This document discusses several key properties of water including its molecular structure, hydrogen bonding abilities, and role in biological systems. Water is a polar molecule that can form up to four hydrogen bonds with neighboring water molecules. This extensive hydrogen bonding is responsible for water's unusually high melting and boiling points. Water also interacts with solutes through hydrogen bonding and electrostatic interactions, determining whether a substance is soluble. These weak interactions are crucial for the structure and function of macromolecules. The document also covers water's role in acid-base chemistry including its autoionization and the pH scale.

1. Water and pH
Biochemistry I
BS Biotechnology (5th Semester)
University of Sargodha

2. Water
• Water is a polar molecule which make up to
70% or more of the weight of most organism
• It has tetrahydron arrangement of 104.5o
which is slightly less than the perfect
tetrahydron of 109.5o. Reason because of
crowding by the nonbonding orbitals of the
oxygen atom.
• Oxygen is more electronegative, that why it
attracts electron pair and develop partial
negative charge and hydrogen develop partial
positive charge, which make two electric
dipoles in water molecule.

3. Water
• Hydrogen bond is weaker than the covalent bond
• Bond dissociation energy
Hydrogen bond (O-H) = 23 kJ/mol
Covalent bond (O-H) = 470 kJ/mol
• The nearly tetrahedral arrangement of the orbitals about the oxygen atom allows
each water molecule to form hydrogen bonds with as many as four neighboring water
molecules.
• In liquid form, water molecules are disorganized and in continuous motion, so that
each molecule forms hydrogen bonds with an average of only 3.4 other water
molecules.
• In ice, each water molecule is fixed in space and forms hydrogen bonds with four
other water molecules and form a regular lattice structure

4. Water
High melting and boiling points of water are due to the hydrogen bonds between
water molecules because more energy is required.
Entropy: dis-orderness/randomness of system
Enthalpy: Overall energy of the system
= free energy required to perform work. It is also known as Gibbs free energy.
= the enthalpy change from making and breaking bonds
= the change in randomness/ entropy
= the absolute temperature
Free energy is the energy which is required to do
work. when bonds break, they release energy,
according to the second law of thermodynamics
some energy is lost in heat which increase
entropy

5. Water forms hydrogen bonds with polar solutes
• Hydrogen bond is formed between electronegative elements and the hydrogen
atom attached to another electro negative atom.
• In biological system, hydrogen bond is usually formed are H-N, and H-O
• H bound with C usually do not participate in hydrogen bonding because carbon
is less electronegative and electron sharing between H and C is nearly equal,
which do not form dipoles.
Common hydrogen bonds in biological systems

6. Some biologically important hydrogen bonds

7. Water interacts with electrostatically charged solutes
Hydrophilic: Interacts with water and other polar substances. Also known as water
loving. E.g. NaCl, sugars
Hydrophobic: poorly interact with water and other polar substances. Also known as
Water hating. E.g. Chloroform, benzene, typical wax
Amphipathic: A chemical compound possessing both hydrophobic and hydrophilic
parts.
Hydrophilic Hydrophobic
Amphipathic

8. Water: Nonpolar gases are poorly soluble in water
Biologically important nonpolar gases
• Molecular oxygen (O2)
• Molecular Nitrogen/atmospheric nitrogen (N2)
• Carbon dioxide (CO2)
• In O2 and N2, electrons are shared equally by both atoms. In CO2, each C=O
bond is polar, but the two dipoles are oppositely directed and cancel each other.

9. Water: Nonpolar gases are poorly soluble in water
How do these biologically important nonpolar gases travel in body of living organisms?
Molecular Oxygen (O2) : It binds with carrier proteins (hemoglobin and myoglobin, for
example) and travel in living body.
Molecular Nitrogen (N2) : Nitrogen is converted to ammonia or other substances by
nitrogen fixing bacteria in soil. These substances are further used for the formation of
amino acid, Nucleotides and nucleic acids etc.
Carbon dioxide (CO2) : It forms carbonic acid (H2CO3) in aqueous solution and is
transported as the HCO3
-(bicarbonate) ion

10. Weak interactions play are crucial to macromolecular
structure and function
• Non-covalent interactions like hydrogen bonds, ionic bond, hydrophobic interactions,
and van der Waals interactions are much weaker than covalent bonds
• The cumulative effect of many such interactions can be very significant
• For macromolecules, the most stable structure is usually that in which weak
interactions are maximized
• Examples:
• Binding of nucleotides in DNA strand
• Binding between enzyme and substrate
• Binding of neurotransmitters to their receptors
• Three dimensional folding of protein polypeptide
• And many more

11. Solutes affect the colligative properties of Aqueous
solution
• Vapor pressure, boiling point, melting point (freezing point), and osmotic pressure are
collectively known as colligative properties
• Colligative properties of water are independent of the chemical properties of the solute
• Colligative properties of water only depends on the number of solute particles in a
given amount of water.

12. Vapor pressure
Vapor pressure is the pressure caused by the evaporation of liquids.
Four common factors that influence vapor pressure are:
• Surface area
• Intermolecular forces
• Temperature
• Liquid is pure or mixed with solute
If the intramolecular forces will be small,
the vapor pressure will be high, but
boiling point will be less because less
amount of energy will be required to
break the intramolecular forces.

13. Osmotic pressure
The minimum pressure that stops the osmosis is equal to the osmotic pressure of solute
Types of solutions with reference to osmolarity of cell’s cytosol
Isotonic: No net water movement
Hypotonic: water moves from solution to the cell (cell swell up)
Hypertonic: water moves from cell to the solution (cell shrinks)

14. Ionization of water, weak
acids and weak bases

15. Water molecules have a slight tendency to undergo reversible ionization to yield a
hydrogen ion (a proton) and a hydroxide ion
At 25 oC, only 2 out of 109 water molecules undergo ionization.
In actual, Ionization of water molecule yields hydronium ion (H3O)
and hydroxide ion (OH).
The ionization of water can be measured by its electrical
conductivity
The high ionic mobility in water is the result of “proton hopping”
Ionization of water

16. Ionization of water
• As a result of the high ionic mobility of H+ , acid-base reactions in aqueous solutions
are exceptionally fast.
• Because reversible ionization is crucial to the role of water in cellular function, we
must have a means of expressing the extent of ionization of water in quantitative
terms.
• The position of equilibrium of any chemical reaction is given by its equilibrium
constant, Keq
• [] indicates concentration
• Concentration is measured in Molar (M)

17. Ionization of water
In pure water at 25 oC , the concentration of water is 55.5 M
The value for Keq, determined by electrical conductivity measurements of pure
water, is 1.8 X 10-16 M at 25 OC.
In pure water, the concentration of H and
OH is exactly equal.
Thus, [H+] = [OH-]
Product of [H+] [OH-] is always equal to
1 X 10-14 M.

18. pH
• Symbol p denotes “negative logarithm of”
• Thus, pH is the negative logarithm of hydrogen ion concentration, this can be expressed
as
• The difference of 1 pH between two solutions means that the concentration of one
solution has 10 times increased number of Hydrogen ions as compared to other
solution.