WaterWater

Dr Una FairbrotherDr Una Fairbrother

SnowSnow

•The temperature at which a snow crystal forms determines its basic shape.The temperature at which a snow crystal forms determines its basic shape.•A snowflake is an agglomerate of many snow crystals.A snowflake is an agglomerate of many snow crystals.•Each snow crystal is symmetrical because its structure reflects the internal order Each snow crystal is symmetrical because its structure reflects the internal order of the water molecules of the water molecules • It eventually grows six evenly spaced branches.It eventually grows six evenly spaced branches.• As more water vapour diffuses onto these branches, the crystal becomes heavy As more water vapour diffuses onto these branches, the crystal becomes heavy and begins to fall from the sky. and begins to fall from the sky. •As it descends, it encounters very complex and variable atmospheric conditions. As it descends, it encounters very complex and variable atmospheric conditions. •This results in each snow crystal having a unique design.This results in each snow crystal having a unique design.

"Biochemistry is primarily the "Biochemistry is primarily the chemistry of water."chemistry of water."

Importance of waterImportance of water

Water is an important but often ignored Water is an important but often ignored biological moleculebiological molecule

Our bodies contain ~ 60% waterOur bodies contain ~ 60% water Our muscles contain~ 75% waterOur muscles contain~ 75% water Edible fruits and vegetables may Edible fruits and vegetables may

contain about 90% watercontain about 90% water This suggests that water may have This suggests that water may have

some importance.some importance.

What accounts for the ubiquitous What accounts for the ubiquitous use of water in living systems?use of water in living systems?

Comparing the structure of water to another substance, Comparing the structure of water to another substance, methane, helps us to understand the unique properties water methane, helps us to understand the unique properties water possesses that make it well suited for biological systems. possesses that make it well suited for biological systems.

Water (HWater (H22O) is made up of 2 hydrogen atoms and one oxygen O) is made up of 2 hydrogen atoms and one oxygen

atom, with a total atomic weight of 18 daltons. atom, with a total atomic weight of 18 daltons. The structure of the electrons surrounding water is tetrahedral, The structure of the electrons surrounding water is tetrahedral,

resembling a pyramid.resembling a pyramid. For comparison, Methane (CH4) is made up of one carbon and For comparison, Methane (CH4) is made up of one carbon and

4 hydrogens.4 hydrogens. Note that methane is similar to water in that it weighs 16 Note that methane is similar to water in that it weighs 16

daltons and also has a tetrahedral structure, yet has very daltons and also has a tetrahedral structure, yet has very different physical propertiesdifferent physical properties

RolesRoles

Important for..Important for.. transporting molecules and ions in living transporting molecules and ions in living

organismsorganisms providing a medium to allow chemical providing a medium to allow chemical

reactions to take place.reactions to take place.

ChemistryChemistry

Although water is a very common molecule it Although water is a very common molecule it has some unusual chemistry.has some unusual chemistry.

  

HH22OO

   If we look at the structure of water is it correct If we look at the structure of water is it correct

to writeto writeH-O-HH-O-H ? ?

  

Structure of waterStructure of water In order to look at the structure we need to In order to look at the structure we need to

consider how the atoms are connected.consider how the atoms are connected. Each hydrogen is connected to oxygen by a Each hydrogen is connected to oxygen by a

covalent bondcovalent bond How many electrons are needed to make a How many electrons are needed to make a

covalent bond?covalent bond? Two, one electron is donated to the bond by Two, one electron is donated to the bond by

the hydrogen atom and one electron is the hydrogen atom and one electron is donated by the oxygen atomdonated by the oxygen atom

How many electrons are left in the How many electrons are left in the outer shell of the oxygen atom?outer shell of the oxygen atom?

Four electrons remain in Four electrons remain in the outer shell and these the outer shell and these are arranged in two are arranged in two pairs of lone electrons. pairs of lone electrons.

