Water and the Fitness of the Environment

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Water and the Fitness of the Environment

• AP Biology Chapter 3

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Importance of Water

• It can be quite correctly argued that life exists
  on Earth because of the abundant liquid water.
  Earth is the only planet in which water exists in
  all three states.
• Other planets have water, but they either have it
  as a gas (Venus) or ice (Mars).
• The chemical nature of water is thus one we must
  examine as it permeates living systems water is
  a universal solvent, and can be too much of a
  good thing for some cells to deal with.

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Water

• Life on Earth began in water and evolved there
  for 3 billion years before spreading onto land
• Most cells are surrounded by water and cells are
  about 70-95 water

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Water

• Water is a tiny V-shaped molecule with the
  molecular formula H2O.

Water has hydrogen bonds, which confers many
characteristics.

• What is the bond angle in H2O?
• 104.5

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Polarity of Water

• Water is polar covalently bonded within the
  molecule.
• This unequal sharing of the electrons results in
  a slightly positive and a slightly negative side
  of the molecule.

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Polarity of Water

• Negative regions of one molecule are attracted to
  positive regions of another molecule, forming a
  hydrogen bond.
• Hydrogen bonds are weak (1/20th as strong as
  covalent bonds)

How many hydrogen bonds can a water molecule
form? 4
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Properties of Water

• Cohesion
• Surface tension
• Adhesion
• High Specific Heat
• Moderates Temperatures on Earth
• High Heat of Vaporization
• Evaporative Cooling
• Ice Floats
• Universal Solvent

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Cohesion

• Cohesion refers to the tendency of water
  molecules to hydrogen bond to each other
• Cohesion contributes to a number of waters
  properties
• These properties include the ability of water to
  be siphoned as well the related property of
  transport of water from the roots to the leaves
  of plants

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Surface Tension

• Surface tension is an emergent property of water
  that results from the tendency of water molecules
  to stick to each other (by hydrogen bonding)
  better than they adhere to air molecules
• Surface tension makes water behave as though it
  were coated with an invisible film.

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Adhesion

• Adhesion is the tendency of water to stick to
  substances other than water.

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Specific Heat

• Water is able to absorb heat without increasing
  much in temperature better than many
  substances.
• Water 1 cal/g/C
• Ethyl alcohol 0.6 cal/g/C
• Iron 0.1 cal/g/C
• This is because for water to increase in
  temperature, water molecules must be made to move
  faster within the water this requires breaking
  hydrogen bonds, and the breaking of hydrogen
  bonds absorbs heat

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• Water is also able to retain heat better than
  many substances (i.e., resist cooling).
• This is because for water to decrease in
  temperature, water molecules must be made to move
  more slowly within the water this requires the
  forming of hydrogen bonds, and the forming of
  hydrogen bonds gives off heat (hence
  counteracting cooling tendencies as heat is lost
  from liquid water).

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Moderate Temperatures

• Waters high specific heat serves to buffer the
  internal temperature of organisms, the
  temperatures of bodies of water, and the
  temperatures of the entire biosphere, all things
  that enhance the ability of life to survive on
  this planet.

You have a house along the coast and a house in
the mountains. Which one will experience more
stable temperatures? Coast Water absorbs heat
during the day, at night and winter the warm
water will warm cooler air.
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High Heat of Vaporization

• Water resists evaporating (i.e., vaporizing)
  because hydrogen bonds must be broken in order
  for water to transition from liquid to the gas
  state
• This high heat of vaporization contributes to the
  ability of water to serve as local heat sinks
  (e.g., organisms, lakes, ponds) and as a global
  heat sink (i.e., oceans) these are regions
  (volumes) that retain heat for longer than
  surrounding substances (such as air or rocks)

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Evaporative Cooling

• The vaporization of water is a consequence of
  individual water molecules escaping the liquid
  state for the gas (or vapor) state.
• Those water molecules that are most energetic
  (i.e., moving fastest) are most likely to escape
  liquid water.
• Faster moving water molecules carry more heat
  than slower moving ones (heat actually is simply
  a measure of degree of molecular motion).

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• It is as if the 100 fastest runners at a college
  transferred to another school the average speed
  of the remaining students would decline.
  (Campbell et al., 1999)
• This results in the average temperature of liquid
  water declining with the loss of each
  more-energetic water molecule to the vapor phase.
• Evaporative cooling contributes to waters
  ability to serve as a temperature buffer.
• We use evaporative cooling when we sweat.

