Biomolecules
Biomolecules
Mineral salts
Inorganic
water
Gases
Biomolecules
Carbohydrates
Organic
Lipids
Proteins
Nucleic Acids
Structure & Properties of Water
STRUCTURE OF WATER
O
The bond that forms water
is a covalent bond
+
+
H
H
O
-
POLAR MOLECULE
H
O
H
The positive hydrogen ends of 1 molecule
are attracted to the negative end of the
oxygen of another molecule.
Hydrogen bonding
• Polar water creates molecular attractions
– attraction between positive H in one H2O
molecule to negative O in another H2O
• Weak bond
creates a sticky molecule
H
H
O
Elixir of Life
• Special properties of water
1.cohesion & adhesion
• surface tension, capillary action
2.good solvent
• many molecules dissolve in H2O
• hydrophilic vs. hydrophobic
3.lower density as a solid
• ice floats!
4.high specific heat capacity
• water stores heat
5.high heat of vaporization
• heats & cools slowly
Ice!
I could use
more ice!
1. Cohesion & Adhesion
• Cohesion
– H bonding between H2O molecules
– water is “sticky”
• Adhesion
– H bonding between H2O & other substances
• meniscus
• water climbs up
paper towel or cloth
Try that
with flour…
or sugar…
Cohesion or adhesion?
How does H2O get to top of trees?
Transpiration is built on cohesion & adhesion
Let’s go to the
videotape!
CAPILLARY ACTION
• The combined force of attraction among
water molecules and with the molecules
of the surrounding material.
• Cohesion + Adhesion
UNIVERSAL SOLVENT
• This is a nickname given to water.
• Water dissolves LOTS of things.
• Remember:
SOLVENT = the thing doing the dissolving
SOLUTE = the thing that dissolves away
• Other polar molecules can be dissolved by
water.
2. Water is the solvent of life
• Polarity makes H2O a good solvent
– polar H2O molecules surround + & – ions
– solvents dissolve solutes creating solutions
Solvation
Solvation is the process in which molecules of
a solvent attract the particles of a solute.
Solvation
As ions dissolve in a solvent, they spread out
and become surrounded by solvent molecules.
What dissolves in water?
• Hydrophilic
– substances have attraction to H2O
– polar or non-polar?
What doesn’t dissolve in water?
• Hydrophobic
– substances that don’t have
an attraction to H2O
– polar or non-polar?
fat (triglycerol)
Oh, look
hydrocarbons!
Density of Water
• Most dense at 4oC
• Contracts until 4oC
• Expands from 4oC to
0oC
The density of water:
1. Prevents water from freezing from the bottom up.
2. Ice forms on the surface first—the freezing of the
water releases heat to the water below creating
insulation.
3. Makes transition between season less abrupt.
Ice floats
LIQUID WATER
ICE
LIQUID WATER
ICE
Why is “ice floating” important?
• Oceans & lakes don’t freeze solid
– surface ice insulates water below
• allowing life to survive the winter
– if ice sank…
• ponds, lakes & even oceans would freeze solid
• in summer, only upper few inches would thaw
– seasonal turnover of lakes
• sinking cold H2O cycles nutrients in autumn
WATER = 24°C
BEACH = 90°C
4. Specific heat capacity
• H2O resists changes in temperature
– high specific heat
– takes a lot to heat it up
– takes a lot to cool it down
• H2O moderates temperatures on Earth
High Specific Heat Capacity
• Amount of heat needed to raise or
lower 1g of a substance 1° C.
• Water resists temperature change,
both for heating and cooling.
• Water can absorb or release large
amounts of heat energy with little
change in actual temperature.
Evaporative cooling
5. Heat of vaporization
Organisms rely on heat of vaporization
to remove body heat
High Heat of Vaporization
• Amount of energy to convert 1g or a
substance from a liquid to a gas
• In order for water to evaporate,
hydrogen bonds must be broken.
• As water evaporates, it removes a lot
of heat with it.
Quick Qs
Question 1. Circle the letter of each sentence that is true about water's specific heat.
a. It is due to the many attractions among water molecules.
b. It makes large bodies of water heat up more quickly than nearby land.
c. It makes large bodies of water cool off more slowly than nearby land.
d. It leads to warmer air over land than over water on summer days.
Question2. Match the terms review.
