Department of Preparatory Year

Level: One

CHEM 105

Introduction to Biochemistry

Lecture: 4

Dr. Fatimah Alsaffar

 Water

Dr. Fatimah Alsaffar

Outline
• Revision

• Introduction to Water

• Key terms

• Water Definition

• Water Properties

• Acid, Base, Buffer

• pH
Write the function of the following organelles.
Organelles Function Organelles Function

1 Cell Wall 7 Golgi complex

2 Cell Membrane 8 Lysosome

3 Cytoplasm 9 Peroxisome

4 Nucleus 10 Mitochondria

5 Ribosome 11 Vacuole

6 ER 12 Chloroplast
Introduction to water:
• Water is a unique and essential biological molecule.

• With 70% of our earth (oceans) and our bodies being made
up of water.

• There is no life without water

• Water is composed of two elements Hydrogen & Oxygen.

• The structure of water plays a key role in its unique

properties.

• Particularly, polarity and hydrogen bonding are two of the

most important biological properties of water.
Is Water Percentage Constant Between Human Ages?
Key terms
• An atom: the smallest unit of matter

that retains all of the chemical

properties of an element.

• Element (s): a pure substance

consisting of only one type of atom

• Example: carbon (C), hydrogen (H),
which all have the same numbers of
nitrogen (N) and oxygen (O).
protons in their nuclei.
Key terms
• Molecule (s): A group of two or more than two atoms of the same or different elements

that are chemically bonded together and have distinctive chemical and physical

properties.

✓Example, two atoms of hydrogen react ✓Example, two atoms of

with each other and form one molecule hydrogen and one atom of

of water. oxygen react with each

other and form one

molecule of water.
Key terms
• Compound (s): a unique substance that consists of two or more elements

combined in fixed proportions (the composition of a compound is always the

same).

✓Example: water (H2O), carbon dioxide (CO2) and table salt (sodium Chloride,

NaCl)
 Key terms
• Bond (s): a force of attraction between atoms

or ions and forms when atoms share or

transfer valence electrons, for example,

covalent, ionic, metallic & hydrogen bonds.

✓Example: The bond between hydrogen and

oxygen atoms to form water is an example of

a covalent bond.
Learning Check

• The following is [Atom – Element • List chemical bonds.

– Compound].
 What is water?

A substance composed of the chemical elements hydrogen and oxygen, that is

a colorless, odorless, and tasteless liquid (at room temperature) that may also

occur in various forms such as gas (water vapor) and solid (ice).
 Water properties
Molar mass 18.01528(33) g/mol

Odor Odorless

Color Colorless
Liquid: 0.99984283(84) g/mL at 0 °C
Density
Solid: 0.9167 g/mL at 0 °C

Melting point 0.00 °C (32.00 °F; 273.15 K)

Boiling point 99.98 °C (211.96 °F; 373.13 K)

pH 7 (Neutral)
Water: Structure
• Chemical formula: H2O

• The molecule has a bent geometry with a bond

distance of 0.958 Å and an bond angle of 104.5°.

• The large electronegativity difference between

H & O confers a 33% ionic character on the
bond.

• Water is clearly a highly polar molecule, a

phenomenon with enormous implications for
living systems.
Water: Interactions
• Water molecules associate through hydrogen bonds.

• A hydrogen bond forms when the negatively

charged oxygen of one water molecule is attracted

to the positively charged hydrogen of a nearby water

molecule.

• Hydrogen bonding between water molecules is the

basis for water’s properties.

How many molecules of H2O in a cup?

• 1 cup is approximately 250ml (i.e. 250cc).

• 1 cup will contain approximately 13.89 moles of

water. (Note: 1 mole of water has 6.02x10(23)

molecules).

• Approximately 8.36 x 10(24) molecules of water.

The Unique properties of water:
1) Water has high heat capacity.

2) Water has high heat of vaporization.

3) Water has cohesive and adhesive properties.

4) Water is less dense as a solid than as a liquid.

5) Water is polar.

6) Water is an excellent solvent.

 The Unique properties of water:
1) Water has high heat capacity.

