Cells

1. The unbroken chain of life
http://www.flickr.com/photos/euthman/216030298/

2. Processes needed for the spontaneous origin of life on Earth
1) The non-living synthesis of simple organic molecules
• Obviously if nothing was alive yet then the source of these molecules had to be abiotic
• We can presume that the early Earth had all of the base elements and compounds required
• They were somehow combined to make simple organic compounds
• Maybe the organic compounds were generated here, maybe they were extra-terrestrial!
2) The assembly of these molecules into polymers
• It makes sense, to make the larger molecules necessary for life, the simple organic
compounds would have to polymerise
3) The origin of self-replicating molecules made inheritance possible
• DNA can’t self replicate, it needs protein enzymes
• However some RNA can self-replicate, it can catalyse
the formation of copies of itself.
• The 64 RNA codons for amino acids is the same in nearly
All forms of life on earth with some variation.
4) The packaging of these molecules into membranes
with internal chemistry different from their
surroundings
• The formation of closed membranes an important step
• Closed membrane vesicles can form spontaneously
from lipids.
• This allowed differentiation between the internal and external environments
http://exploringorigins.org/resources.html

3. The experiments of Miller and Urey into the origin of organic compounds
Earth’s atmosphere was ‘reducing’ in the early days. It did not contain
oxygen gas until after plants started photosynthesising
Can you
identify these
molecules?
All molecules public domain from Wikimedia Commons, Background image http://www.flickr.com/photos/lrargerich/4587244190/

4. The experiments of Miller and Urey into the origin of organic compounds
Earth’s atmosphere was ‘reducing’ in the early days. It did not contain
oxygen gas until after plants started photosynthesising
The atmosphere contained:
Hydrogen
Nitrogen
Water vapour
Methane
Ammonia
Hydrogen sulfide
The gases came from
abundant volcanic activity
All molecules public domain from Wikimedia Commons, Background image http://www.flickr.com/photos/lrargerich/4587244190/

5. These monomers mixed in the ‘primeval soup’, shallow oceans laden with chemicals
where it is thought that they reacted to form biological molecules
Miller and Urey tried to recreate these conditions in the lab in 1953
They were trying to demonstrate ‘chemical evolution’, the formation of more complex
molecules from simpler stock in the primeval soup
They combined the molecules from the previous page in a closed glass vessel
(simulated atmosphere), they heated the water (simulated volcanic activity) and
sparked electricity through the gases (simulated lightning)
http://www.flickr.com/photos/afeman/663646181/

6. Carny http://upload.wikimedia.org/wikipedia/commons/5/59/MUexperiment.png

7. After a week they found:
Thirteen of the twenty naturally occurring amino acids
Around 15% of the carbon was now in
organic compounds

8. D.1.3 State that comets may have delivered organic compounds to Earth
Panspermia is the hypothesis
that life on Earth originated
from material delivered by a
comet, either in the form of
amino acids or as hardy
bacteria
Existing bacteria and archaebacteria have been found in odd
and extreme environments on Earth:
In hot springs, kilometres deep in the crust and even
embedded in ice cores from deep inside Antarctica
Space is so
empty, yet full
of the
potential for
life
It is feasible that they could survive on or in a comet

9. Cosmic radiation could provide
the energy for reactions that lead
to the formation of complex
organic molecules
Analysis of the spectra of light
coming from the comets reveals
the presence of hydrocarbons,
amino acids and peptides
The bombardment of
Earth by comets 4
billion years ago
could have ‘kick
started’ chemical
evolution

10. D.1.3 Discuss possible locations where conditions could have allowed the synthesis of
organic compounds
Problem: The water in the Miller Urey experiment
tends to hydrolyse any polymers as they form and
prevents their formation. The conditions in the
ocean not ideal for polymerisation
Solution: “black smokers”, hydrothermal vents where
superheated steam escapes from within the crust.
The outflow is full of dissolved sulfides that crystallise around
the vent and may be a suitable environment for the formation
and concentration of complex biological compounds

11. Volcanoes may also have played a part:
Gases from above hot lava lakes have been found to
contain a higher than average level of fixed nitrogen Nitrogen fixation is the
formation of ammonia (NH4)
from nitrogen gas (N2).
The Haber process is a
modern industrial way to fix
nitrogen and it requires high
pressures (200 atm) and high
temperatures (400 °C)
Volcanoes and geysers
may have provided a suitable
location for the formation of
biological compounds
The hypothesis that life originated
on Earth is called abiogenesis
(ab bio genesis)
(aboriginal – life – creation)
http://www.flickr.com/photos/storm-crypt/3043902298/

