Ch.3 Life on Earth is Aqueous!
Predominance of Water
-3/4 of earth covered with water (liquid & solid)
-cells are 70-95% water
-all organisms require water for survival
~ 1 week survival time for human without water!
LE 3-2
Hydrogen
bonds
Key properties of water defines behavior
-Polarity: partial positive and negative charges
-Hydrophilic nature: attracted to other water molecules
and charged particles
•
Four of water’s properties
- Cohesive behavior
- Ability to moderate temperature
- Expansion upon freezing
- Versatility as a solvent
LE 3-3
>100 ft
Cohesion & Adhesion
During Transpiration
Water-conducting cells
100 µm
H2O
Cohesion
Water molecules hold together through H-bonds to other water
molecules
Example
Cohesion helps transport water against gravity in plants from roots to
stems during transpiration
Adhesion
Water’s attraction to other charged surfaces
Example
Water’s attraction to cell walls helps upward transport against
gravity
Surface Tension:
Strong ordered film-like structure at interface
of water and atmosphere
Held together through H-bonds
Strength creates surface for small organisms to move across
High specific heat
1 cal/g/oC
Amount of heat gained or lost to change the temperature
of 1g of water by 1ºC
Compare to alcohol: specific heat of 0.6 cal/g/oC
• Consequence
– Lessens temperature fluctuations to within limits
that permit life
– Heat is absorbed to hydrogen bonds break
– Heat is released when hydrogen bonds form
Evaporative Cooling
– transformation of a substance from liquid to gas
– Heat of vaporization
• The amount of heat 1 g of liquid must absorb to be converted to
gas (water: ~580 cal/g at 25oC)
• remaining surface cools during evaporation, a process called
evaporative cooling
• Consequence
• Evaporative cooling of water helps stabilize temperatures in
organisms and bodies of water
• Perspiration: sensation?
Solid water (ice):
less dense than liquid because H-bonds more stable and
ordered; expansion occurs
Consequence:
Ice floats on liquid water
Insulates; prevents temperature fluctuations
Example: ponds and lakes in wintertime
aquatic organisms survive in the liquid water beneath ice
Polar solvent
Dissolves other polar or charged solutes
Examples: salts, polar proteins, nucleic acids
Creates an aqueous solution
-through
Hydration shells
H-bonds
LE 3-6
Hydration shells form around
cations and anions
–
Na+
Causes salt crystals to dissolve
In H2O
+
+
–
–
+
–
–
Na+
–
+
+
Cl–
Cl–
+
–
+
+
–
–
–
–
LE 3-7a
Lysozyme molecule (protein)
in a nonaqueous environment.
LE 3-7b
Can you deduce what regions on lysozyme are positive and negative?
Lysozyme molecule in a aqueous environment.
Concepts of Hydrophilic and Hydrophobic
• Hydrophilic substance
– Attracted to water due to charged or polar nature
e.g. salts (ionic)
• Hydrophobic substance
-Repelled by water due to nonpolar nature
e.g. oils, fats (nonpolar)
Important when considering the plasma membrane.
Aqueous chemistry in biological systems
• Most biochemical reactions occur in water
• Most reactions are highly sensitive to pH
Enzyme
Reactant-1 + Reactant-2
Product
What is pH and how does it relate to water?
LE 3-UN53
Water occasionally produces protons (H+) and hydroxide ions (OH-)
Hydronium
ion (H3O+)
Simplified to H+
Hydroxide
ion (OH–)
Water Dissociation
-Hydrogen involved in H-bonds in H2O can lose
electron
-H+ (proton) can bond with another H2O molecule
Results:
– molecule with the extra proton is now a
hydronium ion (H3O+)
– The molecule that lost the proton is now a
hydroxide ion (OH-)
• Dissociation of water molecules
• Rare in pure water (25oC)
» [H+]=10^-7 M
» [OH-]=10^-7 M
• Changes in concentrations of H+ and OH- drastically affect the
chemistry of a cell
Such changes alter the pH
pH
-reflects the molar concentration of H+ in a
solution
pH= -log[H+]
-increases in [H+] increase acidity
e.g. HCl (hydrochloric acid) readily
dissociates into H+ and Cl-increases in [OH-] raises alkalinity,
decreases in acidity
e.g. the base NaOH (sodium hydroxide)
readily dissociates into Na+ and OH-
The pH Scale
• pH 7 occurs when [H+] =[OH-]
• Acidic solutions pH < 7, [H+] > [OH-]
• Basic solutions pH > 7, [H+] < [OH-]
• Most biological fluids: pH 6-8
LE 3-8
pH Scale
0
Increasingly Acidic
[H+] > [OH–]
1
Neutral
[H+] = [OH–]
Battery acid
2 Digestive (stomach)
juice, lemon juice
3 Vinegar, beer, wine,
cola
4 Tomato juice
5 Black coffee
Rainwater
6 Urine
7 Pure water
Human blood
8
Increasingly Basic
[H+] < [OH–]
Seawater
9
10
Milk of magnesia
11
Household ammonia
12
Household bleach
13
Oven cleaner
14
Calculating pH
Given:
pH= -log[H+]
Constant: Water ion product
10^-14 M^2= [H+][OH-]
What is the pH of a solution containing 10^-7 M H+? 10^-4 M
For the same solutions, what is the concentration of OH-?
Determine the concentration of H+ and OH- at pH 3.
Apparent small changes in pH value are really LARGE
Exponential!
Calculate the difference between pH 7 and pH 4
[H+] is10^3 x larger
Buffers
• pH of most living cells must remain close to pH 7
• Buffers minimize changes in [H+] and [OH- ]in a solution
• Most buffers consist of an acid-base pair that reversibly combines
with H+
•
•
H CO
2 3
carbonic acid
HCO - + H+
3
bicarbonate
The Damage of Acid Precipitation
• Acid precipitation refers to rain, snow, or fog with a pH
lower than 5.6
• Caused by the mixing of different pollutants with water
in the air e.g. sulfur and nitrogen oxides
• Main source: combusted fossil fuels
• Acid precipitation can damage life in lakes and
streams
– Leaches geological buffers from soils
– Solubilizes toxic heavy metals e.g. aluminum
LE 3-9
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
More
acidic
Acid
rain
Normal
rain
More
basic