Chemical Basis of Life
Chapter 2
Bio 160
Atoms
• Atoms – smallest complete unit of an element
Protons (+)
Neutrons (neutral)
Electrons (-)
Atoms
– Nucleus of an atom is comprised of neutrons
and protons and therefore has a + charge;
electrons (-) are attracted to the nucleus and
move around the nucleus in specific areas of
space called shells
1st shell – holds 2 e2nd shell – holds 8 e3rd shell – holds 8 e-
Elements
• Elements – consist of only one kind of atom
arranged in periodic table by symbols, along
with atomic number and weight
– Atomic number = number of protons =
number of electrons (in a neutral atom)
atomic number determines which element is
in question
Elements
– Atomic weight = number of protons +
number of neutrons
(mass (weight) of protons or neutrons is
1840 x larger than that of e-)
– Isotopes - elements with the same # of
protons but a varying # of neutrons, ie,
element with the same atomic number but a
different atomic weight
Elements
atomic weights are not whole numbers, but
merely an average of all isotopes of an
element
unstable isotopes emit energetic particles
and are known as radioisotopes
– All elements with atomic number > 84 are
radioactive but these elements are not
normally present in biological material
Molecules & Compounds
• Molecules - chemical combination of 2 or
more atoms of one or more elements bonded
together by chemical bonds (atoms don't have
to be different) (H2 is a molecule as is H2O)
• Compounds - 2 or more different elements
joined together
Chemical Bonding
• Electrons are the part of an atom that
actively participates in a chemical reaction
(never the nucleus)
• Chemical behavior of atoms can be explained
by behavior of electrons
• Octet rule: atoms react with one another to
achieve 8 e- in outer shell (except 1st shell)
Chemical Bonding
• Atoms with fewer than 8 electrons in outer
shell may transfer or share electrons to
complete outer shell
• When atoms react with one another they are
attracted to each other by chemical bonds
• Ions - atoms that are electrically charged either
by gaining or losing electrons
Types of Chemical Bonding
• Ionic bond – formed when electrons are
transferred from one atom to another; the
bond is therefore the electrical attraction
between 2 oppositely charged ions
compounds containing ionic bonds readily
separate (dissociate) into ions in water and are
called electrolytes because as charged
particles they can conduct an electric current
examples of electrolytes are acids, bases, and salts
Types of Chemical Bonding
if outer shell has 1,2, or 3 e-, the atoms tend
to lose the electron and become + ions =
cations
if outer shell has 6 or 7 e-, the atoms tend to
gain electrons and become - ions = anions
if outer shell has 4 or 5 e-, they don't tend to
form ions; they tend to share electrons and
form covalent bonds
Types of Chemical Bonding
• Covalent bond – results from sharing
electrons; atoms with 4 or 5 electrons in
outer shell tend to share electrons and will
not form ions; therefore covalent
compounds tend to be nonelectrolytes
Carbon has 4 electrons in outer shell and needs 4
more electrons
More compounds of carbon than any other element
Types of Chemical Bonding
– Nonpolar covalent - equal sharing of
electrons; no charged regions
– Polar Covalent - electrons are not equally
shared; results in charged regions
water is a good example of a polar covalent
compound
Types of Chemical Bonding
• Hydrogen bonds - weak attraction between
a slightly + hydrogen and a slightly oxygen or nitrogen
H bonds can be between molecules
(intermolecular; as in water) or within
molecules (intramolecular; as in DNA and
protein)
H bonds are significantly weaker than covalent
or ionic bonds
Chemical Reactions
• Chemical Reactions – the making and
breaking of chemical bonds
– Reactants – starting materials in a reaction
– Products – ending materials in a reaction
Types of Chemical Reactions
• Synthesis - reactions that consume energy to
build complicated molecules from simpler
ones
• Decomposition - reactions that break down
complex molecules into simpler compounds
• Exchange reactions
• Reversible reactions
Acids and Bases
• Acids - substances that release H+ in water
• Bases - release OH- or accept H+
• pH - measure of H+ concentration
• Salts - strong acid + strong base→ salt and
H2O
Organic Compounds
• Organic compounds - any substance that
contains both carbon and hydrogen
– Polymer - long chain of a repeating
molecular unit
monomer + monomer → polymer + H2O
(dehydration synthesis = condensation)
polymer + H2O → monomer + monomer
(digestion reaction = hydrolysis)
Carbohydrates
• Carbohydrates (contain C, H, O) = sugars
and starches
most abundant group of organic compounds
provides most readily available source of
energy
short term storage of energy
Carbohydrates
– Monosaccharides
 Pentose - 5 carbon sugar - ribose/deoxyribose
of RNA/DNA
 Hexose - 6 carbon sugar
glucose - most abundant hexose
galactose
fructose
Carbohydrates
– Disaccharides - Monomer + Monomer =
disaccharide
Sucrose – Table sugar
Lactose - Milk
Carbohydrates
– Polysaccharides
 Starches - principal storage polysaccharide in
plants; ex) potatoes, rice, grain
 Glycogen - principal storage polysaccharide in
animals
Lipids - all tend to be insoluble in
water
Lipids
• Triglycerides (fats) - largest of class of lipids
– glycerol (monomer) + 3 fatty
acids(monomers)→triglyceride
 saturated fats – all bonding sites of the fatty
acid are filled with hydrogen
 unsaturated fats - some double bonds present
so not all bonding sites are filled with
hydrogen and the fatty acid has a kink in its
shape, preventing solidification
Lipids
• Phospholipids - in cell membranes and membranes of
organelles
– consists of glycerol + 2 fatty acids + phosphate group
(- charge)
fatty acid tail - hydrophobic = insoluble in water
phosphate head - hydrophilic = soluble in water
– phospholipids are arranged as a bilayer with phosphate
heads toward water (outside of bilayer in contact with
aqueous solutions inside and outside the cell) and the
fatty acid tails toward the interior of the membrane
Lipids
• Steroids
cholesterol (precursor from which most other
steroids are synthesized)
sex hormones - estrogen, progesterone,
testosterone
cortisol and aldosterone
Vitamin D and bile salts
Proteins
• Protein - most abundant organic molecule in
organism (makes up 50% of dry weight)
Monomer = amino acid; polymer = protein
(polypeptide chain)
Amino acids are linked by peptide bonds
(covalent bonds)
Proteins - contain C, H, O, N and some S
Proteins
• Proteins have four levels of structure which
contribute to their unique three-dimensional
shape, or conformation
• Denaturation - Proteins lose their
conformation, hence their function, due to
exposure to heat, radiation, electricity, pH
changes, or various chemicals
Nucleic Acids
• DNA and RNA
monomer = nucleotide
– pentose sugar (5-carbon monosaccharide) =
ribose/deoxyribose
– phosphate group
– nitrogenous base