IB Biology Option D.1: Origin of life
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IB Biology Option D.1: Origin of life



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Learning support material for Option D.1 Origin of Life



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IB Biology Option D.1: Origin of life IB Biology Option D.1: Origin of life Presentation Transcript

  • IB Biology
    Option D
    D1 Origin of Life on Earth
    All syllabus statements ©IBO 2007
    All images CC or public domain or link to original material,
    Jason de Nys
  • D.1.1 Describe four processes needed for the spontaneous origin of life on Earth
    The non-living synthesis of simple organic molecules
    • Obviously if nothing was alive yet then the source of these molecules had to be abiotic
    • We can presume that the early Earth had all of the base elements and compounds required
    • They were somehow combined to make simple organic compounds
    • Maybe the organic compounds were generated here, maybe they were extra-terrestrial!
    The assembly of these molecules into polymers
    • It makes sense, to make the larger molecules necessary for life, the simple organic compounds would have to polymerise
    The origin of self-replicating molecules made inheritance possible
    • DNA can’t self replicate, it needs protein enzymes
    • However some RNA can self-replicate, it can catalyse
    the formation of copies of itself.
    • They are called Ribozymes and are the basis of the
    RNA World Hypothesis
    The packaging of these molecules into membranes
    with internal chemistry different from their
    • The formation of closed membranes an important step
    • Closed membrane vesicles can form spontaneously
    from lipids.
    • This allowed differentiation between the internal and external environments
  • D.1.2 Outline the experiments of Miller and Urey into the origin of organic compounds
    Earth’s atmosphere was ‘reducing’ in the early days. It did not contain oxygen gas until after plants started photosynthesising
    Can you identify these molecules?
    All molecules public domain from Wikimedia Commons, Background image http://www.flickr.com/photos/lrargerich/4587244190/
  • D.1.2 Outline the experiments of Miller and Urey into the origin of organic compounds
    Earth’s atmosphere was ‘reducing’ in the early days. It did not contain oxygen gas until after plants started photosynthesising
    The atmosphere contained: Hydrogen
    Water vapour
    Hydrogen sulfide
    The gases came from abundant volcanic activity
    All molecules public domain from Wikimedia Commons, Background image http://www.flickr.com/photos/lrargerich/4587244190/
  • These monomers mixed in the ‘primeval soup’, shallow oceans laden with chemicals
    where it is thought that they reacted to form biological molecules
    Miller and Urey tried to recreate these conditions in the lab in 1953
    They were trying to demonstrate ‘chemical evolution’, the formation of more complex
    molecules from simpler stock in the primeval soup
    They combined the molecules from the previous page in a closed glass vessel
    (simulated atmosphere), they heated the water (simulated volcanic activity) and
    sparked electricity through the gases (simulated lightning)
  • Carnyhttp://upload.wikimedia.org/wikipedia/commons/5/59/MUexperiment.png
  • After a week they found:
    Thirteen of the twenty naturally occurring amino acids
    Around 15% of the carbon was now in organic compounds
  • D.1.3 State that comets may have delivered organic compounds to Earth
    Panspermia is the hypothesis that life on Earth originated from material delivered by a comet, either in the form of amino acids or as hardy bacteria
    Space is so empty, yet full of the potential for life
    Existing bacteria and archaebacteria have beenfound in odd and extreme environments on Earth:
    In hot springs, kilometres deep in the crust and even embedded in ice cores from deep inside Antarctica
    It is feasible that they could survive on or in a comet
  • Cosmic radiation could provide the energy for reactions that lead to the formation of complex organic molecules
    Analysis of the spectra of light coming from the comets reveals the presence of hydrocarbons, amino acids and peptides
    The bombardment of Earth by comets 4 billion years ago could have ‘kick started’ chemical evolution
  • D.1.3 Discuss possible locations where conditions could have allowed the synthesis of organic compounds
    Problem: The water in the Miller Urey experiment tends to hydrolyse any polymers as they form and prevents their formation. The conditions in the ocean not ideal for polymerisation
    Solution: “black smokers”, hydrothermal vents where superheated steam escapes from within the crust.
    The outflow is full of dissolved sulfides that crystallise around the vent and may be a suitable environment for the formation and concentration of complex biological compounds
  • Volcanoes may also have played a part:
