Essential idea:
• Water is the medium of life
IB ASSESSMENT STATEMENT
• Draw and label a diagram showing the
structure of water molecules to show their
polarity and hydrogen bond formation.
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
The polarity of water molecules results in hydrogen
bonding
• The water molecule is a polar molecule: The
opposite ends have opposite charges
• Polarity allows water molecules to form hydrogen
bonds with each other
_
O
H
+
H
+
Animation: Water Structure
The polarity of water molecules results in hydrogen
bonding
• This image of water show the covalent
bonds between oxygen and two hydrogen
atoms.
• The nuclei of oxygen is significantly
larger and greater charge (+8) than the
hydrogen nuclei (+1).
• Consequently the electron pair in the
covalent bond is found 'closer' to the
oxygen than the hydrogen nuclei.
• This creates a polar molecule in which
the oxygen carries and additional small
negative dipole and each hydrogen a
small positive dipole.
Hydrogen
bonds
Consequences of Polarity
• Due to the polarity of the water molecules, the small
negative charge on each oxygen atoms is attracted the
hydrogen atoms of other water molecules.
• These attractions are called HYDROGEN BONDS.
• A hydrogen bond in water has a typical strength of 5.0
kcal/mol but these vary in strength in solutions.
• Hydrogen bonds can occur between water molecules and
with other types of polar molecule
IB ASSESSMENT STATEMENT
• Draw and label a diagram showing the
structure of water molecules to show their
polarity and hydrogen bond formation.
IB Assessment Statements
• Outline the thermal, cohesive and solvent
properties of water.
Four emergent properties of water contribute to
Earth’s fitness for life
•
Four of water’s properties that facilitate an
environment for life:
1. Cohesive behavior
2. Ability to moderate temperature
3. Expansion upon freezing
4. Versatility as a solvent
Cohesion
• Collectively, hydrogen bonds hold water
molecules together, a phenomenon called
cohesion
• Cohesion helps the transport of water against
gravity in plants
• Adhesion of water to plant cell walls also helps to
counter gravity
Animation: Water Transport
Water-conducting cells
100 µm
Xylem Cells  Leaves
Cohesion and Adhesion
• Water molecules are weakly
attracted to each other by
hydrogen bonds
(Cohesion). This action
extends down the xylem
creating a 'suction' effect.
• There is also adhesion
between water molecules
and the xylem vessels
Xylem Cells  Leaves
Capillary Action
• The cohesion and
adhesion act together to
maintain the water column
all the way up from the root
to the stomata
• The tendency of water to
rise in a thin tube due to
adhesion & cohesion is
called capillary action
Surface Tension is a function of cohesive
properties of water
• Surface tension is a measure of how hard it is to
break the surface of a liquid
• Surface tension is related to cohesion
Water is a kinetic energy/ heat energy sponge
– Liquid Water can absorb a lot of heat energy
without changing temperature.
Moderation of Temperature
Water has four properties that allow it to
moderate the temperature of living organisms
and the temperature on earth
1. High Specific Heat Capacity
2. High Latent heat of Vaporisation
3. High Latent heat of fusion
4. Ability to Insulate of Bodies of Water by
Floating Ice
Moderation of Temperature (specific heat
capacity)
• Liquid water can absorb or release a large amount
of heat with only a slight change in its own
temperature
• The heat energy required to raise a substances
temperature is called specific heat capacity.
• Liquid Water has a high specific heat capacity.
• Thus Liquid water can absorb a lot of heat
without changing temperature.
Heat Capacity of Liquid Water
Water is a kinetic energy/ heat energy sponge
–
Liquid Water can absorb a lot of heat energy without changing
temperature.
