WATER

As we have already seen that life first originated in water, so no life can exist on the earth without water. All living things are composed of 70% - 90% water. Water provides an environment for the organisms that live in it. Water is an inorganic substance as carbon is absent in H-O-H.

Polar molecule: The oxygen of water contains a negative charge and its hydrogen contains a positive charge, so water molecules are called polar molecules. As a result, the oxygen atom tends to attract the single electron of the hydrogen atom.

Biological importance of water

The water content varies from 65 to 90 percent in different organisms. Human tissues contain about 20% water in bone cells and 85% in brain cells.

Importance of Hydrogen bond in water: A hydrogen bond is much weaker than a covalent bond within a water molecule, but taken together, hydrogen bonds cause water molecules to cling together. Without hydrogen bonding between molecules, water would boil at 80°C and freeze at 100°C, making life impossible. But because of hydrogen bonding, water is a liquid at temperature suitable for life. It boils at 100°C and freezes at 0°C.

Solvent properties: Water is an excellent solvent for polar substances. These include ionic substances like salts which have charged particles i.e. ions and some non-ionic substance like sugar having polar group i.e. slightly negative hydroxyl group (-OH). On contact with water, the ions and the polar groups are surrounded by water molecules which separate the ions or molecules from each other. This is what happens when a substance dissolves in water. Once a substance is in solution, its molecule or ions can move freely, thus making it chemically more reactive than if it were solid. Thus, in the majority of the cells chemical reactions take place in aqueous solutions. Non-polar molecules are hydrophobic. Such hydrophobic interaction is important in the formation of membrane and helps to determine the three dimensional structure of many protein molecules and nucleic acids etc. Water’s solvent properties also mean that it acts as a transport medium e.g. blood and excretory system etc.

High heat capacity: Water has high heat capacity, i.e. a large increase in heat energy results - in a relatively small rise in temperature. The specific heat capacity of water the number of calories required to raise the temperature of 1 g of water from 15 to 16°C, is 1.0. The many hydrogen bonds that link water molecules help water to absorb heat without a great change in temperature and when water cools down, heat is released. However water holds heat and its temperature falls more slowly then other liquids. The property of water is important not only for aquatic organisms but also for all living things. Water protects organisms from rapid temperature change and helps them to maintain their normal internal temperature.

High heat of vaporization: It is expressed as calories absorbed per gram of water vaporized. The specific heat of vaporization of water is 574 k cal/kg. Evaporation of 2m1 out of one liter of water lowers the temperature of remaining 998 ml by l °C. Hydrogen bonds must be broken to change water to steam. For this, a large amount of heat is needed for evaporation. This property of water helps to moderate the temperature of Earth. It also gives animals in hot environment an efficient way to release heat. When an animal sweats its body heat is used to vaporize the sweat, thus cooling the animal.

High heat of fusion: Having high heat capacity, water requires relatively large amount of heat to thaw. Likewise liquid, water must lose a relatively large amount of heat to freeze. Contents of cells and their environment are therefore less likely to freeze.

Density and freezing properties: As water cools down, the molecules come closer together and water expands as it freezes. The density of water decreases below 4°C and ice therefore tends to float, as ice is less dense than liquid water. Water always freezes from the top down. When a body of water freezes on the surface, the ice acts as an insulator to prevent the water below it from freezing. This protects many aquatic organisms so that they can survive in winter.

Water molecules are cohesive and adhesive: Cohesion is the force whereby individual molecules stick together. The water molecules cling together because of hydrogen bonding. Because of cohesion water flows freely. Thus water is an excellent transport agent. They adhere to surface, particularly polar surfaces, therefore water exhibits adhesion. Cohesion and adhesion both contribute to the transport of water in plants.

High surface tension: At the surface of water a force called surface tension exists between the molecules as a result of cohesive forces between the molecules. These cohesive forces are due to hydrogen bonds. A water strider can even walk on the surface of a pond without breaking the surface of water.

Water as a reagent: Water is biologically significant as an essential metabolite, that is, it participates in chemical reactions of metabolism e.g. as a source of hydrogen in photosynthesis, and is used in hydrolysis reaction.

Water ionizes: When water ionizes, it releases an equal number of hydrogen ions (H) and hydroxide ion (OH).

                                   

                                     H-C-H  à  H+OH

       

 This reaction is reversible but equilibrium is maintained at 25°C. The H and 0H ion affect and take part in many of the reactions that occur in cells, e.g. it helps to maintain or change the pH of the medium.