Transcript WELCOME TO BIOLOGY 2002 - University of Indianapolis

A primary question that can be asked about life:
How did life originate?
To ask this question using science (as opposed to other “ways
of knowing”):
What natural processes could account for the origins of life?
"We are Stardust"
The atoms that form our genetics, which form us,...such as carbon, oxygen, iron
and calcium...were formed by fusion in side the cores of massiv e stars b illions of
years ago. We then evolved from these elements here on earth. Our home. The
atoms that w e are made of are 4.6 bill ion years old . We a re stardust.
"Stars have died that w e might l ive." - Preston Cloud, Geo logist
"We are star s tuff." -Carl Sagan
Chemical Evolution

Early earth atmosphere and surface was composed of
inorganic atoms and simple molecules.

Chemical evolution of these molecules produced the
complex organic compounds that are the basis for life.
CHEMICAL EVOLUTION
Light energy
Heat
H
H C O
1. Simple molecules in
atmosphere of ancient
Earth
2. Reduced carboncontaining compounds
H H HOCH2 O OH
C H H C
H C C O
H C C H
H
HOOH
3. First carboncarbon bonds
The Structure of Atoms
-
-
-
Proton
+
+
+
Hydrogen
Electron
-
-
Neutron
Carbon
When the number of protons = the number of electrons:
The positive and negative charges cancel each other out
and the atom is electrically neutral
The Structure of Atoms

Atomic and mass numbers indicate the identity of atoms
Mass number = of
protons
+ neurons
Atomic number =
# of protons
1
1
7
3
23
11
4
2
H
Li
9
4
11
5
Be B
Na 2412 Mg 2713 Al
12
6
28
14
C
14
7
Si
31
15
N
16
8
P
32
16
1 proton ≈ 1 dalton; 1 neutron ≈ 1 dalton
He
O
19
9
F
20
10
Ne
S
35
17
Cl
40
18
Ar
The Structure of Atoms

All atoms of the same element have the same number of
protons, but they may have different numbers of neutrons

Atoms of the same element with different numbers of neutrons
are called isotopes

Example:
•
12C
•
14C
Radioactive Isotopes allow us to determine the age of certain materials
• Naturally-occurring radioactive materials break down into other materials
at known rates. This is known as radioactive decay.
• Once the rate of break down is known, geologists can estimate the length
of time over which decay has been occurring by measuring the amount of
radioactive parent element and the amount of stable daughter elements
Radioactive Parent
Stable Daughter
Half life
Thorium 232
Lead 208
14 billion yrs
Uranium 235
Lead 207
704 million yrs
Uranium 238
Lead 206
4.47 billion yrs
Carbon 14
Nitrogen 14
5730 years
This graph shows how a sample of radioactive atoms decays with time. At the
time a sample forms, the ratio of parent to daughter isotopes is 100:0. After the
amount of time represented by one half-life passes, the ratio is 50:50; after two
half-lives it is 25:75, after three half-lives it is 12.5:87.5, and so on.
Parent Daughter
isotope isotope
(gray)
(red)
Percentage of parent
isotopes remaining
100
50
25
13
6
0
1
2
3
Number of half-lives
4
How electrons are distributed in an atom
electron shells
electron orbital
1
2
3
+
the valence shell
= electron
= empty space
ENERGY
TRANSFORMATION
IN AN ATOM
Ep (higher)
1. An electron in an
outer shell has a
defined amount of
potential energy, Ep-
Ek
2. As the electron falls to a
lower energy shell, its
potential energy is converted
to kinetic energy, Ek-
Heat
or light
3. Once the electron
arrives at a lower electron
Ep (lower) shell, the kinetic energy is
converted to light or heat
Energy is neither created nor
destroyed; it simply changes form.
Ions are atoms that have a charge
A sodium
ion being
formed
A chloride
ion being
formed
Na+
Loss of
electron
Na+
Gain of
electron
Cl
Cl-
Cation
Anion
Ions of opposite charges are held together by ionic
bonds
ClNa+
Table salt is a crystal composed of two ions.
Ionic solids dissolve readily in water
Water
molecule
d+
d-
Na+
d-
d+
Cl -
dd+
d+
The number of electrons in the valence shell of an
atom determines its chemical reactivity.
Distribution of electrons in shells of these elements
Hydrogen
Lithium
Sodium
Helium
Beryllium
Boron
Magnesium Aluminum
Carbon
Nitrogen
Oxygen
Fluorine
Neon
Silicon
Phosphorus
Sulfur
Chlorine
Argon
Sharing electrons between two atoms, in the form of a
covalent bond, produces molecules.
H
+
H
Hydrogen atoms each have
one unpaired electron
H
H
H2 molecule has
two shared electrons
Methane
Ammonia
Water
Oxygen
CH4
NH3
H2O
O2
Molecular formulas:
H
Structural formulas:
H
C H
H
Ball-and-stick models:
Space-filling models:
H
N
H
H
O H
H
O O
How do chemical reactions occur?

Chemical reactions = the formation and breaking of chemical
bonds:
Reactant
+
Reactant

Products
AB
+
CD

AC + BD
When the forward and back reactions occur at the same rate, the
system is stable and is called a chemical equilibrium
Chemical Reactions on Early Earth

It is thought that the early atmosphere was made of gases
ejected from volcanoes

Volcanoes eject carbon dioxide, water, and nitrogen, along with
some H2 and methane (CH4)
• CO2 + H2O  H2CO3
(carbonic acid)