Water and Life

Water,its extraordinary  

properties and its essential 

role in living systems

The origin and maintenance   of life on earth depend critically 

upon water. Water is the most abundant
 of all compounds 

in cells, forming 60% to 90% of most living organisms. Water 

has several extraordinary properties that explain its essential 

role in living systems and their origin. These properties result 

largely from hydrogen bonds that form between its molecules

Geometry of water molecules. Each water molecule is linked 

by hydrogen bonds ( dashed lines) to four other molecules. 

If imaginary  lines connect the water molecules as shown, a 

tetrahedron is obtained.

Water has following important properties that support life

1.High specific heat capacity

Water has a high specific heat capacity: 1 calorie * is required 

to elevate  the temperature of 1 g of water 1° C, a higher thermal 

capacity than any other liquid except ammonia. Much of this heat 

energy is used to rupture some hydrogen bonds in addition to 

increasing the kinetic energy (molecular movement), and thus the 

temperature, of the water. Water’s high thermal capacity greatly 

moderates  environmental temperature changes, thereby protecting living organisms from extreme thermal  fluctuation.
Calorie

*A calorie is defined as the amount of heat required to heat 1 g of water 

from 14.5° to 15.5° C. Although the calorie is the traditional unit of heat
widely used in publications and tables, it is not part of the International
System of Units (the SI system) which uses the joule (J) as the energy
unit (1 cal 4.184 J).

2.High heat of vaporization 

Water also 

has a high heat of vaporization, requiring more than 500 calories to convert 1 g of liquid water to water vapor. All hydrogen 

bonds between a water molecule and its neighbors must be ruptured before that water molecule can escapeاڑجانا the surface and enter 

the air. For terrestrial  animals (and plants), cooling produced by 

evaporation of water is important for expelling  excess heat.

3.Unique Density Behavior 

Another property of water important for life is its unique 

density behavior during changes of temperature. Most liquids 

become denser with decreasing temperature. Water, however, 

reaches its maximum density at 4° C while still a liquid, then 

becomes less dense with further cooling. Therefore, 

ice floats rather than sinking to the bottoms of lakes and ponds. If

ice were denser than liquid water, bodies of water would freeze 

solid from the bottom upward in winter and might not melt completely in summer. Such conditions would severely  limit aquatic 

life. In ice, water molecules form an extensive, open, crystal-like 

network supported by hydrogen bonds that connect all molecules. The molecules in this lattice are farther apart, and thus less 

dense, than in liquid water at 4° C.

 When water freezes at 0° C, the four partial charges of each atom
in the molecule interact with the opposite charges of atoms in other
water molecules. The hydrogen bonds between all the molecules form
a crystal-like lattice structure, and the molecules are farther apart (and
thus less dense) than when some of the molecules have not formed
hydrogen bonds at 4° C

4.High Surface Tension

Water has high surface tension, exceeding that of any 

other liquid but mercury. Hydrogen bonding among water 

molecules produces a cohesiveness important for maintaining 

protoplasmic form and movement. The resulting surface tension creates an ecological niche  for insects, such as 

water striders and whirligig beetles, that skate on the surfaces of 

ponds.

Because of hydrogen bonds between water molecules at the water-air interface, the water molecules cling together and create a high 

surface tension. Thus some insects, such as this water strider, can 

literally walk on water.

5.Low Viscosity 

Despite its high surface tension, water has 

low viscosity, permitting  movement of blood through minute

capillaries and of cytoplasm inside cellular boundaries.

6.Excellent Solvent 

Water is an excellent solvent. Salts dissolve  more extensively 

in water than in any other solvent. This property results from the 

dipolar nature of water, which causes it to orient around charged 

particles dissolved in it. When, for example, crystalline NaCl dissolves in water, the Na  and Cl  ions separate (figure below). The 

negative zones of the water dipoles attract the Na  ions while 

the positive zones attract the Cl  ions. This orientation keeps 

the ions separated, promoting their dissociation. Solvents lacking this dipolar character are less effective at keeping the ions 

separated. Binding of water to dissolved protein molecules is 

essential to the proper functioning of many proteins. 

7.Reactions

1.Hydrolysis

Water also participates in many chemical reactions in living 

organisms. Many compounds are split into smaller pieces by the 

addition of a molecule of water, a process called hydrolysis

Cells disassemble macromolecules into their constituent subunits 

through reactions that are the reverse of dehydration—a molecule 

of water is added instead of removed (figure below) . In this reaction 

called hydrolysis, a hydrogen atom is attached to one subunit and 

a hydroxyl group to the other, breaking a specific covalent bond in 

the macromolecule.

2.Condensation Reaction

Likewise, larger compounds may be synthesized from smaller 

components by the reverse of hydrolysis, called condensation

reactions.

a. Biological macromolecules are polymers formed by linking
monomers together through dehydration reactions. This process
releases a water molecule for every bond formed. b. Breaking the
bond between subunits involves hydrolysis, which reverses the
loss of a water molecule by dehydration.