2.4 Water1

A Level Biology CIE Your notes
2.4 Water
Contents
2.4.1 Water & the Hydrogen Bond
2.4.2 T he Role of Water in Living Organisms
2.4.1 Water & the Hydrogen Bond
Your notes
Water Molecules: Hydrogen Bonds
Water is of great biological importance. It is the medium in which all metabolic reactions take
place in cells. Between 70% to 95% of the mass of a cell is water
As 71% of the Earth’s surface is covered in water it is a major habitat for organisms
Water is composed of atoms of hydrogen and oxygen. One atom of oxygen combines with two
atoms of hydrogen by sharing electrons (covalent bonding)
Although water as a whole is electrically neutral the sharing of the electrons is uneven between
the oxygen and hydrogen atoms
The oxygen atom attracts the electrons more strongly than the hydrogen atoms, resulting in a
weak negatively charged region on the oxygen atom (δ-) and a weak positively charged
region on the hydrogen atoms(δ+), this also results in the asymmetrical shape
This separation of charge due to the electrons in the covalent bonds being unevenly shared is
called a dipole. When a molecule has one end that is negatively charged and one end that is
positively charged it is also a polar molecule
Water is a polar molecule
The covalent bonds of water make it a polar molecule
Hydrogen bonds form between water molecules Your notes

As a result of the polarity of water hydrogen bonds form between the positive and negatively
charged regions of adjacent water molecules
Hydrogen bonds are weak, when there are few, so they are constantly breaking and reforming.
However when there are large numbers present they form a strong structure
Hydrogen bonds contribute to the many properties water molecules have that make them so
important to living organisms:
An excellent solvent – many substances can dissolve in water
A relatively high specific heat capacity
A relatively high latent heat of vaporisation
Water is less dense when a solid
Water has high surface tension and cohesion
It acts as a reagent
The polarity of water molecules allows hydrogen bonds to form between adjacent water molecules
It is important to know where the hydrogen bonds form between water molecules (oxygen of Your notes
2.4.2 The Role of Water in Living Organisms
Your notes
Water Molecules: In Living Organisms
Water has many essential roles in living organisms due to its properties:
The polarity of water molecules
The presence and number of hydrogen bonds between water molecules
Solvent
As water is a polar molecule many ions (e.g. sodium chloride) and covalently bonded polar
substances (e.g. glucose) will dissolve in it
This allows chemical reactions to occur within cells (as the dissolved solutes are more
chemically reactive when they are free to move about)
Metabolites can be transported efficiently (except non-polar molecules which are
hydrophobic)
Your notes
Due to its polarity water is considered a universal solvent
High specific heat capacity

The specific heat capacity of a substance is the amount of thermal energy required to raise the
temperature of 1kg of that substance by 1°C. Water’s specifi c heat capacity is 4200 J/kg°C
The high specific heat capacity is due to the many hydrogen bonds present in water. It takes a lot
of thermal energy to break these bonds and a lot of energy to build them, thus the temperature of
water does not f l u c t u a t e greatly
The advantage for living organisms is that it:
Provides suitable habitats
Allows for constant temperatures within bodies and cells to be maintained (this ensures
enz ymes have the optimal temperatures)
This is because a large increase in energy is needed to increase the temperature of water
Latent heat of vaporisation
In order to change state (from liquid to gas) a large amount of thermal energy must be absorbed
Your notes
by water to break the hydrogen bonds and evaporate
This is an advantage for living organisms as only a little water is required to evaporate for the
organism to lose a great amount of heat
This provides a cooling effect for living organisms, for example the transpiration from leaves or
evaporation of water in sweat on the skin
Properties of water & its role in living organisms table
When discussing the role water has in living organisms remember to mention the ‘why’ in relation
to its properties (ie. it is an excellent solvent due to the polar nature of water molecules).

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A Level Biology CIE Your notes

Hydrogen bonds form between water molecules Your notes

Due to its polarity water is considered a universal solvent

High specific heat capacity

Properties of water & its role in living organisms table

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