biol 1115 chapt 3 notes
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Biology 1115 Outline
Biology 1115 Lecture Notes
Chapter 3: Water and the fitness of the environment
Outline
The polarity of water molecules results in hydrogen bonding
Properties of water contribute to the fitness of earth as an
environment for life
Organisms depend on the cohesion of water molecules
Water contributes to earth's habitability by moderating
temperatures
Water's high specific heat
Evaporative cooling
Ice floats
Water as the solvent of life
Solute concentrations in Aqueous solutions
Organisms are sensitive to pH changes
Acids and Bases
The pH scale
Buffers
All organisms are made mostly of water and live in a world where water dominates climate and many other
features of the environment. Water is the only common substance to exist in the natural environment in all three
physical states: solid, liquid and gas. This chapter develops a conceptual understanding of how water contributes to
the fitness of earth for life.
The polarity of water molecules results in hydrogen bonding
Oxygen is more electronegative than hydrogen. Consequently, the electrons of the polar bonds spend more
time near the oxygen atom. This makes water a polar molecule.(Fig 3.2)
The unique (emergent) properties of water arises from attractions among these polar molecules.
Each water molecule can hydrogen bond (H-bond) to a max of four neighbors.
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H-bond = electrostatic attraction between a hydrogen in a polar bond to an electronegative atom of another
molecule.
The charged regions of a polar molecule are attracted to opposite charges of neighboring polar or ionic
molecules.
4 properties of water contribute to the fitness of earth as an environment for life
1.
2.
3.
4.
Cohesion
Temperature stabilization
Expansion upon freezing
Solvent of life
Organisms depend on the cohesion of water molecules
Water molecules stick together as a result of H-bonding. H-bonds form, break, and reform very frequently. At
any given time, a substantial portion of all molecules are bonded to their neighbors, giving water more
structure than most liquids.
Cohesion = tendency of molecules to stick together. Much stronger for water than for other liquids.
Important in water transport in plants.
Adhesion = the clinging of one substance to another. Also important in water transport in plants.
Surface tension = a measure of how difficult it is to stretch or break the surface of a liquid. Higher for
water than for most liquids.
Water contributes to earth's habitability by moderating temperatures
Water stabilizes air temperature by absorbing heat from air that is warmer and releasing the stored heat to
the air that is cooler.
Water can store a lot of energy (heat) with only a slight increase in its own temperature.
Heat = measure of the total quantity of kinetic energy (energy of motion) due to molecular motion in
a body of matter.
Temperature = measures intensity of heat due to the average kinetic energy of the molecules.
Calorie = amount of heat energy it takes to raise the temp of 1 gram of water by 1°C (Food calorie =
1000 calories)
Water's high specific heat
Specific heat = amount of heat that must be absorbed or lost for 1 gram of that substance to change
temperature by 1 °C.
Compared to most substances, water has an unusually high specific heat (10x that of Fe) This is due to
H-bonding.
A calorie of heat causes a relatively small change in temperature of water because much of that heat energy
is used to disrupt H-bonds before water molecules can begin to move faster.
Conversely, when the temperature of water drops slightly, many additional H-bonds form, releasing a lot of
heat energy.
Water buffers against extreme changes in temperature.
Evaporative cooling
Molecules in a liquid stay close together because they are attracted to one another. Molecules moving fast
enough to overcome these attractions can depart from the liquid and enter into gas state.
Heat of Vaporization = quantity of heat a liquid must absorb for 1 gram of it to be converted from liquid to
gaseous state.
Water's high heat of vaporization helps moderate earth's climate. A considerable amount of solar heat
absorbed by tropical seas is consumed during evaporation of surface water. Thus, as moist tropical air
circulates poleward, it releases heat as it condenses to form rain.
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Evaporative cooling also helps moderate temperature in lakes and ponds, and prevents terrestrial organisms
from overheating.
Ice floats
Water is one of the few substances that is less dense as a solid than as a liquid. (Fig 3.6)
If ice sank, eventually all bodies of water would freeze solid since floating ice insulates liquid water below.
Ice floats because as temperature decreases, there is less energy to break H-bonds, so eventually all water
molecules are H-bonded to one another resulting in a crystal lattice structure in which water molecules are
less densely packed.
Water as the solvent of life
Water dissolves more solutes than any other liquid. Most of life's molecules, such as the nucleic acids,
carbohydrates and proteins are dissolved in water (Fig 3.7 and 8 ). It is the favorable disordering of water
molecules that is the driving force of many biological processes (e.g. protein folding, formation of bilayers,
double helix, viral self-assembly, and more).
