The Chemical Context of Life
Chapter 2
A Chemical Connection to Biology
HCO2H.
Methanoic acid

Ex. Ants maintain Duroia hirsuta “devil’s gardens,”
in Peru by injecting formic acid into other plants
Ants Myrmelachista schumanni
This plant is the only species in this tropical rain forest!
Scientific method
• Two saplings of a common Amazonian tree, Cedrela odorata, or Spanish
cedar, were planted inside each Devil’s garden near the base of a D. hirsuta
tree actively patrolled by worker ants. A sticky insect barrier was applied to
one cedar sapling to exclude ants, while the other sapling was left untreated.
• The results were immediate. Worker ants promptly attacked the untreated
saplings by injecting formic acid into the leaves, which began to die within
24 hours. "Most of the leaves on these saplings were lost within five days,
and the proportion lost was significantly higher than on ant-excluded
saplings,". Cedars treated with the insect barrier fared lived.
•
•
•
•
Independent variable =
Control group =
Controlled variables =
Dependent variable =
Matter, elements, and compounds
• matter - anything that takes up space and
has mass
• Matter is made up of elements
• element - substance that cannot be broken
down to other substances
25 are essential for life in humans
94 elements occur naturally
An element is defined by # protons (atomic number)
compound
-consists of 2 or more elements in fixed ratio
-characteristics different from those of its elements
Na
Sodium and water
Cl
NaCl
http://youtu.be/m55kgyApYrY
• The above link is a video on periodic table
elements that are reactive and are grouped
together on the table.
Essential Elements of Life
About 25 of the 92 elements are essential to life
(humans)
• CHON =96% of living
matter
• Also…….
• Trace elements required
by an organism in minute
quantities
Copper deficiency prevents full opening of leaves
(b) Iodine deficiency
Trace amounts of iodine required for thyroid
function
An element’s properties
depend on structure of its atoms
• atom =smallest unit of matter that still
retains properties of element
• subatomic particles
• Neutrons (no electrical charge)
• Protons (+ charge)
– # protons defines the element
• Electrons (- charge)
Isotopes
• Isotopes
– Atoms of an element that differ in # of neutrons
• Radioactive isotopes
– Isotope decays  particles and energy
decay
– Half life = lifetime for ½ of the isotopes to decay
(atom loses particles)
FYI
• uranium has one or more isotopes with half lives long enough to survive
as remnants of the explosive big bang
•
•
•
•
•
Tritium (3H) low energy beta particle. 12.35 years. Used to tag DNA. No shield necessary.
Wear gloves.
Carbon-14 decays into nitrogen-14 by emission of a beta particle. Half life 5730 years. No
shield, wear gloves, can penetrate skin.
Phosphorus-32 a beta particle. Half life 14.2 days. Use plexiglass.
Iodine-125 gamma rays. 60 day half life. biological assays, nuclear medicine imaging and
in radiation therapy. Thyroid cancer treatment. Use lead shield.
Polonium has 26 isotopes, all radioactive. 250 billion times more toxic than hydrocyanic
acid. It is readily soluble in weak acid. It was the first element discovered by Marie Curie,
in 1898, and named after her native Poland. Her daughter Irene was contaminated with
polonium in a laboratory accident and died of leukemia at the age of 59.
• There are three main kinds of ionizing radiation:
– alpha particles, which include two protons and two neutronsm
– beta particles, which are essentially high-speed electrons
– gamma rays and x-rays, which are pure energy (photons).
• radioactive isotopes in biology:
– Dating fossils
– Medical imaging
– Lab research
Radiolabeled glucose atom used in PET scans
FYI: A PET scan (positron emission tomography) detects locations of intense chemical
activity. Inject patient with radioactive glucose. A scanner measures collisions of the
radioactive glucose with active cells to locate tumor. Gamma rays.
PET scan to:
Locate site of cancer
Determine size of tumor
Differentiate benign from malignant growths
Discover if cancer has spread
Monitor the success of therapy
Detect recurrent tumors
Chemical Formulas
• Structural formula represents atom bonding
H–OH
• Molecular formula abbreviation
H2O
Chemical Bonds
1. Covalent bond – electrons shared by 2 atoms
• Ex. O2, H2O
• Strong bond
2. Ionic bond – one atom loses an electron
–Ion is a charged atom
Na+
Cl– Weaker bond
– Salts
3. Hydrogen Bonds
• H atom covalently bonded to
electronegative atom is attracted to
another atom
Molecular Shape and Function
• molecules have specificity/function based on
shape
• similar shapes can have similar biological effects
Natural endorphin
Key
Carbon
Hydrogen
Morphine
(a) Structures of endorphin and morphine
Natural
endorphin
Brain cell
Morphine
Endorphin
receptors
(b) Binding to endorphin receptors
Nitrogen
Sulfur
Oxygen
Chemical reactions
Chemical reactions make and break chemical bonds
– reactants - starting molecules
– products - final molecules
Photosynthesis
6 CO2 + 6 H20 →
C
H
O
+
6
O
6
12
6
2
light
• Many chemical reactions are reversible
• Chemical equilibrium =when forward
and reverse reaction rates are equal (in a
closed system)
CO2 + H2O  H2CO3
