Fundamentals of
Biochemistry
Third Edition
Donald Voet • Judith G. Voet •
Charlotte W. Pratt
Chapter 2
Water
Copyright © 2008 by John Wiley & Sons, Inc.
Why is Water Important?
• Most biological molecule’s shapes are
influenced by physical & chemical
properties of water
• Most biochemical reactions take place in
water
• Water, or components of water participate
in many biological reactions
Section 1 – Physical Properties of Water
Hydrogen bonding is very important in explaining the properties of
water.
Each water molecule can make up to 4 hydrogen bonds
Solid water
Liquid water
Table 2-1
Why do nonpolar substance not like water?
Emulsions and Surfactants
• Emulsion: One liquid suspended in fine droplets in
another liquid which to not mix.
– Ex: oil and water
– Salad dressing
– Mayonnaise
• Emulsifier: a chemical liaison between two
incompatible liquids to make them stable,
typically a surfactant
• Surfactant: agents that lower the surface tension
of a liquid – amphiphilic – love’s both, contains
both a polar and non-polar group
Surfactants
Detergents
Fabric softener
Emulsifiers
Paints
Adhesives
Inks
Anti-fogging
Soil remediation
Wetting
Ski wax, snowboard wax
Deinking (particularly during the
enzymatic deinking of used paper
during the recycling and repulping
process)
Foaming agents
Defoaming agents
Laxatives
Agrochemical formulations
Herbicides
Insecticides
Quantum dot coating
Biocides (sanitizers)
Hair conditioners (after shampoo)
Spermicide (nonoxynol-9)
Types of surfactants
Non-ionic – Used in drug delivery, cosmetics, and shampoos
Ex: Fatty alcohols – drug delivery
cocamide MEA – foaming agent in shampoo
Anionic – Negatively charged head group, used in drug delivery,
soaps, shampoos
Ex: SDS – sodium dodecyl sulfate – protein separation, car soap,
toothpaste, some aspirins
Cationic – positively charged head group, aniseptics, herbicides
Ex: Cetylpyridinium chloride (CPC) – antiseptic, mouthwash, antisore throat spray
Lipids, Water, and Disease
• Medical surfactant – surface active lipoprotein
– Coats outside of aveoli
– Important to inspiration
• Creates equal surface tension on all aveoli
• Aides in gas exchange
• Acts as a pollution filter
http://trc.ucdavis.edu/biosci10v/bis10v/week10/alveolar.gif
Osmosis – The movement of WATER across a semipermeable
membrane
Diffusion – the movement of other stuff across a concentration
gradient
Osmotic Pressure
Osmotic Pressure = ∏ = the force required to resist water
movement
van‘t Hoff equation: ∏ = icRT
ic = osmolarity of the solution
i = number of solutes per molecule
(NaCl = 2)
c = concentration in molarity
R = gas constant 8.315 J/mol
T = temperature in Kelvin
Osmolarity in Biology
Hypertonic: solution has a
higher osmolarity than the
cytosol
Hypotonic: solution has a
lower osmolarity than the
cytosol
Practice
Which of the following solutions has an osmolarity of 3?
3M Na3PO4
0.43M Na3PO4
0.75 M Na3PO4
3 M NaCl
1.5 M NaCl
Practice
You want to isolate a lysosome to study it in the
lab. Assuming that the only components inside a
lysosome are KCl (0.1M) and NaCl (0.03M),
how much sucrose (342 g/mol) do you need to
make 1 liter of an isotonic solution to isolate the
lysosomes?
Practice
You want to isolate a lysosome to study it in the
lab. Assuming that the only components inside a
lysosome are KCl (0.1M) and NaCl (0.03M),
how much glycogen (18,000 g/mol) do you need
to make 1 liter of an isotonic solution to isolate
the lysosomes?
Figure 2-14
Section 2 – Chemical Properties of Water

H  H 3O


H 2O  H  OH

1
 H   OH  
K
 H 2O 
2
K w   H   OH  
3
1
pH   log  H   log
 H  



HA  H 2O  H 3O  A
General acid reaction


HA  H  A
Abbreviated acid reaction


HB  H  B
Abbreviated acid reaction
4
 H 3O    A 
K
 HA H 2O 
5
 H    A 
K a  K  H 2O  
 HA
6
pK a  pK   log K
Weak acids have pK > 1
7
HA

K
 H 

 A 
HA

   log K
 log  H 

 A 

 A 
pH  pK  log
 HA

8
Buffers are very important to
biological systems
Buffering capacity is +/-1 pH
unit from the pK
Different compounds have
different buffering ranges
Practice
What is the buffer concentration and pH of a
mixture of 0.042M NaH2PO4 (pK = 6.86)
and 0.058 M Na2HPO4?
Practice
What is the pH of a mixture of 75 mL of
0.042M NaH2PO4 (pK = 6.86) and 150 mL
of 0.058 M Na2HPO4?
Synthesis
You are working in the lab and need to make a physiological
phosphate buffer (pH=7.2). You have the following chemicals at your
disposal: Phosphoric acid, monosodium phosphate, and disodium
phosphate whose corresponding pKa’s are 2.15, 6.86, and 12.32. The
molecular weight of each species is 98.0 g/mol, 119.98 g/mol, and
141.96 g/mol. Calculate the mass of each species that you would use
to make a 100 mL of a 0.5M phosphate buffer at pH=7.2.
Can complete all problems at the end of chapter two except
for question 11.