Latent heat of
vaporization
Latent heat of
fusion
Water is a kinetic energy/ heat energy sponge
–
Latent HEAT of FUSION – heat energy that can be released
before something will start melting/ becoming a liquid. Note that
heat is released but the temperature does not change
–
Latent Heat of Vaporization – heat energy that can be absorbed
before something starts to boil and become a gas. Note that heat
is absorbed but the temperature does not change
Latent heat of
vaporization
Latent heat of
fusion
Water’s High Specific Heat (specific heat)
• Water’s high specific heat minimizes
temperature fluctuations to within limits that
permit life
– Heat is absorbed when hydrogen bonds
break
– Heat is released when hydrogen bonds form
Evaporative Cooling (Latent Heat of vaporization)
• Evaporation is transformation of a substance from
liquid to gas
• Latent Heat of vaporization is the heat a liquid
must absorb for 1 gram to be converted to gas
• As a liquid evaporates, its remaining surface
cools, a process called evaporative cooling
• Evaporative cooling of water helps stabilize
temperatures in organisms and bodies of
water
Latent Heat of Fusion
• Latent Heat of Fusion -- The amount of heat
required to be removed in order for a
substance to become a solid.
• The latent heat of fusion is very high for
water.
• Thus when water becomes ice it absorbs an
incredible amount of heat.
Insulation of Bodies of Water by Floating Ice
• Ice floats in liquid water because hydrogen bonds
in ice are more “ordered,” making ice less
dense
• If ice sank, all bodies of water would eventually
freeze solid, making life impossible on Earth
The Solvent of Life
• A solvent is the dissolving liquid
• The solute is the substance that is dissolved
• Water is a versatile solvent due to its polarity
• Water is an effective solvent because it readily
forms hydrogen bonds
• When an ionic compound is dissolved in water,
each ion is surrounded by a sphere of water
molecules, a hydration shell
LE 3-6
–
Na+
+
+
–
–
+
–
–
Na+
–
+
+
Cl–
Cl–
+
–
+
+
–
–
–
–
Hydrophilic and Hydrophobic Substances
• A hydrophilic substance is one that has an
affinity (attraction) for water
• A hydrophobic substance is one that does not
have an affinity (attraction) for water
• Water can also dissolve compounds made of
nonionic polar molecules (sugars and
alcohols)
• Likes dissolves likes
IB ASSESSMENT STATEMENTS
• Explain the relationship between the properties
of water and its uses in living organisms as a
coolant, medium for metabolic reactions and
transport medium
Thermal Properties
• Water absorbs a lot of heat before there is a
change in temperature
• Therefore water is a useful substance for
living things both physiologically and
ecologically.
Thermal Properties
Physiology: Cooling effects
•The heat generated by the body needs to be removed to prevent a
denaturation of enzyme systems.
•Water absorbed a great deal of energy before entering the vapour phase.
This makes it an effective agent for the removal of heat and the
maintenance of body temperature.
•Blood which is mainly water can absorb and carry heat away from hot
parts of the body to the cooler parts.
Ecological effects:
•Oceans, lakes and ponds have fairly stable temperature (since it can
absorb a lot of heat without a temperature change) which means that
poikilothermic organisms need not waste energy on thermoregulation.
Surrounding air temperatures may show marked changes but water water
in the same vicinity will remain relatively stable
Medium for metabolic reactions:
• Most biochemical reactions take place in
solution where water is the solvent.
– The basis of this ability of water molecules to
act as a solvent in covered in section 3.1.5.
– Remember that the cytoplasm is largely water
and all cells exist in a medium which is water
based e.g. tissue fluid, blood, pond.
Transport Medium:
• Transport systems require something to do
the 'transporting', carrying of material from one
place to another.
– Blood carries nutrient, gases and waste all of
which are dissolved in the water of blood
– Once more this is achieved by taking
advantage of the solubility of water.
HELP!!!!
Need more help check out the Basic
Biochemistry Tutorial link below:
•http://www.biology.arizona.edu/biochemistry/t
utorials/chemistry/page3.html
Applications and skills:
• Application: Comparison of the thermal
properties of water with those of methane.
• Application: Use of water as a coolant in sweat.
• Application: Modes of transport of glucose,
amino acids, cholesterol, fats, oxygen and
sodium chloride in blood in relation to their
solubility in water.