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

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

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

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

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

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

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

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

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

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"

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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?

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

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

Where cells come from Francesco Redi

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

Where cells come from Francesco Redi

Where cells come from John Needham

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!

Where cells come from Lazzaro Spallanzani

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

Where cells come from Louis Pasteur

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!

Where cells come from Louis Pasteur

Dave Ferguson (Kobe, JA) MAJOR SOURCES Thank you to my favorite sources of information when making these lectures! Dave Ferguson (Kobe, JA) http://canada.canacad.ac.jp/High/49 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 – www.hhmi.org/biointeractive Mr. Hoye’s TOK Website – http://mrhoyestokwebsite.com And all the contributors at www.YouTube.com