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Lecture 16

2ee3 lecture 16 origin of life.docx

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Jianping Xu

LECTURE 16  Origin of Life o essential elements that make up the organic compounds: hydrogen, carbon, nitrogen, oxygen and phosphorous etc. upto iron. Carbon nitrogen and oxygen produced through nuclear fusion (CNO cycle) and the others till iron were made from a nuclear fusion in the run. Others were generated through supernova (very high temperature explosion) creates heavier elements. o living organisms need energy to sustain otherwise they would decay. Need a constant source of energy  from the sun which is 6000 K degrees and emission of electromagnetic energy (heat) is what drives processes and gives us energy o water is required for all life forms on earth. 0-100 degrees Celsius  needed for liquid water.  Life cycle of a massive star o as a while, the inside of the earth is reduced. it s packed with elemental iron – it can easily donate electrodes o the further you go out, there is more oxidized elements o the crust is mostly silicates  outer surface from about 3 km beneath the surface to 30-40 above the earth’s surface contains living organisms o core: quartz o mantle: iron magnesium silicate o stratosphere: felix (jumper experiment) organisms can survive or get precipitated o biosphere: terrestrial and aquatic environment  Evidence for ancient life on Earth o stromatolites: rocky substances with many microbes growing inside them and cellular shape can be visualized under the microscope. Deposit of silicates and calcium carbonate within cells. o Microfossils: decayed microbial cells with distinct morphology filled with calcium carbonate/silicate. o Isotope ratios: analysis of isotopes 12C:13C ratio. S32:S34 also used. If substances have been processed by biological metabolism, then the ratio of lighter isotope would be enhanced and heavy isotope would be reduced. Heavy isotope depletion. Because current metabolic enzymes have preference for lighter isotopes. o Biosignatures: sterols are very stable due to rings. If they are preserved they can last for hundreds of years in rocks. Evidence that living organisms exist. o Oxidation state: when it is extremely red, it shows iron has been oxidized and you can date the banded iron  this would indicate banded iron was produced in the presence of oxygen that was produced by cyanobacteria at that time point. Earth was in a reduced state (with little oxygen) in the past.  Geological evidence for life on Earth o oldest sedimentary rocks  presence of water. Some have c13 depletion (relatively high ratio of c12 compared to 13)  evidence of life o Recent studies show that even though there is 13C depletion, evidence of microfossil in this rock has been refuted. o 2.5 billion years ago  C13 depletion and hopanoid finding, gradual increase of O2 concentration in the atmosphere due to finding banded iron o Colonies of bacteria (cyanobacteria)  can’t be formed without the presence of oxygen o ½ billion years ago  complex life forms (multicellular organisms) o Cambrian explosion  500,000 years ago  Rocky Mountains Canada has evidence  Isotope fractionation o isotope ratios and potential presence of life o 12,13C are stable and 14C is not stable because it tends to decay o Ratio of 12C and 13C would typically be 20:1 or 19:1 o In most objects on earth, 12C would be greater than 19:1 and 20:1 o The more metabolic processes occur, the more enriched 12C will be  Early metabolism o Oxidation/reduction: Lithotrophs  can use inorganic electron donors to harvest energy and oxidize inorganic compounds (they don’t need oxygen). These were the first things on earth because there was no oxygen present. o Light driven proton pumps  bacteriolodoxin*  retinol pigment can absorb energy and generate a proton gradient that can synthesize ATP o Methanogenesis  combine carbon dioxide and oxygen  Proposed time line o Archaen: C13 or S34 depletion, some uncertainties about when oxygen was formed (presence of oxygenic photosynthesis) o Proterozoic: Cyanobacteria provides robust evidence of oxygen presence and lead to a lot of life forms  Models for early life o probiotic soup  inorganic compounds can produce organic compounds  all amino acids and nucleotides can be abiotically synthesized  with the simple input of energy monomers can be synthesized  thin atmosphere and no filtration of UV light provided earth with a lot of energy, there were a lot of inorganic compounds present  this formed a random inorganic soup. Monomers interacted with each other and formed primitive compounds like phospholipids which could have proceeded to form cells. o metabolist  TCA cycle shared among living organisms  carbon dioxide fixation is integral to living organisms  need certain elements for cell maintenance and energy generation o rna world  nucleotides required for rna  purines, pyrimidines and nucleotides can be synthesized in an abiotic environment  polymerization of nucleotides can happen spontaneously given enough energy  how can RNA act as an enzyme in catabolizing its own synthesis and its own splicing, or genetic material template in replication and transcription?  it is accepted that RNA was most likely the first genetic material  most cellular organisms use DNA, why do we have a switch
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