ASTR/GEOL-2040: Search for life in the Universe, Lecture...
Transcript of ASTR/GEOL-2040: Search for life in the Universe, Lecture...
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ASTR/GEOL-2040: Search for life
in the Universe, Lecture 3
Axel Brandenburg
(Office hours: Mondays 2:30 – 3:30 in X590
and Wednesdays 11-12 in D230)
ad
One-off additional
office hours
this Tuesday: Sep 5
8 – 10 in X490
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• More individuals produced than survive
• Struggle for existence (limited resources)
• Individuals show variation (often subtle)
• Individuals produce similar offspring
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Darwinian evolution
RGS p.2, Lon p.168, BS p.164,181
Works also/especially at the molecular level:
RNA can replicate itself RNA world
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A. the occasional mutations that occur in DNA;
B. the mechanism by which advantageous traits are
preferentially passed on from parents to offspring;
C. the idea that organisms can develop new
characteristics during their lives and then pass
these on to their offspring.
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Natural selection is
BS p.195
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A. the occasional mutations that occur in DNA;
B. the mechanism by which advantageous traits are
preferentially passed on from parents to offspring;
C. the idea that organisms can develop new
characteristics during their lives and then pass
these on to their offspring.
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Natural selection is
BS p.182
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• Replicate &
• Evolve through natural selection
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Again: What is life?
RGS p.2, BS p.164-165
Still unclear how life got started
Chemical vs biological evolution
Where exactly is the threshold?
automatically implies
input of matter & energy
Any definition may be challenged
as we have new observations
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• When left alone, a system
undergoes conversions that lead to
increasing disorder
• Consider life in sealed box: use up
all molecules and energy …
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2nd law of thermodynamics
BS p.157
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A. The maintenance of order in
living cells
B. The ability to evolve over time
C. The ability to violate the second
law of thermodynamics?
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Which of the following is
not a key property of life?
RGS p.3, Lon p.171, BS p.166-167
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A. The maintenance of order in
living cells
B. The ability to evolve over time
C. The ability to violate the second
law of thermodynamics?
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Which of the following is
not a key property of life?
RGS p.3, Lon p.171, BS p.166-167
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The other law of thermodynamic
Lon p.161-163, BS p.157
• 1st law of TD: energy is conserved
– can be transferred between different
reservoirs (thermal, kinetic, chemical)
• 2nd law of TD: each time energy is
transferred, disorder of Universe incr. Life creates order & messes up surroundings
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• Rothery, Gilmour, Sephton (RGS) pp. 4-12
• Longstaff (Lon) pp. 175-183
• Bennett & Shostak (BS) pp. 167-169
• pp. 176-183
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Building blocks of life
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• H and He in the Universe
• Nobel elements He, Ne, Ar, … highly unreactive (inert)
• Rest in Universe O, N, C, Si
• Human body H, O, C, N11
Most abundant elements
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• Proteins (“meat”)
• Lipids (“fats”, cell boundaries)
• Carbohydrates (“sugars”)
• RNA & DNA12
Molecules in human body
Molecule mass sum
Water 65% 65%
Protein 20% 85%
Lipids 12% 97%
Other inorg (eg carbohydrates) 1.5% 98.5%
RNA 1.0% 99.5%
Other org 0.4% 99.9%
DNA 0.1% 100%
The building
blocks of life
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• To dissolve & transport organic
molecules, available for reactions
• Is liquid over wide T range
• Alternatives; lower T, reactions slow
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Why water?
RGS p.4, Lon p.171-173, BS p.246-248
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• Ice floats & insulates deeper layers
• Water is a highly polar molecule hydrogen bonds (very stable)
• Dissolves other polar molecules
– Like dissolves like
• Does not dissolve apolar molecules critical for existence of cells
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Other reasons
RGS p.5, Lon p.172-173, BS p.248
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• Water does not dissolve apolar molecules critical for existence of cells
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Water & fat
RGS p.5, Lon p.172-173, BS p.248
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• Water-insoluble
– Act as cell membranes
• Energy substrates
– Fats (used by migrating birds)
• Fatty acids
– Double bond = unsaturated
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Properties of lipids
RGS p.4, Lon p.171-173, BS p.168
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Cells are made of lipids
hydrophylic
hydrophobic
amphobic
carboxyl
group
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• Only weakly polar
• Cells could dissolve
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Ammonia NH3
RGS p.5, Lon p.172-173, BS p.248
H2O CH4
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• Versatile, because it can bond with
a large range of different atoms
• Carbon compounds can readily
dissolve in water
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Why carbon
RGS p.3, BS p.164-165
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• Si is larger, so bonds are weaker
• Si does not form double or triple
bonds as easily
• SiO2 is solid, not mobile like CO2
• In Earth’s crust, Si is 1000x more
abundant than C, but C won
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Why not silicon?
RGS p.3, Lon p.171, BS p.166-167
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• carbohydrates
• hydrocarbons
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Common carbon compounds
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• Hydrocarbons: made up entirely
of hydrogen & carbon
CH3-CH2-CH2-CH2-CH2-CH3
• Carbohydrates: made up of
hydrogen, carbon, & oxygen
Cm(H2O)n
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Common carbon compounds
H-C-OH
H-C-OH
H-C-OH
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– COOH carboxyl group
– OH hydroxyl group
– CH3 methyl group
– NH2 amino group (=amine group)
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These endings have names
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building macromolecules
+nH2O+energy
endergonic
reaction
condensation
=dehydration
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• Glucose=hexose
=monosaccharide
= C6H12O6
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(CH2O)n = “life”
chemists drop
C & H
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– Why carbon, & not silicon-based
– Why water?
– pp. 3-8, Sects. 1.1 & 1.2
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What we talked about