Ch'3 Life on Earth is Aqueous

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Ch.3 Life on Earth is Aqueous!
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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!
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LE 3-2
Hydrogen bonds
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Key properties of water defines
behavior -Polarity partial positive and
negative charges -Hydrophilic nature attracted
to other water molecules and charged particles
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• Four of waters properties
• - Cohesive behavior
• - Ability to moderate temperature
• - Expansion upon freezing
• - Versatility as a solvent

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LE 3-3
Cohesion Adhesion During Transpiration
gt100 ft
Water-conducting cells
100 µm
H2O
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• 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
• Waters attraction to other charged surfaces
• Example
• Waters attraction to cell walls helps upward
  transport against
• gravity

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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
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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

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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?

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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
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Polar solvent
Dissolves other polar or charged
solutes Examples salts, polar proteins, nucleic
acids Creates an aqueous solution -through Hyd
ration shells H-bonds
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LE 3-6
Hydration shells form around cations and
anions Causes salt crystals to dissolve In H2O

Na







Na



Cl
Cl








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LE 3-7a
Lysozyme molecule (protein) in a nonaqueous
environment.
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LE 3-7b
Can you deduce what regions on lysozyme are
positive and negative?
Lysozyme molecule in a aqueous environment.
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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.

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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?
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LE 3-UN53
Water occasionally produces protons (H) and
hydroxide ions (OH-)
Hydronium ion (H3O)
Hydroxide ion (OH)
Simplified to H
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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-)

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• Dissociation of water molecules
• Rare in pure water (25oC)
• H10-7 M
• OH-10-7 M
• Changes in concentrations of H and OH-
  drastically affect the chemistry of a cell
• Such changes alter the pH

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pH -reflects the molar concentration of H in a
solution pH -logH -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-
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The pH Scale

• pH 7 occurs when H OH-
• Acidic solutions pH lt 7, H gt OH-
• Basic solutions pH gt 7, H lt OH-
• Most biological fluids pH 6-8

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LE 3-8
pH Scale
0
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Battery acid
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Digestive (stomach) juice, lemon juice
Vinegar, beer, wine, cola
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Increasingly Acidic H gt OH
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Tomato juice
Black coffee
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Rainwater
Urine
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Neutral H OH
Pure water
7
Human blood
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Seawater
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Increasingly Basic H lt OH
Milk of magnesia
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Household ammonia
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Household bleach
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Oven cleaner
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Calculating pH
Given pH -logH Constant Water ion
product 10-14 M2 HOH-
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.
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Apparent small changes in pH value are really
LARGE Exponential!
Calculate the difference between pH 7 and pH 4
H is103 x larger
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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
• H2CO3
  HCO3- H
• carbonic acid
  bicarbonate

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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

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LE 3-9
0
More acidic
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2
Acid rain
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4
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Normal rain
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7
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More basic
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