Chapter 3
Water & The Fitness of the Environment
S
You Must Know
S 1) The importance of hydrogen bonding to the properties of
water
S 2) Four unique properties of water, and how each
contributes to life on Earth
S 3) How to interpret the pH scale
S 4) The importance of buffers in biological systems
WATER!!!
S Water is the biological medium on Earth
S All living organisms require water more than any other
substance
S Most cells are surrounded by water, and cells themselves are
about 70–95% water
S The abundance of water is the main reason the Earth is
habitable
3.1
•
The water molecule is a polar molecule: The opposite ends
have opposite charges
•
Oxygen is slightly negative, Hydrogen is slightly positive
S The Polarity of water molecules results in hydrogen bonding
S Hydrogen bonds form between (-) Oxygen & (+) Hydrogen on
ANOTHER water molecule
S
Can form a maximum of 4 hydrogen bonds at a time
S The structure of water is the key to its properties
S One Oxygen & Two Hydrogen’s bonded to form a molecule
3.2
S Four properties of water (the key are the hydrogen bonds)
S
S
S
S
Cohesive behavior
Ability to moderate temperature
Expansion upon freezing
Versatility as a solvent
S 1) Cohesion
S Linking of like molecules
S “water molecule joined to
water molecule”
S Allows for surface tension
S Adhesion
S Clinging of one substance to another
S “water molecule attached to some other molecule”
S Water droplets on a mirror
S Transpiration
S Movement of water molecules up thin tubes (xylem) and
evaporation through the openings in leaves
S Uses Adhesion (xylem) & Cohesion (water to water)
S 2) Ability to moderate temperature
S Kinetic energy is the energy of motion
•
•
Heat is a measure of the total amount of kinetic energy due to
molecular motion
Temperature measures the intensity of heat due to the average
kinetic energy of molecules
S Specific Heat:
S
Amount of heat required to raise or lower the temperature of a
substance by 1 degree Celsius
S
Relative to most materials, the temp of water changes less when
heat is lost or absorbed
S
This causes a high specific heat
S Water’s high specific heat can be traced to hydrogen
bonding
S Heat is absorbed when hydrogen bonds break
S Heat is released when hydrogen bonds form
S This high specific heat results in:
S Stable temperature of Earth’s oceans
S Enables Earth to support vast quantities of both plant &
animal life
S 3) Expansion upon freezing
S
Water is LESS dense as a solid than as a liquid (opposite in most
other substances)
S
This is why ice floats
S
This keeps large bodies of water from freezing solid – this moderates
temperatures & allows for LIFE
S 4) Versatility as a solvent
S A solution is a liquid that is a homogeneous mixture of
substances
S A solvent is the dissolving agent of a solution
S The solute is the substance that is dissolved
S An aqueous solution is one in which water is the solvent
S A hydrophilic substance is one that has an affinity for water
S A hydrophobic substance is one that does not have an
affinity for water
S Oil molecules are hydrophobic because they have relatively
nonpolar bonds
S A colloid is a stable suspension of fine particles in a liquid
S Most biochemical reactions occur in water
S Chemical reactions depend on collisions of molecules and
therefore on the concentration of solutes in an aqueous
solution
3.3
S Acidic & Basic conditions affect living organisms
S An acid is any substance that increases the H+ concentration
of a solution
S A base is any substance that reduces the H+ concentration of a
solution
S A hydrogen atom in a hydrogen bond between two water
molecules can shift from one to the other:
S The hydrogen atom leaves its electron behind and is transferred
as a proton, or hydrogen ion (H+)
S The molecule with the extra proton is now a hydronium ion
(H3O+), though it is often represented as H+
S The molecule that lost the proton is now a hydroxide ion (OH–
)
S Changes in concentrations of H+ and OH– can drastically
affect the chemistry of a cell
S Concentrations of H+ and OH– are equal in pure water
S Adding certain solutes, called acids and bases, modifies the
concentrations of H+ and OH–
S pH Scale
S In any aqueous solution at 25°C the product of H+ and OH–
is constant and can be written as
[H+][OH–] = 10–14
S The pH of a solution is defined by the negative logarithm of
H+ concentration, written as
pH = –log [H+]
S For a neutral aqueous solution
[H+] is 10–7 = –(–7) = 7
S Acidic solutions have pH values less than 7
S Basic solutions have pH values greater than 7
S Most biological fluids have pH values in the range of 6 to 8
S The internal pH of most living cells must remain close to
pH 7
S Buffers are substances that minimize changes in
concentrations of H+ and OH– in a solution
S Most buffers consist of an acid-base pair that reversibly
combines with H+