• It takes a lot of energy to raise the temperature of a certain

amount of water by a degree, so water helps with

regulating temperature in the environment.

• For example, this property allows the temperature of water

in a pond to stay relatively constant from day to night,

regardless of the changing atmospheric temperature.

The Unique properties of water:
2) Water has high heat of vaporization.

• Humans (and other animals that sweat) use water’s high heat of vaporization
to cool off.

• Water is converted from its liquid form to steam when the heat of
vaporization is reached.

• Since sweat is made mostly of water, the evaporating water absorbs excess
body heat, which is released into the atmosphere. This is known as
evaporative cooling.
 The Unique properties of water:
3) Water has cohesive and adhesive properties.

• Water molecules have strong cohesive forces due to

their ability to form hydrogen bonds with one another.

• Cohesive forces are responsible for surface tension,

the tendency of a liquid’s surface to resist rupture
when placed under tension or stress. For example, they allow
nutrients to be
• Water also has adhesive properties that allow it to transported to the top
of a tree against the
stick to substances other than itself. force of gravity.
 The Unique properties of water:
4) Water is less dense as a solid than as a liquid.

• As water freezes, the molecules form a crystalline

structure that spaces the molecules further apart than
in liquid water.

• This means that ice is less dense than liquid water,

which is why it floats.

• This property is important, as it keeps ponds, lakes, and

oceans from freezing solid and allows life to continue to
thrive under the icy surface.
The Unique properties of water:
5) Water is polar

• Water molecules are polar, with partial positive charges

on the hydrogens, a partial negative charge on the

oxygen, and a bent overall structure.

• This is because oxygen is more electronegative, meaning

that it is better than hydrogen at attracting electrons.

The Unique properties of water:

6) Water is an excellent solvent.

• Water has the unique ability to dissolve many polar and ionic substances.

• This is important to all living things because, as water travels through the

water cycle, it takes many valuable nutrients along with it!

Water: The solvent of life
• A versatile solvent:

✓Example: A tablespoon of salt (NaCl) is placed in a

glass of water, the salt will dissolve. The glass will
then contain a uniform mixture of salt and water.

• Water is a solvent because e its polar molecules

are attracted to charged and polar substances
capable of forming hydrogen bonds
Water: The solvent of life

• The dissolving agent of a solution is the Solvent (Water)

• The substance that is dissolved is the Solute (salt)

• An Aqueous Solution is one in which water is the solvent.

Water: The solvent of life
Hydrophilic Hydrophobic

• Substances have an affinity for • Substances do not have affinity for

water, a liquid that is homogeneous water, the heterogeneous mixture
mixture of two or formed is a colloid.

more substance

is called a Solution.
Water: The solvent of life

Acid Base

• A water molecule can transfer an H+ to another water molecule to form

H3O+ (= H+) and OH-
Introduction: Acids, Bases, And Buffers

• Biological molecules, such as proteins and nucleic acids, bear numerous

functional groups, such as carboxyl and amino groups, that can undergo

acid–base reactions.

• Many properties of these molecules therefore vary with the acidities of the

solutions in which they are immersed.

 Introduction: Acids, Bases, And Buffers
Acid Base Buffer

• An Acid is a substance that • A base is a substance that • Buffers in biological fluids

increases the hydrogen ion reduces the hydrogen ion resist changes in pH.
(H+) concentration of a (H+) concentration of a • Consists of an acid-base
solution. solution. pair that combines
reversibly with hydrogen
• Example: Hydrochloride • Example: Sodium hydroxide
ions.
Acid (HCl) (NaOH)
• Example: Blood (pH 7.4)
 pH

Acid Base Buffer

• The concentration of H+ is expressed as pH

• pH= - log [ H+ ]

• pH scale range: 0-14

 pH
• A pH value less than 7, denotes an Acidic Solution.

✓Lower than the number 7, the solution is more acidic.

✓The pH declines as H+ concentration increases.

• Basic Solutions when the pH is above 7.

• The pH scale is based on H+ concentration, and it also

implies OH- concentration

Thank you