Biological Importance of Water
Water is an important part of life: Without it, life on earth would
not exist. Water is a major component in cells, typically forming 70
to 95% of the cell's mass. In humans water is around 80% of our mass.
Water also provides an environment for organisms to live in. One
obvious example of water's biological importance
is that 75% of the
Earth is covered in water.
[IMAGE]Water is one of the most unusual compounds on Earth. It has a
variety of properties not found in any other liquid. These properties
are due to its molecular composition, which is simply just 2 hydrogen
atoms and one oxygen atom, making up H20.
Water is a covalent compound. This means that water has covalent bonds
which are formed by sharing electrons in the outer orbits of the
quantum shells. In the case of water however the large number of
protons in the oxygen nucleus have a stronger attraction for these
shared electrons than the comparatively tiny hydrogen nuclei. This
pulls the electrons slightly closer to the oxygen nucleus and away
from the hydrogen so that the oxygen develops a slight negative charge
and the hydrogen's a slight positive charge. This makes the water
molecule a slightly Polar Molecule.
This slight charge gives water its various properties; the first I
will discuss is its solvent properties.
Water can act as a solvent. "Things" can dissolve in solvents and
therefore "things" can dissolve in water. Substances that dissolve in
water sre known as hydrophilic substances Water can dissolve polar or
ionic substances, because they contain a charge.
[IMAGE]As can be seen from the diagram (right), Ionic substances such
as sodium chloride
, NaCl, are made up of positive and negative ions.
Sodium chloride is held in it's structure by the strong attraction
between it's positive sodium ions and negative chloride ions. Normally
these ionic attractions require a large amount of energy to break but
when put into water the negative oxygen side of the water molecules
cluster around the positive sodium ions Na+ and the positive hydrogen
atoms cluster around the negative chloride ions Cl-. The attraction
between the Na+ and Cl- ions is weakened as the ions are separated. As
the H20 molecules have surrounded the Na+ and Cl- Ions, they prevent
the ions from joining back together and so keeping the substance in
Water's property as a solvent is biologically important to life as
most biochemical reactions such as respiration occur in solution. A
medium, that is water, is required for the transportation, as reaction
of certain substances. Blood plasma is mostly water. Blood is needed
to transport vital substances around organisms. Water cannot dissolve
hydrophobic substances such as fats and oils.
Water also has many thermal properties. It has a high boiling point,
100oC, which is unusual for a compound of such small molecular mass,
(Mr H2O = 18). Other molecules of similar size such as Carbon dioxide
(Mr CO2 = 44) are gas at room temperature where as water is a liquid.
The water molecules have a weak, partial negative charge at one region
of the molecule (the oxygen atom in water) and a partial positive
charge elsewhere (the hydrogen atoms in [IMAGE]water).
Thus when water molecules are close together, their positive and
negative regions are attracted to the oppositely charged regions of
nearby molecules. The force of attraction, shown on the diagram as a
dotted line, is a hydrogen bond. Each water molecule is hydrogen
bonded to four others. Individual bonds are weak but the sheer number
of them means that the total force keeping the molecules together is
As water has millions of hydrogen bonds, it causes it to have a high
heat capacity. This means that water takes substantial heat to raise
the temperature of water significantly but once warm, it cools slowly.
This is essential to life where internal body temperature has to be
maintained at a constant temperature and fluctuations can result in a
breakdown of essential processes. Large bodies of water will remain at
an almost constant temperature with only very gradual changes, which
makes temperature regulations for organisms far more straightforward.
Because of the large number of bonds holding water molecules together,
it takes 2 kJ per gram of water, which is a considerable amount of
energy to separate the bonds and turn the liquid to vapour. Water is
therefore described as having a high latent heat of evaporation.
Animals use this property of water by using excess body heat to
evaporate water from their surfaces, resulting in them transferring a
lot of energy into the environment but only losing a little water.
Sweating and panting are based on this principle.
Water's freezing property is also quite unique. The density of water
when frozen (ice) at 0oC is less than liquid water. Most substances
when turning from liquids to a solid usually become denser, as the
energy in them is reduced, so they are held closer together, but this
is not the case with water. As water cools, its density does increase.
Hydrogen bonds between the water molecules take on a more latticed
formation as ice. Yet ice floats on the surface of water, which means
it's, density must be lower than that of water. Water is at it's most
dense at 4oC which is when its bonds are closest together. When water
freezes the lattice arrangement of its structure move apart slightly
and it floats on the surface. This means that the layer of ice
insulates the water below which stays at 4oC and aquatic life can
[IMAGE]The last important property of water is that it has a high
surface tension and cohesion. Surface tension can be seen in test
tubes where we see a curved meniscus. This is due to hydrogen bonds
causing a film of water to curve in the glass tube. Surface tension is
important for some insects such as water skaters and mosquito larvae
that live on surface film. They are able to survive because the film
allows them to be held above the water. Mosquito larvae hang upside
down with the air passage held above the water by surface tension. Its
ability to survive depends upon the surface tension. Without it, the
larvae would sink and die.
Cohesion is when things stick together, and are attracted to each
other. Water molecules are attracted to each other by hydrogen bonds.
If one molecule is pulled the neighbouring molecules as a result are
also dragged along. Eg, if you apply pressure in a straw, all the
water is pulled up. This is an important property as it enables water
to be transported upwards, such as in plants. The ability of a plant
to dissolve nutrients in roots and move them along the plant depends
on water's property of cohesion.