1. 1.5 Origin of cells
Essential idea: There is an unbroken chain of life from the
first cells on Earth to all cells in organisms alive today.
Two billion year old prokaryotic fossils of cells. https://c1.staticflickr.com/9/8187/8117548368_0615009f66_b.jpg

2. Understandings, Applications and Skills
Statement Guidance
1.5 U.1 Cells can only be formed by division of pre-
existing cells.
Students should be aware that the 64
codons in the genetic code have the same
meanings in nearly all organisms, but that
there are some minor variations that are
likely to have accrued since the common
origin of life on Earth.
1.5 U.2 The first cells must have arisen from non-living
material.
1.5 U.3 The origin of eukaryotic cells can be explained by
the endosymbiotic theory.
Evidence for the endosymbiotic theory is
expected. The origin of eukaryote cilia and
flagella does not need to be included.
1.5 A.1 Evidence from Pasteur’s experiments that
spontaneous generation of cells and organisms
does not now occur on Earth.

3. Use the tutorials to learn about Pasteur’s
experiment.
1.5 A.1 Evidence from Pasteur’s experiments that spontaneous generation of
cells and organisms does not now occur on Earth.
Repeat Pasteur’s experiment and
see the results for yourself.
http://biologyjunction.com/pasteur_experiment.ht
m
http://www.sumanasinc.com/webcontent/animations/content/scientifi
cmethod.html

4. 1.5 A.1 Evidence from Pasteur’s experiments that spontaneous
generation of cells and organisms does not now occur on Earth.
http://bcs.whfreeman.com/thelifewire/content/chp03/0302003.html

5. 1.5 A.1 Evidence from Pasteur’s experiments that spontaneous
generation of cells and organisms does not now occur on Earth.
Method:
• Two experiments were setup
• In both, Pasteur added nutrient broth to
flasks and bent the necks of the flasks into S
shapes
• Each flask was then heated to boil the broth
in order than all existing microbes were
killed.
• After the broth had been sterilized, Pasteur
broke off the swan necks from the flasks in
Experiment 1, exposing the nutrient broth
within them to air from above.
• The flasks in Experiment 2 were left alone.
Results:
• The broth in experiment 1 turned
cloudy whilst the broth in experiment
2 remained clear.
• This indicates that microbe growth
only occurred in experiment 1.
Conclusion: Pasteur rejected
the hypothesis of
spontaneous generation as for
growth of microbes to occur a
source of contamination was
needed.
Louis Pasteur designed an experiment to test
whether sterile nutrient broth could
spontaneously generate microbial life.
Q – was Pasteur correct, could
spontaneous generation of life
never occur?

6. 1.1 U.1 According to the cell theory, living organisms are composed of
cells.
Cell theory states that:
• All living things are composed of cells (or cell products)
• The cell is the smallest unit of life
• Cells only arise from pre-existing cells

7. 1.5 U.1 Cells can only be formed by division of pre-existing cells.
Cells can only be formed by
division of pre-existing cells:
• Cells multiply through division
• Mitosis results in genetically identical
diploid daughter cells
• Meiosis generates haploid gametes
(sex cells)
4-cell stage of a sea biscuit by Bruno Vellutini on Flickr
(CC) http://flic.kr/p/daWnnS

8. What evidence do we have
(other than Pasteur’s
experiments) to support this
theory?
4-cell stage of a sea biscuit by Bruno Vellutini on Flickr
(CC) http://flic.kr/p/daWnnS

9. 1.5 U.1 Cells can only be formed by division of pre-existing cells.
1. Cells are highly complex
structures and no mechanism has
been found for producing cells
from simpler subunits.
http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Eukaryotic_Cell_%28animal%29.jpg/1024px-Eukaryotic_Cell_%28animal%29.jpg

10. 1.5 U.1 Cells can only be formed by division of pre-existing cells.
2. All known examples of growth
from cell to tissue, tissue to organ,
organ to multicellular organism,
are all a result of cell division.
http://upload.wikimedia.org/wikipedia/commons/thumb/d/d3/Onion_root_mitosis.jpg/749px-Onion_root_mitosis.jpg

11. 1.5 U.1 Cells can only be formed by division of pre-existing cells.
3. Viruses are produced from simpler
subunits, but they do not consist of cells,
and they can only be produced inside the
host cells that they have infected.
http://upload.wikimedia.org/wikipedia/commons/3/3a/Influenza_virus_particle_color.jpg

12. 1.5 U.1 Cells can only be formed by division of pre-existing cells.
4. Genetic code is universal each
of the 64 codons (a codon is a
combination of 3 DNA bases)
produces the same amino acid in
translation, regardless of the
organism [2.7.A2]*.
* There are some minor variations that are likely to have accrued since the common origin of
life on Earth, but these are rare and most of the genetic code is universal most of the time.
The logical deduction is that all
cells have arisen as the result of
cell division from a single
common ancestor.

