What determines the different water content in the cell. Water, its role in the cell and the body. Water uptake by plant cells

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 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 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, 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%.

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 cytoplasm, and, consequently, part of the water is osmotically bound.

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 external 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 feasible only for plants growing in water. For land plants, this task is complicated by the fact that water in the plant organism is continuously lost in the process of evaporation. Evaporation of water by the plant reaches enormous proportions. An example can be given: one corn plant evaporates up to 180 kg of water during the growing season, and 1 hectare of forest in South America evaporates an average of 75 thousand kg of water per day. The huge water consumption is due to the fact that most plants have a significant leaf surface located in an atmosphere that is not saturated with water vapor. At the same time, the development of an extensive leaf surface is necessary and developed in the course of a long evolution to ensure normal nutrition with carbon dioxide contained in the air in an insignificant concentration (0.03%). In his famous book "The fight of plants against drought" K.A. Timiryazev pointed out that the contradiction between the need to capture carbon dioxide and reduce water consumption left an imprint on the structure of the entire plant organism.

In order to compensate for the loss of water during evaporation, a large amount of it must continuously enter the plant. Two processes continuously going on in a plant - the inflow and evaporation of water - are called plant water balance. For the normal growth and development of plants, it is necessary that the water consumption approximately correspond to the income, or, in other words, that the plant reduces its water balance without a large deficit. To do this, in the process of natural selection, the plant developed adaptations to absorb water (a colossally developed root system), to move water (a special conductive system), to reduce evaporation (the system of integumentary tissues and the system of automatically closing stomatal openings).

Despite all these adaptations, a water deficit is often observed in the plant, i.e., the intake of water is not balanced by its consumption in the process of transpiration.

Physiological disturbances occur in different plants with varying degrees of water deficiency. There are plants that have developed in the process of evolution various adaptations to tolerate dehydration (drought-resistant plants). The elucidation of the physiological characteristics that determine the resistance of plants to a lack of water is a most important task, the solution of which is of great not only theoretical, but also agricultural practical importance. At the same time, in order to solve it, knowledge of all aspects of the water exchange of a plant organism is necessary.

Water is the most common compound on Earth and in living organisms. The water content in cells depends on the nature of metabolic processes: the more intense they are, the higher the water content.

On average, the cells of an adult human contain 60-70% water. With the loss of 20% of water, organisms die. Without water, a person can live no more than 7 days, while without food no more than 40 days.

Rice. 4.1. The spatial structure of the water molecule (H 2 O) and the formation of a hydrogen bond

The water molecule (H 2 O) consists of two hydrogen atoms that are covalently bonded to oxygen atoms. The molecule is polar because it is bent at an angle and the nucleus of the oxygen atom pulls the shared electrons to this angle, so that the oxygen acquires a partial negative charge, and the hydrogen atoms at the open ends become partially positive charges. Water molecules can be attracted to each other by positive and negative charges, forming hydrogen bond (Fig.4.1.).

Due to the unique structure of water molecules and their ability to bind to each other using hydrogen bonds, water has a number of properties that determine its important role in the cell and organism.

Hydrogen bonds cause relatively high boiling and evaporation temperatures, high heat capacity and thermal conductivity of water, and the property of a universal solvent.

Hydrogen bonds are 15-20 times weaker than covalent ones. In the liquid state, hydrogen bonds are either formed or broken, which causes the movement of water molecules, its fluidity.

The biological role of H 2 O

Water determines the physical properties of the cell - its volume, elasticity (turgor). The cell contains 95-96% free water and 4-5% bound. Bound water forms aqueous (solvate) shells around certain compounds (for example, proteins), preventing their interaction with each other.

free water is a good solvent for many inorganic and organic polar substances. Substances that are highly soluble in water are called hydrophilic. For example, alcohols, acids, gases, most salts of Sodium, Potassium, etc. For hydrophilic substances, the binding energy between their atoms is less than the energy of attraction of these atoms to water molecules. Therefore, their molecules or ions are easily integrated into the general system of hydrogen bonds of water.

