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 options for answers to questions that I would like to receive from students.
Objectives: 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; improving the skills of logical thinking.
Equipment:physical map of the Earth, test tubes, glasses, capillary tubes; table salt, ethyl alcohol, sucrose, vegetable oil, paraffin, egg white, gastric juice, ice; reference books in physics and chemistry.
Organizing time
The teacher informs students about the topic and purpose of the lesson and how to conduct it.
Knowledge check 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.
Work at the blackboard
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) , macrocells, microelements. Indicate their percentage in the cell.
(Students answer: a) organogenic: N, H, C, O; b) macroelements: Ca, Cl, Na, Mn, P, K, Fe, Mg; c) trace elements: F, Zn, J, Cu, Co).
2. To characterize 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 deficiency of any of the necessary elements (organogenic, macroelement, microelement) affect the vital activity of a cell, an organism? How can this be manifested? Give examples.
Option 2 What conclusion can be drawn from the fact that cells have a similar elemental and chemical (molecular) composition?
Option 3What scientific significance do the data on the similarities and differences in the elemental composition (qualitative and quantitative) of animate and inanimate nature have?
Learning New Material
The water content in cells and organisms
1. Read the poetic lines of Mikhail Dudnik and say whether they are true from a biological point of view. (The poem is written on the board.)
They say that eighty percent of the water is made up of people,
From the water, I’ll add, its native rivers,
From the water, I’ll add, it’s rains that they watered him,
From water, I add, from ancient water, springs.
Of which his grandfathers and great-grandfathers drank ...
(Student response. Poetic lines are correct, because more than 2/3 of the people are water.)
2. Looking at a physical map, remember what is the ratio of land to the oceans on our planet.
(Student response. Oceans, i.e. water surrounding continents and islands occupies about 71% of the earth’s surface.)
Teacher commentary. Water not only covers most of the earth’s surface, but also makes up most of all living things: microorganisms, plants, animals, humans.
3. Is water important in human life?
(Student response. A person drinks water, washes it, uses it in various industries, in agriculture. Now many countries of the world lack fresh water, to get it you have to build special plants, treatment facilities.)
Teacher commentary. Water, such a familiar substance, has completely amazing properties. Only thanks to these properties of water, 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 significance of water for biological systems is due even to its mere 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, anatomy and human physiology.
(Student response. Water makes up 80% of the cell mass in the young human or animal body and 60% in the old cells. In the cells of the brain, its 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 enamel cells of her teeth only 10-15%. There is a lot of water in the cells of the pulp of juicy fruits and leaves of plants, but it is very small 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 irrigated 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 intense.)
Water intake in animal and plant organisms
What do you know how to get water from 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, the 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 generated in the body during the oxidation of mainly fats (during the oxidation of 1 g of fat 1.1 g of water is formed); the 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 using root hairs;
e) unusual ways of obtaining 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 leaves, from where it is gradually absorbed through the epidermis.
Molecule structure and water properties
Numerous biological functions performed by water are provided by its unique properties, and the uniqueness of the properties of water is determined by the structure of its molecule.
1. Remember the structural features of a water molecule known to you from a 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 with an oxygen atom at one of its vertices and a hydrogen atom at the other two.)
2. What is the nature of the covalent bond between an oxygen atom and hydrogen atoms?
(Student response. The connection between the oxygen atom and the hydrogen atoms is polar, because oxygen attracts electrons more than hydrogen.)
Teacher commentary. Indeed, an 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. On the whole, the water molecule is electrically neutral, but the electric charge inside the molecule is not evenly distributed, and a positive charge prevails in the region of hydrogen atoms, and a negative charge prevails in the region where oxygen is located (Fig. 1). Therefore, such a molecule is an electric dipole.
Fig. 1. A water molecule in which one oxygen atom is covalently bonded to two hydrogen atoms. Molecule is polar
A negatively charged oxygen atom of one water molecule attracts positively charged hydrogen atoms of two other molecules, so water molecules turn out to be connected by hydrogen bonds. You are already familiar with the concept of hydrogen bonding (Fig. 2).
