BIOL 110
Chapter 3
Water and Life
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Overview: Water
•Water is the biological
medium on Earth
•All living organisms require
water more than any other
substance.
•Most cells are surrounded
by water, & cells
themselves are about 70–
95% water
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Overview: Water
• The abundance of water is
the main reason the Earth
is habitable
• It is the only common
substance to naturally exist
in three physical states –
liquid, solid (ice), and a gas
(water vapor)
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Covalent Bonding
H2O
–
• In a nonpolar covalent bond, the
atoms share the electron equally
O
+
H
H
+
• In a polar covalent bond,
1 atom is more electronegative, &
the atoms don’t share the
electron equally
• Unequal sharing of electrons
causes a partial positive (+) or
partial negative (-) charge for
each atom or molecule
(Slide # 20 from the previous lecture)
Water Molecules – H-bonds
• The water molecule is
a polar molecule: the
opposite ends have
opposite charges
• Polarity allows water
molecules to form
hydrogen bonds with
each other

Hydrogen
bond
+
+
Polar covalent
bonds


+
+

Water Properties
Four of water’s properties that facilitate and allow an
environment for life on earth are:
1.
2.
3.
4.
Cohesive behavior
Ability to moderate temperature
Expansion upon freezing
Versatility as a solvent
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Water: Cohesive Behavior
• Collectively, hydrogen
bonds hold water
molecules together cohesion
Adhesion
Two types of
water-conducting
cells
• Cohesion helps the
transport of water
against gravity in plants
• Adhesion - attraction
between different
substances, Ex: between
water & plant cell walls
Cohesion
Direction
of water
movement
300 m
Water: Cohesive Behavior
•
•
•
Surface tension is a measure of how hard it is to break the surface of a
liquid
Surface tension is related to cohesion – at the water-air interface the
molecules are orderly arranged through hydrogen bonds to one another and
to the molecules below the surface
Water behaves as if covered by an invisible film
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Water: Moderation of Temperature
• Water absorbs heat from warmer air & releases stored
heat to cooler air
• Water can absorb or release a large amount of heat
with only a slight change in its own temperature
• Anything that moves has kinetic energy – for instance
atoms and molecules
•
Ek= mv2/2
• The Ek depends on the mass of the object (m) and the
speed of the movement (v2)
• The Ek of the random movement of atoms and
molecules is called thermal energy
Water: Moderation of Temperature
• Temperature is the average kinetic energy of body of
matter
• The total thermal energy is proportional to the amount
(the mass) of the matter
• 200 ml of boiling water would have temperature of
100oC but less total thermal energy than a swimming
pool with temperature around 20oC
• Thermal energy transferred from one body of matter
to another is defined as heat
• Cub of ice in hot drink cools it by absorbing some of
the thermal energy from the liquid that results in
melting of the ice
Water: Moderation of Temperature
• The Celsius scale is a measure of temperature using Celsius
degrees (C)
• A calorie (cal) is the amount of heat required to raise the
temperature of 1 g of water by 1°C
• The “calories” on food packages are actually kilocalories (kcal),
where 1 kcal = 1,000 cal
• The Joule (J) is another unit of energy where
1 J = 0.239 cal, or 1 cal = 4.184 J
Water’s High Specific Heat
• The specific heat of a substance = the amount of heat that must
be absorbed or lost for 1 g of that substance to change its
temperature by 1ºC
• The specific heat of water is 1 cal/g/ºC; ethyl alcohol has 0.6
cal/g/ºC; the Fe has 0.1 cal/g/ºC
• Water resists changing its temp because of its high specific heat
• Water’s high specific heat can be traced to hydrogen bonding
• Heat is absorbed to break hydrogen bonds
• Heat is released when hydrogen bonds form
• The high specific heat of water minimizes temperature
fluctuations to within limits that permit life. During the winter –
the weather is milder closer to the ocean, and harsher inland.
Water: Evaporative Cooling
• Evaporation is transformation
of a substance from liquid to
gas
• Heat of vaporization is the
heat a liquid must absorb for
1 g to be converted to gas
• To evaporate 1 g of H2O at
25oC about 580 cal are
needed, nearly twice the
amount needed to evaporate
1 g of alcohol or ammonia
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Water: Evaporative Cooling
• As a liquid
evaporates, its
remaining surface
cools, a process
called evaporative
cooling
• Evaporative cooling
of water helps
stabilize
temperatures in
organisms and
bodies of water
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Floating of Ice on Liquid Water
• Ice floats in liquid water because hydrogen bonds in ice are more “ordered,”
making ice 10% less dense than liquid H2O at 4oC
• Water reaches its greatest density at 4°C
• If ice sank, all bodies of water would eventually freeze solid, making life
impossible on Earth
• What is the affect of global warming?
Hydrogen bond
Ice:
Hydrogen bonds
are stable
Liquid water:
Hydrogen bonds
break and re-form
Water: The Solvent of Life
•
•
•
•
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 = solvent
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Water: The Solvent of Life
•
•
Water is a versatile solvent due to its polarity, which allows it to form
hydrogen bonds easily
When an ionic compound is dissolved in water, each ion is surrounded by a
sphere of water molecules called a hydration shell
Na