12. The hypothesis that life came an extraterrestrial source:
As previously mentioned, organic molecules are out there
Mars is smaller than Earth and therefore cooled down more
quickly, life could have begun there while Earth was still scorching
Meteorites and comets impacting on mars could have thrown up debris with
early life attached, this could then have crashed on Earth.
Meteorites of Mars origin have been found in Antarctica
There is no evidence that life has been transferred in this way. Every now and then
there is a news story about “Fossils found in Mars meteorite”
but so far this has not been confirmed
The extraterrestrial hypothesis still doesn’t address how life formed,
just how it could move around the galaxy
http://images.cdn.fotopedia.com/flickr-2406913018-hd.jpg

13. D.1.5 Outline Two properties of RNA that would have allowed it to play a role in the origin of life
RNAs can store, transmit and replicate genetic Information
Ribozymes are RNA molecules that can catalyse reactions
(Hey! You told us that all enzymes are proteins! Liar!)
Some can polymerise nucleotides using ATP
Some can break chemical bonds, including peptide bonds
Ribosomes are themselves Ribozymes (huh?).
The part that catalyses the peptide bonds is RNA, the protein part of a ribosome
seems to have a purely structural function
Evolution by natural selection requires variation and heritability. RNA possesses these traits
http://genetics.mgh.harvard.edu/szostakweb/exploringOriginsDownloads/centralDogma.jpg

14. D.1.6 State that living cells may have been preceded by protobionts,
with an internal chemical environment different from their surroundings
(Proto = first, or precursor)
Coacervates are droplets of polymeric molecules.
Coacervates containing enzyes can absorb
and concentrate substrate molecules
and then release the products to
their surrounds
If they absorb a lot of material they
can divide into two smaller
coacervate droplets
This is not true reproduction though
so they are not alive.
An illustration of a protocell,
composed of a fatty acid membrane
encapsulating RNA ribozymes.
http://exploringorigins.org/protocells.html

15. • Protobionts may have arisen from coacervates.
• Coacervates containing RNA may have started synthesising proteins
• Enzyme controlled binary fission may have arisen.
• The first true cells probably heterotrophic (maybe getting energy from
sulfur chemistry) and anaerobic (there was no free oxygen)
Microspheres: are another candidate
for a structure that might have given
rise to protobionts.
They form when amino acids are
heated and polymerise to form
simple proteins (thermal proteins)
http://www.daviddarling.info/encyclopedia/M/microsphere.html
One milligram of
thermal proteins can
make 100 million
microspheres!
They divide like coacervates
and can catalyse some reactions

16. The contribution of prokaryotes to the creation of an oxygen-rich atmosphere
Remember: there was little free oxygen in the early
atmosphere
Small amounts were made by UV light splitting water
vapour in the atmosphere
After about 2 billion years of prokaryote life (2 billion years ago) there
was an Earth changing event: a form of chlorophyll appeared in
bacteria that allowed oxygenic photosynthesis
The increase in Oxygen led to:
• The breakdown of the chemicals in the ‘chemical soup’ to carbon dioxide
The oxygen concentration rose to 0.45% of the atmosphere
Not much compared to today’s 21%, but it coincides with the rise of the Eukaryotes
COINCIDENCE? Probably not.
and oxidised sediments
• The formation of the ozone layer, which blocked out UV and stopped the
production of more of the ‘soupy’ molecules

17. D.1.8 Discuss the endosymbiotic theory for the origin of eukaryotes
Endosymbiosis is the theory
that chloroplasts and
mitochondria were once free-living
prokaryotes that were
engulfed by larger prokaryotes
and survived to evolve into the
modern organelles
Evidence in support:
1. Mitochondria and Chloroplasts have their
own DNA that is more like bacterial DNA
than what is found in the nucleus
2. The structure and biochemistry of
chloroplasts is similar to cyanobacteria
3. New organelles are made by a process that
resembles binary fission
4. Both organelles have a double membrane
which resembles the structure of
prokaryotic cells
5. Their ribosomes resemble those of bacteria
(70S)
6. DNA analysis suggests that some DNA in
plant nuclei was previously in the
chloroplast
7. Some proteins coded for in the nucleus are
transported to the organelles. The
organelles have lost the DNA to make it
themselves.

18. Further information: