CH. 3

1
CH. 3

• Water and the Fitness of the Environment

2
The Effects of Waters Polarity

• The polarity of water molecules results in
  hydrogen bonding
• Organisms depend on the cohesion of water
  molecules
• Water moderates temperatures on Earth
• Oceans and lakes dont freeze solid because ice
  floats
• Water is the solvent of life

3
The polarity of water molecules results in
hydrogen bonding

• The bonds that hold together the atoms in a water
  molecule are polar covalent bonds, with the
  oxygen region of the molecule having a partial
  negative charge and the hydrogens having a
  partial positive charge.
• Polar molecule-opposite ends of the molecule have
  opposite charges.

• The extraordinary qualities of water are emergent
  properties resulting from the hydrogen bonding
  that orders molecules into a higher level of
  structural organization.

4
Organisms depend on the cohesion of water
molecules

• Water molecules stick to each other as a result
  of hydrogen bonding. In liquid form, water is
  constantly forming, breaking, and reforming, so
  that at any instant a substantial percentage of
  molecules are bonded to their neighbor, making
  water more structured that most liquids.
• Cohesion-the binding together of like molecules,
  often by hydrogen bonds
• Cohesion due to hydrogen bonding contributes to
  the transport of water against gravity in plants.

• Adhesion- the clinging of one substance to
  another
• Adhesion of water to the walls of the vessels
  helps counter the downward pull of gravity.
• Surface Tension- a measure of how difficult it is
  to stretch or break the surface of a liquid
• Water has a greater surface tension than most
  other liquids.

5
Water moderates temperatures on EarthHeat and
Temperature

• Kinetic energy- the energy of motion
• Heat- the measure of total quantity of kinetic
  energy due to molecular motion in a body of
  matter
• Temperature- measures the intensity of heat due
  to the average kinetic energy of the molecules
• Calorie (cal)- the amount of heat energy it takes
  to raise the temperature of 1 g of water by 1
  degree celsius

6
Water moderates temperatures on EarthWaters
High Specific Heat

• The ability of water to stabilize temperature
  stems from its relatively high specific heat.
• Specific heat- the amount of heat that must be
  absorbed or lost for 1 g of that substance to
  change its temperature by 1 degree celsius
• Because of its high specific heat relative to
  other substances, water will change its
  temperature less when it absorbs or loses a given
  amount of heat.
• A calorie of heat causes a relatively small
  change in the temperature of water because much
  of the heat energy is used to disrupt hydrogen
  bonds before the water molecules begin moving
  faster.
• Because of its high specific heat, the water that
  covers most of the Earth keeps temperature
  fluctuations on land and in water within limits
  that permit life. Because organisms are made
  primarily of water, they are more able to resist
  changes in their own temperatures than if they
  were made of a liquid with a lower specific heat.

7
Water moderates temperatures on EarthEvaporative
Cooling

• Heat of vaporization- the quantity of heat a
  liquid must absorb for 1 g of it to be converted
  from the liquid to the gaseous state
• Waters high heat of vaporization helps moderate
  Earths climate.
• Evaporative cooling- as a liquid evaporates, the
  surface of the liquid left behind cools down
• Provides a mechanism that prevents terrestrial
  organisms from overheating.

8
Oceans and lakes dont freeze solid because ice
floats

• Water is one of the few substances that are less
  dense as a solid than a liquid. While other
  materials contract when solidified, water
  expands.
• If ice sank, then eventually all ponds, lakes,
  and even oceans would freeze solid, making life
  as we know it impossible on Earth.

9
Water is the solvent of life

• Solution- a liquid that is a completely
  homogeneous mixture of two or more substances
• Solvent- the dissolving agent
• Solute- the substance being dissolved
• Aqueous solution- solution where water is the
  solvent
• Hydration cell- the sphere of water around each
  dissolved ion

10
Water is the solvent of lifeHydrophilic and
Hydrophobic Substances

• Hydrophilic- substance that has an affinity for
  water
• Hydrophilic is used even if the substance cant
  be dissolved because the molecules are too
  large, for instance.
• Cellulose is hydrophilic but has molecules that
  are too large to be dissolved. Cellulose is also
  present in the walls of water-conducting vessels
  in a plant.
• Hydrophobic- substances that repel water
• Usually because they are nonpolar or non-ionic
• Hydrophobic molecules are major ingredients of
  cell membranes

11
Water is the solvent of lifeSolute Concentration
in Aqueous Solutions

• Most of the chemical reactions that occur in
  organisms involve solutes dissolved in water.
• Mole (mol)- a practical way to measure molecules
  equal in number to the molecular weight of a
  substance
• Molecular weight- the sum of the weights of all
  the atoms in a molecule
• A mole of one substance has the exact same number
  of molecules as a mole of any other substance
• Avogadros number- 6.02x1023
• Measuring in moles makes it convenient for
  scientists working in the laboratory to combine
  substances in fixed ratios of molecules.
• Molarity- the number of moles of solute per liter
  of solution the unit of concentration most often
  used by biologists for aqueous solutions.

12
The Dissociation of Water Molecules

• Organisms are sensitive to changes in pH
• Hydroxide ion (OH-) the water molecule that has
  lost a proton charge of -1
• Hydrogen ion (H) a single proton with a charge
  of 1
• Acid precipitation threatens the fitness of the
  environment

13
Organisms are sensitive to changes in pHAcids
and Bases

• Acid- a substance that increases the hydrogen ion
  concentration of a solution
• Base- substance that reduces the hydrogen ion
  concentration of a solution
• Some bases reduce H concentration directly by
  accepting hydrogen ions. Other bases reduce the
  H concentration indirectly by dissociating to
  form hydroxide ions, which then combine with
  hydrogen ions in the solution to form water.
• A solution in which the H and OH- are equal is
  said to be neutral.

14
Organisms are sensitive to changes in pHThe pH
Scale

• In any aqueous solution the product of H and OH-
  concentrations is constant at 10(-14)
• An acid not only adds hydrogen ions to a
  solution, but also removes hydroxide ions because
  of the tendency of for H to combine with OH- and
  form water. A base has the opposite effect.
• The pH of a solution is defined as the negative
  logarithm (base 10) of the hydrogen ion
  concentration
• The pH of a neutral solution is 7, the midpoint
  of the scale. A pH calue less that 7 denotes an
  acidic solution (the lower the more acidic).The
  pH for basic solutions is above 7.
• Buffers- substances that minimize changes in the
  concentrations of H and OH- in a solution

15
Acid precipitation threatens the fitness of the
environment

• One of the most serious assaults on water quality
  is acid precipitation.
• Acid precipitation- rain, snow, or fog that is
  more acidic than pH 5.6
• It is caused primarily by the presence in the
  atmosphere of sulfur oxides and nitrogen oxides,
  gaseous compounds that react with water to form
  strong acids, which fall to Earth with rain or
  snow.
• A major source of the oxides come from the
  burning of fossil fuels.
• Strong acidity can alter the structure of
  biological molecules and prevent them from
  carrying out the essential chemical processes of
  life.
• Acid rain and snow can bring about profound
  changes in soils by affecting the solubility of
  soil minerals.