Biological functions of water. Chemical composition of the cell

Water (H 2 O) is the most important inorganic substance of the cell. In a cell, in quantitative terms, water ranks first among other chemical compounds. Water performs various functions: maintaining the volume, elasticity of the cell, participating in all chemical reactions. All biochemical reactions occur in aqueous solutions. The higher the metabolic rate in a particular cell, the more water it contains.

Pay attention!

Water in a cell is in two forms: free and bound.

Free water located in intercellular spaces, vessels, vacuoles, and organ cavities. It serves to transport substances from the environment into the cell and vice versa.
Bound water is part of some cellular structures, located between protein molecules, membranes, fibers and is connected to some proteins.
Water has a number of properties that are extremely important for living organisms.

Water molecule structure

The unique properties of water are determined by the structure of its molecule.

Hydrogen bonds are formed between individual water molecules, which determine the physical and chemical properties of water.
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, resulting in a water molecule dipole(has polarity). Each of the two hydrogen atoms has a partially positive charge, and the oxygen atom carries a partially negative charge.

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 connect hydrogen bond with four neighboring water molecules.

Properties of water

Since water molecules are polar, water has the property of dissolving polar molecules of other substances.
Substances that are soluble in water are called hydrophilic(salts, sugars, simple alcohols, amino acids, inorganic acids). When a substance goes into solution, its molecules or ions can move more freely and, therefore, the reactivity of the substance increases.

Substances that are insoluble in water are called hydrophobic(fats, nucleic acids, some proteins). Such substances can form interfaces with water at which many chemical reactions take place. Therefore, the fact that water does not dissolve some substances is also very important for living organisms.

Water has a high specific heat capacity, i.e. the ability to absorb thermal energy with a minimal increase in its own temperature. To break the numerous hydrogen bonds present between water molecules, a large amount of energy must be absorbed. This property of water ensures the maintenance of thermal balance in the body. The large heat capacity of water protects body tissues from rapid and strong temperature increases.
To evaporate water, quite a lot of energy is required. The use of a significant amount of energy to break hydrogen bonds during evaporation helps to cool it. This property of water protects the body from overheating.

Example:

Examples of this include transpiration in plants and sweating in animals.

Water also has high thermal conductivity, ensuring uniform distribution of heat throughout the body.

Pay attention!

High specific heat capacity and high thermal conductivity makes water an ideal liquid for maintaining thermal equilibrium of cells and organisms.

Water practically does not shrink, creating turgor pressure, determining the volume and elasticity of cells and tissues.

Example:

The hydrostatic skeleton maintains shape in roundworms, jellyfish and other organisms.

Thanks to the adhesive forces of molecules, a film is created on the surface of the water, which has such a characteristic as surface tension.

Example:

Due to the force of surface tension, capillary blood flow, ascending and descending currents of solutions in plants occur.

Among the physiologically important properties of water is its ability to dissolve gases(O 2, CO 2, etc.).

Water is also a source of oxygen and hydrogen released during photolysis during the light phase of photosynthesis.

Biological functions of water

• 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.
• Water is an active participant in metabolic reactions.
• Water is involved in the formation of lubricating fluids and mucus, secretions and juices in the body (these fluids are found in the joints of vertebrates, in the pleural cavity, in the pericardial sac).
• Water is part of mucus, which facilitates the movement of substances through the intestines and creates a moist environment on the mucous membranes of the respiratory tract. The secretions secreted by some glands and organs are also water-based: saliva, tears, bile, sperm, etc.

Water plays a vital role in the life of cells and living organisms in general. In addition to the fact that it is part of their composition, for many organisms it is also a habitat. The role of water in a cell is determined by its properties. These properties are quite unique and are associated mainly with the small size of water molecules, with the polarity of its molecules and with their ability to connect with each other through hydrogen bonds.

Water molecules have a nonlinear spatial structure. The atoms in a water molecule are held together by polar covalent bonds, which bind one oxygen atom to two hydrogen atoms. The polarity of covalent bonds (i.e., uneven distribution of charges) is explained in this case by the strong electronegativity of the oxygen atoms relative to the hydrogen atom; The oxygen atom pulls electrons from shared electron pairs.

As a result, a partially negative charge appears on the oxygen atom, and a partially positive charge appears on the hydrogen atoms. Hydrogen bonds occur between the oxygen and hydrogen atoms of neighboring molecules.

Thanks to the formation of hydrogen bonds, water molecules form one another, which determines its initial state under normal conditions.

Water is excellent solvent for polar substances, such as salts, sugars, alcohols, acids, etc. Substances that are highly soluble in water are called hydrophilic.

Water does not dissolve or mix with absolutely non-polar substances such as fats or oils, since it cannot form hydrogen bonds with them. Substances that are insoluble in water are called hydrophobic.

Water has high specific heat capacity. Breaking the hydrogen bonds that hold water molecules together requires the absorption of a large amount of energy. This property ensures the maintenance of the body's thermal balance during significant temperature changes in the environment. In addition, water has high thermal conductivity, which allows the body to maintain the same temperature throughout its entire volume.

