Water and the Fitness of the Environment

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Water and the Fitness of the Environment

• AP Biology
• Campbell Ch. 3

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Whats so special about water?

• Its polarity
• The opposite ends of the water molecule have
  slight, but opposite charges.

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WHY is water polar?

• The atoms of water
• Hydrogen and oxygen
• are held together WITHIN the molecule by
  covalent bonds
• The electrons in these bonds are NOT shared
  equally
• Oxygen is ELECTRONEGATIVE
• Oxygen pulls the electrons more toward itself
• Thus a slight but unequal charge difference
  exists between the two ends (H and O) of the
  molecule

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What effects does polarity have on the behavior
of water molecules

• Water molecules tend to want to stick to each
  other or to other polar (or charged) molecules.
• Sort of like little magnets might stick at their
  opposite poles.
• The places where these slight opposite charges
  meet and stick are called HYDROGEN BONDS

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Hydrogoen Bonds

• Weak bonds that exist between polar molecules
• Weaker than saycovalent bonds, but stronger than
  nothing at all.
• Affect the behavior of many different types of
  molecules.
• Remember
• COVALENT bonds are WITHIN the water molecule
• HYDROGEN BONDS are BETWEEN two separate water
  molecules

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The effects of hydrogen bonding on water

• Waters tendency to form hydrogen bonds with
  itself (and other polar or charged substances)
  causes it to have some very unique and important
  behaviors as compared with substances that are
  non polar and, thus, lack hydrogen bonding.

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Cohesion

• The binding together of like molecules via
  hydrogen bonds
• That is, water likes to stick to itself

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Importance of Cohesion

• Surface Tension
• Transport of water up a plant (against gravity) -
  transpiration

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Adhesion

• Clinging of one substance to another
• Water tends to stick to other polar (or charged)
  molecules

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Importance of Adhesion

• Also aids in transpiration
• Capillary action

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Water has a high specific heat

• Specific heat amount of heat that must be
  absorbed or lost for 1g of a substance to change
  its temperature by 1 degree C.
• Waters specific heat is 1 cal/g/C
• BECAUSE water has a high specific heat, water
  will change its temperature less when it absorbs
  or loses a given amount of heat (as compared to
  other substances)

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Why does water have a high specific heat

• Hydrogen bonds, of course!
• Substances feel hotter because heat energy causes
  them to move faster
• Because of waters h-bonds, heat applied to water
  is first absorbed in breaking hydrogen bonds.
• Only after h-bonds are broken can the water
  molecules begin moving fast.

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Why is a high specific heat relevant to life on
earth?

• Because we have so much water on our planet, it
  acts to stabilize temperatures on our planet
• Also helps maintaining stable temperatures within
  living things
• Living things contain lots of water

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Water has a high heat of vaporization

• Heat of vaporization - The quantity of heat a
  liquid must absorb for 1g to be converted from
  liquid to gas
• Waters heat of vaporization is high compared to
  other liquids - Why?
• Hydrogen bonds, of course

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What is the effect of waters high heat of
vaporization on life?

• Moderates Earths climate
• Allows us to maintain liquid water in large
  amounts on the planet
• Evaporative cooling

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What is evaporative cooling?

• The hottest molecules are the ones that are most
  likely to leave a surface because they are the
  ones moving fast enough to break the hydrogen
  bonds and become vapor.
• These molecules take heat energy with them
  leaving cooler temps behind

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Liquid water is MORE dense than solid water

• Ice floats
• Above 4 degrees C, liquid water becomes more
  dense as it is chilled
• Densest at 4 C
• Below this, hydrogen bonds cause a crystalline
  lattice to form that spreads water molecules AWAY
  from each other
• FROZEN water is thus NOT as tightly packed as
  liquid water
• Ice FLOATS not SINKS.

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Why is this important?

• Keeps lakes/ponds, etc. from freezing solid
• Ice forms an insulating layer on the top allowing
  life to persist underneath.

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Water is an excellent solvent

• of polar or charged stuff
• Remember terms
• Solution
• Solute
• Solvent
• Hydrophilic
• Hydrophobic

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Formation of hydration shells around ions

• Hydration shell
• Positive end of water molecules surround negative
  ion
• Negative ends of water molecules surround
  positive ion
• Separates ionically bound substances like salt

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Water organizes non polar molecules

• Water has no attraction to any molecule that has
  no charge or polarity
• It would rather stick to itself
• Thus, when placed with a non polar substance,
  water will push the non polar molecules away as
  it seeks to bind with other water molecules.
• This results in water creating important cell
  structures called membranes.

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Dissociation of Water Molecules

• Water molecules tend to fall apart into H and
  OH ions
• As long as the water is pure, H and OH are always
  equal pH neutral
• Adding extra H or OH ions to water causes
  formation of acid or base
• Adding Extra H ions results in acid
• Adding Extra OH ions results in base
• Water changes pH immediately when extra ions of
  either type (H or OH) are added

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Dissociation of Water Molecules

• Importance of Buffers
• Buffers are substances that will take up H ions
  if they are in excess or will release H ions if
  there are excess OH ions in the solution
• This acts to neutralize a solution that has had
  excess H or OH added
• WATER IS NOT A GOOD BUFFER!!
• Carbonic acid is a good buffer
• Found in blood
• Keeps blood pH very close to neutral