Water: The Chemistry of Living
Organisms
Water: Not a Universal Solvent, but a
Versatile Solvent
A. Water is a polar
molecule. All of the
properties of water stem
from this fact.
B. This allows water
molecules to interact
with one another and
form up to 4 hydrogen
bonds with oxygen
atoms of neighboring
molecules in liquid
water.
Water has more structure than most
liquids.
C) H bonds form, break, and reform in a few
trillionths of a second.
D) Property that gives water more structure
than most liquids = cohesion.
E) Water molecules can interact with other
polar substances e.g. Ammonia (NH3),
ionic compounds.
Polar vs. Non-Polar Substances
Polar substances are water-loving: hydrophilic
Polar substances are attracted to 1 end or the
other of water molecule and may form weak
hydrogen bonds.
Non-polar substances are water-dreading:
hydrophobic
e.g. increased amounts of covalent bonds
between C and H such as lipids. Lipids are fats,
oils, steroid hormones, and phospholipids which
are the major component of biological
membranes.
Important Point
F. Hydrogen bonds and hydrophobic
interactions underlie the 3 properties of
water that are biologically important.
B. Properties of Water
1. Internal Cohesion: H bonds give
cohesion to water. These bonds resist
rupturing when placed under tension. This
is important at the water-air interface
where H bonds exert an inward pull on the
uppermost molecules. The result:
a) Surface Tension
 Surface of water –
leaves float, insects
lay eggs.
 Surface of alveoli in
vertebrate lung is also
wet so that gases
dissolve in the film;
this requires
surfactant, a wetting
agent.
b) Water Transport: movement of water
and minerals – soil to leaves
 Movement is against gravity and relies on 1)
cohesion of water molecules 2) adhesion of
water to sides of transport vessels (xylem) (think
of the meniscus) and 3) evaporation of water
into air which provides the necessary pull.
2. Solvent Properties
Definition of a solvent: any fluid in which 1
or more substances can be dissolved.
Water is a good solvent for ions and polar
molecules (solutes).
What does dissolve mean? A sphere of
hydration forms around individual ions or
molecules. E.g. NaCl dissolved in water.
Importance: both seawater and living cells
contain many different dissolved ions.
NaCl dissociates: separation into ions
 Partial negative
charge of oxygen is
attracted to the
positive sodium ion.
 Partial positive charge
of hydrogen is
attracted to the
negative chloride ion.
 Spheres of hydration
are formed around
these dissociated ions
Water Soluble Protein
Temperature Stabilizing Effects
 Think about what it’s like
living on the ocean’s
coast.
 The ocean acts as a heat
sink=it warms up slowly
and cools down slowly.
 Moderates weather of
coastal areas. Prevents
severe temperature
fluctuations.
Temperature
 Measure of how fast molecules are moving –
kinetic energy.
 As average speed increases, we record as a
temperature increase.
 When water is heated, its molecules cannot
move faster until hydrogen bonds among them
are broken.
 This takes considerable heat to break H-bonds
and prevent them from reforming.
 Think about boiling point of alcohol vs. water.
High Specific Heat
A) Because it takes a relatively large
amount of energy to increase or decrease
water temperature we say that water has
high specific heat.
Definition of specific heat: amount of heat
that must be absorbed or lost for 1 gram of
that substance to change temperature by
1oC.
High Specific Heat
 High specific heat: a measure of the extent to
which a substance resists changing
temperature.
 Specific heat of water at 25oC is 1
calorie/gram/degree
 Specific heat of most carbon compounds = ½
that of water (think melting sugar)
 Specific heat of iron = 1/9 that of water (think of
heating a cast iron skillet)
 Only ammonia has higher specific heat
 Aid to biological systems because heat
generated by chemical reactions inside cells
would destroy cells if not for high specific heat of
water within it…coolant
b) High heat of vaporization
 Heat of vaporization = the energy to change 1
gram of liquid water to a gas (586 calories).
 Evaporation of water from a surface causes
cooling of that surface because the fastest
moving molecules (i.e. those with the greatest
temperature) evaporate and leave the surface
cooler.
 Many organisms get rid of excess body heat by
sweating, panting, transpiration = called
evaporative cooling.
High Specific Heat: Evaporative Cooling
c) Behavior of water at low temperatures
 Cooling water below 4oC (which is waters
densest point) causes water to become less
dense, turn to ice and float.
 Water molecules are locked into a crystal-like
lattice of hydrogen bonds = ice.
 Molecules no longer move fast enough to break
H bonds.
 Ice is less dense than water because hydrogen
bonds in ice space water molecules far apart.
 10% less dense than liquid water.
Water at Low Temperatures: Ice
Water as Insulation that Provides Habitat
 Ice insulates large
bodies of water and
provides habitat for
living organisms at
low air temperatures.
Water: Ice, Water, Steam
Water As Habitat
C) Wet Chemistry: 1 Mole of Each
 Most biochemical
reactions involve
solutes dissolved in
water e.g. Digestion,
reproduction,
circulation, excretion
 To calculate
concentrations of
solute – measure
substances in moles.
Moles
1 mole (mol; M) = molecular weight of a
substance in grams
H+ = atomic mass = 1
1 mole of hydrogen ions weighs 1 gram
1 mole of any substance is Avocadro’s #
or 6.02 x 1023 molecules of that substance
Water forms ions spontaneously: ionizes
At 25oC/ 1/554 million water molecules
ionizes
Water Dissociation
Pure water and pH scale
Pure water has 1/10,000,000 or 10-7 moles
of hydrogen ions.
Can express hydrogen ion concentration
using pH scale 1-14.
High hydrogen ion concentration = acid
Strong acid: HCl hydrochloric acid
Weak acid: H2CO3 Carbonic acid
Bases
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Low hydrogen ion concentration = base
Strong base: NaOH sodium hydroxide
Weak base: NH3 + H+ yields NH4+ ammonium ion
Convenient way to express hydrogen ion concentration
is to use pH scale
Define pH = -log (H+) where ( ) = concentration.
Log of exponent is a whole number
pH = exponent x (-1); low number < 7 acidic; > 7 basic
pH of water with hydrogen ion concentration of 1 x 10-7
= 7 neutral
Logarithmic scale: difference of 1 number = 10 x (H)+
Simple calculations
If hydrogen ion concentration is 1 x 10-1
than pH = 1
Or if the hydroxide ion concentration is 1 x
10-13, pH = 1
If hydrogen ion concentration is 1 x 10-2
than pH = 2
Or if the hydroxide ion concentration is 1 x
10-12.
pH Scale: Hydrogen ion concentration
pH range of living organisms
Staying in pH range necessary for enzyme
activity. Otherwise the enzyme loses its
3D shape (denaturation).
Main buffer in human blood is an acid
base pair: carbonic acid and bicarbonate
Acid Rain
Effects of Acid Rain on Statuary
Transparency
Last characteristic of water that makes it
important to living organisms is
transparency.
1) Light can penetrate bodies of water and
allows life below water.
2) Light can penetrate cells =
photosynthesis.