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Origin of Life
What is life?
What conditions are required for life to exist on Earth?
Where did the organic building blocks of life come from?
How did these building blocks get together?
How did replication arise?
How does life get energy?
When did life start?
What is life?
1.
2.
3.
4.
Boundaries
Organized structure that is separated from the outside
world.
Replication
Information stored in molecules (DNA, RNA) passed
from generation-to-generation (with occasional mistakes
(mutations) and a set of instructions for interpreting
these molecules (genetic code).
Metabolism
Harvest energy (solar, chemical)
Run bodies and make complex macromolecules
Homeostasis?
Self regulating mechanisms to maintain a constant
internal environment
Metabolic equilibrium
Basic Requirements
Carbon-based “organic” molecules: Most
molecules in a cell have C bonded to either itself or
to H. Tremendous complexity.
Liquid water: Cells are full of water. Main
component of Cytoplasm. Universal solvent.
Energy source:
Basic Requirements
Elemental chemistry of universe is fine for life,
but Earth looks a bit sketchy.
C, H, O, N, P, S
What conditions are required for
life to exist on Earth?
“Planet Goldilocks”
The right stuff: Our sun is a 2nd generation star.
The Big Bang produced only H and He.
Nucleosynthesis and supernova explosions required
for other elements.
E0102-72, is the aftermath of
a star that exploded 190,000
light years away, in a nearby
galaxy known as the Small
Magellanic Cloud. The image
represents a time ~1000 years
after the explosion.
What conditions are required for
life to exist on Earth?
“Planet Goldilocks”
The right size: Earth is large enough to keep N2, O2,
H2O and CO2, but small enough to allow H and He to
escape.
80% N2
20% O 2
86% H2
13% He
What conditions are required for
life to exist on Earth?
“Planet Goldilocks”
The right distance: Incoming radiation from the Sun
heats the surface close to the temperature where
water is liquid. With a little greenhouse warming, get
water vapor and liquid, as well as ice.
Basic Cell
DNA
(lipid bilayer)
Domains of Life
Key Large Biological Molecules
Major Biomolecues
Functions
Proteins
Structural
Enzymes
Support and motion
Catalysis
Building
Block
Amino Acids
Lipids
Bilayers
Fats
Membranes
Energy store in animals
Carbohydrates
Glycogen
Starch
Cellulose, lignin, etc.
Energy store in animals
Energy store in plants
Support in plants
Fatty Acids
Nucleotides &
Nucleic Acids
DNA
RNA (and ribozymes)
ATP
Sugars
Info storage
Info storage, catalysis
Energy store and transfer
Sugar,
phosphate,
&
nitrogenous
base
The Building Blocks
1. Lipids: roughly (CH2)nO
Hydrophobic end, hydrophilic end: water insoluble
Flexible - Membranes
Energy dense - Storage
The Building Blocks
2. Carbohydrates: (CH2O)n
Many -OH groups, polar: water soluble
Polymerize to form Polysacchrides
Sugars, starches: storage in plants
Cellulose, chitin: structure in plants
Glycerol:
Back-bone for fats
The Building Blocks
3. Amino Acids
C, H, R-group, amino (NH2 ) group, carboxyl (COOH) group
Structural proteins, enzymes, transport molecules
20 common amino acids in protein
Carboxyl
Amino
Amino
Carboxyl
Peptide Bond
The Building Blocks
4. Nucleotides
5-C sugar, 1 or more PO4 groups, N-containing base
Nucleotide
Nucleic Acid: formed
by dehydration reaction
Purines
Pyrimidines
DNA
dehydration reactions
Adenosine triphosphate (ATP)
Adenosine diphosphate (ADP)
Adenosine monophosphate
(AMP)
DNA & RNA
Function: Synthesis & Reproduction!!!
Genes code replication proteins, regulatory information
Protein Synthesis Process:
Messenger RNA
1. Copies gene (DNA) sequence as RNA
2. Transport out of nucleus
3. Transcribed by Ribosomes
4. Ribosome assembles proteins
Reproduction Process:
Cell Division
1. Double helix unwinds and splits
2. Free nucleotides (A,T,G,C) pair up
3. Produces 2 new double helices (~errors)
Where did the organic building
blocks of life come from?
Alexander Oparin (1930s): Amino acids, fatty acids, sugars,
nucleic acids produced by atmospheric chemical reactions.
Rain to Earth to make Primoridal Soup.
Miller/Urey Experiments (early 1950s): Ran an electric
current through a simulated atmosphere for early Earth
(water, methane, ammonia, etc.). Made “geo-junk” rich in
amino acids and sugars.
Not clear early atmosphere was
reduced enough to get a good
yield from these types of
reactions.
Where did the organic building
blocks of life come from?
Chyba/Sagan (1990s): Comets are rich in organic molecules.
Extra-terrestrial source for building blocks?
Interplanetary dust
Where did the organic building
blocks of life come from?
Chirality: comets biased left
Amino acids in most Earth
materials are left biased as
well. May be remnant of
early delivery.
How did the building blocks hook up?
