Water

1
Water

• The Solvent of Life

2
Importance of Water

• Folding (structure) of biomolecules is affected
  by the physical and chemical properties of water
• Water is the medium for most biochemical
  reactions
• Water (or its derivatives) participate in many
  biochemical reactions

3
Physical Properties of Water

• Water is a Polar Molecule

4
Structure of Water
Figure 2-1a
5
sp3 Hybridization
Figure 2-1b
6
Permanent Dipole(Electronegativity of Oxygen)
7
Hydrogen Bonding
Figure 2-2
8
Hydrogen Bonding by Functional Groups
Figure 2-7
9
Characteristics of Hydrogen Bonds

• Length 1.8Å (versus 0.96 Å)
• Strength 20kJ/mol (versus 460kJ/mol)
• Most stable when linear
• Example of Weak Interaction

10
Weak Interactions
Table 2-1
11
van der Waals Interactions
Figure 2-5
12
Structure of Ice
Figure 2-3
13
Consequences of Water Structure
14
Cohesiveness of Water Molecules

• High surface tension
• High boiling point
• High heat of vaporization
• High heat of fusion
• Water remains liquid over a wide range of
  conditions of temperature and pressure

15
Ice is Less Dense than Liquid Water

• Ice floats
• Ice layer insulates water below
• Large bodies of water remain liquid providing a
  liquid medium in which life has been able to
  evolve and persist under a range of conditions on
  earth

16
Water has a High Heat Capacity

• Water modulates temperature on earth within a
  range compatible with life

17
Effect of Water on the Structure and Function of
Biomolecules
18
Solvation of Ions
Figure 2-6
19
Orientation of Water Molecules Around a Nonpolar
Solute
Figure 2-8
20
Aggregation of Nonpolar Molelcules in Water
Figure 2-9
21
Fatty Acid Anions(Amphipathic)
Figure 2-10
22
Monolayers(very dilute solutions)
23
Structure of Micelles and Bilayers
Figure 2-11
24
Liposomes
25
Osmosis and Diffusion

• Water moves by Osmosis
• Solutes move by Diffusion

26
Osmosis

• Movement of solvent from a region of high
  concentration (e.g. pure water) to a region of
  relatively low concentration (e.g. solvent plus
  solute)

27
Osmotic Pressure
Figure 2-13
28
Cellular Resistance to Osmotic Pressure

• Iso-osmolar environment
• (complex organisms)
• Rigid Cell Wall
• (plants, bacteria, fungi)

29
Diffusion and Dialysis
Figure 2-14
30
Chemical Properties of Water
31
Water Ionizes to Form H and OH
32
Dissociation Constant (K)
33
Ionization (Ion Product) of Water (Kw)

• KH2O HOH
• K 1.8 x 1016M
• H2O 55.5 M
• 55.5 M x 1.8 x 1016 HOH Kw
• Kw HOH 1 x 1014 M2

34
pH
Kw HOH 1 x 1014 M2 Neutrality H
1 x 107 M pH 7
35
Relationship of pH and H OH
Figure 2-16
36
Solutions
37
Acids and Bases Alter the pH
38
Ionization of a Weak Acid
39
Strength of an Acid(Acid Dissociation Constant,
Ka)
40
Henderson-Hasselbalch Equation
41
Buffers Resist Changes in pH
42
Acetic Acid
43
Buffering
44
Titration Curves and Buffers
Figure 2-17
45
Titration of a Polyprotic Acid
Figure 2-18
46
Biological Fluids are Heavily Buffered
47
Blood Buffering System
48
Disturbances in the Blood Buffer System

• Acidosis
• Alkalosis