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's mass. Water is doubly important for living organisms, because it is necessary not only as a component of cells, but for many also as a habitat.

1. Water determines the physical properties of the 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 external environment in an aqueous solution, and in an aqueous solution, waste products are removed from the cell.

4. Water has 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 turns out to be lighter than liquid water and remains on its surface. This is very important for organisms living in aquatic environment.

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 through hydrogen bonds.

Biological functions of water:

transport. Water ensures the movement of substances in the cell and body, the absorption of substances and the removal of metabolic products. In nature, water carries waste products into soils and water bodies.

metabolic. Water is the medium for all bio chemical reactions, electron donor during photosynthesis; it is necessary for the hydrolysis of macromolecules to their monomers.

Water is involved in the formation of lubricating fluids and mucus, secretions 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 cell metabolism (exchange), since physiological processes occur exclusively in an aqueous 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 during photosynthesis. Water in a cell is in two forms: free and bound. Free water makes up 95% of the total water in the cell and is used primarily as a solvent and as a dispersion medium. colloidal system protoplasm. Bound water, which accounts for only 4% of the total water in the cell, is loosely bonded to proteins by hydrogen bonds.

Due to the asymmetric distribution of charges, the water molecule acts as a dipole and can therefore be bound by both positively and negatively charged protein groups. The dipole property of a water molecule explains its ability to orient itself in an electric field and 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 of the body depends on its age and metabolic activity. It is highest in the embryo (90%) and gradually decreases with age. The water content of different tissues varies depending on their metabolic activity. For example, in the gray matter of the brain there is up to 80% water, and in the 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 of plants) and in the cell. Water serves as a “lubricant”, necessary wherever there are rubbing surfaces (for example, in joints). Water has its maximum density at 4°C. Therefore, ice, which has a lower density, is lighter than water and floats on its surface, which protects the reservoir from freezing. This property of water saves the lives of many aquatic organisms.

1. What structure does water have?

Answer. The water molecule has an angular structure: the nuclei included in its composition form isosceles triangle, which has two hydrogens at the base and an oxygen atom at the top. Internuclear O-H 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 represent two lone pairs of electrons.

A water molecule is a small dipole containing positive and negative charges at its poles. There is a lack of electron density near the hydrogen nuclei, 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 amount of water (in%) is contained in various cells?

The amount of water varies in different tissues and organs. Thus, in humans, its content in the gray matter of the brain is 85%, and in 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 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 varies in different plant organs: sunflower leaves contain 80-83% water, stems - 87-89%, roots - 73-75%. The water content of 6-11% is typical mainly for air-dried seeds, in which vital processes are inhibited. Water is contained in living cells, dead xylem elements and intercellular spaces. In the intercellular spaces, water is in a vapor state. The main evaporative organs of the plant are the leaves. In this regard, it is natural that greatest number water fills the intercellular spaces of leaves. In the 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; it apparently ranges from 30 to 50%. Forms 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 its structural features: water molecules have the shape of a dipole and hydrogen bonds are formed 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% bound to proteins).

Functions of water:

1.Water as a solvent. Many chemical reactions in the cell are ionic and therefore occur only in an aqueous environment. Substances that dissolve in water are called hydrophilic (alcohols, sugars, aldehydes, amino acids), those that do not dissolve are called hydrophobic (fatty acids, cellulose).

2.Water as a reagent. Water is involved in many chemical reactions: polymerization reactions, hydrolysis, and in the process of photosynthesis.

3.Transport function. Movement throughout the body along with water of substances dissolved in it to its various parts and removal of unnecessary products from the body.

4.Water as a thermostabilizer 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 entire 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 this that 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. A water molecule consists of an O atom bonded to two polar H atoms covalent bonds. The characteristic arrangement of electrons in a water molecule gives it electrical asymmetry. The more electronegative oxygen atom attracts the electrons of the hydrogen atoms more strongly, as a result of which the common pairs of electrons in the water molecule are shifted towards it. Therefore, although the water molecule as a whole is uncharged, each of the two hydrogen atoms carries 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 neighboring 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 are also soluble in water, i.e., the molecule of which contains charged (polar) groups, for example 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 occur, and all hydrolysis reactions and numerous redox reactions occur 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 at 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 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 own temperature. The large heat capacity of water protects body tissues from rapid and strong temperature increases. Many organisms cool themselves by evaporating water (transpiration in plants, sweating in animals).

4. Why is it believed that water is an ideal liquid for a cell?

Answer. High water content in the cell - the most important 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 awaken and become active again.

