1. Mr. Christopher Briner Unit 1.5 The Origin of Cells
KIS International School
DP Biology
Unit 1.5
The Origin of Cells

2. 1.5.u1
Cells can only be formed by division of pre-existing cells.

3. 1.5.u2
The first cells must have arisen from non-living material.

4. Origin of Life on Earth Life from Scratch
“If you wish to make an apple pie from scratch, you must first invent the universe” Carl Sagan

5. Origin of Life on Earth Life from Scratch
Four main steps to origin of life:
Producing organic molecules
Polymerization
Forming a genetic material
Producing membranes

6. Origin of Life on Earth Life from Scratch
Process 1: Producing organic molecules
Synthesis of simple organic molecules from environmental precursors

7. Origin of Life on Earth Life from Scratch
Process 1: Producing organic molecules
Using inorganic molecules:
Water
Carbon dioxide
Ammonia

8. Origin of Life on Earth Life from Scratch
Process 1: Producing organic molecules
Chemical reactions to produce simple organic molecules:
Amino acids (20 types)
Nucleotides (Purines and pyrimidines)
Monosaccharides (Glucose and ribose)
Fatty acids and glycerol

9. Origin of Life on Earth Life from Scratch Process 2: Polymerization
Assembly of simple organic molecules into polymers
Amino acids into polypeptides
Nucleotides into nucleic acids
Monosaccharides in polysaccharides

10. Origin of Life on Earth Life from Scratch
Process 3: Forming a genetic material
Formation of polymers that can self- replicate
RNA almost certainly preceded DNA as the genetic material = "RNA world"

11. Origin of Life on Earth Life from Scratch
Process 3: Forming a genetic material
RNA has two key abilities that make it the likely original genetic material
Genetically: Self-replication
RNA has been shown to self-replicate
One molecule can act as the template for another
RNA nucleotide sequence is variable
Allowing for inheritance of information coding for amino acid sequences in polypeptides

12. Origin of Life on Earth Life from Scratch
Process 3: Forming a genetic material
RNA has two key abilities that make it the likely original genetic material
Enzymatically: Catalyzing chemical reactions
RNA ribozymes are found in modern cell

13. Origin of Life on Earth Life from Scratch
Process 4: Producing membranes
Packaging molecules inside membranes
Creating an internal chemistry different from their surroundings
Including self-replicating polymers that held the genetic information

14. Origin of Life on Earth Life from Scratch Result: Protobionts
Protobiont: Primitive cell-like structure
Product of the above four processes is likely to have been cell-like structures
Natural selection likely acted on variants of protobionts competing for resources

15. 1.5.u3
The origin of eukaryotic cells can be explained by the endosymbiotic theory.

16. Origin of Eukaryotes Endosymbiosis
Eukaryotic cells appear to have evolved from prokaryotic ancestry
Small prokaryote inside large prokaryote
In most situations either:
Larger host cell digests smaller invader
Smaller invader multiplies & kills larger host

17. Origin of Eukaryotes Endosymbiosis Endosymbiosis is the third outcome:
Coexistence
Coexistence is only likely if it is mutually beneficial

18. Origin of Eukaryotes Endosymbiosis
Scenario for mutually beneficial coexistence:
Larger host cell (Eater) restricted to anaerobic environments consuming smaller cells
Smaller invader (Eliminator) eliminates oxygen (poison)
Can occupy environments where Eater is absent

19. Origin of Eukaryotes Endosymbiosis
Scenario for mutually beneficial coexistence:
If Eater consumes eliminator, and eliminator avoids digestion
Eater benefits from low oxygen levels
Eliminator benefits from predation avoidance
Resulting host becomes the eukaryotic cell, with eliminator as mitochondria

20. Origin of Eukaryotes Endosymbiosis
Scenario for mutually beneficial coexistence:
Similar process involves coexistence with an additional consumed cell
Photosynthetic Sunshine

21. Origin of Eukaryotes Endosymbiosis
Scenario for mutually beneficial coexistence:
Sunshine produces oxygen as a byproduct
Benefits from coexistence with mitochondria/eliminator
Sunshine also benefits from protection within eater
Which benefits from the food produced by sushine/chloroplast

24. Origin of Eukaryotes Evidence for endosymbiosis
Mitochondria and chloroplasts both have double membranes
A second outer membrane from the host
Mitochondria and chloroplasts both have a loop of naked DNA
Lacking histone proteins
Like prokaryotes

25. Origin of Eukaryotes Evidence for endosymbiosis
Mitochondria and chloroplasts both divide by binary fission
Independent of nuclear division
Mitochondria and chloroplasts both have smaller 70S ribosomes
Similar to prokaryotes
Different from 80S eukaryotic ribosomes

26. Origin of Eukaryotes Evidence for endosymbiosis
Chloroplast thylakoids are similar to cyanobacteria photosynthetic structures
Chlorophyll a is the main photosynthetic pigment for both chloroplasts and prokaryotes
Mitochondrial cristae are similar to bacterial mesosomes

27. TOK QUESTION: What proof?
Can we ever be sure that the theory explains what actually happened in the past?
For something to be a scientific theory, we must also be able to test whether it is false. Can we do this if the theory relates to a past event?
Is a special standard required for claims about events in the past to be scientific?
If they cannot be falsified, is it enough if they allow us to make predictions?

28. 1.5.a1
Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth.

29. Where cells come from Spontaneous generation
Many scientists used to think that life often formed from non-living material
Bacteria from air
Mice from dirty laundry or old bread
Maggots from rotting meat
Fleas from dust
Worms from rain

30. Where cells come from
Francesco Redi

31. Where cells come from Francesco Redi
Disproved spontaneous generation of large animals (flies)
Put meat in covered and uncovered jars
After several days:
Maggots in open jar and on cloth covering closed jar
No maggots in closed jar
Maggots did not grow naturally in meat
Also ‘discovered’ fly eggs

32. Where cells come from
Francesco Redi

33. Where cells come from
John Needham

34. Where cells come from John Needham Argued with Redi
Said spontaneous generation could happen for small organisms like bacteria
Boiled broth in uncovered jar, and after several days bacteria was growing anyway!

35. Where cells come from
Lazzaro Spallanzani

36. Where cells come from Lazzaro Spallanzani Needham was not scientific!
Repeated Needham’s experiment, but with one open and one closed jar.
No bacteria in closed jar
Some then argued that life could only spontaneously generate in ‘fresh’ air

37. Where cells come from
Louis Pasteur

38. Where cells come from Louis Pasteur Disproved spontaneous generation
Boiled broth to kill all bacteria
Used jar with curved neck to allow air to enter, but not let bacteria land
No bacteria grew, so air did not turn into bacteria!

39. Where cells come from
Louis Pasteur

40. Dave Ferguson (Kobe, JA)
MAJOR SOURCES
Thank you to my favorite sources of information when making these lectures!
Dave Ferguson (Kobe, JA)
Andrew Allott – Biology for the IB Diploma
C. J.Clegg – Biology for the IB Diploma
Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate
Howard Hugh’s Medical Institute –
Mr. Hoye’s TOK Website –
And all the contributors at