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Water Dr Una Fairbrother. Snow The temperature at which a snow crystal forms determines its basic shape.The temperature at which a snow crystal forms.

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Presentation on theme: "Water Dr Una Fairbrother. Snow The temperature at which a snow crystal forms determines its basic shape.The temperature at which a snow crystal forms."— Presentation transcript:

1 Water Dr Una Fairbrother

2 Snow 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 of the water moleculesEach snow crystal is symmetrical because its structure reflects the internal order 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 and begins to fall from the sky. 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.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.

3 "Biochemistry is primarily the chemistry of water."

4 Importance of water Water is an important but often ignored biological molecule Water is an important but often ignored biological molecule Our bodies contain ~ 60% water Our bodies contain ~ 60% water Our muscles contain~ 75% water Our muscles contain~ 75% water Edible fruits and vegetables may contain about 90% water Edible fruits and vegetables may contain about 90% water This suggests that water may have some importance. This suggests that water may have some importance.

5 What accounts for the ubiquitous use of water in living systems? 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. 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 (H 2 O) is made up of 2 hydrogen atoms and one oxygen atom, with a total atomic weight of 18 daltons. Water (H 2 O) 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. 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. 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 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

6 Roles Important for.. Important for.. transporting molecules and ions in living organisms transporting molecules and ions in living organisms providing a medium to allow chemical reactions to take place. providing a medium to allow chemical reactions to take place.

7 Chemistry Although water is a very common molecule it has some unusual chemistry. Although water is a very common molecule it has some unusual chemistry. H 2 O If we look at the structure of water is it correct to write If we look at the structure of water is it correct to write H-O-H ?

8 Structure of water In order to look at the structure we need to consider how the atoms are connected. 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 Each hydrogen is connected to oxygen by a covalent bond How many electrons are needed to make 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 Two, one electron is donated to the bond by the hydrogen atom and one electron is donated by the oxygen atom

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

10 H-O-H has a bond angle of o 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 (+)

11 Polar Molecule Therefore water has a partial ionic character and is said to be a polar molecule

12 Hydrogen bonds Opposite partial charges can attract each other. This forms weak bonds between water molecules. These are called hydrogen bonds

13 Methane is not polar Methane molecules do not have a dipole attraction for one another Methane molecules do not have a dipole attraction for one another Thus spaced farther apart. Thus spaced farther apart. Despite its similar size and mass to water, methane is much less dense than water. 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. 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. 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.

14 Hydrogen bonds in ice 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.

15 Properties In the graph above water is extensively hydrogen bonded whereas the other compounds H 2 S (hydrogen sulfide), H 2 Se (hydrogen selenide) and H 2 Te (hydrogen telluride) are not.

16 High melting and boiling points 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. 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.

17 Water has high values for Water has high values for Specific heat - amount of heat require to raise the temp of 1g water by 1 o C Specific heat - amount of heat require to raise the temp of 1g water by 1 o C 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 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 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 Surface tension - due to cohesive forces All of the above are due to hydrogen bonding All of the above are due to hydrogen bonding

18 Water is a good solvent 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.

19 Water makes "solvation shells" around ions Water can also readily dissolve other polar molecules, even if they are not positively or negatively charged. 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. 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. 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). 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. Hydrophobic substances gather together to exclude water as 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.

20 pH: Ionization of Water Sometimes the hydrogen of one water molecule will "jump" to another water molecule: Sometimes the hydrogen of one water molecule will "jump" to another water molecule: H2O + H2O H3O+ + OH- This 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. This 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+ + OH- So remember, H3O+ = H+ So remember, H3O+ = H+ Looking at either of the two chemical equations above, it is important to note that the reverse reaction is also occurring Looking at either of the two chemical equations above, it is important to note that the reverse reaction is also occurring

21 How much H+ and OH- exist in 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 OH-, they rarely meet and neutralize each other. 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-] 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 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: 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 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: 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: 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( M) = 7

22 The 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 have a pH greater than 7. 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. 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. 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. pOH is the negative logarithm of the OH- concentration. One useful thing to remember is: One useful thing to remember is: 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 37 nM to 43 nM without serious metabolic consequences. This corresponds to a [H+] of about 40 nM. This value can only vary from 37 nM to 43 nM without serious metabolic consequences.

23 Buffers 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 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 in a healthy individual. Large changes in pH can be life threatening.

24 How does the body maintain a constant blood 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 conjugate base or a weak base with its conjugate acid. 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+. 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 base. Similarly, a weak base will not accept a H+ as well as a strong 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 carbon dioxide (CO2), in the air. The carbonic acid, H2CO3, in the blood is in equilibrium with the carbon dioxide (CO2), in the air.

25 Protein structural stability As a solvent, water plays an important role in maintaining the correct folding of a protein e.g. an enzyme 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, 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 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. Bound water may play a role in stabilising the structure of single proteins as well as that of complexes.

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

27 Protein (immunoglobulin) as wireframe surrounded by water

28 Summary: Water is an important but often ignored biological molecule Opposite partial charges attract forming weak bonds called hydrogen bonds Opposite partial charges attract forming weak bonds called hydrogen bonds Polar Molecule Polar Molecule High melting and boiling points High melting and boiling points High values for: Specific heat, Latent heat of evaporation, Latent heat of fusion, Surface tension - due to hydrogen bonding 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 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 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 pH Buffers maintain pH Water stabilises proteins Water stabilises proteins


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