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Water, pH and dissociation

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The structure and function of water. Dissociation of weak ... Hydrophobicity/Micelles. From Lehninger, 2nd ed., Ch 4. Summary of water and pH relationship ... – PowerPoint PPT presentation

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Title: Water, pH and dissociation


1
Water, pH and dissociation
  • Lecture 3, Medical Biochemistry

2
Lecture 3 Outline
  • Homeostasis
  • The structure and function of water
  • Dissociation of weak acids and weak bases
  • pH and the Henderson-Hasselbalch equation
  • Buffers, biological/physiological examples

3
HOMEOSTASIS
  • The dynamic that defines the distribution of
    water and the maintenance of pH and electrolyte
    concentrations
  • Water distribution maintained by the kidneys,
    antidiuretic hormone, hypothalamic thirst
    response, respiration and perspiration
  • Clinically, need to be aware of water depletion
    caused by decreased intake (coma, wandering the
    desert) or increased loss (diarrhea, renal
    malfunction, over-exercise), and excess body
    water due to increased intake (too much I.V.) or
    decreased excretion (renal failure)

4
Structure of H20
From Lehninger, 2nd ed., Ch 4
5
WATER
  • Comprises approx 70 of human mass (45-60
    intracellular, 25 extracellular/blood plasma)
  • dipolar partial negative charge on oxygen,
    partial positive charge on hydrogens
  • dipolar nature leads to formation of many low
    energy hydrogen bonds

6
Water Solubility / Hydrophilic
From Lehninger, 2nd ed., Ch 4
7
Hydrophilic/Hydrophobic
From Lehninger, 2nd ed., Ch 4
8
Hydrophobicity
From Lehninger, 2nd ed., Ch 4
9
Hydrophobicity/Micelles
From Lehninger, 2nd ed., Ch 4
10
Summary of water and pH relationship
  • Very low dissociation of H2O to H or OH-
  • The ion product of H2O, Keq X 55.5 M, leads to
    this H OH- 1 X 10-7 M for pure H2O
    which is a constant in biological systems
  • Therefore, if H gt 10-7 M, then OH- must be
    less than 10-7 M, and vice versa.
  • Thus, if the negative logarithm of H is
    derived ( pH -log H ), pure water would be
    pH 7, acids pH lt 7, and bases pH gt 7

11
From Devlin, 3rd ed., Ch 1
12
Dissociation Constant and pH
From Marks, Marks, Smith, Ch 4
13
Henderson-Hasselbalch Equation
14
From Devlin, 3rd ed., Ch 1
15
From Devlin, 3rd ed., Ch 1
16
Sample pH problems
From Devlin, 3rd ed., Ch 1
17
Sample pH Problem (cont)
From Devlin, 3rd ed., Ch 1
18
Buffers
  • Definition A weak acid plus its conjugate base
    that cause a solution to resist changes in pH
    when an acid or base are added
  • Effectiveness of a buffer is determined by 1)
    the pH of the solution, buffers work best within
    1 pH unit of their pKa 2)
    the concentration of the buffer the more
    present, the greater the buffering capacity

19
Physiological Buffers
  • Carbon Dioxide-Bicarbonate System a major
    regulator of blood pH
  • Phosphate System major regulator of cytosolic pH
  • CO2 and HCO3 are much higher than PO4 in
    blood the reverse is true in the cytosol, PO4
    gtgtgt HCO3

20
Examples - Physiological Buffers
From Marks, Marks, Smith, Ch 4
21
From Marks, Marks, Smith, Ch 4
22
pH Titration Curves
From Lehninger, 2nd ed., Ch 4
23
Blood Bicarbonate and Metabolic Acidosis
The bicarbonate blood buffer in a normal
adult maintains the blood pH at about 7.40. If
the blood pH drops below 7.35, the condition is
referred to as an ACIDOSIS. A prolonged blood pH
below 7.0 can lead to death. Clinically for an
acidosis, the acid-base parameters (pH, HCO3-
, CO2 ) of the patients blood should be
monitored. The normal values for these are pH
7.40 HCO3- 24 mM CO2 1.2 mM.
24
Sample Problem Metabolic Acidosis
  • The blood values of a patient were pH 7.03 and
    CO2 1.1 mM. What is the patients blood
    HCO3- and how much of the normal HCO3- has
    been used in buffering the acid causing the
    condition?
  • The pK for HCO3-/CO2 6.10

25
Solution
  • Substitute into Henderson-Hasselbalch equation
  • 7.03 6.10 log HCO3-/1.1 mM, or
  • 0.93 log HCO3-/1.1 mM
  • The anti-log of 0.93 8.5, thus
  • 8.5 HCO3-/1.1 mM, or HCO3- 9.4 mM
  • Since normal HCO3- equals 24 mM, there was a
    decrease of 14.6 mmol of HCO3- per liter of
    blood in this patient. This would be approaching
    the point where, if left untreated, the HCO3-
    buffering capacity would be no longer effective
    in this patient.
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