These two pairs form These two pairs form nonbonding orbitalsnonbonding orbitals

These four orbitals (2 These four orbitals (2 bonding, 2 nonbonding ) bonding, 2 nonbonding ) repel each other so that repel each other so that H-O-H has a bond angle H-O-H has a bond angle of 104.5of 104.5oo

H-O-H has a bond angle of H-O-H has a bond angle of 104.5104.5oo

The two lone pairs of The two lone pairs of electrons and the electrons and the electronegative nature of electronegative nature of oxygen partly draws the oxygen partly draws the electrons away from the electrons away from the hydrogen atoms hydrogen atoms

Results in oxygen end of Results in oxygen end of water has a partial water has a partial negative chargenegative charge (-) (-) and and that the hydrogen end of that the hydrogen end of the molecule has a partial the molecule has a partial positive chargepositive charge (+) (+)

Polar MoleculePolar Molecule

Therefore water has Therefore water has a partial ionic a partial ionic character and is character and is said to be a said to be a polarpolar moleculemolecule

 

Hydrogen bondsHydrogen bonds

Opposite partial Opposite partial charges can charges can attract each attract each other.other.

This forms weak This forms weak bonds between bonds between water molecules.water molecules.

These are called These are called hydrogen bondshydrogen bonds

Methane is not polarMethane is not polar Methane molecules do not have a dipole attraction for one Methane molecules do not have a dipole attraction for one

another another Thus spaced farther apart. Thus spaced farther apart. Despite its similar size and mass to water, methane is much Despite its similar size and mass to water, methane is much

less dense than water. less dense than water. This is the reason that under room temperature situations, water This is the reason that under room temperature situations, water

exists as a liquid while methane is a gas. exists as a liquid while methane is a gas. We know that water can be converted to its gaseous phase, 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 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 disrupt the large attraction of the water molecules for one another.another.

Hydrogen bonds in iceHydrogen bonds in ice

In water (whether ice, liquid In water (whether ice, liquid water or steam) the H--- O water or steam) the H--- O hydrogen bonds are about 20 hydrogen bonds are about 20 times weaker than H----O times weaker than H----O covalent bonds.covalent bonds.

In ice there is extensive In ice there is extensive hydrogen bonding hydrogen bonding

The water molecules are The water molecules are linked together in tetrahedral linked together in tetrahedral arrays ie linked tetrahedrons.arrays ie linked tetrahedrons.

In liquid water there is less In liquid water there is less extensive hydrogen bonding extensive hydrogen bonding so it has a weaker structure so it has a weaker structure than ice .than ice .

PropertiesProperties

In the graph above In the graph above water is extensively water is extensively hydrogen bonded hydrogen bonded whereas the other whereas the other compounds Hcompounds H22S S (hydrogen sulfide), (hydrogen sulfide), HH22Se (hydrogen Se (hydrogen selenide) and Hselenide) and H22Te Te (hydrogen telluride) (hydrogen telluride) are not.are not.

High melting and boiling High melting and boiling pointspoints

The extensive hydrogen bonding in The extensive hydrogen bonding in water makes it more difficult to water makes it more difficult to separate one molecule from separate one molecule from another and therefore results in another and therefore results in abnormally high melting and abnormally high melting and boiling points.boiling points.

  Water has high values forWater has high values for

Specific heat - amount of heat require to raise Specific heat - amount of heat require to raise the temp of 1g water by 1 the temp of 1g water by 1 ooC C 

Latent heat of evaporation - amount of heat that Latent heat of evaporation - amount of heat that water absorbs without a rise in temp as it water absorbs without a rise in temp as it changes from a liquid to a gaschanges from a liquid to a gas

Latent heat of fusion - amount of heat that water Latent heat of fusion - amount of heat that water absorbs without a rise in temp as it changes absorbs without a rise in temp as it changes from a solid to a liquidfrom a solid to a liquid

Surface tension - due to cohesive forcesSurface tension - due to cohesive forces All All of the above are due to hydrogen bondingof the above are due to hydrogen bonding

Water is a good solventWater is a good solvent

The polar nature of water, The polar nature of water, with its partial positive and with its partial positive and partial negative dipole, partial negative dipole, allows it to dissolve charged allows it to dissolve charged molecules (ions) easily.molecules (ions) easily.

Water is thus an excellent Water is thus an excellent solvent for charged solvent for charged compounds. compounds.

