Ch. 2: Chemistry of Life
The Chemical Context of Life
Matter
• Matter consists of chemical elements in pure
form and in combinations called compounds.
Matter is anything that takes up space and has
mass.
• An element is a substance that cannot be
broken down to other substances by chemical
reactions.
• A compound is a substance consisting of 2 or
more elements combined in a fixed ratio.
Elements
• About 25 of the 92 natural elements are known
to be essential to life. Trace elements are those
required by an organism in only minute quantities
[e.g. iron (Fe) and iodine (I)].
• An element’s properties depend on the structure
of its atoms. Atoms are the smallest unit of an
element that still retains the property of the
element. Atoms are made of protons, neutrons,
and electrons.
Subatomic Particles
• p+ and n are close together in the nucleus of
the atom, and e- move quickly in a cloud
around the nucleus.
• The number of protons an element possesses
is referred to as its atomic #, and this number
is unique to every element. The mass # of an
element is the sum of its protons and
neutrons.
Molecules
• The formation and function of molecules
depend on chemical bonding between atoms.
Chemical bonds are defined as interactions
between the valence electrons of different
atoms.
• Atoms are held together by chemical bonds to
form molecules.
• A covalent bond occurs when valence
electrons are shared by two atoms.
Bonding
• Nonpolar covalent bonds occur when the
electrons beings shared are shared equally
between the two atoms.
• In polar covalent bonds, one atom has greater
electronegativity than the other, resulting in
an unequal sharing of the electrons
Ionic Bonds
• Ionic bonds are ones in which the two bonded
atoms attract the shared electrons so
unequally that the more electronegative atom
steals the electron away from the less
electronegative atom.
• Ionic bonds form ionic compounds, or salts.
• A charged atom or molecule is called an ion.
Anions and Cations
• If the charge is positive, the atom is called a
cation.
• If the charge is negative, it is called an anion.
Water
• In hydrogen bonds, the positively charged
hydrogen atom of one molecule is attracted to
the negatively charged atom of another
molecule.
• The polarity of water molecules results in
hydrogen bonding. The properties of water
are the key to its special properties.
Water
Water is made
up of one atom
of oxygen and
two atoms of
hydrogen,
bonded to form
a molecule like
this:
Water
• The fact that water molecules are bent means
that the opposite ends of the individual
molecules have opposite charges. In other
words, they are polar.
Water
• The end bearing the oxygen atom has a partial
negative charge, whereas the end bearing the
hydrogen atoms has a partial positive charge.
This is why water molecules form H bondsthe negatively charged oxygen atom from one
water molecule is attracted to the partially
positively charged hydrogen end of another
water molecule.
Hydrogen Bonds
• Each water molecule can form 4 hydrogen
bonds at a time.
• Four emergent properties of water contribute
to earth’s fitness for life. Water molecules stay
close to each other as a result of hydrogen
bonding.
Hydrogen Bonds
• When water is in its liquid form, its hydrogen
bonds are very fragile. They break and re-form
with high frequency.
• Thus, at any instant, a substantial percentage
of all the water molecules are bonded to their
neighbors, making water more structured
than most other liquids.
Hydrogen Bonds
• Collectively, the hydrogen bonds hold the
substance together, a phenomenon called
cohesion.
• Cohesion due to hydrogen bonding
contributes to the transport of xylem sap form
the roots to the stem of plants.
• Adhesion is the clinging of one substance to
another.
Hydrogen Bonds
• Water is very adhesive because of its
hydrogen bonds. It is this property of adhesion
that allows water to travel up the stems of
plants.
• Adhesion of water to the walls of the cells
helps counter the downward pull of gravity.
Properties of Water
• Specific heat is the amount of heat required to
raise or lower the temperature of a substance
by 1 degree Celsius. The specific heat of water
is 1 cal/gC.
• Relative to other materials, the temperature
of water changes less when a given amount of
heat is lost or absorbed.
Properties
of water
This high specific
heat makes the
temperature of the
earth’s oceans
relatively stable
and able to
support vast
quantities of both
plant and animal
life.
Water
• Water’s solid state is less dense than its liquid
state, whereas the opposite is true of most
other substances.
• Solid water forms a regular crystal lattice
structure, in which each water molecule is
hydrogen bonded to four other water
molecules.
• Because ice is less dense than liquid water, ice
floats.
Water
• Water is also an important solvent.
• The substance that something is dissolved in is
called the solvent, while the substance being
dissolved is called the solute. Together they
are called the solution.
• Solutions in which water is the solvent are
called aqueous solutions.
Water
• Substances that are water-soluble are
hydrophilic substances. These include ionic
compounds, polar molecules like sugars and
alcohols and some proteins,
• Oils, however, are hydrophobic and nonpolar,
meaning they do not dissolve in water.
pH
• Dissociation of water molecules leads to acidic
and basic conditions that affect living
organisms.
• When a hydrogen atom is transferred from
one molecule to another, it leaves its electron
and is transferred as a hydrogen ion, H+ which
is a proton with a charge of +1.
pH
• This transfer or dissociation makes the water
molecule that lost its proton the hydroxide ion
(OH-) and the molecule that gains the proton
is a hydronium ion (H3O-).
• Hydronium and hydroxide ion quantities are
about equal in pure water.
• But if acids or bases are added to water, this
equilibrium shifts.
pH
• Water has a pH of 7 which means it is neutral.
• Buffers are substances that minimize changes
in pH. They accept H+ from solution when
they are in excess and donate H+ when they
are depleted.