Solution = liquid that is a homogenous mix of 2 or more substances
Solvent = dissolving agent
Solute = substance that is dissolved
Aqueous solution = one in which water is solvent
The versatility of water as a solvent is based on its polarity.
-
Ions and polar water molecules have a mutual affinity through electrical attractions. E.g. Cl attracted and
+
surrounded by positive part of water and Na attracted and surrounded by negative part of water molecules.
A compound does not have to be an ion to be dissolved by water. Polar compounds are also water-soluble (
E.g. proteins, carbohydrates, nucleic acids).(refer to Fig 3.8: hydration of soluble protein)
Hydrophilic = any substance with an affinity for water (ions, polar molecules), even if that substance does
not dissolve (E.g. cellulose)
Hydrophobic = any substance that neither dissolves nor has an affinity for water (nonpolar). E.g fats,
waxes, etc...
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Solute concentrations in Aqueous solutions
The biological activity of molecules is more related to the number of molecules rather than their mass. i.e. its
the concentration that is important. Knowing concentrations is important in biology. This allows the
combinations of substances in fixed ratios to make chemical solutions.
Molarity = Number of moles/Liter
A mole (mol) = equal to the molecular weight of a substance, but scaled to units of grams rather than
daltons. Rescaling makes weighing more practical.
Molecular weight = sum of the weight of all the atoms in a molecule.
23
Avogadros Number = number of molecules in a mole (6.02 x 10 ).
23
e.g. 1 mole of sucrose has 6.02 x 10
23
6.02 x 10
molecules and weighs 342 g. 1 mole of ethanol also has
molecules, but weighs only 46 g.
Organisms are sensitive to pH changes
Dissociation of water molecules
Occasionally a hydrogen atom shared between two water molecules in a hydrogen bond shifts from one
+
-
molecule to another to produce a hydrogen ion (H ) and a hydroxide ion (OH ).
Dynamic equilibrium = forward and reverse reaction rates are equal. For water at equilibrium,
+
-
concentration of water molecules greatly exceeds the concentration of H and OH .
Hydrogen and hydroxide ions are very reactive. Even small changes in their concentrations profoundly affect
biological molecules and processses.
Acids and Bases
+
-
-7
In pure water, [H ] = [OH ] = 10
+
Acid = substance that increases the [H ] of a solution. A strong acid, such hydrochloric acid, dissociates
completely when mixed with water.
+
Base = A substance that reduces the [H ] in a solution. A strong base, such as sodium hydroxide,
dissociates completely when mixed with water.
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+
Some bases, such as ammonia, reduce [H ] directly by accepting hydrogen ions
.
Weak acids and bases do not dissociate completely.
The pH scale
Refer to Fig 3.9
+
-
-14
In any aqueous solution [H ][OH ] = 10
+
-
2
M
-7
In a neutral aqueous solution [H ] = [OH ] = 10
M.
+
-5
-
If enough acid is added to aa aqueous solution to increase its [H ] to 10 , then [OH ] will decline by an
-9
equivalent amount to 10 .
+
-
An acid not only adds H ions to a solution, but also removes OH ions from solution since they will react to
form water. A base has the opposite effect.
+
-
Because [H ] and [OH ] can vary by a factor of 100 trillion or more, this variation has been more
conveniently expressed as a pH scale, which ranges from 0 to 14.
+
pH = -log [H ]
+
-7
For a neutral solution [H ] is 10
M,
-7
therefore -log 10 = -(-7) = 7
Each pH unit represents a 10 fold
change in concentration.
Buffers
Because biological systems are very sensitive to pH, they need to minimize changes in pH. They do this with
buffers.
Buffers = compounds that resist changes to their own pH when acids or bases are introduced.
A buffer works by accepting hydrogen ions from the solution when they are in excess and donating hydrogen
ions to the solution when they have been depleted. Most buffers are weak acids or bases. (acid-base pairs)
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Related Links
Water science for schools
Environmental protection agency
The water page
The world’s water
Canada Water Act
Sierra Legal Defence on water
The Health of our Water: Toward sustainable agriculture in Canada
Open Door Web on the pH scale
The power of hydrogen
This Page last updated Monday, Sept 17, 2007 11:08 AM. For more information contact Mário Moniz de Sá
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