Water is an extra-ordinary compound. It is the only substance that occurs in nature in three phases i.e. solid, liquid and vapour, within the normal range of earth’s temperature.
Water is of extreme importance to living organisms. It plays an important role in the sustenance of life as:
(i) It is an important active constituent of the living matter, the protoplasm.
(ii) It serves as a metabolite in many metabolic processes.
(iii) It is a universal solvent and a medium of transport.
12.1.1 Water as a component of cell
In living cells, water is invariably present. It normally constitutes 85-90 percent of the protoplasm.
However, there are certain structures produced by living organisms, such as seeds, spores, cysts etc. in which the water content may be as low as 10 percent. The reason for the low percentage of water in them is that they are produced under special conditions and are meant for special purpose i.e. reproduction or presentation. They have a very low physiological activity rate, which does not require much water.
Water is present within the cells and also bathes the cells, all around within the body of the organism.
Do you know that the existence of life on our earth is due to the presence of water? If there were no water, life would not have originated or evolved on our earth, no water, no life. Water is not everywhere, but wherever there is water, there is life.
12.1.2 Water as a Solvent
Water is an excellent solvent for the substances, which are very important to all processes of life. The salts dissolve to a much greater degree in water than in any other solvent. The solvent potential of water is due to the dipolar nature of its molecules, which causes it to orient itself around charged particles dissolved in it. When, for instance, NaCl (table salt) is dissolved in water, its Na+and Cl- disperse as independent ions. The negative zones of the water dipoles will arrange themselves around the positively charged Na+ ions, while the positive zones of the water molecules will do so around the negatively charged Cl- ions.
This alignment of the salt ions in this way keeps them separated and thus promotes a high degree of dissociation. Substances, when dissolved in water, become chemically reactive.
12.1.3 Water as a Metabolite
Water acts as metabolite as all the chemical reactions, occurring in the cells, take place in an aqueous medium. Those reactions often consume or produce water molecules. Water is required for hydrolysis and it is also an essential ingredient for the manufacture of food by photosynthesis. For hydrolysis and it is also an essential ingredient for the manufacture of food by photosynthesis.
Water is also essential for transporting the solutes, for cooling the plants and animals (by evaporation from their surface), and for developing the internal pressure that provides support and rigidity to the bodies of plants and many animals.
In fact, the life that exists on our earth would not have originated and evolved if there had been no water. The maintenance of constant aqueous internal environment is a principal physiological requirement for all terrestrial as well as aquatic organisms.