Water and Life
•The Molecule That Supports Life
Water is the biological medium
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¾ of the Earth is covered by water
Exists in all 3 phases naturally on Earth
Life began in water over 3 billion years ago
Living cells require water and are made up of
70-95% water
•Water is Made of Polar Covalent Bonds
Intramolecular force:
polar covalent bond –
unequal electron sharing
The overall molecule is also
polar – overall distribution of
electrons is unequal.
Intermolecular force:
Hydrogen bonds  the slightly
negative O of one water
molecule attract the slightly
positive H ends of neighboring
water molecules.
4 H bonds per molecule!
Last for trillionths of a second
Sheer quantity implies many
bonds at a given time.
•Cohesion of Water Molecules
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H bonds keep water molecules linked
creating a more structured liquid than
most.
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This phenomenon is know as cohesion
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Cohesion helps drive water
transport in plants
Adhesion – the attraction between
2 different substances also helps
(water to cells)
Water also has a high surface
tension!
•Water Moderates Temperature
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Water can absorbs and store heat
from the air
Water has a very high Specific Heat
relative to other substances.
1cal/g×°C
Means it takes a great deal of energy
to change the temperature of water!
What are some of the impacts on life
of this specific heat?
•Evaporative Cooling
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Water also has a high heat of vaporization (energy
needed to convert 1g of liquid to gas).
Regulates temperature stability of bodies of water.
“hottest” molecules vaporize first – leaving the
remaining surface slightly cooler.
•Density of Water vs. Ice
Liquid water is more dense than
solid water
This is directly caused by the hydrogen
bonding of water.
Above 4°C water behaves like most liquids.
From 4-0°C water starts to freeze due to lack
of movement.
0°C locked in crystalline lattice.
•Water: The Solvent of Life
Water can dissolve Ionic bonds due
to the positive and negative regions
of the molecule.
As long as a molecule has a polar
region the water can generate H
bonds with it.
Hydrophobic: Water-fearing. Nonionic and Non-polar. Do not mix
with water!
Hydrophilic: Water-loving. Does not
need to dissolve, but will generate
bonds with the water.
•Solubility: Dissolving and Solutions
•Water is a Polar Covalent Solvent
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•Water as an Acid or Base
Water is amphoteric
ACID
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Acids increase H ions in a
solution.
Acids increase the pH
BASE
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Bases reduce H ions
concentrations.
Bases decrease the pH
pH measures the negative log of the hydrogen ion concentration.
•Water is a Buffer
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Most living things require an optimal pH of 7 (neutral).
A buffer is a substance that minimizes changes in H
and OH concentrations.
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