Biological Molecules
Learning Intention: To understand the nature of
water as a biological molecule.
Biological molecules are those which are found in
living things. Biochemistry is a branch of biology
which deals with the study of these molecules. This
is fundamental to our understanding of how living
things work. The human body is mainly composed
of the following substances:
Substance
Percentage %
Water
Lipids
Proteins
Carbohydrates
Inorganic ions
70
15
12
0.15
2.5
Carbohydrates, lipids and proteins are collectively
known as organic molecules. They are produced
inside living things and are characterised by the
fact that they contain carbon in their structure.
Water is a very important inorganic molecule. It is
perhaps the most important molecule for the
survival and life of organisms. On the surface of
the planet there is obviously much more water than
land which shows its significance. To understand
the importance of water both as a habitat for living
things and as a vital component of living things we
must first consider the structure of the molecule. It
is the chemical structure gives it specific properties
which make it crucial as a component of living
things.
Water has a chemical formula H² O.This means that
water is a compound composed of two atoms of
hydrogen and one of oxygen. These atoms are
bonded together by strong covalent bonds where an
electron is shared by the oxygen and the hydrogen.
However due to the nucleus of the oxygen carrying
a greater number of protons and hence a greater
positive charge it pulls the negatively charged
shared electrons away from the hydrogen and
towards itself. This results in an uneven
distribution of charge due to the electrons being
drawn towards the oxygen. The oxygen side of the
molecule becomes more negative which results in
an uneven distribution of charge around the
molecule. For this reason water is described as a
polar molecule.
Copy fig. 3.3 p.62
Hydrogen Bonds:
Within a body of water the polarity of the water
molecules gives rise to attractions between
molecules where the positive hydrogen of one
molecule sticks to the negative oxygen of another
molecule. These areas of attraction are formed by
hydrogen bonds.
Copy Fig.3.5 P.63
It is the polar nature of water and the hydrogen
bonding that exists between the molecules that give
water its unique properties. It allows for water to
move as a continuous column in transpiration. It
causes it to have a high specific heat capacity
which means it requires a lot of energy to raise the
temperature of water. For this reason it offers a
very stable environment to aquatic cold blooded
animals.
Water makes up 70% of the human body. Cell
cytoplasm is mainly water. It is ideal as a solvent
due to its polar nature. The majority of chemical
reactions which occur within cells do in aqueous
solution. The polar nature of water results in
dissociation of substances such as salts eg. NaCl.
The sodium and chloride ions are pulled apart and
are able to move freely in solution and as such they
are able to take part in metabolic reactions.
Inorganic ions Copy table P.65
Essay: How does the chemical nature of water
make it ideal as an environment for and a
component of living things?