Water also has high heat of vaporization, i.e. the ability of molecules to carry away a significant amount of heat, cooling the body. This property of water is used in sweating in mammals, thermal shortness of breath in crocodiles and transpiration in plants, preventing them from overheating.

It is exclusively characteristic of water high surface tension. This property is very important for adsorption processes, for the movement of solutions through tissues (blood circulation, ascending and descending currents in the body of plants). Many small organisms benefit from surface tension: it allows them to float on water or glide across its surface.

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.

Metabolic. Water is the medium for all biochemical reactions in the cell. Its molecules participate in many chemical reactions, for example in the formation or hydrolysis of polymers. In the process of photosynthesis, water is an electron donor and a source of hydrogen atoms. It is also a source of free oxygen.

Structural. The cytoplasm of cells contains from 60 to 95% water. In plants, water determines the turgor of cells, and in some animals it performs supporting functions, being a hydrostatic skeleton (round and annelids, echinoderms).

Water is involved in the formation of lubricating fluids (synovial in the joints of vertebrates; pleural in the pleural cavity, pericardial in the pericardial sac) and mucus (which facilitate the movement of substances through the intestines and create a moist environment on the mucous membranes of the respiratory tract). It is part of saliva, bile, tears, sperm, etc.

Mineral salts. Salt molecules in an aqueous solution dissociate into cations and anions. The most important cations are: K +, Na +, Ca 2+, Mg 2+ and anions: Cl -, H 2 PO 4 -, HPO 4 2-, HCO 3 -, NO 3 -, SO 4 2-. Not only the content, but also the ratio of ions in the cell is significant.

The difference between the amounts of cations and anions on the surface and inside the cell ensures the occurrence of an action potential, which underlies nerve and muscle excitation. The difference in ion concentrations on different sides of the membrane is associated with the active transfer of substances through the membrane, as well as energy conversion.

Phosphoric acid anions create a phosphate buffer system that maintains the pH of the body's intracellular environment at 6.9.

Carbonic acid and its anions create a bicarbonate buffer system that maintains the pH of the extracellular environment (blood plasma) at 7.4.

Some ions are involved in the activation of enzymes, the creation of osmotic pressure in the cell, in the processes of muscle contraction, blood clotting, etc.

Some cations and anions can be included in complexes with various substances (for example, phosphoric acid anions are part of phospholipids, ATP, nucleotides, etc.; Fe 2+ ion is part of hemoglobin, etc.).

Water is a universal solvent for polar molecules - salts, sugars, simple alcohols. Water has the unique property of breaking all types of molecular and intermolecular bonds and forming solutions.

A solution is a liquid molecular disperse system in which molecules and ions of dissolved substances interact with each other. There are solutions of electrolytes, non-electrolytes, and polymers.

Body fluids are complex solutions - polyelectrolytes. When dissolved in water, hydration occurs, and the substances formed are called hydrates. In this case, intermolecular bonds are broken.

Electrolyte solutions are characterized by electrolytic dissociation of the solute to form ions. In the liquid media of the body, according to the nature and mechanisms of hydration, there are no actual salts, acids and bases, but there are their ions.

Solutions of biopolymers - proteins, nucleic acids - are polyelectrolytes and do not pass through most biological membranes.

Non-polar substances, such as lipids, do not mix with water.

Water is a solvent for many substances and transports them through the blood, lymphatic and excretory systems.

The fluid media of the body - blood, lymph, cerebrospinal fluid, tissue fluid, washing cellular elements and taking part in the metabolic process, together form the internal environment of the body. The term “internal environment” or “internal sea” was proposed by the French physiologist C. Bernard.

Biological functions of water

About 60% of an adult’s body weight (for men - 61%, for women - 54%) is water. In a newborn child, the water content reaches 77%, in old age it decreases to 50%.

Water is part of all tissues of the human body: about 81% in the blood, 75% in the muscles, 20% in the bones. Water is associated in the body mainly with connective tissue.

Water is a universal solvent of inorganic and organic compounds. In a liquid environment, food is digested and nutrients are absorbed into the blood.

Water is the most important factor ensuring the relative constancy of the internal environment of the body. Due to its high heat capacity and thermal conductivity, water participates in thermoregulation, promoting heat transfer (sweating, evaporation, thermal shortness of breath, urination).

Water is a participant in many metabolic reactions, in particular hydrolysis. It stabilizes the structure of many high-molecular compounds, intracellular formations, cells, tissues and organs, provides the supporting functions of tissues and organs, preserving their turgor, forlysis and
position (hydrostatic skeleton). Water is a carrier of metabolites. hormones, electrolytes, and is involved in the transport of substances across cell membranes and the vascular wall as a whole. With the help of water, toxic metabolic products are removed from the body.

Sources of water and routes of excretion from the body

An adult consumes an average of 2.5 liters of water per day. Of these, 1.2 are in the form of drinking water, drinks, etc.; 1 liter with incoming food; 0.3 liters is formed in the body as a result of the metabolism of proteins, fats and carbohydrates, the so-called metabolic or endogenous water. The same amount of water is removed from the body.

