Importance Water: Water is the medium of life. It is the most abundant compound in all organisms. It varies from 65 to 89 percent in different organisms. Human tissues contain about 20 per cant water in bone cells and 85 per cent in brain cells. Almost all reaction of a cell occurs in the presence or water. It also takes part in many biochemical reactions such as hydrolysis of macromolecules. It is also used as a raw material in photosynthesis.
Solvent properties
Due to its polarity, water is an excellent solvent for polar substances. Ionic substances when dissolved in water, dissociate into positive and negative inos. Non-ionic substances having charged groups in their molecules are dispersed in water. When in solution, ions and molecules move randomly and are in a more favorable state to react with other molecules and ions. It is because of this property of water that almost all reactions in cells occur in aqueous media. In cells all chemical reactions are catalyzed by enzymes which work in aqueous environment. Nonpolar organic molecules, such as fats, are insoluble in water and help to maintain members which make compartments in the cell.
Heat capacity
Water has great ability of absorbing heat with minimum of change in its own temperature. The specific heat capacity of water—the number of calories required to raise the temperature of 1g of water form 15 to 16C is 1. 0. This is because much of the energy is used to break hydrogen bonds. Water thus works as temperature stabilizer for organisms in the environment and hence protects living material against sudden thermal changes.
Heat of vaporization
Water absorbs much heat as it changes from liquid to gas. Heat of vaporization is expressed as calories absorbed per gram vaporized. The specific heat of vaporization of water is 574 kcal/kg, which plays an important role in the regulation of heat produced by oxidation. It also provides cooling effect to plants when water is transpired or to animals when water is perspired. Evaporation of only two ml out of one liter of water lowers the temperature of the remaining 998 ml by 1C.
Ionization of water
The water molecules ionize to form H+ and OH ions:
This reaction is reversible but an equilibrium is maintained. At 25 degreeC the concentration of each of H+ and OH ions in pure water is about 10 -7 mole/ litere. the H+ and OH- ions affect, and take part in many of the reactions that occur in cells.
Protection
Water is effective lubricant that provides protection against damage resulting from friction. For example, tears protect the surface of eye from the rubbing of eyelids, water also forms a fluid cushion around organs that helps to protect them from trauma.