Page 7 - 11-Bio-2 Biological Molecules

2. Biological Molecules eLearn.Punjab

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 1°C.

Ionization of water

The water molecules ionize to form H+ and OH- ions:
H20 m H+ + OH-

This reaction is reversible but an equilibrium is maintained. At 25°C the concentration of each of

H+ and OH- ions in pure water is about 10-7 mole/litre. 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.

CARBOHYDRATES

Carbohydrates occur abundantly in living organisms. They are found in all organisms and in almost
all parts of the cell. Cellulose of wood, cotton and paper, starches present in cereals, root tubers, cane
sugar and milk sugar are all examples of carbohydrates. Carbohydrates play both structural and
functional roles. Simple carbohydrates are the main source of energy in cells. Some carbohydrates
are the main constituents of cell walls in plants and micro-organisms.

The word carbohydrate literally means hydrated carbons. They are composed of carbon, hydrogen
and oxygen and the ratio of hydrogen and oxygen is the same as in water. Their general formula
is Cx (H20 )y where(x) is the whole number from three to many thousands whereas y may be the
same or different whole number. Chemically, carbohydrates are defined as polyhydroxy aldehydes
or kentones, or complex substances which on hydrolysis yield polyhydroxy aldehyde or ketone
subunits. (Hydrolysis involves the break down of large molecules into smaller ones utilizing water
molecules).

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