1. Cohesion ____ 2. capillary action ____ 3. adhesion _____4. polarity _____
5. hydrogen bond _____6. surface tension ______ 7. specific heat
a. tendency to stick to other substances
b. tightness caused by the pulling of water molecules on each other
c. the amount of heat needed to raise 1kg of a substance 1o
d. weak bonds formed between water molecules
e. tendency to stick to other water molecules
f. uneven distribution of charges in a molecule
g. tendency to climb up due to cohesion and adhesion properties of water
Property of water
Ice is less dense than water
Significance for life
Answers
The high surface tension of water
means that it can form a habitat on
the surface of the water
High surface tension
Can be used for cooling organisms
by evaporation of for example sweat
Strong cohesive properties and high
tensile strength
Colourless with a high transmission
Liquid at room temperature
In order to evaporate it must absorb a
large amount of energy (high latent
heat of vaporisation)
Water can absorb a lot of energy for
only a small rise in temperature (high
specific heat capacity)
Ice forms an insulating layer over
water
Conditions are stable in cells and
aquatic environments
Can be used for transport and a
medium for reactions
Light can pass through cells for
photosynthesis
Water can be pulled through plants
in a column as the water molecules
are held together by H-bonds
Property of water
Significance for life
Ice is less dense than water
Ice forms an insulating layer over
water
High surface tension
The high surface tension of water
means that it can form a habitat on
the surface of the water
Strong cohesive properties and high
tensile strength
Colourless with a high transmission
Liquid at room temperature
Water can be pulled through plants
in a column as the water molecules
are held together by H-bonds
Light can pass through cells for
photosynthesis
Can be used for transport and a
medium for reactions
In order to evaporate it must absorb a
large amount of energy (high latent
heat of vaporisation)
Can be used for cooling organisms
by evaporation of for example sweat
Water can absorb a lot of energy for
only a small rise in temperature (high
specific heat capacity)
Conditions are stable in cells and
aquatic environments
Next
To do
What’s happening here?
When water dissociates, one of the hydrogen nuclei leaves its
electron behind with the oxygen atom to become a hydrogen ion,
while the oxygen and other hydrogen atoms become a hydroxide
ion.
• The hydrogen ion attaches to the oxygen atom
of a second un-ionized water molecule to form
a hydronium ion (H3O +)
The overall dissociation of water
Degree of Dissociation
• The degree of dissociation of a substance is
defined as the fraction of its molecules
dissociating at a given time
• For every 10000000 molecules of water only
one is ionised.
• ie. Concentration of H+ in water is 1x10-7/l
• Leads to large and rapid pH changes
Ions:
Acids & Bases
An acid is any ionic compound that
releases hydrogen _____ (H+) in
solution.
A base is any ionic compound that
releases hydroxide _____
(-OH) in solution.
From the Virtual Cell Biology Classroom on ScienceProfOnline.com
48
Q1: An acid is a compound that in
aqueous solution will readily:
A. Shed a proton.
B. Shed an electron.
C. Gain a proton.
D. Gain an electron.
E. None of the above.
49
Acidity can be measured on a scale called pH
Measurements of Acidity & Alkalinity (pH)
Acidity of a solution > measured by
concentration of hydrogen ions
(H+).
pH ranges: 0
basic).
(very acidic)
to 14 (very
Change in just one unit of scale =
tenfold change in H+
concentration.
If concentration of H+ = OH - …
neutral.
Images: pH scale, Edward Stevens, Wiki
From the Virtual Cell Biology Classroom on ScienceProfOnline.com
What’s the difference?
52
Weak acids
• Weak acids have only a modest tendency to shed their
protons (definition of an acid).
• When they do, the corresponding negatively charged
anion becomes a willing proton acceptor, and is called
the conjugate base.
53
Weak acid
Conjugate base
conjugate acid
the substance that results when a base gains (or
accepts) a proton
conjugate base
the substance that results when an acid loses (or
donates) a proton
54
55
Conjugate base
Conjugate acid
56
Conjugate base
Conjugate base
57
Why am I talking about weak acids?
58
When running really hard—let’s look at some of the effects:
• blow gently through the straw into the pink
solution for 15-30 seconds.
• What happens?
• What does the “indicator” tell you?
• Why did it happen?
59
RESULTS
• The solution went from pink to clear,
• indicating a decrease in pH (the colorchanging dye is phenolphthalein),
• i.e., an increase of acidity.
What could this be due to?
60
The Hydration of Carbon Dioxide in Water
CO2 + H2O
H2CO3
⇌
HCO3- + H+
As carbon dioxide goes into solution, carbonic acid is
formed, which partially dissociates, liberating protons (H+)
and thus causing the solution to become more acidic, i.e.,
lowering the pH.
61
CO2 + H2O
H2CO3
What if you could reverse
this…
by removing carbon
dioxide?
HCO3- + H+
As carbon dioxide leaves the solution, carbonic acid is used
up, which by the Law of Mass Action shifts the equilibrium to
the left, using up protons (H+) and thus causing the solution
to become less acidic, i.e., raising the pH.
62
How Does This Work?
CO2 + H2O
H2CO3
⇌
HCO3 + H+
Excess bicarbonate will soak up many of the free protons,
and drive the equilibrium to the left. This will reduce the
acidity, increasing the pH, and the carbon dioxide produced
will be blown off in the lungs.
63
64
Carbonic acid-bicarbonate buffer
65
BUFFERS
Contain a weak acid and it’s conjugate base
Solutions that maintain pH constant when small amounts
of acid or base are added.
Phosphate buffer
dihydrogen
phosphate
Ion
monohydrogen
phosphate
ion
Add acid H+