Biology30:UnderstandingLifeResearchAssignmentANSWERSContributionsofscientiststounderstandingoflife:

1. FrancescoRedi(Italian:1626-1697)a. Oftenreferredtoasthefounderofexperimentalbiology(made

statementsbasedonobservationsandexperiments–notrandomguessing)

b. Challengedtheideaofspontaneousgeneration(theideathatlifecoulddevelopfrominanimateobjects–likedust)

c. Hedidthisbyshowingthatmaggotsdidnotspontaneouslygenerate–insteadtheycamefromtheeggsofflies

d. Hisfirstpaperwasaboutviperswherehedispelledanumberofpopularmythsbydoingexperiments.Heprovedthat:

i. Vipersdidnotdrinkwineii. Vipersdidnotshatterglassesiii. Vipervenomisnotpoisonousifswallowediv. Aviperheadisnotanantidotev. Theviper’svenomisnotproducedinthegallbladder

e. Continuedthismethodofexperimentationtodisprovespontaneousgeneration.Theideapopularatthetimewasthatmaggotsarosespontaneouslyfromrottingmeat.Heusedexperimentswithmeatwhereheputitinjars,somecoveredandsomeuncovered.Thecoveredjars(wherefliescouldnotland)producednomaggots,whereastheuncoveredmeat(wherefliescouldland)producedmaggots.Redithencapturedthemaggots,allowedthemtomature,anddiscoveredthattheydevelopedintoflies.Healsoshowedthatwhendeadflieswereputinsealedjarswithmeat,nomaggotsformed,butwhenthiswasdonewithliveflies,maggotsdidform.

2. LouisPasteur(French:1822-1895)a. Mademanycontributionstoscience,includingvaccinations(forrabiesand

anthrax),pasteurization(heatmilkandwinetokillbacteriasothatmilk(etc.)canbestoredforlonger),andprovidedevidenceinsupportofgermtheoryofdiseasetransmission.Alsoprovidedfurtherevidenceagainstspontaneousgeneration.

i. Insterilizedandsealedflasks,nothinggrew.Whileinsterilizedbyopenedflasks,microorganismsdeveloped.

b. Thedominanttheoryofdiseasetransmission(cholera,chlamydia,BlackDeath)inthe19thcenturywasthroughmiasma(akamiasmictheory).Themiasmictheoryproposedthatthesediseaseswerecausedbybreathinginbadair,knownasnightair.Somescholarsatthetimeextendedthisideaandthoughtthatyoucouldgetfatbybreathinginfoododours.

c. Pasteurwasinstrumentalindisprovingthemiasmictheory,andinsteadprovidingevidenceforthegermtheoryofdisease(firstproposedin1546byGirolamoFracastoro).Thegermtheoryheldthatdiseasewerecausedbygerms(microorganismslikebacteria,protists,fungi,viruses,prions,viroids,etc.),andthisiswhatspreadandmadetheminfectious.

i. Provedthatfermentationiscausedbythegrowthofmicroorganisms–thissupportedbiogenesis(lifecomesfromlife).Didthisdoingexperimentswithmilkandwine,respectively.

ii. Alsoprovedthatdiseasestosilkwormswerecausedbymicrobesattackingsilkwormeggs,andnotbadair.

3. StanleyMiller(American:1930-2007)andHaroldUrey(American:1893-1981)a. MillerwasagraduatestudentofUreyattheUniversityofChicago,

whodiscovereddeuteriumandhadplayedaroleinthedevelopmentoftheatomicbomb.

b. ThepairarefamousfortheMiller-Ureyexperiment(1952)inwhichtheyshowedthatfrominorganicmaterial(materialthatisnotfromlivingmatter)itwaspossibletocreateorganiccompounds,underconditionssimilartothosethoughttobepresentwhentheEarthdeveloped.ThissupportedtheideathatlifecouldhavebeencreatedonEarththroughsomeothermethodthandivineintervention,andthatevolutionaryprocesseshaveledtothedevelopmentoflifeasweknowit.

c. TheexperimentprovidedproofforthePrimordialSoupTheory(lifearosefromnon-livingmatterbetween3.8and4.1billionyearsago).

d. Intheexperiment,MillerattemptedtorecreatetheatmosphereofEarthbillionsofyearsagobymixingtogethermethane,ammonia,andhydrogen.Thegaseousmixturethenhadanelectricaldischargerunthroughit.Afterthisreactionwasrunforaweek,Millerdetectedaminoacidsinthemixture.Aminoacidsarethebasicstructuralandfunctionalunitsofcellularlife–thereforetheconclusionwasthatlifecouldhavedevelopedonEarthinaprocessverysimilartothis.

e. Theexperimentwasrepeatedin1972.33aminoacidswereproduced(10ofwhichareknowntonaturallyoccurinorganisms).

