Download First cells ppt The first cells ppt

The First Cells
Conditions on Earth
• Planet formed 4.6 billion years ago
• For the first few hundred million years. . .
•
•
•
•
Huge chunks of ice and rock bombarded the planet
Intense heat vaporized all available water
Asteroids began to melt the Earth’s crust
Radioactive core produced intense volcanic eruptions spewing molten lava
and gases
So then what happened?
• Gaseous H20 from meteorites and the Earth’s
crust began to rise and combine with CO2
creating dense clouds
• These shielded the Earth from the sun’s
radiation eventually causing cooling
• Then. . .RAIN!
• Creating lakes and rivers
Chemical makeup of the
atmosphere
• Most early scientists believed that the
atmosphere was “reducing” (electron adding)
made up of:
– Very little O2
– Thick water vapor
– Volcanic gases such as methane (CH4) and
ammonia (NH4)
Abiogenesis
• The idea that some small organic molecules
(the basic building blocks of living organisms)
could have been created from inorganic
compounds under the right conditions.
• These then went on to evolve into diverse life
forms over MANY years.
Scientists hypothesize that simple
cells could have been produced
through a series of 4 main stages.
1. The abiotic (nonliving) synthesis of small
organic molecules (amino acids and
nitrogenous bases).
2. The joining of these into macromolecules
(proteins and nucleic acids).
3. Packaging these into protocells = droplets
with membranes that maintained an internal
chemistry different from that of their
surroundings.
4. The origin of self-replicating molecules that
made inheritance possible
The Miller- Urey Experiment
• Assumed:
– A reducing atmosphere
– Energy supplied by lightning and intense UV
radiation
• Let’s look
– https://www.youtube.com/watch?v=mF9U5x6Nxn
w
Controversy
Not everyone agrees about the composition of
early Earth’s atmosphere.
Was it more “neutral”?
Were there only “pockets” of reducing
substances such as near volcanoes and deep sea
vents?
In 2008, the experiments were
repeated under
volcanic-atmosphere conditions. . .
Number of
amino acids
20
10
Mass of
amino acids (mg)
Figure 24.3
0
1953
2008
200
100
0
1953
2008
Alien invasion?
• 2nd source of organic molecules may have
been meteorites.
• Fragments of a 4.5 billion year old meteorite
that fell in 1969 was found to contain 80
amino acids, some lipids, simple sugars and
nitrogenous bases!
Small organic molecules to
macromolecules
• 2009 experiment showed spontaneous
synthesis of RNA monomers.
• By dripping RNA nucleotides or solutions of
amino acids on hot clay, polymers formed.
• These could then have acted as catalysts for
further chemical reactions.
The next step
• Vesicle formation
• Vesicles form spontaneously when lipids or
other organic materials are added to water.
• Think oil or soap
in water!
• Furthermore, it has been found that these
vesicles:
– Can reproduce by simple division
– Grow
– Absorb particles on which RNA and other organic
materials may have been attached.
Figure 24.4b
20 m
(b) Reproduction
• The rate of vesicle assembly and polymer
formation increases on montmorillonite- a soft
mineral clay derived from volcanic ash!
Relative turbidity, an
index of vesicle number
Figure 24.4a
0.4
Precursor molecules plus
montmorillonite clay
0.2
Precursor
molecules only
0
0
(a) Self-assembly
40
20
Time (minutes)
60
Homework
• In 50 words, or fewer (total for both), answer
each of the 2 questions below:
– What is the difference between abiogenesis and
spontaneous generation?
– What do scientists believe was the first genetic
material? RNA or DNA? Why?
You will turn this in at the beginning of the period.