Campbell and Reece
Chapter 2
THE CHEMICAL CONTEXT OF LIFE
ELEMENTS & COMPOUNDS
Element: substance that cannot be broken
down to other substances by chemical
reactions
 92 natural-occurring elements

COMPOUNDS


2 or more different elements combined in a fixed
proportion
compound has different properties than
properties of individual elements that make up
the compound
COMPOUNDS ARE PURE SUBSTANCES
ESSENTIAL ELEMENTS OF LIFE
those of the 92 that organisms must have to
live & reproduce
 species specific;

 humans
need 25 elements
 plants need 17
ESSENTIAL ELEMENTS
96% of living matter (by mass) made up of:
 Oxygen
 Carbon
 Hydrogen
 Nitrogen

4% of living matter (by mass) made up of:
 Calcium
 Phosphorus
 Potassium
 Sulfur
 Chlorine
 Magnesium

Trace Elements make up <1%
 organisms require these in minute amounts
 to name only a few:
 Iron
 Iodine (in vertebrates only)
 Zinc
 Selenium
 Manganese

TOXIC ELEMENTS
Arsenic:
 Mercury

PROPERTIES OF ELEMENTS
ATOMIC MASS
1 proton or 1 neutron = 1 amu
 1 amu = 1.7 x 10² g = 1 dalton
 mass of e- ignored

MASS NUMBER

#protons + #neutrons
ISOTOPES

different forms of same element:
 same
# protons
 different # of neutrons
occurs naturally
Atomic Mass see on periodic table is weighted
mass average of all isotopes
RADIOACTIVE ISOTOPES
nucleus spontaneously decays
 giving off particles & energy
 used medically to measure an organ’s
function, monitor tumor growth

RADIOACTIVE ISOTOPES

decay hazardous to living organisms by
damaging cellular molecules
ENERGY LEVELS OF ELECTRONS
electrons only subatomic particle involved
in chemical reactions
 e- have PE due to their position around
nucleus of atom
 electron’s (-) charge attracted to (+) proton
 the farther out e- is from nucleus, the more
PE that e- has

ELECTRON ENERGY LEVELS
VALENCE ELECTRONS
electrons in outermost shell
 if valence shell completely filled: atom is
nonreactive (inert like noble gases)

ELECTRON ORBITALS
CHEMICAL BONDS

formed by interactions between atoms with
incomplete valence orbitals
COVALENT BONDS
sharing of a pair of valence electrons by 2
atoms
 2 or more atoms held together by covalent
bonds = molecule

TYPES OF COVALENT BONDS
1 pair e- shared = single covalent bond
 2 pair e- shared = double covalent bond
 3 pair e- shared = triple covalent bond

hydrogen gas

NONPOLAR COVALENT BOND

shared pair of electron are shared evenly
between the 2 atoms
ELECTRONEGATIVITY
attraction of a particular atom for the
electrons in covalent bond
 greater the electronegativity on 1 atom over
the other in the covalent bond, the more
likely the bond will be a polar covalent
bond

POLAR COVALENT BONDS
electrons of the bond are not shared equally
 bonds vary in their polarity: depends on the
relative electronegativity of the 2 atoms

IONIC BONDS
transfer of e- from metal  nonmetal
 creates cations (+) & anions (-)
 opposite charges attract forming ionic bond
 compounds formed by ionic bonds called
ionic compounds or salts
 as solids form crystals, very strong bonds
 as liquids completely dissociate

WEAK CHEMICAL BONDS
in living organisms: most of the strongest
chemical bonds are covalent
 weak bonds also important:

 hydrogen
bonds, van der Waals, ionic bonds in
water
 maintain shape of proteins, DNA
 reversibility key to their importance
HYDROGEN BONDS
water: H held to O in polar covalent bonds
 partial (+) charge on H makes it attract the
partial (-) charge on O from another water
molecule

VAN DER WAALS FORCES
nonpolar covalent bonds may not have
electrons symmetrically distributed
 @ any given instant in time a region of the
molecule may be slightly (-) or (+)  everchanging regions of (+) or (-) charge
 occurs only when atoms or molecules are
very close together

MOLECULAR SHAPE & FUNCTION
shape of a molecule key to its function in
cell
 shape of a molecule with >2 atoms
determined by positions of atom’s orbitals

MOLECULAR SHAPE
determines how biological molecules
recognize & respond to each other
with specificity
 2 molecules temporarily forming
weak bond can only happen if their
shapes are complimentary to each
other

MOLECULAR SHAPE

because morphine and endorphins
have same shape that fits into
endorphin receptors morphine is able
to create same response as
endorphins: pain relief & euphoria
during stress
ENDORPHINS & MORPHINE
CHEMICAL REACTIONS

chemical bonds of reactants broken,
chemical bonds of products formed in
a chemical reaction
CHEMICAL EQUILIBRIUM