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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Water: The molecule that supports
life on this planet!
• Most cells are
surrounded by water,
and cells themselves
are about 70–95%
water
• The abundance of
water is the main
reason the Earth is
habitable
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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? Go, ahead…guess!
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How does water
moderate temperature?
• Yep! Hydrogen bonding again!
The fact that each water
molecule can make TWO
H-bonds each means they are
really, really attracted to each
other, thus more ENERGY must
be added to the water sample to
increase their molecular motion.
• Remember, temperature is a
measure of the average kinetic
energy of a sample of molecules!
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How does water
moderate temperature?
• We should also discuss this
effect as it relates to “air” with
regard to the amount of water
vapor in the air or humidity.
The more water vapor in the
air, the more heat the air can
absorb.
• Large bodies of water absorb
heat from warmer air and
release stored heat to cooler
air.
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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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Care to examine some data?
Formulate an explanation for the temperature data
presented below. Your explanation should address the
properties of water as they relate to thermoregulation.
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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: stable hydrogen bonds
Liquid water: transient
hydrogen bonds
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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Buffers in Action
Acidification: A Threat to Water Quality
• Human activities such as burning fossil fuels
threaten water quality
• CO2 is the main product of fossil fuel
combustion
• About 25% of human-generated CO2 is
absorbed by the oceans
• CO2 dissolved in sea water forms carbonic acid;
this process is called ocean acidification
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Buffers in Action
• As seawater acidifies, H+
ions combine with
carbonate ions to produce
bicarbonate
• Carbonate is required for
calcification (production of
calcium carbonate) by
many marine organisms,
including reef-building
corals
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Acid Rain
• The burning of fossil fuels is also a major source
of sulfur oxides (SOx) and nitrogen oxides (NOx)
• These “socks and knocks” compounds react
with water in the air to form strong acids that fall
within rain or snow
• Acid precipitation is rain, fog, or snow with a
pH lower than 5.2
• Acid precipitation damages life in lakes and
streams and changes soil chemistry on land
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Acid Rain: Before & After
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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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Created by:
René McCormick
National Math & Science
Dallas, TX