Water as a universal solvent plays an extremely important role, since most chemical reactions occur in aqueous solutions. The penetration of substances into the cell and the removal of waste products from it in most cases is possible only in dissolved form.

Water does not dissolve non-polar (non-charged) substances, since it cannot form hydrogen bonds with them. Substances that are insoluble in water are called hydrophobic . These include fats, fat-like substances, polysaccharides, rubber.

Some organic molecules have dual properties: in some areas they are polar groups, and in others - non-polar. Such substances are called amphipathic, or amphiphilic. These include proteins, fatty acids, phospholipids, nucleic acids. Amphiphilic compounds play an important role in the organization of biological membranes, complex supramolecular structures.

Water is directly involved in the reactions hydrolysis- breakdown of organic compounds. At the same time, under the action of special enzymes, OH ions are added to the free valences of organic molecules. - and H + water. As a result, they form new substances with new properties.

Water has a high heat capacity (i.e., the ability to absorb heat with minor changes in its own temperature) and good thermal conductivity. Due to these properties, the temperature inside the cell (and the body) is maintained at a certain level with significant changes in ambient temperature.

An important biological significance for the functioning of plants and cold-blooded animals is that under the influence of dissolved substances (carbohydrates, glycerol) water can change its properties, in particular the freezing and boiling point.

The properties of water are so important for living organisms that it is impossible to imagine the existence of life, as we know it, not only on Earth, but on any other planet without an adequate supply of water.

MINERAL SALT

They can be in a dissolved or undissolved state. Molecules of mineral salts in an aqueous solution decompose into cations and anions.

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 for organic and inorganic compounds involved in metabolism, and acts as a background environment in which all biochemical processes take place. Easily penetrating through the shells and membranes of cells, water circulates freely throughout the plant, ensuring the transfer 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, 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 radicals of organic molecules. 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. In this case, the vacant hydrogen bonds of the organic substances of the protoplast are filled, and water actively enters the cell from the environment. 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 the chemical composition of the 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, protein seeds (peas, beans, beans) absorb the largest amount of water per unit weight by solvation, starch seeds (wheat, rye) the intermediate one, and oilseeds (flax, sunflower) the smallest.

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

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 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 external conditions. These physiological correlations are not always observed.

A plant cell absorbs water according to the laws of osmosis. Osmosis is observed in the presence of two systems with different concentrations of substances, when they communicate with a semipermeable membrane. In this case, according to the laws of thermodynamics, the concentrations equalize due to the substance for which the membrane is permeable.

When considering two systems with different concentrations of osmotically active substances, it follows that the 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 real flow will be zero.

The plant cell can be considered as an osmotic system. The cell wall surrounding the cell has a certain elasticity and can be stretched. Water-soluble substances (sugars, organic acids, salts) that have osmotic activity accumulate in the vacuole. The tonoplast and plasmalemma perform the function of a semipermeable 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 the cell enters the environment, where the concentration of 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 the water potential (Ψw). According to the laws of thermodynamics, water always moves from an area with a higher water potential to an area with a lower potential.

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

The units of water potential are units of pressure: 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. Quantitatively, this decrease is expressed by a quantity 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 the solution, the lower the osmotic potential.

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

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

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

Ψ in. = Ψ osm.

As water enters the cell, the backpressure of the cell membrane appears, the water potential will be equal to the difference between the osmotic potential and the pressure potential:

Ψ in. = Ψ osm. + Ψ pressure

The difference between the osmotic potential of the cell sap and the backpressure 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 counterpressure of the cell membrane, the water potential becomes zero, and water ceases to flow into the cell:

- Ψ osm. = Ψ pressure , Ψ c. = 0

Water always flows in the direction of a more negative water potential: from the system where the energy is greater to the system where the energy is less.

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 components of the cell. The matrix potential is always negative. Large value of Ψ mat. has when water is absorbed by structures in which there are no vacuoles (seeds, meristem cells).

Water is the most common chemical compound on Earth, its largest mass in a living organism. It is estimated that water makes up 85% of the total mass of the average statistical cell. Whereas in human cells water is on average about 64%. However, the water content in different cells can vary significantly: from 10% in the cells of tooth enamel to 90% in the cells of the mammalian embryo. Moreover, young cells contain more water than old ones. So, in the cells of an infant, water is 86%, in the cells of an old person, only 50%.