Cl




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

Na
Cl











Water: The Solvent of Life
• Water can also dissolves compounds made of non-ionic polar
molecules
• Even large polar molecules (proteins) can dissolve in water if
they have ionic & polar regions
• Biological fluid as blood, cytoplasm, maple syrup are water-based
solutions
+


+
Water: The Solvent of Life
• A hydrophilic substance is one
that has an affinity for water
• Some hydrophilic substances do
not dissolve in water - cotton
• A hydrophobic substance is one
that does NOT have an affinity
for water
• Oil molecules - hydrophobic
because they have relatively
non-polar bonds
• Fatty acids are main building
blocks of the cell membranes
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Water: The Solvent of Life
Solute
Concentration in
Aqueous Solutions
• Most biochemical reactions
occur in water
• Chemical reactions depend
on collisions of molecules
and therefore on the
concentration of solutes in
an aqueous solution
chemwiki.ucdavis.edu
Water: The Solvent of Life
• When carrying out experiments, we use mass to
calculate the number of molecules
• Molecular mass = sum of all masses of all atoms in a
molecule
• Numbers of molecules are usually measured in moles,
where 1 mole (mol) = 6.02 x 1023 molecules (Avogadro)
• Avogadro’s number and the unit dalton were defined
such that 6.02 x 1023 daltons = 1 g
• Molarity (M) = number of moles of solute
liter of solution
Molarity and Avogadro's number
Sucrose = C12H22O11 The molecular weight of
sucrose will be:
12 times the atom mass of carbon(12) + 22 times
the atom mass of H (1) + 11 times the atom mass of
O (16) = 342 Daltons
Because the way Daltons and Avogadros number
are connected, to have 1 mol (6.02 x 1023 molecules)
of sucrose we need to measure 342 g.
Mol. Weight of NaCl = Na (23) + Cl (35.5) = 58.5
Daltons
Molarity and Avogadro's number
To prepare 1 M of Sucrose we need to take 342 g
of that substance and dissolve it in 1 l of water.
Workout: take 342 g sucrose, put in a flask and
start adding water with constant stirring, until the
entire amount of sucrose was dissolved. Then
adjust the volume to 1 l
1 M of NaCl: measure 58.5 g solute, dissolve in 1 l
of water.
In both cases in the 1 l of solution we will have
6.02 x 1023 molecules in 1 l of sucrose or NaCl
Acids & Bases and pH
• A hydrogen atom in a hydrogen bond between 2 water molecules
can shift from one to the other
•
•
•
•
•
The hydrogen atom leaves its electron behind & 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–)
Water is in a state of dynamic equilibrium in which water molecules
dissociate at the same rate at which they are being reformed
Only one out of every 554, 000, 000 is dissociated, the [] of each ion
at 25oC is 10-7 M
+
2 H 2O
Hydronium
ion (H3O+)