4. LynnMargulis(American:1938-2011)a. Providedevidenceandwasamajorproponentofsymbiogenesis

(theorythateukaryoticcellsdevelopedfromasymbiosis(combining)ofsingle-celledprokaryoticcells.

b. Theevolutionarytheoryofsymbiogenesisstatesthatcellorganelleslocatedineukaryoticcells(suchasmitochondria,plastids,andchloroplasts)areformerlyfree-livingprokaryoticcellsthatweretakeninsideadifferentprokaryoticcell.Thetheoryisthatthisadaptationhappenedapproximately1.5billionyearsagobetweenbacteria.(Note:symbiosisisarelationshipbetweentwoorganismsthatisbeneficialtoboth.)

c. Margulis’scelltheorythatproposedthatorganelleslikemitochondriawereonceindependentbacteriawasignoredfordecadesbeforeitwassubstantiallybackedupthroughgeneticevidence.

i. ItwasfoundthatthegeneticmaterialofmitochondriaandchloroplastswassubstantiallydifferentthantheDNAinthenucleusofacell.

d. Margulisalsoarguedthatsymbiosisbetweenorganismsofoftendifferentphylaorkingdoms(notnaturalselection)isthedrivingforcebehindevolutionandgeneticvariations.Itisproposedthatgeneticvariationoccursbecauseofthetransferofnuclearmaterialbetweenbacteria,viruses,andeukaryoticcells.Thisisnotcurrentlyawidelyacceptedtheorytoexplaingeneticvariation.ThisisincontracttotheideaofNeo-Darwinismwheregeneticvariationhappensbecauseoftheslowaccrualofmutationsthatgiveanadvantageduringtheprocessofnaturalselection.

AdvancesinMicroscopy/CellTheoryAdvancesinourabilitytoseesmallobjectswithgreaterclarityhasledtoincreasedknowledgeofwhathappensatthecellularlevel.

• RobertHooke(1635-1703):In1665RobertHookepublishedapapercalledMicrographia;init,Hookeiscreditedwithfirstusingtheterm“cells.”Hookewasusingacrudemicroscopetoviewcork,andnotedthatthereappearedtobebox-likestructuresthat

weresimilartothesleepingquartersofmonks,knownascells.Hookeborrowedthetermtodescribethesestructuresofthecork.Hookedidnotviewanyoftheinternalstructuresofthecell;hesimplyviewedtheouterstructure.

• AntonyVanLeeuwenhoek(1632-1723)wasabletocreateamicroscopethathada

magnificationofatleast250x.Compoundmicroscopesavailableduringthesametimeperiodweresodistortedthattheycouldonlyproduceimages30xto40xbigger.ThisallowedLeeuwenhoektobethefirsttoobserveanddescribesingle-celledorganisms(bacteria/microorganisms)in1674.

• MatthiasJakobSchleiden(1804-1881):In1838Schleidenwasdoingworkobservingplant

tissueandstatedthatallplantsweremadeupofcells,anddeclaredthatcellswerethebasicbuildingblocksofallplantmatter.

• TheodorSchwann(1810-1882):In1839Schwanncomesupwiththesameconclusionabout

animaltissuesasSchleidendidforplanttissues–thatisthatthebasicunitofanimaltissuewasalsothecell.Fromthisobservation,Schwannstatedthatcellsarethebasicunitoflife.

• RudolfVirchow(1821-1902):In1855Virchownotedthatcellsdevelopfromothercells.This

addedtothecelltheorydevelopedbySchleidenandSchwann.Virchowalsonotedthatdiseasedcellsoriginatedfromhealthycellsofnormaltissue.

• TheobservationsofSchleiden,Schwann,andVirchowcametobeorganizedintoTheCellTheory.This

theorystates:1. Alllivingorganismsarecomposedofoneormorecells.2. Thecellisthebasicunitofstructureandorganizationoforganisms.3. Cellsarisefrompre-existingcells.

• Thedevelopmentofthecelltheorydisprovedideasofhowlifeformed,suchasspontaneousgeneration.• Furtheradvancesinmicroscopy(suchasthedevelopmentoftheelectronmicroscope)haveallowed

scientiststoviewcellularorganellesthatwerenotvisiblebefore,suchas:mitochondria,ribosomes,endoplasmicreticulum,golgiapparatus,andlysomes.