Fig. 2. Hydrogen bonds (lines) between water molecules; oxygen atoms (white circles) carry partial negative charges, therefore they form hydrogen bonds with hydrogen atoms (black circles) of other molecules carrying partial positive charges
In liquid water, these weak bonds are rapidly formed and are also quickly destroyed by random collisions of molecules. Due to the ability of water molecules to bind to each other using hydrogen bonds, water has a number of properties that are important for life.
Student group assignments
The class is divided into five groups, each of which, using pre-prepared equipment, works according to the instruction card containing the assignment.
Task 1st group
You are offered a number of substances: table salt, ethyl alcohol, sucrose, vegetable oil, paraffin. Try to sequentially dissolve these substances in water. Which of the proposed 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 get acquainted with?
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 watching? What reaction has occurred and thanks to which enzyme of gastric juice? What water property did you meet?
Task 3rd group
Dip the pieces of ice into a glass of water. What are you watching? What can you say about the density of water and ice? Specific information on the density of water and ice can be obtained from the Handbook of Elementary Physics (Enokhovich). What features of water have you met?
Task to the 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 the results.
Task 5th group
Try pouring water into the top glass. Why is this possible? In a glass of water, slowly lower the glass tube of small diameter. What are you watching? Explain the results of the experiment. What water property did you meet?
Group 1 Report
Dissolved in water from the proposed substances: table salt, ethyl alcohol, sucrose (cane sugar). Not soluble: vegetable oil and paraffin. From the results obtained, it can be concluded that substances with an ionic chemical bond (sodium chloride), as well as nonionic compounds (sugars, alcohols), in the molecules of which there are probably charged (polar) groups, dissolve in water. Water is one of the most versatile solvents: almost all substances dissolve in it, even in trace amounts.
Teacher commentary. If the energy of attraction between the molecules of water and the molecules of a substance is greater than the energy of attraction between the molecules of water, then the substance dissolves. Water-soluble substances are called hydrophilic (salts, alkalis, acids, etc.). Nonpolar (non-charge-carrying) compounds practically do not dissolve in water. They are called hydrophobic (fats, fat-like substances, rubber, etc.).
Group 2 Report
Insoluble flakes of egg white under the action of pepsin of gastric juice dissolve. Enzymatic hydrolysis (cleavage) of proteins to amino acids takes place with the addition of a water molecule when each peptide bond is broken. 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 medium in which all biochemical processes occur. Easily penetrating the membranes and membranes of cells, water freely circulates throughout the plant, providing transport of substances and thereby 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, 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 radicals of molecules of organic substances. Especially easily hydrogen bonds form such radicals:
This form of water is called constitutional . It is contained in a cell with a strength of up to 90 thousand barrels.
Due to the fact that water molecules are dipoles, they form integral aggregates with charged molecules of organic substances. Such water, associated with molecules of organic substances of the cytoplasm by the forces of electric attraction, is called solvate . Depending on the type of plant cell, solvate water accounts for 4 to 50% of its total amount. Solvate water, like constitutional water, has no mobility and is not a solvent.
A significant portion of the water in the cell is capillary , since it is placed in the cavities between the 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 held by the cell with a force that is determined by the magnitude of the osmotic potential of the vacuolar content.
Water absorption by plant cells
Since there are no active carriers for water molecules in the 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, the concentration gradients of dissolved substances are the main engines 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 . During seed germination, it begins to absorb water through the mechanism of imbibition. In this case, the vacant hydrogen bonds of the protoplast organic substances are filled, and water actively enters the cell from the environment. Compared to other forces acting in the cells, the imbibition forces are colossal. For some hydrogen bonds, they reach a value of 90 thousand barrels. At the same time, seeds can swell and germinate in relatively dry soils. After filling all the vacant hydrogen bonds, the imbibition stops and the next mechanism of water absorption is activated.
Solvation . In the process of solvation, water absorption occurs by building hydration layers around the molecules of organic substances of the protoplast. The total water content of the cell continues to increase. The intensity of solvation substantially depends on the chemical composition of the protoplast. The more hydrophilic substances in the cell, the more fully solvation forces are used. Hydrophilicity decreases in the series: proteins -\u003e carbohydrates -\u003e fats. Therefore, the largest amount of water per unit weight by solvation absorbs protein seeds (peas, beans, beans), the intermediate - starchy (wheat, rye), and the smallest - oilseeds (flax, sunflower).
