1. 1.5 Origin of cells
Two billion year old prokaryotic fossils of cells.
Essential idea: There is an unbroken chain of life from the first cells on Earth to all cells in organisms alive today.

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. 1.5 A.1 Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth (Page 13)
Use the tutorials to learn about Pasteur’s experiment.
Repeat Pasteur’s experiment and see the results for yourself.

4. 1.5 A.1 Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth.

5. 1.5 A.1 Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth.
Louis Pasteur designed an experiment to test whether sterile nutrient broth could spontaneously generate microbial life.
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.
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.
Conclusion: Pasteur rejected the hypothesis of spontaneous generation as for growth of microbes to occur a source of contamination was needed.
Q – was Pasteur correct, could spontaneous generation of life never occur?

6. Remember this theory from 1.1 Introduction to cells?
1.1 U.1 According to the cell theory, living organisms are composed of cells. (Page 13 and 1)
Remember this theory from 1.1 Introduction to 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 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)

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)

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.

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.

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.

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]*.
The logical deduction is that all cells have arisen as the result of cell division from a single common ancestor.
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.

13. Some of the key problems are:
1.5 U.2 The first cells must have arisen from non-living material. (page 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.
Some of the key problems are:
Non-living synthesis of simple organic molecules, e.g. sugars and amino acids
Assembly of these organic molecules into polymers
Formation of membranes to package the organic molecules
Compartmentalization by formation of polymers that can self-replicate (enabling inheritance)

14. Prebiotic Chemistry/Earth
1.5 U.2 The first cells must have arisen from non-living material.
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)

15. Non-living synthesis of simple organic molecules
1.5.U.2 The first cells must have arisen from non-living material.
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

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

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 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

21. 1.5 U.2 The first cells must have arisen from non-living material.
3. Formation of polymers that can self-replicate (enabling inheritance)
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

22. and major coding molecule
Molecular Reproduction
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
RNA
self-replicates
Proteins take
over catalysis
Packaging evolves
- RNA codes
and catalyses
DNA becomes
long term storage
and major coding molecule

23. 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

24. 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
Microsphere
Ocean froth

25. 1.5 U.3 The origin of eukaryotic cells can be explained by the endosymbiotic theory. (page 13)
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,

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

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

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

29. Theory of Endosymbiosis
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
Lynn Margulis
Example:
Divide by binary fission independently of host cell
Have 70 S ribosomes
Have circular DNA
Have 2 membranes