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Water - Biochemistry Lecture
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BIOCHEMISTRY – LECTURE THREE PART ONEbiochemistry of water
In water each hydrogen atom is covalently bonded with an oxygen atom. The
oxygen atom also has two pairs of electrons not involved in bonding. Because of
the pairs of unbonded electrons (lone pairs) the shape of the molecule is bent
due to electron repulsion. The electronegativity of oxygen relative to hydrogen
causes water to be a polar molecule. It has areas of relative positivity and an
area of relative negativity.
o Because of the polarity, there is an electrostatic force of attraction
between the + hydrogen part of the molecule and the - oxygen part of
another molecule. This is known as a hydrogen bond.
Hydrogen bonds are intermolecular bonds, unlike covalent and ionic bonds which
are intramolecular. The unique properties of water are due to the hydrogen
bonds between the molecules. Hydrogen bonding can occur between a hydrogen
atom attached to nitrogen or an oxygen atom and oxygen or nitrogen of a
different molecule.
Water has many properties:
Liquid at physiological temperatures – this is key to waters importance as
a medium of reactions in cells.
Solid is less dense than liquid – the property of the solid form of a
substance being less dense than the liquid is very unusual. There are a
few other substances that have this property, but they are mainly metallic.
Involves itself in reactions – condensation polymerisation involves the
elimination of water and hydrolysis requires water.
High temperature of evaporation – for a molecule of its size, it takes a lot
of energy to turn liquid water into vapour. This is why sweating is an
effective mechanism of cooling down, when sweat leaves the skin a lot of
heat is dissipated.
High surface tension – because of the hydrogen bonds between the water
molecules, water has a cohesive nature. The sticking together of
molecules at the surface gives it surface tension.
Superb solvent – water is almost capable of dissolving any substance in it.
Because of the polar nature of water, it can dissolve a wide range of ionic
and polar molecules. Because water is such an excellent solvent, it is the
medium for almost all chemical reactions in the body.
Hydrogen bonds are not permanent because of the kinetic energy of the
molecules, they move around, constantly breaking and making new hydrogen
bonds between them. If the water is heated enough molecules of water may
have enough energy to break all bonds and leave the liquid state. If the
temperature drops, the water molecules move less until their movement is no
longer able to break the hydrogen bonds and the bonds become fixed. This fixing
increases the space between them, hence why ice is more dense than liquid
water.
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