UNIT 1: Biochemistry
1.2: Water: Life’s Solvent
pg. 19 – 24
All living things contain water. Many live within water.
Water is a universal solvent, more substances dissolve in water than ay other
substance.
Our body relies on water for survival.
60% of our body weight comes from water.
(70% brain, 90% lungs, and 22% bone tissue)
All cellular processes occur in water. Cellular components are dissolved,
suspended, and surrounded by water.
Properties of Water
Water is special because of its size, shape, polar structure, and ability to
associate with each other because of hydrogen bonding.
Fluid property of water is created by the lattice it forms between water
molecules. The hydrogen bonds between water molecules constantly break
and reform in new positions. Only 3.4 hydrogen bonds are created between
neighbouring molecules.
Ice is created when the water lattice becomes a rigid crystalline structure.
Each water molecule forms 4 hydrogen bonds with its neighbouring
molecule.
Specific Heat is the mount of thermal energy required to raise the
temperature of a given quantity of a substance by 1oC. Most of the heat
energy is used to break the bonds between water molecules to create water
vapour. This maintains that water is a liquid from 0oC to 100oC.
Cohesion between water molecules creates surface tension, which is the
measure of how difficult it is to stretch or break the surface of a liquid.
Property of Adhesion occurs between water molecules and other polar
molecules.
These two properties support the movement of water in the vascular tissue
(xylem) in plants.
Table 1: The Unique Properties of Water
Aqueous Solutions
Water molecules form hydration shells around polar molecules and ions of
other substances.
Water causes the dissociation of other molecules forming solutions, and
prevents the re-association, by surrounding them with a hydration shell.
Hydrophilic molecules - are polar or charged molecules that are strongly
attracted to water.
Hydrophobic molecules - are non-polar molecules that are not strongly
attracted to water.
Cells and blood are made up of water molecules, therefore they have the
ability to dissolve thousands of solutes necessary for life, and allow
chemical reactions to occur.
Ionization and pH
Autoionization - is the process in which a molecule spontaneously
dissociates into ions.
Water molecules can react with each other forming hydronium ions (H3O+)
and hydroxide ions (OH-). When this happens there is an equal umber of
hydronium and hydroxide ions produced.
2H2O →
H3O+ + OH-
Acids and Bases
When the hydronium and hydroxide ion concentration is altered, acids or
bases are created. When the hydronium ion concentration is greater then the
hydroxide ion concentration, the properties of an acid is created, and vice
versa creates a base.
Acid
HCl (g) + H2O (l) → H3O+ (aq) + Cl- (aq)
Base (dissociate in water)
NaOH (s) + H2O (l) → Na+ (aq) + OH- (aq)
Base (combine with H+ ions)
NH3 (g) + H2O (l) →
NH4+ (aq) + OH- (aq)
pH scale is a numerical scale used to determine the acidity of a solution. The
scale is measured from 0 to 14.
Logarithm: pH = -log10 [H+]
The equation represents the concentration of H+ or H3O+ ions in an aqueous
solution.
Strong and Weak Acids and Bases
The strength of an acid or base depends on the degree to which it ionizes
when dissolved in water.
Strong acids and bases are completely dissociated in water, creating an
aqueous solution.
Weak acids and bases partially ionize in water, such as: Acetic acid and
Ammonia.
Neutralization Reactions and Buffers
Neutralization occurs when an acid is combined with a base. Water and an
ionic salt are created. Water and salt are neutral creating a solution with a pH
of 7.
HCl (aq) + NaOH (aq) → H2O (l) + NaCl (aq)
Buffer - is a chemical that compensates for pH change in a solution by
accepting or donating H+ ions.
Organisms have control over their internal pH of their cells. Buffers are used
to compensate for the release of hydrogen ions (H+ ions) during chemical
reactions. Also if there aren’t enough hydrogen ions present, a buffer will
release some into the solution. A buffer can also release hydroxide ions
(OH-), to control pH values.