    Gases from above hot lava lakes have been found to contain a higher than average level of fixed nitrogen
    Nitrogen fixation is the formation of ammonia (NH4) from nitrogen gas (N2).
    The Haber process is a modern industrial way to fix nitrogen and it requires high pressures (200 atm) and high temperatures (400 °C)
    Volcanoes and geysers
    may have provided a suitable location for the formation of biological compounds
    The hypothesis that life originated
    on Earth is called abiogenesis
    (ab bio genesis)
    (aboriginal – life – creation)
  • The hypothesis that life came an extraterrestrial source:
    As previously mentioned, organic molecules are out there
    Mars is smaller than Earth and therefore cooled down more quickly, life could have begun there while Earth was still scorching
    Meteorites and comets impacting on mars could have thrown up debris with
    early life attached, this could then have crashed on Earth.
    Meteorites of Mars origin have been found in Antarctica
    There is no evidence that life has been transferred in this way. Every now and then
    there is a news story about “Fossils found in Mars meteorite”
    but so far this has not been confirmed
    The extraterrestrial hypothesis still doesn’t address how life formed,
    just how it could move around the galaxy
  • D.1.5 Outline Two properties of RNA that would have allowed it to play a role in the origin of life
    RNAs can store, transmit and replicate genetic Information
    Ribozymes are RNA molecules that can catalyse reactions
    (Hey! You told us that all enzymes are proteins! Liar!)
    Some can polymerise nucleotides using ATP
    Some can break chemical bonds, including peptide bonds
    Ribosomes are themselves Ribozymes (huh?).
    The part that catalyses the peptide bonds is RNA, the protein part of a ribosome
    seems to have a purely structural function
    Evolution by natural selection requires variation and heritability. RNA possesses these traits
  • D.1.6 State that living cells may have been preceded by protobionts,
    with an internal chemical environment different from their surroundings
    (Proto = first, or precursor)
    Coacervatesare droplets of polymeric molecules.
    Coacervates containing enzyes can absorb
    and concentrate substrate molecules
    and then release the products to
    their surrounds
    If they absorb a lot of material they
    can divide into two smaller
    coacervate droplets
    This is not true reproduction though
    so they are not alive.
    An illustration of a protocell, composed of a fatty acid membrane encapsulating RNA ribozymes.
    • Protobionts may have arisen from coacervates.
    • Coacervates containing RNA may have started synthesising proteins
    • Enzyme controlled binary fission may have arisen.
    • The first true cells probably heterotrophic (maybe getting energy from
    sulfur chemistry) and anaerobic (there was no free oxygen)
    Microspheres:are another candidate
    for a structure that might have given
    rise to protobionts.
    They form when amino acids are
    heated and polymerise to form
    simple proteins (thermal proteins)
    One milligram of thermal proteins can make 100 million microspheres!
    They divide like coacervates
    and can catalyse some reactions
  • D.1.7 Outline the contribution of prokaryotes to the creation of an oxygen-rich atmosphere
    Remember: there was little free oxygen in the early atmosphere
    Small amounts were made by UV light splitting water vapour in the atmosphere
    After about 2 billion years of prokaryote life (2 billion years ago) there was an Earth changing event: a form of chlorophyll appeared in bacteria that allowed oxygenic photosynthesis
    The increase in Oxygen led to:
    • The breakdown of the chemicals in the ‘chemical soup’ to carbon dioxide and oxidised sediments
    • The formation of the ozone layer, which blocked out UV and stopped the production of more of the ‘soupy’ molecules
    The oxygen concentration rose to 0.45% of the atmosphere
    Not much compared to today’s 21%, but it coincides with the rise of the Eukaryotes
    COINCIDENCE? Probably not.
  • D.1.8 Discuss theendosymbiotictheory for the origin of eukaryotes
    Evidence in support:
    Mitochondria and Chloroplasts have their own DNA that is more like bacterial DNA than what is found in the nucleus
    The structure and biochemistry of chloroplasts is similar to cyanobacteria
    New organelles are made by a process that resembles binary fission
    Both organelles have a double membrane which resembles the structure of prokaryotic cells
    Their ribosomes resemble those of bacteria (70S)
    DNA analysis suggests that some DNA in plant nuclei was previously in the chloroplast
    Some proteins coded for in the nucleus are transported to the organelles. The organelles have lost the DNA to make it themselves.
    Endosymbiosis is the theory that chloroplasts and mitochondria were once free-living prokaryotes that were engulfed by larger prokaryotes and survived to evolve into the modern organelles
  • Further information: