Water & pH
lecture-2
Water
 Water is essential for life, it covers 2/3 of the
earth's surface and every living thing is dependent
upon it.
 The human body is comprised of approximately
70% water, and it is a major component of many
body fluids including blood, urine, saliva, and
others.
A. Structure of Water
• A water molecule is composed of two hydrogen
atoms covalently bonded to an oxygen atom.
• With a total atomic weight of 18 amu (Dalton).
• The hydrogen atoms form an angle of about
105°, so the molecule is “bent”.
• The structure of the electrons surrounding water
is tetrahedral, resembling a pyramid.
• A water molecule is formed when two atoms
of hydrogen bond covalently with an atom of
oxygen.
• In a covalent bond electrons are shared
between atoms. In water the sharing is not equal,
the oxygen atom attracts the electrons more
strongly than the hydrogen.
• This gives water an asymmetrical distribution of
charge.
• Molecule that have ends with partial negative and
positive charges are known as polar molecule.
• Water is a polar solvent, it readily dissolves most
biomolecules, which are generally charged or
polar compounds.
• Each hydrogen bears a partial positive charge,
while the oxygen atom bears a partial negative
charge.
• As a result, there is an electrostatic attraction
between the oxygen atom of one water molecule
and the hydrogen of another, called a hydrogen
bond.
• The bent geometry of water and the attraction
between water molecules gives rise to unique
properties that are essential for its role in living
organisms and the environment.
B. Ionization of Water
• Consider a glass of pure water:
– In a tiny fraction of the water molecules, one of
the hydrogen nuclei is completely pulled off the
molecule.
– This forms two ions:
• A hydrogen ion (H+)
• And a hydroxyl ion (OH–)
– This is caused by the attraction of the water
molecules for each other.
• Water molecules are continuously splitting into
ions and rejoining to form water molecules.
Thus, water molecules exist in equilibrium
with hydrogen ions and hydroxide ions.
• In chemically pure water, the number of H+ and
OH– ions are the same.
• Certain chemical substances, when dissolved in
water, can change the amounts of H+ or OH –.
• Acid:
– A substance that increases the amount of H+
(and decreases the amount of OH–) or it can be defined
as substance that can donate hydrogen ions (protons).
• Base:
– A substance that increases the amount of OH–
(and decreases the amount of H+) or it can be defined
as substance compounds accept hydrogen ions.
• Neutral substance:
– A substance that does not change the amounts of H+ and
OH– (so H+ remains equal to OH–)
• Acidity and alkalinity are represented by a value
called “pH”
–
–
–
–
–
–
–
pH = -log[H+]
The pH scale measures how acidic or basic a substance is.
The pH scale ranges from 0 to 14.
Acids: pH value is less than 7
Bases: pH value is greater than 7
Neutral substances: pH value is equal to 7
Each pH value represents a 10-fold change in the amount
of H+ in the solution.
– So a substance with pH = 5 has a 10 times greater amount
of H+ than a substance with pH = 6.
• The acidity of the aqueous solution is usually
reported using the logarithmic pH scale.
• Buffers usually maintain the pH of the extracellular
fluid between 7.35-7.45
• pH values in the cell and in the extracellular fluid
are kept constant within narrow limits.
• In the blood, the pH value normally ranges only
between 7.35 and 7.45
• The pH value of cytoplasm is slightly lower than
that of blood at 7.0–7.3
• In lysosomes pH is 4.5–5.5
• Extreme values are found in the stomach pH 2 and
in the small bowel > 8.
• Since the kidney can excrete either acids or bases,
depending on the state of the metabolism, the pH of
urine has a particularly wide range of variation 4.8–
7.5.
• Many biomolecules are amphoteric in
aqueous solution that is they can accept or
donate protons.
• For example, at physiologic pH all the amino
acids have a positive group and a negative
group. Thus they can act as an acid or a base.
Such substances are called Ampholytes.
Importance of pH
• Measurement of pH is one of the most
important and frequently used procedures in
biochemistry. The pH affects the structure and
activity of biological macromolecules;
1. For example, the catalytic activity of
enzymes is strongly dependent on pH.
2. Measurements of the pH of blood and urine
are commonly used in medical diagnoses.
C. Solvent Properties of Water
• Solution:
– A mixture of two (or more) different substances in
which the particles of one substance are completely
interspersed with the particles of the other substance.
– Solvent: The substance that is present in the largest
amount.
– Solute: The substance that are present in smaller
amount.
• Hydrophilic substances:
– Substances that can be dissolved in water.
– Water molecules are attracted to ions or to other
molecules that have partial positive and
negative charges.
– Examples of hydrophilic substances:
• Sodium chloride (table salt): This substance consists
of sodium ions and chloride ions.
• Sucrose (table sugar): This substance is a compound
with many -OH groups in its structure, with many
partial positive and negative charges.
• Hydrophobic substances:
– Substances that cannot be dissolved in water but
can be dissolved in nonpolar solvents such as
chloroform and benzene.
– Water molecules have difficulty interacting with
uncharged molecules, these substances tend to
separate from water.
– Example of a hydrophobic substance:
• Cooking oil: The molecules of cooking oil have long
chains of carbon atoms bonded to hydrogen.
The atoms do not have the “bent” geometry of water, so
there are no partial charges to attract water molecules.
Therefore, oil and water don’t mix!
• Amphipathic substances:
– Substances in which part of the molecule is
hydrophobic, and part of the molecule is
hydrophilic.
– When amphipathic substances are mixed with
water, its molecules form into clusters called
micelles:
• with the hydrophilic part on the outside of the
micelle in contact with water.
• and the hydrophobic part on the inside of the
micelle, away from the water .
• Amphipathic substances (cont.):
– Example of an amphipathic substance:
• Soap: Soap molecules have an ionic group attached
to one end, and an oily hydrocarbon chain attached
to the other end. When soap is mixed with water, it
forms micelles that trap oily dirt molecules.
D. Thermal Properties of Water
• Molecules are in constant motion due to the
heat energy (kinetic energy) they contain.
• Phases of matter:
– Solid
• Limited movement of molecules; non-fluid.
– Liquid
• Molecules can move freely around each other; fluid.
– Gas
• Molecules have greatest freedom of movement;
substance can expand to fill the available space.
• Water has unusual thermal properties
because of the attraction of water molecules
for each other.
1-Water has relatively high boiling point.
2-Water remains in a liquid state over a wide
temperature range.
3-Water has a high heat capacity: it can absorb a
large amount of heat with a small change in
temperature.
4-The solid form of water (ice) is less dense than
the liquid, so ice floats on water.
E. Monomers and Polymers
• Monomer:
– An organic molecule that serves as a “building
block” to build larger organic molecules.
• Polymer:
– An organic molecule composed of two or more
monomer units linked together by covalent bonds.
• Condensation reaction:
– Polymers are often formed by the process of
condensation.
– In this process, two hydrogen atoms and an oxygen
atom are removed from two monomer units.
– And a covalent bond forms between the monomers.
• Hydrolysis reaction:
– Polymers are often broken down by the process of
hydrolysis.
– In this process, a water molecule is inserted
between the monomer units of a polymer to split
the polymer into its monomer units.