1. Faculty of medicine, U of D
Water, PH ,PK and Buffers
By Dr. Rehab Omer
Faculty of medicine, U of D

2. ( وجعلنا من الماء كل شيء حي )

3. Biomedical importance of water
It provides the medium for biochemical reactions.
It participates in biochemical reactions (H+ or OH-).
It transports nutrients & waste products within and between cells.
It is able to solvate a wide range of organic and inorganic molecules.
It has a slight tendency to dissociate into H+ and OH-.
6) Its shape & the polarity of O-H bond → H-boding capability → its special properties → biomolecules dissolved in it assume specific shapes.

4. ⅔ of total body water is intracellular,
It is the most abundant biomolecule in living organisms making up to 60% or more of the weight of the body.
⅔ of total body water is intracellular,
⅓ is extracellular (blood plasma the intravascular is 25% the rest is interstitial )

5. Role of water in the body
1- Water as solvent (solvent properties of water is the bases of life on earth).
2-Water as a reactant (hydrolysis and hydration reaction).
3-Ionization of water(is the bases of pH scale)

6. 1-Water is the ideal biologic solvent
Water has chemical properties which make it ideal solvent.
chemical properties:
Dipolar molecule (partially –ve charge on one side ,and partially +ve on other site

7. Water is dipolar Molecule
In the oxygen atom the outer orbital contain 2 pairs of unshared electron
This make oxygen nucleus attracts electrons more strongly than does the hydrogen nucleus.

8. O atom is more electronegative than H atom.
So an uneven distribution of charge occurs within each O-H bond of the water molecule (Oδ-―Hδ+).
This uneven distribution of charge within a bond is known as a dipole, and the bond is said to be polar.
A water molecule is V-shaped (not linear).
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10. The sharing of electrons between H and O is therefore unequal.
The result of this unequal sharing is two electric dipoles in the water molecule; the oxygen atom bears a partial negative charge
and each hydrogen a partial positive charge.

13. The resulting electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another constitutes a hydrogen bond.
Hydrogen bond is sharing of hydrogen between 2 electronegative atom (O ,N)
A hydrogen bond is formed between an electronegative atom O or N (H acceptor) & a H atom covalently bonded to another electronegative atom (H donor) in the same or another molecule.

15. The physical properties of water like:
liquid at room temp.
High melting point
High boiling point
High heat of vaporization
are a consequence of strong attractions between adjacent water molecules, which give liquid water great internal cohesion

16. Each water molecule forms hydrogen bonds with as many as four neighboring water molecules.

20. Water forms hydrogen bonds with solutes containing electronegative atoms such as oxygen, nitrogen
Solubility is the surrounding of the biomolecule by water.

21. Biomolecules such as carbohydrates are soluble in water because they form hydrogen bonds
Alcohols, aldehydes, amines, and ketones form hydrogen bonds with water…solube in water.
Salts are soluble in water NaCl…electrostatic interactions with water
(like dissolved like,polar solvent dissolve polar compound and vice-versa)

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25. Hydrogen bonds can be formed between water and solutes, or between solute molecules themselves.

26. Hydrogen atoms bonded to carbon do not form hydrogen bonds (there is small difference in electronegative between carbon and H.
Non polar hydrocarbon such as oil ,benzene or hexane when mixed with water they form separate phase; niether liquid is soluble in the other.

28. When amphipathic compounds are mixed with water the :-
Amphipathic compounds contain regions that are polar and regions that are non polar.
When amphipathic compounds are mixed with water the :-
The polar or charged hydrophilic region interacts favorably with the solvent and tends to dissolve, but the non polar, hydrophobic region avoid contact with water. .

29. Thus, the non polar regions of the molecule cluster together to present the smallest hydrophobic area to the solvent.
The forces that hold the non polar regions of the molecules together are called hydrophobic interactions.

30. Many biomolecules are amphipathic, membrane lipids and proteins
Formation of biological membrane depends on the properties of water

31. Biomolecule fold to position their charges and polar group on their surface
Most biomolecules are amphipathic.
phospholipid bilayer, protein, DNA ,polar region contacts water while the nonpolar part is away from water. .

