Transcript Water

Water
Dr Una Fairbrother
Snow
•The temperature at which a snow crystal forms determines its basic shape.
•A snowflake is an agglomerate of many snow crystals.
•Each snow crystal is symmetrical because its structure reflects the internal order
of the water molecules
• It eventually grows six evenly spaced branches.
• As more water vapour diffuses onto these branches, the crystal becomes heavy
and begins to fall from the sky.
•As it descends, it encounters very complex and variable atmospheric conditions.
•This results in each snow crystal having a unique design.
"Biochemistry is primarily the
chemistry of water."
Importance of water
Water is an important but often ignored
biological molecule
 Our bodies contain ~ 60% water
 Our muscles contain~ 75% water
 Edible fruits and vegetables may
contain about 90% water
 This suggests that water may have
some importance.
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What accounts for the ubiquitous
use of water in living systems?
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Comparing the structure of water to another substance,
methane, helps us to understand the unique properties water
possesses that make it well suited for biological systems.
Water (H2O) is made up of 2 hydrogen atoms and one oxygen
atom, with a total atomic weight of 18 daltons.
The structure of the electrons surrounding water is tetrahedral,
resembling a pyramid.
For comparison, Methane (CH4) is made up of one carbon and
4 hydrogens.
Note that methane is similar to water in that it weighs 16
daltons and also has a tetrahedral structure, yet has very
different physical properties
Roles
Important for..
 transporting molecules and ions in living
organisms
 providing a medium to allow chemical
reactions to take place.
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Chemistry
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Although water is a very common molecule it
has some unusual chemistry.
H2O
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If we look at the structure of water is it correct
to write
H-O-H ?
Structure of water
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In order to look at the structure we need to
consider how the atoms are connected.
 Each hydrogen is connected to oxygen by a
covalent bond
 How many electrons are needed to make a
covalent bond?
 Two, one electron is donated to the bond by
the hydrogen atom and one electron is
donated by the oxygen atom
How many electrons are left in the
outer shell of the oxygen atom?
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Four electrons remain in
the outer shell and these
are arranged in two
pairs of lone electrons.
 These two pairs form
nonbonding orbitals
 These four orbitals (2
bonding, 2 nonbonding )
repel each other so that
H-O-H has a bond angle
of 104.5o
H-O-H has a bond angle of
o
104.5
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The two lone pairs of
electrons and the
electronegative nature of
oxygen partly draws the
electrons away from the
hydrogen atoms
Results in oxygen end of
water has a partial
negative charge (-) and
that the hydrogen end of
the molecule has a partial
positive charge (+)
Polar Molecule
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Therefore water has
a partial ionic
character and is
said to be a polar
molecule
Hydrogen bonds
Opposite partial
charges can attract
each other.
 This forms weak bonds
between water
molecules.
 These are called
hydrogen bonds
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Methane is not polar
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Methane molecules do not have a dipole attraction for one
another
Thus spaced farther apart.
Despite its similar size and mass to water, methane is much less
dense than water.
This is the reason that under room temperature situations, water
exists as a liquid while methane is a gas.
We know that water can be converted to its gaseous phase,
steam, but only by applying a lot of energy in the form of heat to
disrupt the large attraction of the water molecules for one
another.
Hydrogen bonds in ice
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In water (whether ice, liquid
water or steam) the H--- O
hydrogen bonds are about 20
times weaker than H----O
covalent bonds.
In ice there is extensive
hydrogen bonding
The water molecules are
linked together in tetrahedral
arrays ie linked tetrahedrons.
In liquid water there is less
extensive hydrogen bonding
so it has a weaker structure
than ice .
Properties
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In the graph above
water is extensively
hydrogen bonded
whereas the other
compounds H2S
(hydrogen sulfide),
H2Se (hydrogen
selenide) and H2Te
(hydrogen telluride)
are not.
High melting and boiling
points
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The extensive hydrogen bonding in water
makes it more difficult to separate one
molecule from another and therefore results
in abnormally high melting and boiling
points.
Water has high values for
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Specific heat - amount of heat require to raise
the temp of 1g water by 1 oC
Latent heat of evaporation - amount of heat that
water absorbs without a rise in temp as it
changes from a liquid to a gas
Latent heat of fusion - amount of heat that water
absorbs without a rise in temp as it changes
from a solid to a liquid
Surface tension - due to cohesive forces
All of the above are due to hydrogen bonding
Water is a good solvent
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The polar nature of water,
with its partial positive and
partial negative dipole,
allows it to dissolve charged
molecules (ions) easily.
Water is thus an excellent
solvent for charged
compounds.
The positive side of water
surrounds negatively
charged molecules,
the negatively charged side
of water surrounds positively
charged molecules.
