IB Biology - PowerPoint Presentation

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IB BiologyOption DD1 Origin of Life on EarthAll syllabus statements ©IBO 2007All images CC or public domain or link to original material,Jason de NysAISHK

http://www.flickr.com/photos/euthman/216030298/Slide2

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 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 possibleDNA can’t self replicate, it needs protein enzymesHowever 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 HypothesisThe packaging of these molecules into membranes with internal chemistry different from their surroundingsThe formation of closed membranes an important stepClosed membrane vesicles can form spontaneouslyfrom lipids.This allowed differentiation between the internal and external environments

http://exploringorigins.org/resources.htmlSlide3

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

All molecules public domain from Wikimedia Commons, Background image http://www.flickr.com/photos/lrargerich/4587244190/Can you identify these molecules?Slide4

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: HydrogenNitrogenWater vapourMethaneAmmoniaHydrogen sulfide

All molecules public domain from Wikimedia Commons, Background image

http://www.flickr.com/photos/lrargerich/4587244190/

The gases came from abundant volcanic activitySlide5

These monomers mixed in the ‘primeval soup’, shallow oceans laden with chemicals where it is thought that they reacted to form biological moleculesMiller 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 soupThey 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)http://www.flickr.com/photos/afeman/663646181/Slide6

Carny http://upload.wikimedia.org/wikipedia/commons/5/59/MUexperiment.pngSlide7

After a week they found:Thirteen of the twenty naturally occurring amino acidsAround 15% of the carbon was now in organic compoundsSlide8

http://www.flickr.com/photos/12057715@N00/354536849/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 bacteriaExisting 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 Antarctica

It is feasible that they could survive on or in a cometSpace is so empty, yet full of the potential for lifeSlide9

Cosmic 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 peptides

The bombardment of Earth by comets 4 billion years ago could have ‘kick started’ chemical evolution

http://www.flickr.com/photos/jpstanley/2030855518/Slide10

D.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

http://www.flickr.com/photos/noaaphotolib/5014975047/sizes/l/in/photostream/Slide11

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 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 geysers

may have

provided a suitable location for the formation of biological compoundsThe hypothesis that life originated on Earth is called abiogenesis (ab bio genesis)(aboriginal – life – creation)http://www.flickr.com/photos/storm-crypt/3043902298/Slide12

The hypothesis that life came an extraterrestrial source:http://images.cdn.fotopedia.com/flickr-2406913018-hd.jpgAs 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 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 confirmedThe extraterrestrial hypothesis still doesn’t address how life formed, just how it could move around the galaxySlide13

D.1.5 Outline Two properties of RNA that would have allowed it to play a role in the origin of lifehttp://genetics.mgh.harvard.edu/szostakweb/exploringOriginsDownloads/centralDogma.jpgRNAs 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 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 ribosome seems to have a purely structural functionEvolution by natural selection requires variation and heritability. RNA possesses these traitsSlide14

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 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 dropletsThis is not true reproduction though so they are not alive.http://exploringorigins.org/protocells.htmlAn illustration of a protocell, composed of a fatty acid membrane encapsulating RNA ribozymes.Slide15

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 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)http://www.daviddarling.info/encyclopedia/M/microsphere.htmlOne milligram of thermal proteins can make 100 million microspheres!

They divide like coacervates

and can catalyse some reactionsSlide16

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 atmosphereThe 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.

The 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’ moleculesAfter 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 Slide17

D.1.8 Discuss the endosymbiotic theory for the origin of eukaryotesEndosymbiosis 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

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 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.Slide18

Further information: