The Significance of Water to Living
Organisms
• Water is of immense
importance to all living
organisms
• It is used by them in many
different ways
• These uses can be explained by
referring to the properties of
water
• Water is used a coolant
– Refer to thermal properties
• Water is used as a transport
medium
– Refer to cohesion, solvent
properties, and thermal properties
• Water is used as a habitat
– Hydrogen bonding, refer to
cohesion, transparency, and
thermal properties
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Polarity and Hydrogen Bonding in Water
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Water molecules consist of 2 hydrogen
atoms bonded to an oxygen atom
The hydrogen atoms have a slight
positive (+) charge and the oxygen atom
has a slight negative (-) charge
So, water molecules have 2 poles (they
are dipoles) – a positive hydrogen pole
and a negative oxygen pole
This feature of a molecule is called
polarity
A bond can form between the positive
pole of one water molecule and the
negative pole of another
This is called a hydrogen bond
In liquid water, many of these bonds
form
Hydrogen bonds provide water with
much of its basic properties
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Water molecules form Hydrogen bonds
slightly positive
charge
hydrogen bond
between (+) and (-)
areas of different
water molecules
slightly negative
charge
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Thermal Properties:
Heat Capacity
• Water has a high heat capacity
• This means that large amounts of energy
are needed to raise its temp
• The energy is needed to break one of the
hydrogen bonds
• This heat energy is given out again when
the water is cooled
• The temperature of water tends to
remain quite stable
• This is useful for organisms such as fish
that use water as a habitat
• Blood, which is mainly composed of
water, can carry heat from warmer parts
of the body to cooler parts
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The Importance of Ice Floating
• If ice sank to the bottom of
a body of water as it
cooled…
– Ponds and lakes would freeze
up from the bottom
– Fish and other organisms
would be trapped in the
small amount of water that is
left
– There would be no access to
the nutrients are the muddy
bottom
• Ice atop a body of water,
insulates the water below
– Life persists under the frozen
surface
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Thermal Properties:
Boiling and freezing points
• The boiling point of water is relatively high
because to change it from a liquid to a gas
all of the hydrogen bonds between the
water molecules have to be broken
– In natural habitats on Earth, water rarely boils
– Living organisms could not survive if the water
inside them boiled
• Water also freezes at a relatively high
temperature but because it becomes less
dense as it cools to freezing point, ice
forms at the surface first
– The ice that forms on the surface of lakes or
seas insulates the water underneath, so living
organisms can survive there
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Thermal Properties:
The Cooling Effect of Evaporation
• Water can evaporate at temps
below boiling point
• Hydrogen bonds have to be broken
to do this
• The heat energy needed to break
the bonds is taken from the liquid
water, cooling it down
• Evaporation of water from plant
leaves (transpiration) and from
human skin (sweat) has useful
cooling effects
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Cohesion
• Water molecules
stick to each
other because of
the hydrogen
bonds that form
between them
• Strong pulling
forces can be
exerted to suck
columns of water
up to the tops of
the tallest trees in
their transport
systems
– These columns of
water rarely
break
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• Trees have specialized structures to transport water: xylem
and phloem “plumbing”
• Water molecules are “dragged” from the roots to the top of
the tree by capillary action (adhesion) and cohesion:
hydrogen bonds help water molecules hydrogen bond to
each other
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Adhesion
• Adhesion refers to
attraction to other
substances.
– Water is adhesive to
any substance with
which it can form
hydrogen bonds.
– Think water climbing
up capillary tubes…
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Cohesion
• At a surface, the cohesion of
water molecules can make it
difficult for small objects to
break through
– surface tension
• a measure of the force necessary to
stretch or break the surface of a liquid
• Some animals such as mosquito
larvae use the surface of water
as a habitat
• Though they are denser than
water they remain on the
surface and do not sink because
of the high surface tension of
water caused by cohesion
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Solvent Properties
• Many different
substances dissolve in
water because of its
polarity
• Inorganic particles such
as sodium ions and
organic substances
such as glucose can
Water’s polarity “pulls” apart the ions of a
dissolve
particular substance (above NaCl). The positive
hydrogen ends are attracted to the anions of a
substance and the negative oxygen ends are
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attracted to the cations of a substance
Like dissolves like:
water can interact with other polar molecules
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• Water transports molecules dissolved in it
– Blood, a water-based solution, transports molecules of
nutrients and wastes within organisms
– Nutrients dissolved in water get transported through
plants
– Unicellular organisms that live in water absorb needed
dissolved substances
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Transparency
• The fact that water is clear allows light to
pass through it
– Aquatic plants can receive sunlight
– Light can pass through the eyeball to receptor
cells in the back
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