The water molecule is electrically neutral. But electric charge inside the molecule it is distributed unevenly: in the region of hydrogen atoms (more precisely, protons), positive charge predominates, in the region where oxygen is located, the density of negative charge is higher. Therefore, a water particle is a dipole. The dipole property of a water molecule explains its ability to orient itself in an electric field and 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 solvent properties. If the energy of attraction of water molecules to molecules of a substance is greater than the energy of attraction between water molecules, then the substance dissolves. Depending on this, a distinction is made between 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, difficult 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 back, as well as from one part of the cell to another.

Most reactions occurring in a cell can only occur 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 equilibrium of cells and organisms.

Water is the main medium for the 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 transport of 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 cell loss of water are wilting of leaves, drying out of fruits;

Acceleration of chemical reactions by dissolving 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 slowly heat up and slowly cool down.

6. What structural and physicochemical characteristics water determine its biological role in the cell?

Answer. The structural physicochemical 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 large heat capacity of water protects body tissues from rapid and strong temperature increases. Many organisms cool themselves by evaporating water (transpiration in plants, sweating in animals).

Water also has high thermal conductivity, ensuring uniform distribution of heat throughout the body. Consequently, high specific heat capacity and high thermal conductivity make water an ideal liquid for maintaining the thermal equilibrium of cells and organisms.

Water practically does not compress, creating turgor pressure, determining the volume and elasticity of cells and tissues. Thus, it is the hydrostatic skeleton that maintains the shape of roundworms, jellyfish and other organisms.

Water is characterized by optimal biological systems the value of surface tension, 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, ascending and descending currents of solutions in plants occur.

In certain biochemical processes, water acts as a substrate.

IN earth's crust occurs around 100 chemical elements, but only 16 of them are necessary for life. The four most common elements in plant organisms are hydrogen, carbon, oxygen, nitrogen, which form various substances. The main components of a plant cell are water, organic and mineral substances.

Water- the basis of life. The water content in plant cells ranges from 90 to 10%. It is a unique substance due to its chemical and physical properties. Water is necessary for the process of photosynthesis, transport of substances, cell growth, it is a medium for many biochemical reactions, a universal solvent, etc.

Minerals (ash)– substances that remain after burning a piece of an organ. The content of ash elements ranges from 1% to 12% of dry weight. Almost all the elements that make up water and soil are found in the plant. The most common are potassium, calcium, magnesium, iron, silicon, sulfur, phosphorus, nitrogen (macroelements) and copper, aluminum, chlorine, molybdenum, boron, zinc, lithium, gold (microelements). Minerals play important role in the life of cells - they are part of amino acids, enzymes, ATP, electron transport chains, are necessary for stabilizing membranes, participate in metabolic processes, etc.

Organic matter plant cells are divided into: 1) carbohydrates, 2) proteins, 3) lipids, 4) nucleic acids, 5) vitamins, 6) phytohormones, 7) products of secondary metabolism.

Carbohydrates make up up to 90% of the substances that make up a plant cell. There are:

Monosaccharides (glucose, fructose). Monosaccharides are formed in leaves during photosynthesis and are easily converted into starch. They accumulate in fruits, less often in stems and bulbs. Monosaccharides are transported from cell to cell. They are an energy material and participate in the formation of glycosides.

Disaccharides (sucrose, maltose, lactose, etc.) are formed from two particles of monosaccharides. They accumulate in roots and fruits.

Polysaccharides are polymers that are very widespread in plant cells. This group of substances includes starch, inulin, cellulose, hemicellulose, pectin, and callose.

Starch is the main storage substance of the plant cell. Primary starch is formed in chloroplasts. In the green parts of the plant, it is broken down into mono- and disaccharides and transported along the phloem of the veins to the growing parts of the plant and storage organs. In the leucoplasts of storage organs, secondary starch is synthesized from sucrose in the form of starch grains.

The starch molecule consists of amylose and amylopectin. Linear amylose chains, consisting of several thousand glucose residues, are capable of helically branching and thus taking on a more compact form. In the branched polysachpride amylopectin, compactness is ensured by intensive chain branching due to the formation of 1,6-glycosidic bonds. Amylopectin contains approximately twice as many glucose units as amylose.

With Lugol's solution, an aqueous suspension of amylose gives a dark blue color, a suspension of amylopectin gives a red-violet color, and a suspension of starch gives a blue-violet color.

Inulin is a polymer of fructose, a storage carbohydrate of the asteraceae family. Found in cells in dissolved form. Does not stain with iodine solution; it turns red with β-naphthol.

Cellulose is a polymer of glucose. Cellulose contains about 50% of the carbon found in the plant. This polysaccharide is the main material of the cell wall. Cellulose molecules are long chains consisting of glucose residues. Many OH groups protrude from each chain. These groups are directed in all directions and form hydrogen bonds with neighboring chains, which ensures rigid cross-linking of all chains. The chains are combined with each other, forming microfibrils, and the latter combine into more large structures– macrofibrils. The tensile strength of this structure is very high. Macrofibrils, arranged in layers, are immersed in a cementing matrix consisting of pectin substances and hemicelluloses.

Cellulose does not dissolve in water; with iodine solution it gives a yellow color.

Pectins consist of galactose and galacturonic acid. Pectic acid is a polygalacturonic acid. They are part of the cell wall matrix and provide its elasticity. Pectins form the basis of the middle plate formed between cells after division. Form gels.

Hemicelluloses are high-molecular compounds of mixed composition. They are part of the cell wall matrix. They do not dissolve in water, hydrolyze in an acidic environment.

Callose is an amorphous polymer of glucose found in different parts plant organism. Callose is produced in the sieve tubes of the phloem and is also synthesized in response to damage or adversity.

Agar-agar is a high molecular weight polysaccharide found in seaweed. Dissolves in hot water, and after cooling it hardens.

Squirrels high molecular weight compounds consisting of amino acids. Elemental composition – C, O, N, S, P.

Plants are able to synthesize all amino acids from simpler substances. 20 basic amino acids form the entire variety of proteins.

The complexity of the structure of proteins and the extreme diversity of their functions make it difficult to create a single, clear classification of proteins on any one basis. Based on their composition, proteins are classified into simple and complex. Simple - consist only of amino acids, complex - consist of amino acids and non-protein material (prosthetic group).

Simple proteins include albumins, globulins, histones, prolamins, and glutenins. Albumins are neutral proteins, soluble in water, and rarely found in plants. Globulins are neutral proteins, insoluble in water, soluble in dilute salt solutions, distributed in seeds, roots, and stems of plants. Histones are neutral proteins, soluble in water, localized in the nuclei of all living cells. Prolamins are soluble in 60-80% ethanol and are found in cereal grains. Gluteins are soluble in alkali solutions and are found in grains of cereals and green parts of plants.

Complex proteins include phosphoproteins (prosthetic group - phosphoric acid), lycoproteins (carbohydrates), nucleoproteins (nucleic acid), chromoproteins (pigment), lipoproteins (lipid), flavoproteins (FAD), metalloproteins (metal).

Proteins play an important role in the life of the plant organism and, depending on the function they perform, proteins are divided into structural proteins, enzymes, transport proteins, contractile proteins, storage proteins.

Lipids – organic matter insoluble in water and soluble in organic solvents (ether, chloroform, benzene). Lipids are divided into true fats and lipoids.

True fats - esters fatty acids and any alcohol. They form an emulsion in water and hydrolyze when heated with alkalis. They are reserve substances that accumulate in seeds.

Lipoids are fat-like substances. These include phospholipids (part of membranes), waxes (form a protective coating on leaves and fruits), sterols (part of protoplasm, participate in the formation of secondary metabolites), carotenoids (red and yellow pigments, necessary to protect chlorophyll, impart color fruits, flowers), chlorophyll (the main pigment of photosynthesis)

Nucleic acids - genetic material all living organisms. Nucleic acids (DNA and RNA) consist of monomers - nucleotides. A nucleotide molecule consists of a five-carbon sugar, a nitrogenous base, and phosphoric acid.

Vitamins– complex organic substances of varied chemical composition. They have high physiological activity - they are necessary for the synthesis of proteins, fats, for the functioning of enzymes, etc. Vitamins are divided into fat-soluble and water-soluble. Fat-soluble vitamins include vitamins A, K, and E; water-soluble vitamins include vitamin C and B vitamins.

Phytohormones– low molecular weight substances with high physiological activity. They have a regulatory effect on the processes of plant growth and development in very low concentrations. Phytohormones are divided into stimulants (cytokinins, auxins, gibberellins) and inhibitors (ethylene and abscisins).

Water is contained in living cells, dead xylem elements and intercellular spaces. In the intercellular spaces, water is in a vapor state. The main evaporative organs of the plant are the leaves. 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: cell membrane, vacuole, 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; it apparently 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 bound mainly 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 the protein molecule is tightly bound and does not have a regular hexagonal structure (colloidally bound water). In addition, there is a certain amount of ions in the protoplasm, and therefore 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 free water content. There is a direct correlation between the content of bound water and the resistance of plants against unfavorable external conditions. These physiological correlations are not always observed.

A plant cell absorbs water according to the laws of osmosis. Osmosis occurs when two systems with different concentrations of substances are present when they are connected using a semi-permeable membrane. In this case, according to the laws of thermodynamics, the equalization of concentrations occurs due to the substance for which the membrane is permeable.

When considering two systems with different concentrations of osmotically active substances, it follows that equalization of concentrations in systems 1 and 2 is possible only due to the movement of water. In system 1, the concentration of water is higher, so the flow of water is directed from system 1 to system 2. When equilibrium is reached, the actual flow will be zero.

A plant cell can be considered as an osmotic system. The cell wall surrounding the cell has a certain elasticity and can stretch. Water-soluble substances (sugars, organic acids, salts) that have osmotic activity accumulate in the vacuole. The tonoplast and plasma membrane perform the function of a semi-permeable membrane in this system, since these structures are selectively permeable, and water passes through them much more easily than substances dissolved in cell sap and cytoplasm. In this regard, if a cell enters an environment where the concentration is osmotically active substances will be less than the concentration inside the cell (or the cell is placed in water), water, according to the laws of osmosis, must enter the cell.

The ability of water molecules to move from one place to another is measured by water potential (Ψw). According to the laws of thermodynamics, water always moves from an area of higher water potential to an area of lower potential.

Water potential(Ψ in) is an indicator of the thermodynamic state of water. Water molecules have kinetic energy; in liquids and water vapor they move randomly. The water potential is greater in the system where the concentration of molecules is higher and their total is greater kinetic energy. Pure (distilled) water has the maximum water potential. The water potential of such a system is conventionally taken to be zero.

The unit of measurement of water potential is pressure units: atmospheres, pascals, bars:

1 Pa = 1 N/m 2 (N- newton); 1 bar=0.987 atm=10 5 Pa=100 kPa;

1 atm = 1.0132 bar; 1000 kPa = 1 MPa

When another substance is dissolved in water, the concentration of water decreases, the kinetic energy of water molecules decreases, and the water potential decreases. In all solutions, the water potential is lower than that of pure water, i.e. under standard conditions it is expressed as a negative value. This decrease is expressed quantitatively by a value called osmotic potential(Ψ osm.). Osmotic potential is a measure of the reduction in water potential due to the presence of solutes. The more solute molecules in a solution, the lower the osmotic potential.

When water enters a cell, its size increases, and hydrostatic pressure inside the cell increases, which forces the plasmalemma to press against the cell wall. The cell membrane, in turn, exerts back pressure, which is characterized by pressure potential(Ψ pressure) or hydrostatic potential, it is usually positive and the greater the more water in a cage.

Thus, the water potential of the cell depends on the concentration of osmotically active substances - osmotic potential (Ψ osm.) and on pressure potential (Ψ pressure).

Provided that water does not put pressure on the cell membrane (state of plasmolysis or wilting), the back pressure of the cell membrane is zero, the water potential is equal to the osmotic one:

Ψ c. = Ψ osm.

As water enters the cell, back pressure from the cell membrane appears; the water potential will be equal to the difference between the osmotic potential and the pressure potential:

Ψ c. = Ψ osm. + Ψ pressure

The difference between the osmotic potential of the cell sap and the back pressure of the cell membrane determines the flow of water at any given moment.

Under the condition that the cell membrane is stretched to the limit, the osmotic potential is completely balanced by the back pressure of the cell membrane, the water potential becomes zero, and water stops flowing into the cell:

- Ψ osm. = Ψ pressure , Ψ c. = 0

Water always flows towards a more negative water potential: from the system where there is more energy to the system where there is less energy.

Water can also enter the cell due to swelling forces. Proteins and other substances that make up the cell, having positively and negatively charged groups, attract water dipoles. The cell wall, which contains hemicelluloses and pectin substances, and the cytoplasm, in which high-molecular polar compounds make up about 80% of the dry mass, are capable of swelling. Water penetrates into the swelling structure by diffusion; the movement of water follows a concentration gradient. The force of swelling is denoted by the term matrix potential(Ψ mat.). It depends on the presence of high molecular weight components of the cell. The matrix potential is always negative. Great importanceΨ mat. occurs when water is absorbed by structures that lack vacuoles (seeds, meristem cells).

1.3 Water distribution in the cell

The physiological significance of free and bound water is different. Most researchers believe that the intensity of physiological processes, including growth rates, depends primarily on the free water content. There is a direct correlation between the content of bound water and the resistance of plants against unfavorable external conditions. These physiological correlations are not always observed.

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