2The non-living synthesis of simple organic molecules D.1.1 Describe four processes needed for the spontaneous origin of life on EarthThe non-living synthesis of simple organic moleculesObviously if nothing was alive yet then the source of these molecules had to be abioticWe can presume that the early Earth had all of the base elements and compounds requiredThey were somehow combined to make simple organic compoundsMaybe the organic compounds were generated here, maybe they were extra-terrestrial!The assembly of these molecules into polymersIt makes sense, to make the larger molecules necessary for life, the simple organic compounds would have to polymeriseThe origin of self-replicating molecules made inheritance possibleDNA can’t self replicate, it needs protein enzymesHowever some RNA can self-replicate, it can catalysethe formation of copies of itself.They are called Ribozymes and are the basis of theRNA World HypothesisThe packaging of these molecules into membraneswith internal chemistry different from theirsurroundingsThe formation of closed membranes an important stepClosed membrane vesicles can form spontaneouslyfrom lipids.This allowed differentiation between the internal and external environments
3Can you identify these molecules? D.1.2 Outline the experiments of Miller and Urey into the origin of organic compoundsEarth’s atmosphere was ‘reducing’ in the early days. It did not contain oxygen gas until after plants started photosynthesisingCan you identify these molecules?All molecules public domain from Wikimedia Commons, Background image
4The atmosphere contained: Hydrogen Nitrogen Water vapour Methane D.1.2 Outline the experiments of Miller and Urey into the origin of organic compoundsEarth’s atmosphere was ‘reducing’ in the early days. It did not contain oxygen gas until after plants started photosynthesisingThe atmosphere contained: HydrogenNitrogenWater vapourMethaneAmmoniaHydrogen sulfideThe gases came from abundant volcanic activityAll molecules public domain from Wikimedia Commons, Background image
5Miller and Urey tried to recreate these conditions in the lab in 1953 These monomers mixed in the ‘primeval soup’, shallow oceans laden with chemicalswhere it is thought that they reacted to form biological moleculesMiller and Urey tried to recreate these conditions in the lab in 1953They were trying to demonstrate ‘chemical evolution’, the formation of more complexmolecules from simpler stock in the primeval soupThey combined the molecules from the previous page in a closed glass vessel(simulated atmosphere), they heated the water (simulated volcanic activity) andsparked electricity through the gases (simulated lightning)
7After a week they found: Thirteen of the twenty naturally occurring amino acidsAround 15% of the carbon was now in organic compounds
8Space is so empty, yet full of the potential for life D.1.3 State that comets may have delivered organic compounds to EarthPanspermia is the hypothesis that life on Earth originated from material delivered by a comet, either in the form of amino acids or as hardy bacteriaSpace is so empty, yet full of the potential for lifeExisting bacteria and archaebacteria have been found in odd and extreme environments on Earth:In hot springs, kilometres deep in the crust and even embedded in ice cores from deep inside AntarcticaIt is feasible that they could survive on or in a comet
9Cosmic radiation could provide the energy for reactions that lead to the formation of complex organic moleculesAnalysis of the spectra of light coming from the comets reveals the presence of hydrocarbons, amino acids and peptidesThe bombardment of Earth by comets 4 billion years ago could have ‘kick started’ chemical evolution
10D.1.3 Discuss possible locations where conditions could have allowed the synthesis of organic compoundsProblem: 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 polymerisationSolution: “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
11Volcanoes may also have played a part: Gases from above hot lava lakes have been found to contain a higher than average level of fixed nitrogenNitrogen 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 geysersmay have provided a suitable location for the formation of biological compoundsThe hypothesis that life originatedon Earth is called abiogenesis(ab bio genesis)(aboriginal – life – creation)
12The hypothesis that life came an extraterrestrial source: As previously mentioned, organic molecules are out thereMars is smaller than Earth and therefore cooled down more quickly, life could have begun there while Earth was still scorchingMeteorites and comets impacting on mars could have thrown up debris withearly life attached, this could then have crashed on Earth.Meteorites of Mars origin have been found in AntarcticaThere is no evidence that life has been transferred in this way. Every now and thenthere is a news story about “Fossils found in Mars meteorite”but so far this has not been confirmedThe extraterrestrial hypothesis still doesn’t address how life formed,just how it could move around the galaxy
13Ribozymes are RNA molecules that can catalyse reactions D.1.5 Outline Two properties of RNA that would have allowed it to play a role in the origin of lifeRNAs can store, transmit and replicate genetic InformationRibozymes are RNA molecules that can catalyse reactions(Hey! You told us that all enzymes are proteins! Liar!)Some can polymerise nucleotides using ATPSome can break chemical bonds, including peptide bondsRibosomes are themselves Ribozymes (huh?).The part that catalyses the peptide bonds is RNA, the protein part of a ribosomeseems to have a purely structural functionEvolution by natural selection requires variation and heritability. RNA possesses these traits
14Coacervates are droplets of polymeric molecules. 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)Coacervates are droplets of polymeric molecules.Coacervates containing enzyes can absorband concentrate substrate moleculesand then release the products totheir surroundsIf they absorb a lot of material theycan divide into two smallercoacervate dropletsThis is not true reproduction thoughso they are not alive.An illustration of a protocell, composed of a fatty acid membrane encapsulating RNA ribozymes.
15One milligram of thermal proteins can make 100 million microspheres! Protobionts may have arisen from coacervates.Coacervates containing RNA may have started synthesising proteinsEnzyme controlled binary fission may have arisen.The first true cells probably heterotrophic (maybe getting energy fromsulfur chemistry) and anaerobic (there was no free oxygen)Microspheres: are another candidatefor a structure that might have givenrise to protobionts.They form when amino acids areheated and polymerise to formsimple proteins (thermal proteins)One milligram of thermal proteins can make 100 million microspheres!They divide like coacervatesand can catalyse some reactions
16Remember: there was little free oxygen in the early atmosphere D.1.7 Outline the contribution of prokaryotes to the creation of an oxygen-rich atmosphereRemember: there was little free oxygen in the early atmosphereSmall amounts were made by UV light splitting water vapour in the atmosphereAfter 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 photosynthesisThe increase in Oxygen led to:The breakdown of the chemicals in the ‘chemical soup’ to carbon dioxide and oxidised sedimentsThe formation of the ozone layer, which blocked out UV and stopped the production of more of the ‘soupy’ moleculesThe oxygen concentration rose to 0.45% of the atmosphereNot much compared to today’s 21%, but it coincides with the rise of the EukaryotesCOINCIDENCE? Probably not.
17New organelles are made by a process that resembles binary fission D.1.8 Discuss the endosymbiotic theory for the origin of eukaryotesEvidence in support:Mitochondria and Chloroplasts have their own DNA that is more like bacterial DNA than what is found in the nucleusThe structure and biochemistry of chloroplasts is similar to cyanobacteriaNew organelles are made by a process that resembles binary fissionBoth organelles have a double membrane which resembles the structure of prokaryotic cellsTheir ribosomes resemble those of bacteria (70S)DNA analysis suggests that some DNA in plant nuclei was previously in the chloroplastSome 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