In males, the water content in the cells is on average 63%, in females - slightly less than 52%. What caused it? It turns out that everything is simple. In the female body, there is a lot of adipose tissue, in the cells of which there is little water. Therefore, the water content in the female body is approximately 6-10% lower than in the male.

The unique properties of water are due to the structure of its molecule. From the course of chemistry, you know that the different electronegativity of hydrogen and oxygen atoms is the cause of the occurrence of a covalent polar bond in the water molecule. The water molecule has the shape of a triangle (87), in which the electric charges are located asymmetrically, and is a dipole (remember the definition of this term).

Due to the electrostatic attraction of the hydrogen atom of one water molecule to the oxygen atom of another molecule, hydrogen bonds arise between the water molecules.

The features of the structure and physico-chemical properties of water (the ability of water to be a universal solvent, variable density, high heat capacity, high surface tension, fluidity, capillarity, etc.), which determine its biological significance, are considered.

What functions does water perform in the body? Water is a solvent. The polar structure of the water molecule explains its properties as a solvent. Water molecules interact with chemicals, the elements of which have electrostatic bonds, and decompose them into anions and cations, which leads to chemical reactions. As you know, many chemical reactions occur only in aqueous solution. At the same time, the water itself remains inert, so it can be used in the body repeatedly. Water serves as a medium for transporting various substances within the body. In addition, the end products of metabolism are excreted from the body mainly in dissolved form.

There are two main types of solutions in living beings. (Remember the classification of solutions.)

The so-called true solution, when the molecules of the solvent are the same size as the molecules of the solute, they dissolve. As a result, dissociation occurs and ions are formed. In this case, the solution is homogeneous and, in scientific terms, consists of one liquid phase. Typical examples are solutions of mineral salts, acids or alkalis. Since such solutions contain charged particles, they are capable of conducting electric current and are electrolytes, like all solutions found in the body, including the blood of vertebrates, which contains many mineral salts.

A colloidal solution is the case when the solvent molecules are much smaller in size than the molecules of the solute. In such solutions, particles of a substance, which are called colloidal, move freely in the water column, since the force of their attraction does not exceed the force of their bonds with solvent molecules. Such a solution is considered heterogeneous, that is, consisting of two phases - liquid and solid. All biological fluids are mixtures that include true and colloidal solutions, since they contain both mineral salts and huge molecules (for example, proteins) that have the properties of colloidal particles. Therefore, the cytoplasm of any cell, the blood or lymph of animals, and the milk of mammals simultaneously contain ions and colloidal particles.

As you probably remember, biological systems obey all the laws of physics and chemistry, therefore, in biological solutions, physical phenomena are observed that play a significant role in the life of organisms.

Water properties

Diffusion (from Latin Difusio - spreading, spreading, dispersion) in biological solutions manifests itself as a tendency to equalize the concentration of structural particles of dissolved substances (ions and colloidal particles), which ultimately leads to a uniform distribution of the substance in solution. It is thanks to diffusion that many unicellular creatures are fed, oxygen and nutrients are transported through the body of animals in the absence of circulatory and respiratory systems in them (remember what kind of animals they are). In addition, the transport of many substances to cells is carried out precisely due to diffusion.

Another physical phenomenon is osmosis (from the Greek. Osmosis - push, pressure) - the movement of a solvent through a semipermeable membrane. Osmosis causes the movement of water from a solution having a low concentration of solutes and a high content of H2O in a solution with a high concentration of solutes and a low water content. In biological systems, this is nothing more than the transport of water at the cell level. That is why osmosis plays a significant role in many biological processes. The power of osmosis ensures the movement of water in plant and animal organisms, so that their cells receive nutrients and maintain a constant shape. It should be noted that the greater the difference in the concentration of a substance, the greater the osmotic pressure. Therefore, if the cells are placed in a hypotonic solution, they will swell and burst due to a sharp influx of water.