normal shape. In plants, water maintains turgor (the elasticity of the endoplasmic membrane), in some animals it serves as a hydrostatic skeleton (jellyfish)
Questions after § 7
1. What is the peculiarity of the structure of the water molecule?
Answer. The unique properties of water are determined by the structure of its molecule. The water molecule consists of an O atom bonded to two H atoms by polar covalent bonds. The characteristic arrangement of electrons in a water molecule gives it an electrical asymmetry. The more electronegative oxygen atom attracts the electrons of the hydrogen atoms more strongly, as a result, the common pairs of electrons in the water molecule are shifted towards it. Therefore, although the water molecule as a whole is not charged, each of the two hydrogen atoms has a partially positive charge (denoted 8+), and the oxygen atom carries a partially negative charge (8-). The water molecule is polarized and is a dipole (has two poles).
The partially negative charge of the oxygen atom of one water molecule is attracted by the partially positive hydrogen atoms of other molecules. Thus, each water molecule tends to hydrogen bond with four adjacent water molecules.
2. What is the importance of water as a solvent?
Answer. Due to the polarity of molecules and the ability to form hydrogen bonds, water easily dissolves ionic compounds (salts, acids, bases). Some non-ionic but polar compounds also dissolve well in water, i.e., in the molecule of which there are charged (polar) groups, such as sugars, simple alcohols, amino acids. Substances that are highly soluble in water are called hydrophilic (from the Greek hygros - wet and philia - friendship, inclination). When a substance goes into solution, its molecules or ions can move more freely and, therefore, the reactivity of the substance increases. This explains why water is the main medium in which most chemical reactions take place, and all hydrolysis reactions and numerous redox reactions take place with the direct participation of water.
Substances that are poorly or completely insoluble in water are called hydrophobic (from the Greek phobos - fear). These include fats, nucleic acids, some proteins, and polysaccharides. Such substances can form interfaces with water, on which many chemical reactions take place. Therefore, the fact that water does not dissolve non-polar substances is also very important for living organisms. Among the physiologically important properties of water is its ability to dissolve gases (O2, CO2, etc.).
3. What is the thermal conductivity and heat capacity of water?
Answer. Water has a high heat capacity, i.e., the ability to absorb thermal energy with a minimal increase in its own temperature. The high heat capacity of water protects the tissues of the body from a rapid and strong increase in temperature. Many organisms cool themselves by evaporating water (transpiration in plants, sweating in animals).
4. Why is it considered that water is an ideal liquid for a cell?
Answer. High water content in the cell - essential condition her activities. With the loss of most of the water, many organisms die, and a number of unicellular and even multicellular organisms temporarily lose all signs of life. This state is called suspended animation. After hydration, the cells wake up and become active again.
The water molecule is electrically neutral. But electric charge inside the molecule it is unevenly distributed: in the region of hydrogen atoms (more precisely, protons), a positive charge predominates, in the region where oxygen is located, the negative charge density is higher. Therefore, a particle of water is a dipole. The dipole property of a water molecule explains its ability to orient itself in an electric field, to attach to various molecules and sections of molecules that carry a charge. As a result, hydrates are formed. The ability of water to form hydrates is due to its universal dissolving properties. If the energy of attraction of water molecules to the molecules of a substance is greater than the energy of attraction between water molecules, then the substance dissolves. Depending on this, hydrophilic (Greek hydros - water and phileo - love) substances that are highly soluble in water (for example, salts, alkalis, acids, etc.) and hydrophobic (Greek hydros - water and phobos - fear) substances are distinguished, hardly or not at all soluble in water (fats, fat-like substances, rubber, etc.). The composition of cell membranes includes fat-like substances that limit the transition from the external environment to cells and vice versa, as well as from one part of the cell to another.
Most of the reactions that take place in a cell can only take place in an aqueous solution. Water is a direct participant in many reactions. For example, the breakdown of proteins, carbohydrates and other substances occurs as a result of their interaction with water catalyzed by enzymes. Such reactions are called hydrolysis reactions (Greek hydros - water and lysis - splitting).
Water has a high heat capacity and at the same time relatively high thermal conductivity for liquids. These properties make water an ideal liquid for maintaining the thermal balance of the cell and organism.
Water is the main environment for the flow of biochemical reactions of the cell. It is a source of oxygen released during photosynthesis, and hydrogen, which is used to restore the products of carbon dioxide assimilation. And finally, water is the main means of transporting substances in the body (blood and lymph flow, ascending and descending currents of solutions through the vessels of plants) and in the cell.
5. What is the role of water in the cell
Ensuring cell elasticity. The consequences of the loss of water by the cell are wilting of leaves, drying of fruits;
Acceleration of chemical reactions due to the dissolution of substances in water;
Ensuring the movement of substances: the entry of most substances into the cell and their removal from the cell in the form of solutions;
Ensuring the dissolution of many chemical substances(a number of salts, sugars);
Participation in a number of chemical reactions;
Participation in the process of thermoregulation due to the ability to slow heating and slow cooling.
6. What are the structural and physiochemical properties water determine its biological role in the cell?
Answer. Structural physical and chemical properties of water determine its biological functions.
Water is a good solvent. Due to the polarity of molecules and the ability to form hydrogen bonds, water easily dissolves ionic compounds (salts, acids, bases).
Water has a high heat capacity, i.e., the ability to absorb thermal energy with a minimal increase in its own temperature. The high heat capacity of water protects the tissues of the body from a rapid and strong increase in temperature. Many organisms cool themselves by evaporating water (transpiration in plants, sweating in animals).
Water also has a high thermal conductivity, ensuring an even distribution of heat throughout the body. Consequently, the high specific heat capacity and high thermal conductivity make water an ideal liquid for maintaining the thermal equilibrium of the cell and organism.
Water practically does not compress, creating turgor pressure, determining the volume and elasticity of cells and tissues. So, it is the hydrostatic skeleton that maintains the shape of roundworms, jellyfish and other organisms.
Water is characterized by the optimal value of the surface tension force for biological systems, which arises due to the formation of hydrogen bonds between water molecules and molecules of other substances. Due to the force of surface tension, capillary blood flow occurs, ascending and descending currents of solutions in plants.
In certain biochemical processes, water acts as a substrate.
1.3 Distribution of water in the cell
The water content in various plant organs varies within fairly wide limits. It varies depending on environmental conditions, age and type of plants. Thus, the water content in lettuce leaves is 93-95%, corn - 75-77%. The amount of water is not the same in different plant organs: sunflower leaves contain 80-83% of water, stems - 87-89%, roots - 73-75%. The water content, equal to 6-11%, is typical mainly for air-dry seeds, in which vital processes are inhibited.
Water is contained in living cells, in the dead elements of the xylem and in the intercellular spaces. In the intercellular spaces, water is in a vapor state. Leaves are the main evaporating organs of a plant. In this regard, it is natural that the largest amount of water fills the intercellular spaces of the leaves. In a liquid state, water is found in various parts of the cell: the cell membrane, vacuoles, and protoplasm. Vacuoles are the most water-rich part of the cell, where its content reaches 98%. At the highest water content, the water content in the protoplasm is 95%. The lowest water content is characteristic of cell membranes. Quantitative determination of water content in cell membranes is difficult; apparently, it ranges from 30 to 50%.
The forms of water in different parts of the plant cell are also different. The vacuolar cell sap is dominated by water retained by relatively low molecular weight compounds (osmotically bound) and free water. In the shell of a plant cell, water is mainly bound by high-polymer compounds (cellulose, hemicellulose, pectin substances), i.e., colloidally bound water. In the cytoplasm itself there is free water, colloidally and osmotically bound. Water located at a distance of up to 1 nm from the surface of a protein molecule is firmly bound and does not have a regular hexagonal structure (colloidal-bound water). In addition, there is a certain amount of ions in the protoplasm, and, consequently, part of the water is osmotically bound.
