Chapter 6Energy and Nutrient Relations.docx

Chapter 6Energy and Nutrient Relations.docx

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Chapter6EnergyandNutrientRelations

Chapeter6EnergyandNutrientRelations

Ascorpionfishlieshalfburiedinthesandneartheedgeofacoralreef;theonlycluestoitspresencearethetelltalemovementsofitsgillcovers.Itsheadlookssomuchlikeanalgae-coveredstonethatseveraltinyshrimpgatheroveritandswimlazilyinthecurrent.Asmallfishonthenearbyreefseestheshrimpanddartsovertofeedonthem.Thescorpionfishopensitsmouthandswallowsthesmallfishinalightningquickmovement.However,beforethescorpionfishcansettlebackintothesand.agreenmorayeel,nearly2mlong,dartsfromthereef,grabsthescorpionfishwithitsrazor-sharpteeth,andswallowsit（fig.6.1）.

FIGURE6.1Themorayeelmeetsitsenergyandnutrientneedsbybeinganeffectivepredator.

Anherbaceousplantwithbroadleavesandslenderstemsgrowsinthehalflightoftherainforestfloor.Itisdifficulttounderstandhowitcanliveinsuchdimlight.However,asyouwatch,asmallshaftofintensesunlightpiercesanunseenholeintherainforestcanopyandshinesononeoftheplant’sleaves.Thephotosyntheticmachineryoftheplanttakesadvantageofthesituation,andforafewminutestheplantusestheenergyofthetinysunfleck.Nearbyisthebuttressedtrunkofagigantictreethathasgrowntallenoughtoemergefromtheforestcanopyandcountitselfamongtherainforestgiants,seeminglyamomsecurepositionthanthatoftheunderstoryherb.However,asmallvinehasbeguntogrowupthesideofthetree.Itwillgrowquicklyupward,windingitswaytowardthesunandexploitingthewoodysupportofthetree.Soonthevinewilloverwhelmandkillthetree,whichwillbereducedtoatrellisforthevine.

Whetheroncoralreef,rainforest,orabandonedurbanlot,organismsengageinanactivesearchforenergyandnutrients.Formustorganisms,lifeboilsdowntoconvertingenergyandnutrientsintodescendants.Theenergyusedbydifferentorganismscomesintheformoflight,organicmolecules,orinorganicmolecules.Nutrientsaretherawmaterialsanorganismmustacquirefromtheenvironmenttolive.Becauseorganismsacquireenergyandnutrientsindiverseways,weneedtoorganizeourdiscussionundertheumbrellaofmajorconcepts.Inchapter6,wefocusonthree.

CONCEPTS

●Organismsuseoneofthreemainsourcesofenergy:

light,organicmolecules,orinorganicmolecules.

●Therateatwhichorganismscantakeinenergyislimited.

●Optimalforagingtheoryattemptstomodelhoworganismsfeedasanoptimizingprocess.

CASEHISTORIES:

energysources

Organismsuseoneofthreemainsourcesofenergy:

light,organicmolecules,orinorganicmolecules.

     Howdowegrouporganisms?

Wegenerallygrouporganismsonthebasisofsharedevolutionaryhistories,creatingtaxasuchasvertebrateanimals,insects,coniferoustrees,andorchids.However,wecanalsoclassifyorganismsbyhowtheyobtainenergy--thatis,bytheirtrophic（feeding）biology.Organismsthatuseinorganicsourcesofbothcarbonandenergyarecalledautotrophs（"self-feeders"）andareoftwotypes,photosyntheticandchemosynthetic.Photosyntheicautotrophsusecarbondioxide（CO2）asasourceofcarbonandlightasasourceofenergy.Thisgroupincludestheplants,photosyntheticprotists,andphotosyntheticbacteria.Chemosyntheticautotrophsuseinorganicmoleculesasasourceofcarbonandenergy.Thesearemadeupofahighlydiversegroupofchemosyntheticbacteria.Heterotmphs（"other-feeders"）areorganismsthatuseorganicmoleculesbothasasourceofcarbonandasasourceofenergy.Theheterotrophsincludebacteria,fungi,protists,animals,andparasiticplants.

Bacteriashowmoretrophicdiversitythantheotherbiologicalkingdoms（fig.6.2）.Theprotistsareeitherphotosyntheticorbeterotrophic,mostplantsarephotosynthetic,andallfungiandanimalsareheterotrophic.Incontrast,thebacteriaincludephotosynthetic,chemosynthetic,andheterotrophicspecies,makingthem,asagroup,themosttrophicallydiverseorganismsinthebiosphere.

FIGURE6.2Trophicdiversityacrossthebiologicalkingdoms.

UsingLightandCO2

    Becausephotosyntheticorganismsuselightasasourceofenergy,weneedtolearnaboutlight.WealsoneedtounderstandhowphotosyntheticorganismsuseCO2.Thesearetopicsweinvestigatenext.

TheSolar-PoweredBiosphere

    Aswesawinchapters2and3,solarenergypowersthewindsandoceancurrents,andannualvariationinsunlightintensitydrivestheseasons.Inchapter4,wealsodiscussedhoworganismsusesunlighttoregulatebodytemperature.Here,buildingonthosediscussions,welookatlightasasourceofenergyforphotosynthesis.