13. There were times in the history of the Earth when cells did not exist.
The first cells must have arisen from non-living material or life was
transported here from elsewhere in the universe. As illustrated below,
an asteroid may have carried the first cells (panspermia). We will
focus on the organic evolution from materials on Earth.
1.5 U.2 The first cells must have arisen from non-living material.
Some of the key problems are:
1. Non-living synthesis of simple organic
molecules, e.g. sugars and amino
acids
2. Assembly of these organic molecules
into polymers
3. Formation of membranes to package
the organic molecules
4. Compartmentalization by formation
of polymers that can self-replicate
(enabling inheritance)

14. Prebiotic Chemistry/Earth
• Little or no free oxygen
Oxygen very reactive, would remove electrons needed for
reactions
• Considerable energy
Heat, light, electrical energy needed to make/break bonds
Vulcanism, sun provided much UV, lightning
• Chemical ‘building blocks’ to be built into more complex molecules
• The reactions required time ~ 0.8 billion years to cause life to come
about. This is a long time, but not long compared to the life of the
Universe (~10-20 billion years)
1.5 U.2 The first cells must have arisen from non-living material.

15. 1. Non-living synthesis of simple organic molecules
(Materials available in the Prebiotic Earth)
• All living things have the same overall chemistry
– Suggests a common origin
– However, likely to be multiple similar origins
• Earth ~ 4.5 billion years old
• Early atmosphere:
» H2O vapor
» NH3
» N2
» H2
» H2S
» CH4
» Virtually no O2
1.5.U.2 The first cells must have arisen from non-living material.

16. Water vapor
Condensed
liquid with
complex,
organic
molecules
Condenser
Mixture of gases
("primitive
atmosphere")
Heated water
("ocean")
Water
CH4
NH3
H2
Electrodes discharge
sparks
(lightning simulation)

17. 2. Assembly of these organic
molecules into polymers
(Miller / Urey Experiment)
• Created an ancient earth
atmosphere in globes
– H2O vapor
– H2
– NH3
– CH4
• Electrical discharge
• Heated via mantle in lower
chamber
• Paper chromatography showed
that 13 of the 20 amino acids had
formed
• Similar experiments have shown
that all 20 amino acids, lipids,
nucleotide bases, and ATP, if
phosphate present.

19. 1.5 U.2 The first cells must have arisen from non-living material.
Primitive ‘Soup’ (ocean surface)
• Haldane and Oparin (~1920s)
proposed that life arose from
nonliving conditions by means of a
series of changes (and increasing
complexity) in molecular
composition as a result of the
reducing, high energy environment
that existed on early Earth
(between 4.6-3.8 bybp (billion
years before present)
• Proposed the Prebiotic Broth
Hypothesis
• Not tested until 1950s
• Tested by Stanley Miller and
Harold Urey

20. • An addition to the theory that life
may have formed close to the
oceans surface, early life may have
formed at hydrothermal vents on
the ocean floor
• Today, hydrothermal vents provide
basic materials to support living
organisms in environments where
there is no light, very little energy,
and little other perturbation
• Ammonia is produced in
abundance at such locations,
suggesting that it could possibly
be built into nucleic acid bases
and the possibly into amino acids
http://faculty.cascadia.edu/jvanleer/astro%20sum01/Hyrothremal%20Ven
t%20Final/hydrothermal-vent.jpg