The physiological significance of free and bound water is different. Most researchers believe that the intensity physiological processes, including growth rates, depends primarily on the content of free water. There is a direct correlation between the content of bound water and the resistance of plants against adverse external conditions. These physiological correlations are not always observed.
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The lesson is designed for 80-90 minutes. The theme of the lesson allows students to demonstrate the relationship of such subjects as biology, geography, chemistry, physics. In parentheses are answers to questions that I would like to receive from students.
Goals: familiarization of students with data on the water content in the cells of various tissues and water metabolism in different organisms, with modern ideas about the structure and properties of water, its biological functions; improvement of logical thinking skills.
Equipment: physical map of the Earth, test tubes, glasses, capillary tubes; salt, ethyl alcohol, sucrose, vegetable oil, paraffin, egg white, gastric juice, ice; reference books on physics and chemistry.
Organizing time
The teacher tells the students the topic and objectives of the lesson and the order of its conduct.
Check of knowledge students on the topic "Elemental and chemical (molecular) composition of the cell." Three students work at the blackboard, the rest (according to options) work on cards.
Whiteboard work
1. A list of elements is written on the board: F, Zn, N, Ca, J, Cl, Na, H, Mn, Cu, P, C, K, Fe, O, Mg, Co, from which you need to choose organogenic (biogenic) , macroelements, microelements. Indicate the percentage of them in the cell.
(Students response: a) organogenic: N, H, C, O; b) macronutrients: Ca, Cl, Na, Mn, P, K, Fe, Mg; c) trace elements: F, Zn, J, Cu, Co).
2. Give a description of organogenic elements. Explain why, during the development of life on Earth, these elements turned out to be “convenient” for the chemistry of life.
3. Write on the board information about the chemical (molecular) composition of the cell, indicating the percentage of the main classes of substances.
Card work
Answer the question in writing.
Option 1. How does the lack of any of the necessary elements (organogenic, macroelement, microelement) affect the vital activity of a cell or organism? How can this manifest itself? Give examples.
Option 2. What conclusion can be drawn from the fact that cells have a similar elemental and chemical (molecular) composition?
Option 3. What is the scientific significance of data on the similarities and differences in the elemental composition (qualitative and quantitative) of living and inanimate nature?
Learning new material
Water content in cells and organisms
1. Read the poetic lines of Mikhail Dudnik and say if they are correct from a biological point of view. (The poem is written on the blackboard.)
They say that eighty percent of the water is man,
From the water, I will add, his native rivers,
From the water, I will add, - the rains that they gave him to drink,
From water, I will add, from ancient water, springs.
From which his grandfathers and great-grandfathers drank ...
(Student response. The poetic lines are correct, because more than 2/3 of the person consists of water.)
2. Looking at physical map, remember what is the ratio of land and ocean areas on our planet.
(Student response. World Ocean, i.e. the water surrounding the continents and islands occupies about 71% of the earth's surface.)
Teacher's comment. Water not only covers most of the earth's surface, but also makes up the majority of all living beings: microorganisms, plants, animals, humans.
3. Is the importance of water in human life great?
(Student response. A person drinks water, washes with it, uses it in various industries, in agriculture. Many countries in the world are currently experiencing a shortage fresh water, to obtain it, it is necessary to build special plants, treatment facilities.)
Teacher's comment. Water, such a familiar substance, has absolutely amazing properties. It is only thanks to these properties of water that life on Earth became possible. When looking for life on other planets, one of the most important questions is whether there is enough water there. The unique importance of water for biological systems is even due to its quantitative content in living organisms.
4. Give examples of the water content in the cells of different organisms, their tissues and organs, known to you from the courses of botany, zoology, human anatomy and physiology.
