Biochemistry
Water and the Fitness of the Environment
Water: The molecule that supports
life on this planet!
• Water is the biological
medium for all life on
Earth
• All living organisms
require water more
than any other
substance
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FOUR Emergent Properties of water
contribute to Earth’s suitability for life
1. Water’s cohesive &
adhesive behavior
2. Water’s ability to
moderate temperature
3. Water’s expansion
upon freezing
4. Water’s versatility as a
solvent.
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What causes water molecules to both
cohere and adhere?
• Yep! Our good buddy
the intermolecular force
(IMF) named hydrogen
bonding
• Cohesion is when water
molecules stick to each
other.
• Adhesion is when water
molecules stick to some
other type of substance
like plant cell walls.
Animation by scrolling over image
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What causes water molecules to both
cohere and adhere?
• Surface tension is a measure of how hard it is to
break the surface of a liquid.
• Surface tension is a consequence of cohesion.
What causes it? Go ahead, guess!
Hydrogen bonding, yet again!
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How does water
moderate temperature?
• Water has a high heat capacity.
• Heat capacity is the amount of heat required to raise
1 gram of water by 1o Celsius (1 calorie = 4.184 J)
• What does that mean? It means water can absorb
large quantities of heat without much change in its
own temperature, thus it’s a good thermo regulator.
• But, why? Guess again!
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It’s a 2-way street!
• Water’s high specific heat can be
traced to hydrogen bonding
(IMFs)
– Heat is absorbed when hydrogen
bonds break
– Heat is released when hydrogen
bonds form
• The high specific heat of water
minimizes temperature
fluctuations to within limits that
permit life
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Ever sweat?
Ever perspire?
• Evaporation is transformation of a substance from
liquid to vapor
• Heat of vaporization is the heat a liquid must absorb
for 1 g to be converted to vapor
• As a liquid evaporates, its remaining surface cools, a
process called evaporative cooling
• Evaporative cooling of water helps stabilize
temperatures in organisms (sweating, panting, etc.)
and bodies of water
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Water is a freak! It expands upon freezing!
• Ice floats in liquid water because hydrogen
bonds in ice are more “ordered” forming a
hexagonal shape with a hole in the middle,
making ice less dense.
• Water reaches its greatest density at 4°C,
which is excellent news if you’re a fish!
• If ice sank, all bodies of water would
eventually freeze solid, making life impossible
on Earth
Water is a freak! It expands upon freezing!
Hydrogen bond
Ice:
Hydrogen bonds
are stable
Liquid water:
Hydrogen bonds
break and re-form
Water’s Versatility as a Solvent
• A solution is a liquid that is a homogeneous
mixture of substances
• A solvent is the dissolving agent of a solution or
“dissolver”
• The solute is the substance that is dissolved or
“disolvee”
• An aqueous solution is one in which water is
the solvent
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Water’s Versatility as a Solvent
• Water is a versatile
solvent due to its
polarity, which allows it
to form hydrogen bonds
easily.
• When an ionic
compound is dissolved
in water, each ion is
surrounded by a sphere
of water molecules
called a hydration shell
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Water’s Versatility as a Solvent
• Water can also dissolve compounds made of
nonionic polar molecules
• Even large polar molecules such as proteins can
dissolve in water if they have ionic and polar
regions
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Hydrophilic vs. Hydrophobic
• A hydrophilic substance is one that has an
affinity for water
• A hydrophobic substance is one that does
not have an affinity for water; “fears water”
• Oil molecules are hydrophobic because they
have relatively nonpolar bonds
• A colloid is a stable suspension of fine
particles in a liquid (like milk—fat suspended
in water—Yummy!)
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Acids, Bases and Buffers. Oh, my!
ACIDIC AND BASIC CONDITIONS
AFFECT LIVING ORGANISMS
• A bonded hydrogen atom within a water molecule
can shift between two water molecules (from one
molecule to the other).
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Acids, Bases and Buffers. Oh, my!
– The hydrogen atom leaves its electron behind
and is transferred as a proton, or hydrogen ion
(H+)
– The molecule with the extra proton is now a
hydronium ion (H3O+), though it is often
represented as H+
– The molecule that lost the proton is now a
hydroxide ion (OH–)
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Acids, Bases and Buffers. Oh, my!
• Though statistically rare, the dissociation of water
molecules has a great effect on organisms
• About 2 in every billion water molecules exist as
H+ and OH–
• Changes in concentrations of H+ and OH– can
drastically affect the chemistry of a cell
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Acids, Bases and Buffers. Oh, my!
• Concentrations of H+ and OH– are equal in
pure water
• Adding certain solutes, called acids and
bases, modifies the concentrations of H+ and
OH–
• Biologists use something called the pH scale
to describe whether a solution is acidic or
basic (the opposite of acidic; also known as
alkaline)
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Acids, Bases and Buffers. Oh, my!
Simply put,
• An acid is any substance that
increases the H+
concentration of a solution
• A base is any substance that
reduces the H+ concentration
of a solution
Bleach
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Acids, Bases and Buffers. Oh, my!
Buffers
• The internal pH of most living cells must remain
close to pH 7
• Buffers are substances that RESIST changes
in concentrations of H+ and OH– in a solution,
therefore they RESIST a change in pH
• Most buffers consist of an acid-base pair that
reversibly combines with H+
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The pH Scale
• An acid is any substance
that increases the H+
concentration of a solution
• A base is any substance
that reduces the H+
concentration of a solution
• The scale was designed to
compare WEAK acids and
bases.
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The pH Scale
• In any aqueous solution at 25°C the product of
H+ and OH– is constant and can be written as the
autoionization constant of water
[H+][OH–] = 10–14
• The pH of a solution is defined by the negative
logarithm of H+ concentration, written as
pH = –log [H+]
• For a neutral aqueous solution, [H+] is 10–7, so
pH = –(–7) = 7
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The pH Scale
• Acidic solutions
have pH values
less than 7
• Basic solutions
have pH values
greater than 7
• Most biological
fluids have pH
values in the range
of 6 to 8
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The pH Scale
• Each increase
(toward more acidic
or more basic –
away from 7 in
either direction) on
the pH scale
increases by a
factor of 10. So an
acid with a pH of 2
is 100 times more
acidic than an acid
with a pH of 4.
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Created by:
René McCormick
National Math & Science
Dallas, TX