21. Sea Vents
http://upload.wikimedia.org/wikipedia/commons/b/b8/Deep_sea_vent_chemistry_diagram.jpg

22. 1.5 U.2 The first cells must have arisen from non-living material.
• DNA though very stable
and effective at storing
information is not able to
self-replicate – enzymes
are required
• However RNA can both
store information and self-
replicate - it can catalyze
the formation of copies of
itself.
• In ribosomes RNA is found
in the catalytic site and
plays a role in peptide
bond formation
For more detail research the RNA World Hypothesis
3. Formation of polymers that can self-replicate (enabling inheritance)

23. •The current “most accepted“ theory of life evolving hypothesizes an
RNA world
•RNA in the early world would have functioned as a self replicating
molecule, eventually developing a number of minimal catalytic
properties
self-replicates Proteins take
over catalysis
DNA becomes
long term storage
and major coding molecule
Packaging evolves
- RNA codes
and catalyses
RNA
Molecular Reproduction

24. The RNA World Hypothesis
• Proposed by Walter Gilbert, mid 1980s
• Concept of RNA catalyzing critical pre-biotic and early biological
reactions
• RNA would give rise to RNA as an informational molecule
• RNA would give rise to protein
• RNA might bind amino acids proteins
• RNA might give rise to DNA
• Proteins might take over some functions
• DNA would take over informational functionality
• Eventually giving rise to the DNA RNA protein scheme
found today

25. 1.5 U.2 The first cells must have arisen from non-living material.
4. Compartmentalization
The formation of membranes to
package the organic molecules.
Experiments have shown that
phospholipids natural assemble
into bilayers, if conditions are
correct.
Formation of the bilayer creates
an isolated internal environment.
The formation of an internal
environment means that optimal
conditions, e.g. for replication or
catalysis can be maintained.
Two ideas
A. Microsphere
B. Ocean froth
http://upload.wikimedia.org/wikipedia/commons/thumb/0/
01/Liposome_scheme-en.svg/220px-Liposome_scheme-
en.svg.png

26. Origin of Cells (Protobionts)
• Bubbles  separate inside from outside
 metabolism & reproduction

27. Microspheres (coacervates)
Formed from heated mixtures of amino acids
Subsequently mixed with salty water
Some are excitable
Have some catalytic activity
Could provide the separation for increasing the concentration of
material required for early prebiotic chemistries

28. Ocean Froth
• The waves crashing on
the shore consisted of
bubbles (froth)
• The “membranes” of the
bubble were either
made of
– hydrocarbon chains/lipids
– Protenoids (chains of
amino acids)

29. http://youtu.be/q71DWYJD-dI
1.5 U.3 The origin of eukaryotic cells can be explained by the
endosymbiotic theory.
Endosymbiotic theory explains the existence of several organelles of eukaryotes.
The theory states that the organelles (e.g. mitochondria and chloroplasts)
originated as symbioses between separate single-celled organisms,
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter26/animation_-_endosymbiosis.html

30. First Eukaryotes (about 2 billion years ago)
• Development of internal membranes
– create internal micro-environments
– advantage: specialization = increase efficiency
• natural selection!
infolding of the
plasma membrane
DNA
cell wall
plasma
membrane
Prokaryotic
cell
Prokaryotic
ancestor of
eukaryotic
cells
Eukaryotic
cell
endoplasmic
reticulum (ER)
nuclear envelope
nucleus
plasma
membrane

31. Endosymbiosis
Ancestral
eukaryotic cell
Eukaryotic cell
with mitochondrion
internal membrane
system
aerobic bacterium mitochondrion
Endosymbiosis
• Evolution of eukaryotes
– origin of mitochondria
– engulfed aerobic bacteria, but did not digest them
– mutually beneficial relationship
• natural selection!

32. mitochondrion
chloroplast
Eukaryotic cell with
chloroplast & mitochondrion
Endosymbiosis
photosynthetic
bacterium
• Evolution of eukaryotes
– origin of chloroplasts
– engulfed photosynthetic bacteria,
but did not digest them
– mutually beneficial relationship
• natural selection!
Eukaryotic
cell with
mitochondrion

33. • Evidence
– structural
• mitochondria & chloroplasts
resemble bacterial structure
– genetic
• mitochondria & chloroplasts
have their own circular DNA, like
bacteria
– functional
• mitochondria & chloroplasts
move freely within the cell
• mitochondria & chloroplasts
reproduce independently
from the cell
Theory of Endosymbiosis
lynn margulis

34. Bibliography / Acknowledgments
Jason de Nys