(Student response. Water makes up 80% of the mass of a cell in a young human or animal body and 60% in the cells of an old one. In the cells of the brain, it is 85%, and in the cells of the developing embryo - 90%. If a person loses 20% of water, then death occurs. True, not all human cells have such a high water content. Say, in the cells of tooth enamel it is only 10-15%. There is a lot of water in the cells of the pulp of succulent fruits and plant leaves, but there is very little of it in the cells of dry seeds or spores of plants and microorganisms, so they can be stored for a very long time until they are again watered under conditions conducive to their germination.)
5. What determines the differences in the water content in cells?
(Student response. There is more water in those cells in which the metabolism is more intensive.)
The flow of water into the organisms of animals and plants
What do you know about the ways in which water is obtained by different organisms?
(Student response. The ways in which water enters the body are very diverse:
a) through the surface of the body - in unicellular organisms, lower plants, larvae of some insects, frogs, fish and other aquatic organisms;
b) with food and drink - in most animals;
c) there are animals that hardly drink or drink very little. This is possible due to: metabolic water, i.e. water formed in the body during the oxidation, mainly of fats (when 1 g of fat is oxidized, 1.1 g of water is formed); economical use of water, which in some is ensured by the presence of waterproof covers, in others by a high concentration of urine (for example, in camels, urine is 8 times more concentrated than plasma); water reserves (for example, in larvae);
d) plants absorb water from the soil with the help of root hairs;
e) unusual ways receive water have: epiphytes - plants that settle mainly on the trunks, branches of other trees - absorb water from the air; many umbrella plants retain moisture in the cup-shaped sheaths of the leaves, from where it is gradually absorbed through the epidermis.
The structure of the molecule and the properties of water
Numerous biological functions performed by water are provided by its unique properties, and the uniqueness of water properties is determined by the structure of its molecule.
1. Recall the structural features of the water molecule known to you from the chemistry course.
(Student response. In a water molecule (empirical formula H 2 O), one oxygen atom is covalently bonded to two hydrogen atoms. The molecule has the shape of a triangle, at one of the vertices of which there is an oxygen atom, and at the other two - by a hydrogen atom.)
2. What is the character covalent bond between an oxygen atom and a hydrogen atom?
(Student response. The bond between the oxygen atom and the hydrogen atoms is polar, because oxygen attracts electrons more strongly than hydrogen.)
Teacher's comment. Indeed, the oxygen atom, due to its greater electronegativity, attracts electrons more strongly than hydrogen atoms. The consequence of this is the polarity of the water molecule. In general, the water molecule is electrically neutral, but the electric charge inside the molecule is distributed unevenly, and in the region of hydrogen atoms, a positive charge prevails, and in the region where oxygen is located, a negative charge prevails (Fig. 1). Therefore, such a molecule is an electric dipole.
Rice. 1. A water molecule in which one oxygen atom is covalently bonded to two hydrogen atoms. The molecule is polar
The negatively charged oxygen atom of one water molecule attracts the positively charged hydrogen atoms of the other two molecules, so the water molecules are bound friend with each other by hydrogen bonds. You are already familiar with the concept of a hydrogen bond (Fig. 2).
Rice. 2. Hydrogen bonds (lines) between water molecules; oxygen atoms (white circles) carry partial negative charges, so they form hydrogen bonds with hydrogen atoms (black circles) of other molecules that carry partial positive charges
In liquid water, these weak bonds are quickly formed and just as quickly destroyed by random collisions of molecules. It is due to the ability of water molecules to bind to each other using hydrogen bonds that water has a number of properties that are important for life.
Tasks for groups of students
The class is divided into five groups, each of which, using pre-prepared equipment, works on an instructive card containing a task.
Task 1st group
You are offered a number of substances: table salt, ethyl alcohol, sucrose, vegetable oil, paraffin. Try to consistently dissolve these substances in water. Which of the following substances are soluble in water and which are not? Try to explain why some substances can dissolve in water, while others cannot. What property of water did you learn about?
Task 2nd group
In a test tube with white flakes of insoluble egg white, heated in a water bath to 37 ° C, add gastric juice. What are you observing? What reaction took place and thanks to what enzyme of gastric juice? What property of water are you familiar with?
Task 3rd group
Dip the ice cubes into a glass of water. What are you observing? What can you say about the density of water and ice? Specific information about the density of water and ice can be obtained from the Handbook of Elementary Physics (Enochovich). What are the features of water?
Task 4th group
You know that water boils and passes into a vapor state at a temperature of 100 °C. Using the Handbook of Elementary Physics, compare the boiling point of water with the boiling point of other liquids. Try to explain your results.
Task 5th group
Try pouring water into a topped glass. Why is this possible? Slowly lower a glass tube of small diameter into a glass of water. What are you observing? Explain the results of the experiment. What property of water are you familiar with?
Report of the 1st group
In the water of the proposed substances dissolve: table salt, ethyl alcohol, sucrose (cane sugar). Insoluble: vegetable oil and paraffin. From the results obtained, it can be concluded that substances with ionic chemical bond(salt), as well as non-ionic compounds (sugars, alcohols), in the molecules of which, probably, charged (polar) groups are present, dissolve in water. Water is one of the most versatile solvents: almost all substances dissolve in it, at least in trace amounts.
Teacher's comment. If the energy of attraction between water molecules and the molecules of a substance is greater than the energy of attraction between water molecules, then the substance dissolves. Substances soluble in water are called hydrophilic (salts, alkalis, acids, etc.). Non-polar (non-charge-bearing) compounds practically do not dissolve in water. They are called hydrophobic (fats, fat-like substances, rubber, etc.).
Report of the 2nd group
Insoluble egg white flakes dissolve under the action of gastric pepsin. There is a reaction of enzymatic hydrolysis (cleavage) of proteins into amino acids with the addition of a water molecule at the break of each peptide bond. Similar reactions occur in the gastrointestinal tract of humans and animals:
Thus, water can enter into chemical reactions, i.e. is a reagent.
The vital activity of cells, tissues and organs of plants is due to the presence of water. Water is a constitutional substance. Determining the structure of the cytoplasm of cells and its organelles, due to the polarity of the molecules, it is a solvent of organic and inorganic compounds involved in the metabolism, and acts as a background environment in which all biochemical processes occur. Easily penetrating through the shells and membranes of cells, water circulates freely throughout the plant, ensuring the transfer of substances and thus contributing to the unity of the metabolic processes of the body. Due to its high transparency, water does not interfere with the absorption of solar energy by chlorophyll.
The state of water in plant cells
Water in the cell is presented in several forms, they are fundamentally different. The main ones are constitutional, solvate, capillary and reserve water.
Some of the water molecules entering the cell form hydrogen bonds with a number of radical molecules organic matter. Hydrogen bonds are especially easy to form such radicals:
This form of water is called constitutional . It is contained by a cell with a strength of up to 90 thousand barr.
Due to the fact that water molecules are dipoles, they form solid aggregates with charged molecules of organic substances. Such water, associated with the molecules of organic substances of the cytoplasm by the forces of electrical attraction, is called solvate . Depending on the type of plant cell, the solvate water accounts for 4 to 50% of its total amount. Solvate water, like constitutional water, has no mobility and is not a solvent.
Much of the cell's water is capillary , because it is located in the cavities between macromolecules. Solvate and capillary water is held by the cell with a force called the matrix potential. It is equal to 15-150 bar.
Reserve called the water inside the vacuoles. The content of vacuoles is a solution of sugars, salts and a number of other substances. Therefore, the reserve water is retained by the cell with a force that is determined by the magnitude of the osmotic potential of the vacuolar content.