33. Covalent & noncovalent bonds stabilize biomolecules
The covalent bond is the strongest force that holds molecules together.
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34. Noncovalent bonds stabilize biomolecules
Noncovalent forces (interactions, bonds) are weaker than covalent bonds.
They also contributes to the structure, stability and function of biomolecules.
They involve interactions a) within the biomolecule b) between biomolecule & H2O.
Noncovalent bonds are:
Hydrogen bonds.
Hydrophobic interaction.
Electrostatic interaction.
Van der Waal forces.

35. Hydrophobic interaction
Hydrophilic (water loving): polar or water soluble.
Hydrophobic (water hating): nonpolar or water insoluble.
Hydrophobic interaction is the association of nonpolar molecules with each other, in an aqueous solution, to minimize contact with water.
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36. It is an entropy-driven process due to a net decrease in order among the H2O molecules.
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37. In cell membrane , the polar groups are in contact with water, and the nonpolar (hydrophobic) tails lie in the interior of the membrane to minimize contact with water.
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38. Electrostatic interaction
It is interactions between charged groups within or between biomolecules also called salt bridges.
They facilitate the binding of charged molecules and ions to proteins and nucleic acids.
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39. Van der Waal forces
Arise from attractions between transient dipoles generated by the rapid movement of electrons of all neutral atoms.
They are weaker than hydrogen bonds.
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42. 2-Water as a reactant (hydrolysis and hydration reaction).
Hydrolysis reaction is the cleavage of bonds (C-O or C-N) by addition of water eg hydrolases enzymes in digestion lipases, proteases ,glucosidases
Hydration reaction addition of water to break (hydratase) and removal of water to make double bond (dehydratase) respectively.

43. Beta-Oxidation of Fatty Acids
hydration reaction
Adds water across the trans C=C bond.
Forms a hydroxyl group (—OH) on the  carbon.
 

44. The elimination of water creates a double bond.

45. 3-Ionization of water(the bases of pH scale)
There is very small degree of ionization of water to hydrogen ion and hydroxide ion.
H2O H+ + OH-

46. How much H+ and OH- exist in water. Very, very little
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!
To predict the state of ionization of water, we must take into account the relevant equilibrium constants for the reaction.

47. The equilibrium constant, Keq describes the ionization equilibrium of water:
Keq = [H+][OH-]
[H2O]
Keq= 1.8x 10-16
[H2O] =W⁄ MW = 1000⁄18 =55.5

48. cross multiplication of the equation:- Keqx[H2O] = [H] [OH]
Keqx[H2O] = Kw
Kw the ion product of water
Kw= x x[55.56] = [H+][OH-]
Kw=1* M
[H+][OH-]= 10-14

49. For neutral water, the Kw is 1 x M and the concentrations of [H+] and [OH-] are each 1 x 10-7 M .

50. Let's look at that number
10-7 without the exponent M
This is obviously a very small number.

51. -log(0.0000001 M) or -log 10-7 = 7 Which is known as pH
A more easy way to discuss small numbers such as this is to take the negative logarithm For the concentration of [H+],
-log( M) or
-log 10-7 = 7
Which is known as pH
So the ion product of water is the bases of the pH scale
1x10-7

52. pH
pH is the negative logarithm of the hydrogen ion concentration. It was introduced by Sorensen in 1909.
pH=-log [H+].
High pH correspond to low concentrations of hydrogen ion while low pH indicates high concentrations of hydrogen ions.

53. The pH of a solution is simply the negative logarithm of [H+].
The pH of a solution describes the acidity of a solution.
A solution, like H2O, with a pH = 7 is neutral.
Acidic solutions are those with a pH of less than 7.
Basic solutions have a pH greater than 7.

54. A difference of 1 in pH is a 10-fold difference in [H+].
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.
So maintenance of pH of extracellular fluid in this range is essential for health.

55. Disturbances of pH is known as acidosis and alkalosis
Disturbances of pH is known as acidosis and alkalosis. Causes of acidosis(pH below 7.35) include Diabetic Keto Acidosis or lactic acidosis. Causes of alkalosis (pH above 7.45) include vomiting of acidic gastric or treatment using certain diuretic.

