Chapter : 2, Topic: WATER

 Importance of water:     

            Water is the medium of life. It is the most abundant compound in all organisms. About 70% of all organisms are made up of water. It varies from 65 to 89% in different organisms. Human tissue including bones made of about 20% water. Human brain cells are made up of 85% of water. The seeds also contain 20% of water. Jellyfish have 99% water in its body. Water also takes part in many biochemical reactions. Almost all reactions occur in cells in the presence of aqueous media. Water is also used as raw material in photosynthesis All biochemical reactions that occur in cells are catalyzed by an enzyme that works in aqueous media. 

 Solvent properties:

                  Water has high polarity due to the mutual sharing of electrons between hydrogen and oxygen atoms, forming a covalent bond. The covalent bond is polar in water and all polar substances are soluble in water Ionic substances when entering the water, dissociate into two different ions. Non-ionic substances dispersed in water. Non-polar substances are not soluble in water, therefore are used to make the cell membrane and cell wall of the cells. Nonpolar substances having charged groups are also soluble in water. 

Heat capacity:

            The number of calories required to raise the temperature of water from 15C to 16C  is "1".  Water has the great ability of an absorbing heat with the minimum change in its own temperature. This is because much of heat is required to break the hydrogen bonds of water. Water has high heat capacity than any other. So water acts as a temperature stabilizer or regulator in organisms and protects them against sudden thermal changes.

The heat of vapourization:

            Water has a high heat of vapourization as a large amount of heat is lost with when it changes from liquid to gas. The heat of vaporization of the water is 574kcal/kg, which plays important role in the regulation of the heat produced by oxidation. It also provides a cooling effect to the plants when water is transpired or to the animals when water is perspired. Because of this property, animals release an excess of heat into the hot environment.  Evaporation of only 2 ml of water reduces the temperature of the remaining 998 ml of water by 1C. 

Ionization of water:

        The water molecule ionizes in H+ and OH- ions. At 25C, the concentration of these ions in pure water is about 10^-7 moles/liter. This reaction is reversible, and these ions affect and take part in many biochemical reactions. It also helps to maintain the PH.

Protection:

    Water also provides protection to the animals from friction and acts as a lubricant. for example, tears protect the eye from the  rubbing of the eyelids and also form cushions around organs to protect them from trauma

Cohesion and Adhesion:

Cohesion is the attraction among the water molecules. Water flows freely due to cohesion. The movement of water through the xylem of the plants is due to cohesion. Water has also the attraction to the polar molecules. this property is also called adhesion. Both adhesion and cohesion are due to the hydrogen bonding of the water molecules. Both these properties enable the water to act as a transport medium and to circulate in living bodies.

Hydrophobic exclusion:

. It is defined as the reduction of the contact area between water and hydrophobic substances which are placed in water. It plays a key role in maintaining the integrity of the lipid bilayer membranes. for example, if you add some drops of oils in water, all drops come to from one large drop.