There are many sources for building blocks, but linking them
into chains is hard work. Building blocks are dilute in a 3D
medium and most chemical linkages between building blocks
are dehydration reactions (i.e., spit out a water molecule).
• Prebiotic Pizza: Clay minerals concentrate building blocks out
of solution and catalyze reactions.
• Proto-metabolism: Dehydration occurs on mineral surfaces
where other energy releasing chemical reactions are occurring.
• Prebiotic foam: Lipid and protein
bubbles trap and concentrate
molecules.
• Ice beer model: Freezing
concentrates molecules
The Replication Paradox
RNA World: earliest "living" form was an energyharvesting RNA molecule
RNA carries genetic information
RNA can function as an enzyme “Ribozymes”
catalyze its own replication
Problems
Hooking up still an issue.
How do we make long RNA molecules?
Energy Options for Life
Energy for life comes from high energy electrons passed
from molecule to molecule in a cell until they reach a
lower energy state. This energy is used to “charge up”
ADP to ATP by adding a phosphate ion. This energy-rich
bond stores the energy from the electron.
Energy source can be either solar or chemical.
Metabolism: Study of how organisms trap electrons, and how they
release this trapped energy to fuel activity and tissue building.
Autotrophs - organisms that directly capture electrons from either
light or energy-rich inorganic chemicals to generate ATP and to make
complex, energy-rich compounds. Ultimate source of C for building
body is usually CO2.
Heterotrophs - organisms that obtain electrons and C by consuming
complex, energy-rich compounds synthesized by autotrophs.
Redox Chemistry
Oxidation - loss of e- (e.g., Fe2+ ⇒ Fe3+ + e-)
Reduction - addition of e- (e.g., Fe3+ + e- ⇒ Fe2+)
Available energy generating reactions
e- dnr + e- actr
4H2 + CO2 → CH4 (e rich)+ 2H2O + heat
CH2O + O2 → H2O + CO2 + heat
Energy Options for Life
In an anaerobic (O2 poor world), encounter metabolisms that seem odd to us.
Chemo-autotrophs - energy from inorganic chemical reactions
e.g., Methanogens (Archaea)
4H2 (e- dnr) + CO2 (e- acptr) → CH4 (methane, e- rich)+ 2H2 O + ?ATP
In cow guts, 500 liters methane/day/cow
Heterotrophs - consume organic compounds (C-H bonds)
e.g., Fermenters (Eubacteria)
Alcohol production
C6 H12O6 (glucose) → 2CH3 CH2 OH (ethanol, e - rich)+2CO2 + 2ATP
Lactic acid production
C6 H12O6 (glucose) → 2C3 H6 O3 (lactate, e - rich) + 2ATP
Both metabolisms were likely common on early Earth
Early life was anaerobic.
Not very energy efficient.
Energy Options for Life
New options needed as hydrogen gas and prebiotic organic compounds
were consumed by early organisms.
Photo-autotrophs - absorb light energy using molecules with organic-metal
complexes (chlorophylls). Use trapped energy to charge up ATP, and then to
make energy-rich compound (sugar).
Anaerobic Photosynthesis (Eubacteria)
ν + 2H2 S (e - dnr) + CO2 (e - acptr) → CH2 O (sugar: e - rich)+ H2 O + 2S
Oxygenic Photosynthesis (Eubacteria, Plants)
6CO2 (e - acptr) + 6H2 O (e- dnr) + ν  C6 H12O6 (glucose: e- rich) + 6O2
1) Catalyzed by the enzyme RUBISCO.
2) Almost limitless source of food.
3) O2 was toxic to earlier organisms. Drove them to anoxic refuges or to invent
detox (anti-oxidant) mechanisms.
4) Higher O2 levels allowed new type of heterotrophy.
Aerobic respiration - “burn” organic matter with O 2 (Eubacteria, Eukarya)
C6 H12O6 (Glucose; e- dnr) + 6O2 (e- acptr)  6CO2 + 6H2 O + 36 ATP
Much more efficient than fermentation. Leaves no energy-rich waste (e.g., ethanol).
Self-sustaining ecosystem as long as light and nutrients to make enzymes and
chlorophyll are present.
Where did
biosynthesis
first occur?
Sea surface
Requires Energy
- electrical discharge
- solar
- geothermal
• Surface Ocean shallow pools
• Crust?
• Black smokers?
– Chemosynthesis Fe2 S redox reactions
yield energy
Deep sea
How do we recognize life in the
rock record?
1.
2.
3.
Body fossils – mineral or organic remains of the bodies of organisms
Trace fossils – structures formed by interaction of organisms with
sediment or each other
Chemical fossils
a.
b.
c.
Biomarkers – organic molecules characteristic of particular groups
Pristane and phytane: breakdown products of chlorophyll
Sterols: resistant molecules in eukaryotic cells walls.
Isotope tracers – distinctive isotope sorting by biochemical reactions.
RUBSICO moves 12C faster than 13C.
Photosynthetic products are enriched in 12C.
Environmental Chemistry – certain sediments are likely products
of biological activity.
Oxidized iron (Fe3+) suggests that oxygenic photosynthesis (or
Fe-oxidizing bacteria).
Massive deposits of elemental S would suggest chemoautotrophs.