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!
Key properties of water defines behavior
-Polarity: partial positive and negative charges
-Hydrophilic nature: attracted to other water moleculesand charged particles
• Four of water’s properties - Cohesive behavior
- Ability to moderate temperature
- Expansion upon freezing
- Versatility as a solvent
CohesionWater molecules hold together through H-bonds to other water molecules
ExampleCohesion helps transport water against gravity in plants from roots to stems during transpiration
Adhesion Water’s attraction to other charged surfaces
ExampleWater’s attraction to cell walls helps upward transport againstgravity
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
• Consequence– Lessens temperature fluctuations to within limits
that permit life
– Heat is absorbed to hydrogen bonds break– Heat is released when hydrogen bonds form
High specific heat
1 cal/g/oCAmount 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
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 solutesExamples: salts, polar proteins, nucleic acids
Creates an aqueous solution-through
Hydration shellsH-bonds
LE 3-6
Na+
Na+
Cl–Cl–
+
+
+
+
+
+
++
–
–
–
–
–
–
–
–
–
–
–
Hydration shells form aroundcations and anions
Causes salt crystals to dissolveIn H2O
LE 3-7b
Lysozyme molecule in a aqueous environment.
Can you deduce what regions on lysozyme are positive and negative?
• 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.
Concepts of Hydrophilic and Hydrophobic
• Most biochemical reactions occur in water
• Most reactions are highly sensitive to pH
Aqueous chemistry in biological systems
What is pH and how does it relate to water?
Reactant-1 + Reactant-2 Product
Enzyme
LE 3-UN53
Hydroniumion (H3O+)
Hydroxideion (OH–)
Simplified to H+
Water occasionally produces protons (H+) and hydroxide ions (OH-)
Results:– molecule with the extra proton is now a
hydronium ion (H3O+)
– The molecule that lost the proton is now a hydroxide ion (OH-)
Water Dissociation-Hydrogen involved in H-bonds in H2O can lose electron
-H+ (proton) can bond with another H2O molecule
• 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 aciditye.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 3-8pH Scale
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14Oven cleaner
Household bleach
Household ammonia
Milk of magnesia
Seawater
Pure waterHuman blood
UrineRainwater
Black coffee
Tomato juice
Vinegar, beer, wine,cola
Digestive (stomach)juice, lemon juice
Battery acid
Neutral[H+] = [OH–]
Inc
rea
sin
gly
Ac
idic
[H+]
> [
OH
–]
Inc
rea
sin
gly
Ba
sic
[H+]
< [
OH
– ]
Given:pH= -log[H+]
Constant: Water ion product 10^-14 M^2= [H+][OH-]
Calculating pH
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+
• H2CO
3 HCO
3- + H+
• carbonic acid 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