Water uptake by plant cells
Since there are no active carriers for water molecules in cells, its movement into and out of cells, as well as between neighboring cells, is carried out only according to the laws of diffusion. Therefore, solute concentration gradients turn out to be the main drivers for water molecules.
Plant cells, depending on their age and condition, absorb water using the sequential inclusion of three mechanisms: imbibition, solvation and osmosis.
imbibition . When seeds germinate, it begins to absorb water due to the imbibition mechanism. At the same time, the vacant hydrogen bonds of the organic substances of the protoplast are filled, and water is actively supplied from environment in a cell. Compared to other forces operating in cells, the imbibition forces are colossal. For some hydrogen bonds, they reach a value of 90 thousand barr. At the same time, the seeds can swell and germinate in relatively dry soils. After all vacant hydrogen bonds are filled, imbibition stops and the following mechanism of water absorption is activated.
solvation . In the process of solvation, water absorption occurs by building hydration layers around the molecules of protoplast organic substances. The total water content of the cell continues to increase. The intensity of solvation essentially depends on chemical composition protoplast. The more hydrophilic substances in the cell, the more fully the solvation forces are used. Hydrophilicity decreases in the series: proteins -> carbohydrates -> fats. Therefore, the largest amount of water per unit weight by solvation absorbs protein seeds (peas, beans, beans), intermediate - starch (wheat, rye), and the smallest - oilseeds (flax, sunflower).
The solvation forces are inferior in power to the imbibition forces, but they are still quite significant and reach 100 bar. By the end of the solvation process, the water content of the cell is so great that capillary moisture settles down, and vacuoles begin to appear. However, from the moment of their formation, solvation stops, and further absorption of water is possible only due to the osmotic mechanism.
Osmosis . The osmotic mechanism of water uptake only works in cells that have a vacuole. The direction of water movement in this case is determined by the ratio of the osmotic potentials of the solutions included in the osmotic system.
The osmotic potential of the cell sap, denoted by R, is determined by the formula:
R = iRct,
where R - osmotic potential of cell sap
R- gas constant equal to 0.0821;
T - temperature on the Kelvin scale;
i- isotonic coefficient indicating the nature of the electrolytic dissociation of dissolved substances.
The isotonic ratio itself is equal to
and= 1 + α ( n + 1),
where α - degree of electrolytic dissociation;
P - the number of ions into which the molecule dissociates. For non-electrolytes P = 1.
The osmotic potential of a soil solution is usually denoted by the Greek letter π.
Water molecules always move from a medium with a lower osmotic potential to a medium with a higher osmotic potential. So, if the cell is in the soil (external) solution at R>π, then water enters the cells. The flow of water into the cell stops when the osmotic potentials are completely equalized (the vacuolar juice is diluted at the entrance of water absorption) or when the cell membrane reaches the limits of extensibility.
Thus, cells receive water from the environment only under one condition: the osmotic potential of the cell sap must be higher than the osmotic potential of the surrounding solution.
If R< π, there is an outflow of water from the cell into the external solution. In the course of fluid loss, the volume of the protoplast gradually decreases, it moves away from the membrane, and small cavities appear in the cell. Such a state is called Plasmolysis . The stages of plasmolysis are shown in fig. 3.18.
If the ratio of osmotic potentials corresponds to the condition P = π, then diffusion of water molecules does not occur at all.
A large amount of factual material indicates that the osmotic potential of the cell sap of plants varies within fairly wide limits. In agricultural plants, in root cells, it usually lies in an amplitude of 5-10 bar, in leaf cells it can rise up to 40 bar, and in fruit cells - up to 50 bar. In solonchak plants, the osmotic potential of cell sap reaches 100 bar.
Rice. 3.18.
A - a cell in a state of turgor; B - angular; B - concave; G - convex; D - convulsive; E - cap. 1 - shell; 2 - vacuole; 3 - cytoplasm; 4 - core; 5 - Hecht threads