Water makes "solvation shells"
around ions
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Water can also readily dissolve other polar molecules, even if
they are not positively or negatively charged.
The solvent properties of water allow for dissolved metals and
buffering systems that are very important for the workhorses of
life, enzymes.
However, the saying "oil and water don't mix" is true--water
cannot dissolve oil.
This is because oily substances are non-polar. Non-polar
substances (which lack dipoles) are also called hydrophobic
(water fearing).
Hydrophobic substances gather together to exclude water as
best they can.
This is why you see oil droplets in water.
This is also important for the stability and structure of enzymes.
pH: Ionization of Water
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Sometimes the hydrogen of one water molecule will "jump" to
another water molecule:
H2O + H2O H3O+ + OHThis proton hopping is called the ionization of water (an ion is a
positively or negatively charged atom or molecule). This ionization
creates a H3O+ and a OH- molecule. The H3O+ is often written as
simply H+. This is because a H3O+ is just a H+ that jumps from one
water molecule to another.
H2O H+ + OHSo remember, H3O+ = H+
Looking at either of the two chemical equations above, it is
important to note that the reverse reaction is also occurring
How much H+ and OH- exist in
water?
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Very, very little! The ratio of either H+ or OH- to H2O in neutral water
is 1:1,000,000,000! Since this is such a small amount of either H+ or
OH-, they rarely meet and neutralize each other.
The equilibrium constant, Keq describes the ionization equilibrium of
water:
Keq = [H+][OH-]
Because of this relationship it is important to note that if the [H+]
goes up then the [OH-] must go down, and vice-versa, for the value
for the Keq of water must remain constant. For neutral water, the Keq
is 1 x 10-14 M and the concentrations of [H+] and [OH-] are each 1 x
10-7 M . Let's look at that last number without the exponent:
0.0000001 M
This is obviously a very small number. A more manageable way to
discuss small numbers such as this is to take the negative logarithm.
For the concentration of [H+], this is called the pH. In this case:
-log(0.0000001 M) = 7
The pH of a solution
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The pH of a solution is simply the negative logarithm of [H+].
It describes the acidity of a solution.
Acidic solutions are those with a pH of less than 7 and basic solutions
have a pH greater than 7.
A solution, like H2O, with a pH = 7 is neutral.
Similarly, the pOH could be used to describe a solution in terms of its
OH- concentration.
pOH is the negative logarithm of the OH- concentration.
One useful thing to remember is:
pH + pOH = 14.
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In the body, the pH of blood is 7.4.
 This corresponds to a [H+] of about 40 nM. This value can only vary from
37 nM to 43 nM without serious metabolic consequences.
Buffers
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The pH of a solution is dependent on
the concentration of H+ ions.
Addition or removal of H+ ions, then,
can greatly affect the pH of a solution.
In the body, the pH of cells and
extracellular fluids can vary from pH 8
in pancreatic fluid to pH 1 in stomach
acids.
The average pH of blood is 7.4, and of
cells is 7 - 7.3.
Although there is great variation in pH
between the fluids in the body, there is
little variation within each system.
E.g. blood pH only varies between
7.35 - 7.45 in a healthy individual.
Large changes in pH can be life
threatening.
How does the body maintain a
constant blood pH?
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The body uses a buffer system to withstand changes in pH.
Buffers are made up of a mixture of a weak acid with its
conjugate base or a weak base with its conjugate acid.
An acid donates a H+.
A weak acid does not donate its H+ as easily.
Similarly, a weak base will not accept a H+ as well as a strong
base.
Buffers maintain pH by binding H+ or OH- ions.
This stabilizes changes in pH.
The bicarbonate buffer system maintains blood pH near pH 7.4.
The carbonic acid, H2CO3, in the blood is in equilibrium with the
carbon dioxide (CO2), in the air.
Protein structural stability
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As a solvent, water plays an important role in maintaining the
correct folding of a protein e.g. an enzyme
Proteins unfold from their native fold upon the addition of urea or
other denaturants,
The presence of alpha helicies increases upon addition of some
solvents, including alcohol
Bound water may play a role in stabilising the structure of single
proteins as well as that of complexes.
Water and ions within the pore of the
nicotinic acetylcholine receptor
Protein (immunoglobulin) as
wireframe surrounded by water
Summary: Water is an important but
often ignored biological molecule
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Opposite partial charges attract forming weak bonds called hydrogen
bonds
Polar Molecule
High melting and boiling points
High values for: Specific heat, Latent heat of evaporation, Latent heat of
fusion, Surface tension - due to hydrogen bonding
Water makes "solvation shells" around ions
The ratio of either H+ or OH- to H2O in neutral water is 1:1,000,000,000
The pH of a solution is simply the negative logarithm of [H+].
Buffers maintain pH
Water stabilises proteins