56. pKa
In living systems, much of the biochemical reactions involve interactions between acids and bases.
Acids are proton (H+ ) or hydrogen donors and bases are proton (H+) or hydrogen acceptors.
HA H+ + A-

57. Acid and base reactions are made up of conjugate acid-base pairs.
Strong acids are those that readily give up a H+.
Weak acids do not readily give up a H+.

58. Strong acids totally dissociate in water:
HCl  H+ + Cl
Weak acids partially dissociate in water:

59. How readily a weak acid gives up its H+ is expressed by the acid dissociation constant, or Ka:
Ka = [H+][A-]\[HA] {3)
Since the value of Ka is very small we can replace it by pKa (like [H+] and pH)

60. The pKa is the negative logarithm of the Ka.
The pKa expresses the relative strength of weak acid.
Strong acids have small pKa. While weak acids have large pKa.

61. Equation (3) can be rearranged and solving to:
[H +] = Ka([HA] / [A-] )
and by taking the log10 of both sides and multiplying each side by -1, we get:
-log10 [H +] = -log10 Ka - log ([HA] / [A-] ) or
pH = pKa - log 10 ([HA] / [A-] ), and rearranged,
pH = pKa + log10 ([A-] / [HA] )
this equation is known as the Henderson-Hasselbalch equation

62. Henderson Hasselbalch equation is an equation that relate the pH of the solution to the pK of the acid and the extent of weak acid dissociation . .
Henderson Hasselbalch equation describe the behavior of weak acids and buffers.

63. Looking at the above equation, it can be seen that when [A-] = [HA], then Ka = [H+]. Then the pKa = pH The pKa can be defined as the pH at which half of the acid is dissociated or the pH at which the proton donor[HA]= proton acceptor[A-]

64. What is a buffer? A buffer is a substance that resist change in pH
A buffer is a molecule that tends to either bind or release hydrogen ions in order to maintain a particular pH.
for example, needs to maintain the pH of blood 7.4.  Buffers help occur.
Good buffers are weak acids and their conjugate bases

65. Buffer can either accept or donate hydrogen ions (proton), depending on the solution they are in.  Since the buffers will accept proton in acidic and donate proton in basic solution. 
Two factors determine the effectiveness of buffer:
1.Its pKa relative to the pH of the solution
2.Its concentration .

66. There are three important buffer systems in our bodies:
bicarbonate buffer system (plasma)
phosphate buffer system (intracellular)
protein buffer system

67. All three work similarly—
if they found in a solution with a lot of free hydrogen ions , they act as bases and suck up the excess hydrogen ions. 
And if they found in a solution lacking free hydrogen ions (a base), they donate their hydrogen ions to the solution.

68. Abuffer works best within 1 pH unit of its pKa (i. e
Abuffer works best within 1 pH unit of its pKa (i.e. between 1 pH unit below and 1 pH unit above the pKa ).
Body fluids must be protected against change in pH because enzymes are very pH sensitive .
Body buffers acts to maintain blood pH between 7.37 and 7.43

69. Importance of bicarbonate buffer is
High concentration
Disposal by the lung
The bicarbonate buffer, so important in blood, has its equilibrium right at the pH of 7.4.  
pK of HCO3 is 6.4

71. The phosphate buffer system consist of:
dihydrogen phosphate ions (H2PO4-) as hydrogen donor (acid) and hydrogen phosphate ions (HPO4--) as hydrogen accepter .
If additional hydrogen ions entre the cellular fluid ,they are consumed with (HPO4--) and the equilibrium will shifts to the left .
In vice versa the equilibrium will shifts to the right .
pK of H2PO4 is 7.2
Intracellular buffer high conc

72. The protein buffer system
Plasma protein buffer
Plasma protein specially albumin account for 95% of nonbicarbonate buffering capacity
Hemoglobin

73. Amino acids have a central carbon with four groups off of it:
  Proteins are made up of amino acids.  
Amino acids have a central carbon with four groups off of it:

74. 1 carboxyl group (COOH)
2 an amino group (NH2)
3 a hydrogen atom
4 an R group
The carboxyl and amino groups are what enable proteins to act as buffers.

76. Diagramatic Overview of amino acids providing buffering functions

77. So, amino acids can accept or donate hydrogen ions, making them excellent buffers.  And any given protein typically has hundreds of amino acids.  So, proteins make super buffers.  Remember, they are found in very high concentration in intracellular solutions and in blood