Lightpropagatesthroughspaceasawave,withallthepropertiesofwavessuchasfrequencyandwavelength.Whenlightinteractswithmatter,however,itactsnotasawavebutasaparticle.Particlesoflight,calledphotons,bearafinitequantityofenergy.Longerwavelengths,suchasinfraredlight,carrylessenergythanshorterwavelengths,suchasvisibleandultravioletlight.

Infraredlight,aswesawinchapter4（seefig.4.13）,isveryimportantfortemperatureregulationbyorganisms.Thisisbecausethemaineffectofinfraredlightonmatteristoincreasethemotionofwholemolecules,whichwemeasureasincreasedtemperature.However,infraredlightdoesnotcarryenoughenergytodrivephotosynthesis.Attheotherendofthesolarspectrum,ultravioletlightcarriessomuchenergythatitbreaksthecovalentbondsofmanyorganicmolecules.Becauseitcanbreakdownorganicmolecules,ultravioletlightcandestroythecomplexbiochemicalmachineryofphotosynthesis.Betweentheseextremesisthelightwecansee,so-calledvisiblelight,whichisalsocalledphotosyntheticallyactiveradiation,orPAR.PAR,withwavelengthsbetweenabout400and700nm,carriessufficientenergytodrivethelight-dependentreactionsofphotosynthesisbutnotsomuchastodestroyorganicmolecules.PARmakesupabout45%ofthetotalenergycontentofthesolarspectrumatsealevel,whileinfraredlightaccountsforabout53%andultravioletlightfortheremainder.

MeasuringPAR

    EcologistsquantifyPARasphotonfluxdensity.Photontluxdensityisthenumberofphotonsstrikingasquaremetersurfaceeachsecond.Thenumberofphotonsisexpressedasmicromoles（μmol）,where1moleisAvagadro'snumberofphotons,6.023×1023.Togiveyouapointofreference,aphotonfluxdensityofabout4.6μmolpersquaremeterpersecondequalsalightintensityofabout1wattpersquaremeterMeasuringlightasphotosyntheticphotonfluxdensitymakessenseecologicallybecausechlorophyllabsorbslightasphotons.

Lightchangesinquantityandqualitywithlatitude,withtheseasons,withtheweather,andwiththetimeofday.Inaddition,landscapes,water,andevenorganismsthemselveschangetheamountandqualityoflight.Forexample,inaquaticenvironments（seechapter3）,onlythesuperficialeuphoriczonereceivessufficientlighttosupportphotosyntheticorganisms.Inaddition,lightchangesinquality,aswellasquantity,withintheeuphoticzone,whichrangesindepthfromafewmeterstoabout100m（seefig.3.7）.

Asinthesea,sunlightchangesasitshinesthroughthecanopyofaforest.Amaturetemperateortropicalforestcanreducethetotalquantityoflightreachingtheforestfloortoabout1%to2%oftheamountshiningontheforestcanopy（fig.6.3）.However,forestsalsochangethequalityofsunlight.Withintherangeofphotosyntheticallyactiveradiation,leavesabsorbmainlyblueandredlightandtransmitmostlygreenlightwithawavelengthofabout550nm.Asinthedeepsea,theorganismsontheforestfloorliveinakindoftwilight.Onlyhere,thetwilightisgreen（seefig.2.9）.

FIGURE6.3Photosyntheticallyactiveradiation（PAR）inaborealforest（datafromLarcher1995.afterKairiukstis1967）.

AlternativePhotosyntheticPathways

    Duringphotosynthesis,thephotosyntheticpigmentsofplants,algae,orbacteriaabsorblightandtransfertheirenergytoelectrons.Subsequently,theenergycarriedbytheseelectronsisusedtosynthesizeATPandNADPH.Thesemolecules,inturn,serveasdonorsofelectronsandenergyforthesynthesisofsugars.Inthisway,photosyntheticorganismsconverttheelectromagneticenergyofsunlightintoenergy-richorganicmolecules,thefuelthatfeedsmostofthebiosphere.Withinphotosyntheticorganisms,specificbiochemicalpathwayscarryoutthisenergyconversion;threedifferentbiochemicalpathwaysareknown:

C3photosynthesis,C4photosynthesis,andCAMphotosynthesis.Thesearefoundinecologicallydifferentorganisms.

Biologistsoftenspeakofphotosynthesisas"carbonfixation,''whichreferstothereactionsinwhichCO2becomesincorporatedintoacarbon-containingacid.Inthephotosyntheticpathwayusedbymostplantsandallalgae,theCO2firstcombineswithafive-carboncompoundcalledribulosebisphosphate,orRuBP.Theproductofthisinitialreaction,whichiscatalyzedbytheenzymeRuBPcarboxylase,isphosphoglycericacid,orPGA,athree-carbonacid.Therefore,thisphotosyntheticpathwayisusuallycalledC3photosynthesisandtheplantsthatemployitarecalledC3plants（fig.6.4）.

FIGURE6.4C3photosynthesis.

Tofixcarbon,plantsmustopentheirstomatatoletCO2intotheirleaves,butasCO2enters,waterexits.WatervaporflowsoutfasterthanCO2flowsin.ThemovementofwaterismorerapidbecausethegradientinwaterconcentrationfromtheleaftotheatmosphereismuchsteeperthanthegradientinCO2concentrationfromtheatmospheretotheleaf.InCO3plants,thereisanotherfactorthatcontributestoalowrateof