Chapter 3:
WATER AND THE FITNESS OF
THE ENVIRONMENT
Overview: The Molecule
That Supports All of Life
• Water is the biological medium on Earth
• All living organisms require water more
than any other substance
• 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
Polarity of Water Molecules
Results in Hydrogen Bonding
• Water is a polar molecule
– The two ends have opposite charges
– O is partially (-), H is partially (+)
• Polarity allows water molecules to
form hydrogen bonds with each
other
Animation: Water Structure
–
Hydrogen
bond
+
H
+
O
–
–
+
H
+
–
Four emergent properties of water
contribute to Earth’s fitness for life
1. Cohesion
2. Moderation of Temperature
3. Insulation of Bodies of Water by
Floating Ice (expansion upon
freezing)
4. Water as a Solvent
1. Cohesion
• Cohesion: the binding together of
like molecules by hydrogen bonds
– Most often seen in water molecules
– helps the transport of water against
gravity in plants
• Adhesion: an attraction between
different kinds of molecules
– Ex: between water and plant cell walls
Animation: Water Transport
Adhesion
Water-conducting
cells
Direction
of water
movement
Cohesion
150 µm
• Surface tension: a measure of how hard
it is to stretch or break the surface of a
liquid
– Related to cohesion
– Water has a high surface tension because
of hydrogen bonding of surface molecules
2. Moderation of Temperature
• Water absorbs heat from warmer air
and releases stored heat to cooler air
• Water can absorb or release a large
amount of heat with only a slight
change in its own temperature
Water’s High Specific Heat
• Specific Heat: amount of heat that must
be absorbed or lost for 1 g of a
substance to change its temperature by
1ºC
• The specific heat of water is 1 cal/g/ºC
– Water resists changing its temperature
because of its high specific heat
• Water’s high specific heat linked to
hydrogen bonding
– Heat is absorbed when hydrogen bonds
break
– Heat is released when hydrogen bonds
form
• Water’s high specific heat helps
keep temperature steady, within
limits that permit life
Evaporative Cooling
• Evaporation: phase change from liquid
to gas at the surface of a liquid
• Evaporative Cooling: As a liquid
evaporates, its remaining surface cools
– Occurs because “hottest” molecules (those
with greatest kinetic energy) are most likely
to leave as gas
– Stabilizes temperatures in organisms and
bodies of water
• Example: Sweating
3. Insulation of Bodies of
Water by Floating Ice
• Ice floats in liquid water
– Water expands when it freezes
– Ice is less dense than liquid water
• Occurs because hydrogen bonds in ice
are more “ordered” and form a
crystalline structure
– This keeps large bodies of water from
freezing solid and therefore moderate s
temperature
Hydrogen
bond
Ice
Hydrogen bonds are stable
Liquid water
Hydrogen bonds break and re-form
4. Water as a Solvent
• Solution: a liquid that is a homogeneous
mixture of substances
• Solvent : the dissolving agent of a
solution
• Solute: the substance that is dissolved
• Aqueous solution: one in which water is
the 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
_
Na+
+
_
_
Na+
+
Cl-
_
Cl-
+ _
+
_
_
+
+ _
+ _
+ _
_
Hydrophilic and Hydrophobic
Substances
• Hydrophilic substance: has an affinity for
water
– Polar molecules, ionic compounds
• Hydrophobic substance: does not have
an affinity for water
– Nonpolar molecules
– Ex: Oil molecules
• A colloid is a stable suspension of fine
particles in a liquid
– Ex: cotton
Acidic and basic conditions
affect living organisms
• A hydrogen atom in a hydrogen bond
between two water molecules can shift
from one to the other:
– The H 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–)
• Water is in a state of dynamic
equilibrium in which water molecules
dissociate at the same rate at which
they are being reformed
H
O
H
H
O
H
2H2O
H
O H
H
Hydronium
ion (H3O+)
O
H
Hydroxide
ion (OH–)
Effects of Changes in pH
• Concentrations of H+ and OH– are
equal in pure water
• Adding acids or bases modifies the
concentrations of H+ and OH–
• Biologists use the pH scale to
describe whether a solution is acidic
or basic
Acids and Bases
• Acid: any substance that increases
the H+ concentration of a solution
• Base: any substance that reduces
the H+ concentration of a solution
The pH Scale
• 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 = –(–7) = 7
Buffers
• The internal pH of most living cells
must remain close to pH 7
• Buffers: substances that minimize
changes in concentrations of H+ and
OH– in a solution
– Accept H+ from solution when they are
in excess and donate H+ when they are
depleted
Threats to Water Quality on Earth
• Acid precipitation: rain, snow, or fog
with a pH lower than 5.6
– Caused mainly by the mixing of different
pollutants with water in the air and can fall
at some distance from the source of
pollutants
– Can damage life in lakes and streams
– Effects on soil chemistry are contributing to
the decline of some forests
• Human activities such as burning
fossil fuels threaten water quality
• CO2 is released by fossil fuel
combustion and contributes to:
– A warming of earth called the
“greenhouse” effect
– Acidification of the oceans
• Leads to a decrease in the ability of corals
to form calcified reefs
Chapter 3 “Need to Knows”
1. The importance of hydrogen
bonding to the properties of water.
2. Four unique properties of water,
and how each contributes to life
on Earth.
3. How to interpret the pH scale.
4. The importance of buffers in
biological systems.