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Ice Floats

• Unlike most substances, solid water (ice) has a
  lower density than liquid water
• As a consequence, solid water floats upon liquid
  water, rather than sinking beneath it
• The lower density of ice is a result of the water
  solid phase containing on average more hydrogen
  bonds per water molecule (i.e., approaching 4)
  than does liquid water at any given moment
• More hydrogen bonds results in more structure
  which, in waters case, results in more
  unoccupied space, i.e., a lower density upon
  freezing
• Because ice floats, bodies of water freeze from
  the top down rather than the bottom up
• Since ice serves as an insulator, this property
  of water assures that the complete freezing of
  bodies of water does not occur

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Structures of Liquid Water and Ice
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Solvent

• The most important property of water to the
  existence of life has to do with the ability of
  water to dissolve some substances and not others
• Water dissolves substances to which it can
  readily hydrogen bond and polar covalent
  molecules.

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Solvent

• When a crystal of salt (NaCl) is placed in
  water, the Na cations form hydrogen bonds with
  partial negative oxygen regions of water
  molecules.
• The Cl- anions form hydrogen bonds with the
  partial positive hydrogen regions of water
  molecules.

Eventually the water dissolves all the ion,
resulting in two solutes sodium and
chloride. Large molecules, like proteins, can
dissolve in water if they have ionic and polar
regions.
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• Hydrophilic
• substance that has an affinity for water (
• has ionic or polar bond
• or do not dissolve in water because the molecule
  is too large and too tightly held together.
• cotton has numerous polar covalent bonds in
  cellulose H bonds form in those areas.
• Hydrophobic
• Substance that has no affinity for water
• Non-ionic and nonpolar covalent bonds no H bonds
  can form
• Oils and cell membranes

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pH

• To understand the chemistry of water we need to
  know about pH.
• pH is the Log Scale Unit of Measure used to
  express the degree of acidity of a substance.
• A Water Molecule has one oxygen atom and two
  hydrogen atoms.
• In pure water, most of the water molecules remain
  intact. However, a very small amount of them
  react with each other in the following manner.
• H2O H2O ? H3O OH

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• H2O H2O ? H3O
  OH

• A simpler way to look at it is
• H20 ?? H OH-
• This reaction is reversible

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Acids

• When pure water reacts, it produces an equal
  amount of H3O and OH. Thus, it does not have an
  excess of either ion. It is therefore called a
  neutral solution.
• If a strong acid, such as hydrochloric acid ( HCl
  ) is added to water, it reacts with some of the
  water molecules as follows
• HCl H2O ?? H3O Cl
• Thus, the addition of HCl to water increases the
  H3O or acid concentration of the resulting
  solution.

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Bases

• If a strong base, such as sodium hydroxide, is
  added to water, it ionizes as follows
• NaOH ?? Na OH
• Thus, the addition of NaOH to water increases the
  OH or alkali concentration of the resulting
  solutions.

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Acids II

• Acids are ionic compounds ( a compound with a
  positive or negative charge) that break apart in
  water to form a hydrogen ion (H).
• The strength of an acid is based on the
  concentration of H ions in the solution. The
  more H the stronger the acid.Example HCl
  (Hydrochloric acid) in water

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Bases II

• Bases are ionic compounds that break apart to
  form a negatively charged hydroxide ion (OH-) in
  water.
• The strength of a base is determined by the
  concentration of Hydroxide ions (OH-). The
  greater the concentration of OH- ions the
  stronger the base.

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pH Scale

• The strength of an acid or base in a solution is
  measured on the pH scale.
• The pH scale is a measure of the hydrogen ion
  concentration. It spans from 0 to 14 with the
  middle point (pH 7) being neutral.
• Any pH number greater than 7 is considered a base
  and any pH number less than 7 is considered an
  acid.

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pH Scale
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Buffers

• Chemical processes in the cell can be disrupted
  by changes in H and OH- concentrations.
• Buffers resist changes to the pH
• Buffers accept hydrogen ions from the solution
  when they are in excess and donate hydrogen ions
  when they have been depleted.
• In human blood and other biological solutions
  carbonic acid bicarbonate are important pH
  regulators.

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Acid Rain

• Rain more acidic than 5.6
• What causes it?
• Burning of fossil fuels produces sulfur oxides
  and nitrogen oxides these react with water to
  form strong acids.
• What is the impact?
• Can wash away key soil buffers and plant
  nutrients (calcium and magnesium)
• Increase solubility of compounds like aluminum to
  toxic levels
• Affect early developmental stages of aquatic
  organisms as acidic snow melts entering lakes and
  streams all at once.

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• May the Force Be With YOU!

Ppt courtesy of Tracy Jackson