Transcript Chapter 12 Section 1

Chapter 12 Section 1
How Did Life Begin?
Grade 10 Biology
Spring 2011
Objectives
Summarize how radioisotopes can be
used in determining Earth’s age
 Compare two models that describe how
the chemicals of life originated
 Describe how cellular organization might
have begun
 Recognize the importance that a
mechanism for heredity has to the
development of life

The Age of Earth

Earth formed 4.5 billion years ago
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Was a fiery ball of molten rock
Eventually cooled and formed a rocky crust
Water vapor cooled and formed oceans
Scientists think life first evolved in oceans and
evolution of life occurred over hundreds of
millions of years
Measuring Earth’s Age

Radiometric dating: estimation of the
age of an object by measuring the content
of certain radioactive isotopes
◦ Scientists have measured the earth this way
Measuring Earth’s Age
Isotope: form of an element whose
atomic mass (mass of each individual
atom) differs from that of other atoms
of the same element
 Radioisotopes: unstable isotopes that
break down and give off energy in the
form of charged particles (radiation)

Measuring Earth’s Age
Radioactive decay: results in other
isotopes that are smaller and more stable
 Half-life: the time it takes for one-half of
a given amount of a radioactive to decay
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Formation of the Basic Chemicals of
Life
It is thought that the path to the
development of living things began when
molecules of nonliving matter reacted
chemically during the first billion years of
Earth’s history
 Chemical reactions produce many
different, simple organic molecules
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Formation of the Basic Chemicals of
Life
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Sun and volcanic heat caused simple
organic molecules to form more complex
molecules that became building blocks of
first cells
Primordial Soup Model
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Primordial Soup Model: early Earth’s
ocean contained large amounts of organic
molecules
◦ Oceans filled with many different organic
molecules like a soup
◦ Hypothesized that these molecules formed
spontaneously in chemical reactions activated
by energy from solar radiation, volcanic
eruptions, and lightning
Primordial Soup Model
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Scientists proposed Earth’s early
atmosphere lacked oxygen and rich in
nitrogen gas, hydrogen gas, and hydrogen
containing gases such as water vapor,
ammonia, and methane
Primordial Soup Model
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Electrons in these gases would have been
frequently pushed to higher energy levels by
light particles from sun or electrical energy
in lightning
Primordial Soup Model
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Today, high energy electrons are socked
up by oxygen, without oxygen high energy
electrons would have been free to react
with hydrogen rich molecules, forming
organic compounds
Miller-Urey Experiments
Placed gases they thought
existed on early Earth
into a device
 Provided electrical sparks
to simulate lightning
 After a few days, found
complex collection of
organic molecules
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◦ Amino acids, fatty acids,
hydrocarbons
Miller-Urey Experiments
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Results support hypothesis that some
basic chemicals of life could have formed
spontaneously under conditions like those
in experiment
Reevaluating Miller-Urey Model
Now know that the reductant molecules
used in Miller’s experiment could not
have existed in abundance on early Earth
 No ozone present, without ozone UV
light would have destroyed any ammonia
and methane present
 When these gases are absent, key
biological molecules are not made
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Reevaluating Miller-Urey Model
If chemicals needed to form life were not
in the atmosphere, where did they come
from?
 Could be produced in ocean bubbles, or
arose in deep sea vents
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Bubble Model
Bubble Model
1.
Ammonia, methane, and other gases
resulting from numerous eruptions of
undersea volcanoes were trapped in
underwater bubbles
Bubble Model
2.
Inside bubbles, methane and ammonia
needed to make amino acids might have
been protected from damaging UV
radiation. Chemical reactions would
take place must faster in bubbles (where
reactants would be concentrated) than
in primordial soup.
Bubble Model
3.
4.
Bubbles rose to surface and burst,
releasing simple organic molecules into
the air
Carried upward by winds, simple organic
molecules were exposed to UV
radiation and lightning, which provided
energy for further reactions
Bubble Model
5.
More complex organic molecules that
formed by further reactions fell into the
ocean with rain, starting another cycle
Precursors of First Cells
In the lab, scientists have not been able to
make either proteins or DNA form
spontaneously in water
 Short chains of RNA, have been made to
form on their own in water
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Precursors of First Cells
Inorganic molecules
RNA nucleotides
Self-Replication
RNA macromolecules
RNA molecules catalyze
protein synthesis
Proteins
Possible Role As Catalyst
RNA molecules can act like enzymes
 RNA’s 3-dimensional structure provides a
surface on which chemical reactions can
be catalyzed
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Possible Role As Catalyst
RNA was the first self-replicating
information storage molecule and it
catalyzed the assembly of the first
proteins
 Would have been capable of changing
from one generation to the next
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Microspheres and Coacervates
Lipids which make up cell membranes,
tend to gather together in water
 Certain lipids, when combined with other
molecules can form a tiny droplet whose
surface resembles a cell membrane
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Microspheres and Coacervates
Microspheres: in water, short chains of
amino acids can gather into tiny droplets
 Coacervates: composed of molecules of
different types, including amino acids and
sugars, gather into tiny droplets
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Microspheres and Coacervates
Formation of microspheres may have
been the first step toward cellular
organization
 Microspheres formed and dispersed
 Those that could persist longer and
incorporate molecules and energy would
become more common than shorter
lasting ones
 Need to have characteristic of heredity to
be considered living things
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Origin of Heredity
Double stranded DNA evolved after
RNA and RNA “enzymes” catalyzed the
assembly of earliest proteins
 Hypothesis that some microspheres or
similar structures that contained RNA
developed a means of transferring their
characteristics to offspring
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Origin of Heredity
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Do not yet understand how DNA, RNA,
and heredity mechanisms first developed