Hydroxide
ion (OH)
Acids & Bases and pH
• Though statistically rare, the dissociation of water molecules has
a great effect on organisms
• 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–
• Biologists use the pH scale to describe whether a solution is
acidic or basic
• 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
Acids & Bases and pH
• What and how the strong HCl does to the pH
• HCl
H+ + Cl- - results in increase of the H+
• What and how the strong NaOH does to the pH
• NaOH
Na+ + OH- - decrease of the H+
• Weak base: ammonia form ammonium ion
• NH3 + H+
NH4+ - decrease of the H+
• Weak acid: carbonic acid form bicarbonate ion
• H2CO3
HCO3- + H+ - increase of the H+
Acids & Bases and pH
• In any aqueous solution at 25°C the product of H+ and OH–
is constant & can be written as
[H+][OH–] = 10–14
• In neutral solution (water) the [H+] = [OH–] = 10–7
• If acid is added and the [H+] goes up to 10–4 this implies
that the [OH–] goes down to 10–10
• If base is added and the [OH–] goes up to 10–3 this implies
that the [H+] goes down to 10–11
• Whenever we know the [H+] we can calculate the [OH–]
Acids & Bases and pH
Actually the concentrations are factor of 100 trillions or
more
This makes it unpractical for everyday use.
In science we use the pH scale
• The pH of a solution is defined by the negative logarithm
of H+ concentration, written as
• For a neutral aqueous solution
pH = –log [H+]
In neutral solution [H+] is 10–7
pH = -log 10-7 = –(–7) = 7
Each pH value (the [H+]) also implies the [OH-] value
If the pH is 4, the [H+] is 10-4 and the [OH-] is 10-10
H+
H+

H+ OH
+
OH H
+
H H+
H+
H+
Acidic
solution
Increasingly Acidic
[H+] > [OH]
pH Scale
0
1
Battery acid
2
Gastric juice, lemon juice
3
Vinegar, wine,
cola
4
Tomato juice
Beer
Black coffee
5
6
Neutral
solution
OH
OH
OH
H+ OH

OH OH
OH
+
H
Basic
solution
Neutral
[H+] = [OH]
7
8
Increasingly Basic
[H+] < [OH]
OH
OH
H+ H+ OH

OH OH +
H
H+
+
H
Rainwater
Urine
Saliva
Pure water
Human blood, tears
Seawater
Inside of small intestine
9
10
Milk of magnesia
11
Household ammonia
12
13
Household
bleach
Oven cleaner
14
pH Scale
• 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
Acids & Bases and pH
0
Acidic
[H+] > [OH]
Acids donate H+ in
aqueous solutions.
Neutral
[H+] = [OH]
7
Basic
[H+] < [OH]
Bases donate OH
or accept H+ in
aqueous solutions
14
When the pH
changes from 4 to 3
this is 10X increase
in the concentration
of the H+.
Change from 3 to 6
is 1000X decrease
of the H+.
Acids, Bases, Buffers, and pH
• 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 a weak acid and its corresponding base,
which combine reversibly with H+
• Ex. buffer in human blood and other biological solutions is
carbonic acid (H2CO3), formed when CO2 reacts with water in
blood plasma
– Carbonic acid dissociates to yield a bicarbonate ion (HCO3-) and
a hydrogen ion (H+)
– The chemical equilibrium between carbonic acid and
bicarbonate acts as a pH regulator
– H2CO3
HCO3- + H+
– The equilibrium shifts left or right as other metabolic
processes add or remove H+ from the solution
Acidification of the
Oceans
• CO2 and the global warming
• CO2 - H2CO3-
• CaCO3
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