Detailed notes on biological molecules. Notes written using OCR Biology A textbooks, past papers and more. Written by a student with all A*s at GCSE with an offer for Natural Sciences at Cambridge.
2.1.2 Biological molecules
(a) How hydrogen bonding occurs between water molecules, and relate this,
and other properties of water, to the roles of water for living organisms
In covalent bonds, electrons are shared between two atoms,
but often the electrons are closer to one atom than the
other. In water, there are two hydrogen atoms covalently
bonded to an oxygen atom, but because oxygen has more
positive protons in its nucleus, it exerts a stronger attraction
on the shared electrons, so the
electrons are closer to the
oxygen nucleus. This means the
oxygen atom gains a slightly
negative charge where its
unshared electrons accumulate
and the hydrogen atom a slightly positive one, on the opposite side
to where the electrons are shared with the oxygen atom. This
means the molecule is ‘polar’.
The negatively charged oxygen atoms attract the positively charged
hydrogen atoms of other water molecules. This attraction is called
‘hydrogen bonding’. Hydrogen bonds are not as strong as covalent
bonds but give water some interesting properties.
Properties of water:
High Specific Heat Capacity
Specific heat capacity is the amount of energy required to raise the temperature of 1g of water
by 1˚C (4.2J). Water molecules are held together tightly by hydrogen bonds, so they require lots
of energy to heat up, meaning they don’t experience rapid temperature changes. This is
important for:
Living things such as prokaryotes and eukaryotes that need a stable temperature for
enzyme-controlled reactions to occur in their body. Enzymes only work in a narrow
temperature range and the water buffers heat change
Aquatic organisms need a stable environment in which to live
Gases remain soluble
High Latent Heat of Vaporisation
A lot of energy is used up when water evaporates, as large amounts of energy are required to
break the hydrogen bonds into a gas. This is known as a high latent heat of vaporisation, and it
allows a great cooling effect, as the molecules with the most kinetic energy escape during
evaporation, lowering the average KE. This is useful for:
Cooling mammals when they sweat
Cooling plants when water evaporates from mesophyll cells
High Cohesion
Cohesion is attraction between molecules of the same type. Water molecules have high cohesion
due to their polarity, meaning it moves as one mass. This helps:
Plants draw up water from roots to leaves in transpiration stream through ‘mass flow’
Surface Tension
Water molecules at the surface of the water are all hydrogen bonded to the water molecules
beneath them, so are more attracted to the water below, than the air. This means the surface of
the water contracts, giving it the ability to resist force applied to it. This allows:
Insects like pond-skaters to walk on water
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