The Chemical Basis of Life
I.
The emergence of biological function starts at the chemical level
a. HOLISTIC – the big picture
b. REDUCTIONIST – individual components
c. Different approaches, but agree on the following
i. Each level in the hierarchy builds on the one below it and
with each step upward, novel properties emerge that were
not present at simpler levels of organization.
II.
Life requires about 25 chemical elements
a. MATTER – anything that occupies space and has mass
b. CHEMICAL ELEMENT – a substance that cannot be broken
down to other substances by ordinary chemical means
c. 92 Naturally occurring elements
i. 25 are essential for life
1. 96 % is Carbon, Hydrogen, Oxygen, and Nitrogen
ii. TRACE ELEMENTS – necessary for life by some, or all,
organisms but only in minute quantities
1. Iodine in humans; necessary for proper thyroid
function or else GOITER (growth in neck)
III.
Elements can combine to form compounds
a. COMPOUND – substance containing 2 or more elements in a
fixed ratio
b. ATOM – smallest unit of matter that still retains the properties
of an element
c. SUBATOMIC PARTICLES
i. PROTON – subatomic particle with a single positive
electric charge (+)
ii. ELECTRON – subatomic particle with a single negative
electric charge (-)
iii. Neutron – subatomic particle with no electrical charge
iv. NUCLEUS – atom’s central core (contains the neutrons
and protons)
1. Electrons orbit the nucleus
d. Differences in elements
i. ATOMIC NUMBER – number (#) of protons of a
particular element
ii. MASS NUMBER – sum of the number (#) of protons
and neutrons in the nucleus of a particular element
e. ISOTOPES – variant forms of elements with different number
of neutrons but same chemical properties
i. RADIOACTIVE ISOTOPE – nucleus decays, giving off
particles and energy
IV.
Radioactive isotopes can help or harm us
a. Basic Research
i. Often use radioactive tracers to follow molecules to
determine pathways (chemical)
b. Medical Diagnosis
i. Tracers are also used in medicine to diagnose disorders
c. Dangers
i. Uncontrolled exposure can damage molecules, especially
DNA
V.
Electron arrangement determines the chemical properties of an
atom
a. ELECTRON SHELLS – electrons only occur at certain energy
levels
i. Electrons in outermost shell determine the chemical
properties of an atom
b. CHEMICAL BONDS – attraction that holds atoms together
i. Accomplished by atom “filling” its outermost shell
VI.
Ionic bonds are attractions between ions of opposite charge
a. ION – an atom or molecule with an electrical charge resulting
from the gain or loss of one or more elections
b. IONIC BOND – two ions with opposite charges attract each
other and are held together
c. SALT – synonym for ionic compound
VII. Covalent bonds, the sharing of electrons, join atoms into molecules
a. COVALENT BOND – two atoms share one or more pairs of
outer-shell electrons
b. MOLECULE – two or more atoms held together by covalent
bonds
c. DOUBLE BOND – sharing two pairs of electrons
VIII. Water is a polar molecule
a. ELECTRONEGATIVITY – an atom’s attraction for the shared
electrons
b. NONPOLAR – equal sharing
c. POLAR – UNequal sharing
d. POLAR MOLECULE – slightly negative pole, slightly positive
pole as a result of polar covalent bond
IX.
Overview: Water’s polarity leads to hydrogen bonding and other
unusual properties
a. HYDROGEN BOND – charged regions on one molecule are
attracted to oppositely charged regions on neighboring
molecules
i. This plus polarity give water unique properties which
make it essential to supporting life on Earth
X.
Hydrogen bonds make liquid water COHESIVE
a. COHESION – tendency of molecules of same substance to
stick together
b. SURFACE TENSION – measure of how difficult it is to break
or stretch the surface of a liquid
XI.
Water’s hydrogen bonds moderate temperature
a. HEAT - the amount of energy associated with the movement of
the atoms and molecules in a body of matter
b. TEMPERATURE – measures intensity of the heat; average
speed of molecules
i. Because of H-bonds, water can absorb and retain large
amounts of heat
1. Moderating climates
2. Evaporative cooling – hottest water evaporates
first, leaving behind cold water
XII. Ice is less dense than liquid water
a. Bonds are stable in crystal solid
b. Liquid molecules are more squished together
c. Results in less molecules in same volume for solid (ice)
XIII. Water is a versatile solvent
a. SOLUTION  homogeneous mixture of two or more
substances
b. SOLVENT  dissolving agent
c. SOLUTE  dissolved substance
d. When water is the solvent it is called “aqueous” (Latin for
water)
e. Versatility as a solvent results from polarity
XIV. The chemistry of life is sensitive to acidic and basic conditions
a. When water molecules break apart, they may form H+ or OHions
b. A chemical that donates H+ ions is an ACID
c. A chemical that accepts H+ ions and removes them from
solution is a BASE (alkali)
d. pH scale is used to describe the acidity of a solution (0-14)
e. Change of 1 unit is a ten-fold change in H+ concentration
f. BUFFERS  resist pH changes by accepting or donating H+
ions to a solution
XV. Acid precipitation threatens the environment
a. May effect aquatic life
i. Changing pH of water during breeding periods
b. May affect plants through changes in soil
XVI. Chemical reactions rearrange matter
a. Structure denotes function
b. H2 + O2  H2O
c. CHEMICAL REACTION  a process that leads to a chemical
change in matter
i. Left of arrow are REACTANTS
ii. Right of arrow are PRODUCTS
d. Organisms can’t make water, but utilize principle of chemical
reactions by rearranging other compounds to their benefit.
i. C40H56 + O2 + 4H  2C20H30O
1. Beta-carotene  Vitamin A