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LAMC
BIOLOGY

LAMC Bio 3 Lecture Notes: Chapter 2 – Chemical Basis of Life

Reading Assignment: Chapter 2

Learning Outcomes:
By the end of today’s lecture, you should be able to:
- Explain the properties of an atom’s protons and electrons.
- Explain the helpful and harmful effects of radioactive isotopes.
- Identify the difference between covalent & ionic bonds.
- List/explain the unique properties of water.
- Explain the differences between an acid, base and buffer
- Explain what the pH scale is
- Explain the effects of acid precipitation.
 

I. Atoms and Elements:

All organisms are composed of matter, which is anything that has mass and takes up space.
Element – a substance that cannot be broken down to other substances by chemical reactions.
E.g. sodium, chlorine, gold, copper

Each element is composed of atoms that are different from the atoms of any other element.
Atom – smallest unit of matter that retains the properties of the element.
Subatomic particles: neutrons (0), protons (+), electrons (-)

Isotope – different atomic forms of an element (have same # of protons, but different # of neutrons)
Radioactive isotope – an unstable isotope where the nucleus decays spontaneously to form a different element.
Some radioactive isotopes are harmless, and can be useful for studying cells, but others can cause damage to living cells.
 

II. Molecules and Compounds:

Molecule – two or more atoms held together by a covalent bond.
E.g. oxygen (O₂), nitrogen (N₂), glucose (C₆H₁₂O₆)

Compound – a substance consisting of two or more elements combined in a fixed ratio.
E.g. sodium chloride (NaCl), calcium chloride (CaCl₂)

A compound often has emergent (novel) properties that are different from those of its component elements.
Every molecule has a characteristic shape and size.  The precise shape of a molecule is usually very important to its function in the cell.
 

III. Covalent and Ionic Bonds:

Covalent bond – a bond between atoms where the electrons are shared. E.g. methane (CH₄), water (H₂O)
Ionic bond – a bond where one atom donates an electron to another atom. Eg. Sodium chloride (NaCl)
The number of electrons an atom contains determines the kind of bond it forms with other atoms.
 

IV. Properties of Water:

Water is a polar molecule; even though the electrons are shared between the atoms, they are not shared equally, so the opposite ends of the molecule have opposite charges.  This unequal distribution of charge attracts water molecules to each other through hydrogen bonds.  These hydrogen bonds give water a higher level of structural organization.

Effects of hydrogen bonds in water:

a) cohesion of water molecules – water molecules stick to each other.  This allows the movement of water against gravity in plants.

b) high surface tension – it is difficult to stretch or break the surface of water.  This allows small animals to live on the surface of water.

c) high specific heat – water requires a large amount of energy input/loss before its temperature will change.  As a result, water helps moderate temperatures on Earth.
Specific heat: amount of heat that must be absorbed or lost to change the temperature by 1^o Celsius.

d) high heat of vaporization – allows evaporative cooling of a surface.
Heat of vaporization: the amount of heat a liquid must absorb before it is converted to a gas. 

e) Ice (solid state) is less dense than water (liquid state) – the hydrogen bonds cause the water molecules to be spread apart as ice forms.  This causes ice to float on the surface rather than sinking, which allows the persistence of life during the cold seasons.

V. The pH Scale (Acidity/Basicity):

Water can dissociate in solution to form: H⁺ and OH^- ions (ions are charged particles).
In pure water, the concentrations of H⁺ and OH^- ions are equal (the solution is neutral)

Acid – a substance that increases the hydrogen ion concentration of a solution. E.g. HCl à H⁺ + Cl^-
Base – a substance that reduces the hydrogen ion concentration of a solution. E.g. NaOH à Na + OH^-

The pH of a solution depends on the H+ concentration; pH decreases as H+ concentration increases.
The pH of a neutral solution is 7; a pH < 7 is an acidic solution, a pH > 7 is a basic solution.

Buffers – substances that minimize changes in the concentrations of H⁺ and OH^- in a solution.
E.g. carbonic acid (H₂CO₃) in blood

Acid precipitation – refers to rain, snow or fog that is more acidic than pH 5.6.  The main source of acid precipitation is the burning of fossil fuels which releases gases that react with the water in the air to form strong acids that fall to earth with rain or snow.

©2003-2004 Dilek Sanver-Wang, MS
Los Angeles, CA USA.  All Rights Reserved.