1.5 liters of saliva, 3.5 liters of gastric juice, 0.7 liters of pancreatic juice, 3 liters of intestinal juices and about 0.5 liters of bile are secreted into the cavity of the digestive tract per day.

About 1-1.5 liters are excreted by the kidneys in the form of urine, 0.2-0.5 liters - with sweat through the skin, about 1 liter - through the intestines with feces. The set of processes of water and salts entering the body, their distribution in internal environments and excretion is called water-salt metabolism.

Types of water in the body

There are three types of water in the human and animal bodies - free, bound and constitutional.

Free, or mobile water, forms the basis of extracellular, intracellular and transcellular fluids.

Bound water is retained by ions in the form of a hydration shell and by hydrophilic colloids (proteins) of the blood and tissue proteins in the form of swelling water.

constitutional (intramolecular) water is part of molecules, proteins, fats and carbohydrates and is released during their oxidation. Water moves between different parts of the body's fluids due to the forces of hydrostatic and osmotic pressure.

Intracellular and extracellular fluids are electrically neutral and osmotically balanced.

Water is a unique substance. It is distributed everywhere on our planet. Try to imagine what our life would be like without the H2O molecule? And there is nothing to imagine - there would be no life on our planet. Humans are 70% water. The younger the body, the more it contains, and with age this amount decreases. For example, let's take an embryo - the percentage of H2O in it is 90%.

In the article, we invite you to highlight everything in the cell and consider each in detail. It is important to mention that it is contained there in two forms: free and bound. We'll deal with this a little later.

Water

Everyone knows that water plays a very important, or rather, key role in our lives. Without it, our planet would be a dead, lifeless desert. Scientists are still studying water and its role in the human body.

We have already said that water is found in our cells in free and bound forms. The first serves to distribute substances - to transfer them into and out of the cell. And the last one is observed:

• between fibers;
• membranes;
• protein molecules;
• cellular structures.

Both free and bound water in the cell necessarily perform some functions, which we will talk about later. And now a few words about how the H2O molecule itself is organized.

Molecule

To begin with, let's denote the molecular formula of water: H2O. This is a very common substance on the planet, and you should remember it, because the molecular formula of water is found quite often in different fields of knowledge. By the way, it is found in all human organs, even in tooth enamel and bones, although its percentage there is very small - 10% and 20%, respectively.

As we have already said, the younger the body, the more water it contains. Scientists have suggested that we age because protein cannot bind large amounts of water. But this, however, is only a hypothesis.

Functions

Now let's highlight more of them clearly from the list below:

• H2O can act as a solvent, since almost all chemical reactions are ionic and occur in water. It should be noted that there are hydrophilic substances (which dissolve, for example, alcohol, sugar, amino acids, and so on), but there are also hydrophobic ones (fatty acids, cellulose, and others).
• Water can act as a reagent.
• Performs transport, thermoregulatory and structural functions.

We propose to consider each of them separately. Let's go in order, the first on our list is the solvent function.

Solvent

The functions of water in the cell are numerous, but one of the most important is to help facilitate many reactions. The H2O molecule can act as a solvent. Almost all reactions occurring in a cell are ionic, that is, the medium in which they can take place is water.

Reagent

The next functions of water in a cell are its participation in chemical reactions taking place in the body as a reagent. These include:

• hydrolysis;
• polymerization;
• photosynthesis and so on.

Now a little about that. In chemistry, this is the name for a substance participating in some chemical reactions. The most important thing is that although it participates in the reaction, it is not the object of processing. Reagents in the laboratory (also called reagents) are a fairly common phenomenon.

Water, as a reagent, is involved in the composition of other substances needed by the body.

Transport function

Why do we live? Our body exists only because the cells of which it consists are alive. And they should thank their unique structure and some of the capabilities of the H2O molecule. We have already mentioned that water is an integral part of our body, and each cell contains these unique molecules, or rather, is in first place in its composition.

The transport function of water in the cell is another purpose of H2O in our body. Water has a certain feature - penetration into the intercellular space, thanks to which nutrients enter the cell.

It is also worth knowing that blood and lymph also contain water, and its lack leads to some consequences: hemorrhages or thrombosis.

Thermoregulation

What functions of water in a cell have we not yet figured out? Of course, thermoregulation. We said that water can absorb heat and retain it for a long time. Thus, H2O can protect the cell from hypothermia or overheating. The function of thermoregulation is needed not only for individual cells, but also for the entire organism as a whole.

Structural function

We have already listed them, but one more purpose remains to be discussed - maintaining the structure of cells.

Have you ever tried to compress liquid water? Even in laboratory conditions this is extremely difficult to achieve. This property of water is necessary in order to maintain the shape and structure of each cell.

Remember forever: without water life is impossible. We experience thirst when the body loses about 3% of water, and with a loss of 20%, the cells die, and, consequently, the person too. Watch how much water you drink.

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