Water (H2O) 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 take place in aqueous solutions. The higher the metabolic rate in a particular cell, the more water it contains.
The water in the cell is in two forms: free and bound.
Free water is in the intercellular spaces, blood vessels, vacuoles, and organ cavities. It serves to transfer substances from the environment to the cell and vice versa.
Bound water is a part of some cellular structures, located between protein molecules, membranes, fibers, and is connected with some proteins.
Water has a number of properties that are of exceptional importance 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, as a result the water molecule is a 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 seeks to hydrogen bond with four neighboring water molecules.
Since water molecules are polar, water has the property of dissolving polar molecules of other substances.
Substances 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 insoluble in water are called hydrophobic (fats, nucleic acids, some proteins). Such substances can form interfaces with water, on 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 minimum increase in its own temperature. To break the numerous hydrogen bonds that exist between water molecules, it takes a lot of energy to be absorbed. This property of water ensures the maintenance of heat balance in the body. The high heat capacity of water protects body tissues from rapid and strong temperature rise.
Quite a lot of energy is needed to evaporate water. The use of a significant amount of energy to break hydrogen bonds during evaporation contributes to its cooling. This property of water protects the body from overheating.
Example: Examples of this would be transpiration in plants and perspiration in animals.
Water also has a high thermal conductivity, ensuring an even distribution of heat throughout the body.
High specific heat capacity and high thermal conductivity make water an ideal liquid for maintaining thermal equilibrium between the cell and the body.
Water practically does not compress, creating turgor pressure, determining the volume and elasticity of cells and tissues.
Example: The hydrostatic skeleton maintains its shape in roundworms, jellyfish, and other organisms.
Due to the adhesion forces of molecules on the surface of the water, a film is created that 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 (O2, CO2, 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 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.
- Water is an active participant in metabolic reactions.
- Water participates 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 a part of mucus, which facilitate the movement of substances through the intestines, create a moist environment on the mucous membranes of the respiratory tract. The secrets secreted by some glands and organs are also water-based: saliva, tears, bile, semen, etc.