The positive side of water The positive side of water surrounds negatively surrounds negatively charged molecules, charged molecules,

the negatively charged side the negatively charged side of water surrounds positively of water surrounds positively charged molecules.charged molecules.

Water makes "solvation shells" Water makes "solvation shells" around ionsaround ions

Water can also readily dissolve other polar molecules, even if Water can also readily dissolve other polar molecules, even if they are not positively or negatively charged. they are not positively or negatively charged.

The solvent properties of water allow for dissolved metals and The solvent properties of water allow for dissolved metals and buffering systems that are very important for the workhorses of buffering systems that are very important for the workhorses of life, enzymes. life, enzymes.

However, the saying "oil and water don't mix" is true--water However, the saying "oil and water don't mix" is true--water cannot dissolve oil. cannot dissolve oil.

This is because oily substances are non-polar. Non-polar This is because oily substances are non-polar. Non-polar substances (which lack dipoles) are also called hydrophobic substances (which lack dipoles) are also called hydrophobic (water fearing). (water fearing).

Hydrophobic substances gather together to exclude water as Hydrophobic substances gather together to exclude water as best they can. best they can.

This is why you see oil droplets in water. This is why you see oil droplets in water. This is also important for the stability and structure of enzymes.This is also important for the stability and structure of enzymes.

pH: Ionization of WaterpH: Ionization of Water

Sometimes the hydrogen of one water molecule will "jump" to Sometimes the hydrogen of one water molecule will "jump" to another water molecule:another water molecule:

H2O + H2O H3O+ + OH-H2O + H2O H3O+ + OH- This proton hopping is called the ionization of water (an ion is a This proton hopping is called the ionization of water (an ion is a

positively or negatively charged atom or molecule). This ionization positively or negatively charged atom or molecule). This ionization creates a H3O+ and a OH- molecule. The H3O+ is often written as 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 simply H+. This is because a H3O+ is just a H+ that jumps from one water molecule to another.one water molecule to another.

H2O H+ + OH-H2O H+ + OH- So remember, H3O+ = H+So remember, H3O+ = H+ Looking at either of the two chemical equations above, it is Looking at either of the two chemical equations above, it is

important to note that the reverse reaction is also occurringimportant to note that the reverse reaction is also occurring

How much H+ and OH- exist in How much H+ and OH- exist in water?water? Very, very little! The ratio of either H+ or OH- to H2O in neutral water 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 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.OH-, they rarely meet and neutralize each other.

The equilibrium constant, Keq describes the ionization equilibrium of The equilibrium constant, Keq describes the ionization equilibrium of water: Keq = [H+][OH-]water: Keq = [H+][OH-]

Because of this relationship it is important to note that if the [H+] 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 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 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 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:each 1 x 10-7 M . Let's look at that last number without the exponent:

0.0000001 M0.0000001 M This is obviously a very small number. A more manageable way to This is obviously a very small number. A more manageable way to

discuss small numbers such as this is to take the negative logarithm. 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:For the concentration of [H+], this is called the pH. In this case:

-log(0.0000001 M) = 7-log(0.0000001 M) = 7

The pH of a solutionThe pH of a solution

The pH of a solution is simply the negative logarithm of [H+]. The pH of a solution is simply the negative logarithm of [H+]. It describes the acidity of a solution. It describes the acidity of a solution. Acidic solutions are those with a pH of less than 7 and basic solutions Acidic solutions are those with a pH of less than 7 and basic solutions

have a pH greater than 7. have a pH greater than 7. A solution, like H2O, with a pH = 7 is neutral.A solution, like H2O, with a pH = 7 is neutral. Similarly, the pOH could be used to describe a solution in terms of its Similarly, the pOH could be used to describe a solution in terms of its

OH- concentration. OH- concentration. pOH is the negative logarithm of the OH- concentration. pOH is the negative logarithm of the OH- concentration. One useful thing to remember is:One useful thing to remember is:

pH + pOH = 14.pH + pOH = 14.