The solvation forces are inferior in power to the forces of imbibition, 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 is assimilated, vacuoles begin to appear. However, from the moment of their formation, solvation ceases, and further absorption of water is possible only due to the osmotic mechanism.
Osmosis . The osmotic mechanism of water absorption only works in cells that have a vacuole. The direction of water movement is determined by the ratio of the osmotic potentials of the solutions included in the osmotic system.
The osmotic potential of cell juice is indicated by R, determined by the formula:
R = iRcT,
where R - osmotic potential of cell juice
R - gas constant equal to 0.0821;
T - Kelvin temperature
i - isotonic coefficient indicating the nature of the electrolytic dissociation of dissolved substances.
The isotonic coefficient itself is equal to
and \u003d 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 large osmotic potential. So, if the cell is in the soil (external) solution at P\u003e π, then water enters the cells. The flow of water into the cell ceases when the osmotic potentials are completely equalized (the vacuolar juice is diluted at the inlet of water absorption) or when the cell membrane reaches its extensibility limits.
Thus, cells receive water from the environment only under one condition: the osmotic potential of cell juice 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. During water loss, the protoplast volume gradually decreases, it moves away from the membrane, and small cavities appear in the cell. This condition is called Plasmolysis . The stages of plasmolysis are shown in Fig. 3.18.
If the ratio of osmotic potentials corresponds to the condition P \u003d π, diffusion of water molecules does not occur at all.
A large amount of factual evidence indicates that the osmotic potential of the plant cell sap varies widely. 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.
Fig. 3.18.
A - a cell in a state of turgor; B - angular; B - concave; G is convex; D - convulsive; E - cap. 1 - shell; 2 - vacuole; 3 - cytoplasm; 4 - core; 5 - Hecht threads
The water content in various organs of plants varies widely. It varies depending on environmental conditions, age and type of plants. So, the water content in lettuce is 93-95%, corn - 75-77%. The amount of water varies in different organs of plants: in the leaves of sunflower water contains 80-83%, in the stems - 87-89%, in the roots - 73-75%. A water content of 6–11% is characteristic mainly of air-dried seeds, in which vital processes are inhibited.
Water is contained in living cells, in the dead elements of xylem and in intercellular spaces. In the intercellular spaces, water is in a vaporous state. The main evaporative organs of the plant are leaves. In this regard, it is natural that the greatest amount of water fills the intercellular spaces of the leaves. In the liquid state, water is in various parts of the cell: the cell membrane, vacuoles, and 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. Quantifying the 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. In vacuolar cell juice, water predominates, retained by relatively low molecular weight compounds (osmotically bound) and free water. In the membrane of a plant cell, water is bound mainly by high polymer compounds (cellulose, hemicellulose, pectin substances), i.e., colloidal bound water. In the cytoplasm itself there is free water, colloidal and osmotic bound. Water located at a distance of 1 nm from the surface of the protein molecule is firmly bound and does not have the correct hexagonal structure (colloid-bound water). In addition, in the cytoplasm there is a certain amount of ions, and, therefore, part of the water is osmotically connected.
The physiological significance of free and bound water is different. According to most researchers, the intensity of 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 to adverse environmental conditions. These physiological correlations are not always observed.
For their normal existence, cells and the plant organism as a whole must contain a certain amount of water. However, this is easily practicable only for plants growing in water. For land plants, this task is complicated by the fact that water in the plant body is continuously lost during the evaporation process. The evaporation of water by a plant reaches enormous proportions. You can give an example: one corn plant evaporates up to 180 kg of water during the growing season, and 1 ha of forest in South America evaporates an average of 75 thousand kg of water per day. The huge flow of water is due to the fact that most plants have a significant leaf surface in the atmosphere, not saturated with water vapor. At the same time, the development of a vast leaf surface is necessary and developed during a long evolution to ensure a normal supply of carbon dioxide contained in air at an insignificant concentration (0.03%). In his famous book “Plant Control with Drought,” K.A. Timiryazev pointed out that the contradiction between the need to capture carbon dioxide and reduce the consumption of water left an imprint on the structure of the whole plant organism.
In order to compensate for the loss of water during evaporation, a large quantity of it must continuously enter the plant. Two processes that continuously go on in a plant — intake and evaporation of water — are called water balance of plants.For the normal growth and development of plants, it is necessary that the water flow approximately corresponds to the income, or, in other words, that the plant reduces its water balance without a large deficit. To do this, adaptations to water absorption (colossally developed root system), to the movement of water (special conductive system), to the reduction of evaporation (the system of integumentary tissues and the system of automatically closed stomatal openings) were developed in the plant during natural selection.
Despite all these adaptations, a water deficit is often observed in the plant, i.e., the flow of water is not balanced by its expenditure during transpiration.
Physiological disorders occur in different plants with varying degrees of water deficiency. There are plants that have developed in the process of evolution various adaptations for the transfer of dehydration (drought-resistant plants). Clarification of the physiological characteristics that determine the resistance of plants to lack of water is the most important task, the resolution of which is of great theoretical and agricultural practical importance. At the same time, in order to solve it, knowledge of all aspects of the water exchange of the plant organism is necessary.
Properties of water and its role in the cell:
In the first place among the substances of the cell is water. It makes up about 80% of the cell mass. Water is doubly important for living organisms, because it is necessary not only as a component of cells, but for many and as a habitat.
1. Water determines the physical properties of a cell - its volume, elasticity.
2. Many chemical processes occur only in aqueous solution.
3. Water is a good solvent: many substances enter the cell from the external environment in an aqueous solution, and in the aqueous solution, waste products are removed from the cell.
4. Water has a high heat capacity and thermal conductivity.
5. Water has a unique property: when it is cooled from +4 to 0 degrees, it expands. Therefore, ice is lighter than liquid water and remains on its surface. This is very important for aquatic organisms.
6. Water can be a good lubricant.
The biological role of water is determined by the small size of its molecules, their polarity and ability to connect with each other by hydrogen bonds.
Biological functions of water:
transport. Water provides the movement of substances in the cell and the body, the absorption of substances and the excretion of metabolic products. In nature, water carries waste products to the soil and to water bodies.
metabolic. Water is the medium for all biochemical reactions, an electron donor in photosynthesis; it is necessary for the hydrolysis of macromolecules to their monomers.
water is involved in the formation of lubricating fluids and mucus, secrets and juices in the body.
With very few exceptions (bone and tooth enamel), water is the predominant component of the cell. Water is necessary for the metabolism (exchange) of the cell, since physiological processes occur exclusively in the aquatic environment. Water molecules are involved in many enzymatic reactions of the cell. 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.
Water serves as a source of hydrogen ions in photosynthesis. Water in the cell is in two forms: free and bound. Free water makes up 95% of all water in the cell and is used mainly as a solvent and as a dispersion medium of the colloidal system of protoplasm. Bound water, which accounts for only 4% of all cell water, is loosely coupled to proteins by hydrogen bonds.
Due to the asymmetric charge distribution, the water molecule acts as a dipole and therefore can be connected both positively and negatively by protein groups. The dipole property of a water molecule explains its ability to navigate in an electric field, to attach to various molecules and sections of molecules that carry a charge. As a result, hydrates are formed.
Due to its high heat capacity, water absorbs heat and thereby prevents sudden temperature fluctuations in the cell. The water content in the body depends on its age and metabolic activity. It is highest in the embryo (90%) and gradually decreases with age. The water content in various tissues varies depending on their metabolic activity. For example, in the gray matter of the brain, water is up to 80%, and in bones up to 20%. Water is the main means of moving substances in the body (blood flow, lymph, ascending and descending currents of solutions through the vessels in plants) and in the cell. Water serves as a “lubricant” material, necessary wherever there are rubbing surfaces (for example, in joints). Water has a maximum density at 4 ° C. Therefore, ice with a lower density is lighter than water and floats on its surface, which protects the pond from freezing. This property of water saves the life of many aquatic organisms.