Water

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Chapter 3

• Water The Fitness of the Environment

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You Must Know

• 1) The importance of hydrogen bonding to the
  properties of water
• 2) Four unique properties of water, and how each
  contributes to life on Earth
• 3) How to interpret the pH scale
• 4) The importance of buffers in biological systems

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WATER!!!

• 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 7095 water
• The abundance of water is the main reason the
  Earth is habitable

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3.1

• The water molecule is a polar molecule The
  opposite ends have opposite charges
• Oxygen is slightly negative, Hydrogen is slightly
  positive
• The Polarity of water molecules results in
  hydrogen bonding
• Hydrogen bonds form between (-) Oxygen ()
  Hydrogen on ANOTHER water molecule
• Can form a maximum of 4 hydrogen bonds at a time
• The structure of water is the key to its
  properties
• One Oxygen Two Hydrogens bonded to form a
  molecule

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3.2

• Four properties of water (the key are the
  hydrogen bonds)
• Cohesive behavior
• Ability to moderate temperature
• Expansion upon freezing
• Versatility as a solvent

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• 1) Cohesion
• Linking of like molecules
• water molecule joined to water molecule
• Allows for surface tension

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• Adhesion
• Clinging of one substance to another
• water molecule attached to some other molecule
• Water droplets on a mirror
• Transpiration
• Movement of water molecules up thin tubes (xylem)
  and evaporation through the openings in leaves
• Uses Adhesion (xylem) Cohesion (water to water)

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• 2) Ability to moderate temperature
• Kinetic energy is the energy of motion
• Heat is a measure of the total amount of kinetic
  energy due to molecular motion
• Temperature measures the intensity of heat due to
  the average kinetic energy of molecules

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• Specific Heat
• Amount of heat required to raise or lower the
  temperature of a substance by 1 degree Celsius
• Relative to most materials, the temp of water
  changes less when heat is lost or absorbed
• This causes a high specific heat

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• Waters high specific heat can be traced to
  hydrogen bonding
• Heat is absorbed when hydrogen bonds break
• Heat is released when hydrogen bonds form

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• This high specific heat results in
• Stable temperature of Earths oceans
• Enables Earth to support vast quantities of both
  plant animal life

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• 3) Expansion upon freezing
• Water is LESS dense as a solid than as a liquid
  (opposite in most other substances)
• This is why ice floats
• This keeps large bodies of water from freezing
  solid this moderates temperatures allows for
  LIFE

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• 4) Versatility as a solvent
• A solution is a liquid that is a homogeneous
  mixture of substances
• A solvent is the dissolving agent of a solution
• The solute is the substance that is dissolved
• An aqueous solution is one in which water is the
  solvent

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• A hydrophilic substance is one that has an
  affinity for water
• A hydrophobic substance is one that does not have
  an affinity for water
• Oil molecules are hydrophobic because they have
  relatively nonpolar bonds
• A colloid is a stable suspension of fine
  particles in a liquid

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• Most biochemical reactions occur in water
• Chemical reactions depend on collisions of
  molecules and therefore on the concentration of
  solutes in an aqueous solution

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3.3

• Acidic Basic conditions affect living organisms
• An acid is any substance that increases the H
  concentration of a solution
• A base is any substance that reduces the H
  concentration of a solution

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• A hydrogen atom in a hydrogen bond between two
  water molecules can shift from one to the other
• The hydrogen atom leaves its electron behind and
  is transferred as a proton, or hydrogen ion (H)
• The molecule with the extra proton is now a
  hydronium ion (H3O), though it is often
  represented as H
• The molecule that lost the proton is now a
  hydroxide ion (OH)

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• Changes in concentrations of H and OH can
  drastically affect the chemistry of a cell
• Concentrations of H and OH are equal in pure
  water
• Adding certain solutes, called acids and bases,
  modifies the concentrations of H and OH

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• pH Scale
• In any aqueous solution at 25C the product of H
  and OH is constant and can be written as
  
  HOH 1014
• The pH of a solution is defined by the negative
  logarithm of H concentration, written as
  pH log H
• For a neutral aqueous solution
  H is 107 (7)
  7

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• Acidic solutions have pH values less than 7
• Basic solutions have pH values greater than 7
• Most biological fluids have pH values in the
  range of 6 to 8

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• The internal pH of most living cells must remain
  close to pH 7
• Buffers are substances that minimize changes in
  concentrations of H and OH in a solution
• Most buffers consist of an acid-base pair that
  reversibly combines with H