In the body, the pH of blood is 7.4. 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 This corresponds to a [H+] of about 40 nM. This value can only vary from

37 nM to 43 nM without serious metabolic consequences.37 nM to 43 nM without serious metabolic consequences.

BuffersBuffers

The pH of a solution is dependent on The pH of a solution is dependent on the concentration of H+ ions. the concentration of H+ ions.

Addition or removal of H+ ions, then, Addition or removal of H+ ions, then, can greatly affect the pH of a solution. can greatly affect the pH of a solution.

In the body, the pH of cells and In the body, the pH of cells and extracellular fluids can vary from pH 8 extracellular fluids can vary from pH 8 in pancreatic fluid to pH 1 in stomach in pancreatic fluid to pH 1 in stomach acids. acids.

The average pH of blood is 7.4, and of The average pH of blood is 7.4, and of cells is 7 - 7.3. cells is 7 - 7.3.

Although there is great variation in pH Although there is great variation in pH between the fluids in the body, there is between the fluids in the body, there is little variation within each system.little variation within each system.

E.g. blood pH only varies between 7.35 E.g. blood pH only varies between 7.35 - 7.45 in a healthy individual. - 7.45 in a healthy individual.

Large changes in pH can be life Large changes in pH can be life threatening.threatening.

How does the body maintain a How does the body maintain a constant blood pH?constant blood pH?

The body uses a buffer system to withstand changes in pH. The body uses a buffer system to withstand changes in pH. Buffers are made up of a mixture of a weak acid with its Buffers are made up of a mixture of a weak acid with its conjugate base or a weak base with its conjugate acid.conjugate base or a weak base with its conjugate acid.

An acid donates a H+. An acid donates a H+. A weak acid does not donate its H+ as easily. A weak acid does not donate its H+ as easily. Similarly, a weak base will not accept a H+ as well as a strong Similarly, a weak base will not accept a H+ as well as a strong

base.base. Buffers maintain pH by binding H+ or OH- ions. Buffers maintain pH by binding H+ or OH- ions. This stabilizes changes in pH. This stabilizes changes in pH. The bicarbonate buffer system maintains blood pH near pH 7.4.The bicarbonate buffer system maintains blood pH near pH 7.4. The carbonic acid, H2CO3, in the blood is in equilibrium with the The carbonic acid, H2CO3, in the blood is in equilibrium with the

carbon dioxide (CO2), in the air.carbon dioxide (CO2), in the air.

Protein structural stabilityProtein structural stability

As a solvent, water plays an important role in maintaining the As a solvent, water plays an important role in maintaining the correct folding of a protein e.g. an enzymecorrect folding of a protein e.g. an enzyme

Proteins unfold from their native fold upon the addition of urea Proteins unfold from their native fold upon the addition of urea or other denaturants, or other denaturants,

The presence of alpha helicies increases upon addition of some The presence of alpha helicies increases upon addition of some solvents, including alcohol solvents, including alcohol

Bound water may play a role in stabilising the structure of single Bound water may play a role in stabilising the structure of single proteins as well as that of complexes. proteins as well as that of complexes.

Water and ions within the pore of the Water and ions within the pore of the nicotinic acetylcholine receptornicotinic acetylcholine receptor

Protein (immunoglobulin) as Protein (immunoglobulin) as wireframe surrounded by waterwireframe surrounded by water

Summary: Summary: Water is an important Water is an important but often ignored biological moleculebut often ignored biological molecule Opposite partial charges attract forming weak bonds called hydrogen Opposite partial charges attract forming weak bonds called hydrogen

bonds bonds Polar MoleculePolar Molecule High melting and boiling pointsHigh melting and boiling points High values for: Specific heat, Latent heat of evaporation, Latent heat High values for: Specific heat, Latent heat of evaporation, Latent heat

of fusion, Surface tension - due to hydrogen bondingof fusion, Surface tension - due to hydrogen bonding Water makes "solvation shells" around ionsWater makes "solvation shells" around ions The ratio of either H+ or OH- to H2O in neutral water is 1:1,000,000,000The 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+].The pH of a solution is simply the negative logarithm of [H+]. Buffers maintain pHBuffers maintain pH Water stabilises proteinsWater stabilises proteins