LE 10-2PlantsUnicellular protistMulticellular algae CyanobacteriaPurple sulfurbacteria10 µm1.5 µm40 µmLE 10-3Leaf cross sectionVeinMesophyllStomataCO2O2Mesophyll cellChloroplast5 µmOutermembraneIntermembranespaceInnermembraneThylakoidspaceThylakoidGranumStroma1 µmCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsTracking Atoms Through Photosynthesis:Scientific Inquiry• Photosynthesis can be summarized as thefollowing equation:6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2 OCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Splitting of Water• Chloroplasts split water into hydrogen and oxygen,incorporating the electrons of hydrogen into sugarmoleculesLE 10-4Reactants:Products:6 CO212 H2OC6H12O66 H2O 6 O2Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsPhotosynthesis as a Redox Process• Photosynthesis is a redox process in which wateris oxidized and carbon dioxide is reducedCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Two Stages of Photosynthesis: A Preview• Photosynthesis consists of the light reactions (thephoto part) and Calvin cycle (the synthesis part)• The light reactions (in the thylakoids) split water,release O2, produce ATP, and form NADPH• The Calvin cycle (in the stroma) forms sugar fromCO2, using ATP and NADPH• The Calvin cycle begins with carbon fixation,incorporating CO2 into organic moleculesLE 10-5_1H2OLIGHTREACTIONSChloroplastLightLE 10-5_2H2OLIGHTREACTIONSChloroplastLightATPNADPHO2LE 10-5_3H2OLIGHTREACTIONSChloroplastLightATPNADPHO2NADP+CO2ADPP+iCALVINCYCLE[CH2O](sugar)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 10.2: The light reactions convert solarenergy to the chemical energy of ATP and NADPH• Chloroplasts are solar-powered chemical factories• Their thylakoids transform light energy into thechemical energy of ATP and NADPHCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Nature of Sunlight• Light is a form of electromagnetic energy, alsocalled electromagnetic radiation• Like other electromagnetic energy, light travels inrhythmic waves• Wavelength = distance between crests of waves• Wavelength determines the type ofelectromagnetic energy• Light also behaves as though it consists ofdiscrete particles, called photonsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• The electromagnetic spectrum is the entire rangeof electromagnetic energy, or radiation• Visible light consists of colors we can see,including wavelengths that drive photosynthesisLE 10-6Visible lightGammaraysX-raysUVInfraredMicro-wavesRadiowaves10–5 nm10–3 nm1 nm103 nm 106 nm1 m(109 nm)103 m380450500 550 600650700750 nmLonger wavelengthLower energyShorter wavelengthHigher energyCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsPhotosynthetic Pigments: The Light Receptors• Pigments are substances that absorb visible light• Different pigments absorb different wavelengths• Wavelengths that are not absorbed are reflectedor transmitted• Leaves appear green because chlorophyll reflectsand transmits green lightLE 10-7ChloroplastLightReflected lightAbsorbed lightTransmitted lightGranumCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• A spectrophotometer measures a pigment’s abilityto absorb various wavelengths• This machine sends light through pigments andmeasures the fraction of light transmitted at eachwavelengthLE 10-8aWhitelightRefractingprismChlorophyllsolutionPhotoelectrictubeGalvanometerThe high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.GreenlightSlit moves topass lightof selectedwavelength0100LE 10-8bWhitelightRefractingprismChlorophyllsolutionPhotoelectrictubeThe low transmittance(high absorption)reading indicates thatchlorophyll absorbsmost blue light.BluelightSlit moves topass lightof selectedwavelength0100Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Below is an absorption spectrum for an unknownpigment molecule. What color would this pigmentappear to you?– A. violet– B. blue– C.green– D.yellow– E. redCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• An absorption spectrum is a graph plotting apigment’s light absorption versus wavelength• The absorption spectrum of chlorophyll asuggests that violet-blue and red light work bestfor photosynthesis• An action spectrum profiles the relativeeffectiveness of different wavelengths of radiationin driving a processLE 10-9aChlorophyll aChlorophyll bCarotenoidsWavelength of light (nm)Absorption spectraAbsorption of light bychloroplast pigments400500 600700LE 10-9bAction spectrumRate of photo-synthesis (measuredby O2 release)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• The action spectrum of photosynthesis was firstdemonstrated in 1883 by Thomas Engelmann• In his experiment, he exposed different segmentsof a filamentous alga to different wavelengths• Areas receiving wavelengths favorable tophotosynthesis produced excess O2• He used aerobic bacteria clustered along the algaas a measure of O2 productionLE 10-9cEngelmann’s experiment400500 600 700Aerobic bacteriaFilament of algaeCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Chlorophyll a is the main photosynthetic pigment• Accessory pigments, such as chlorophyll b,broaden the spectrum used for photosynthesis• Accessory pigments called carotenoids absorbexcessive light that would damage chlorophyllLE 10-10CH3CHOin chlorophyll ain chlorophyll bPorphyrin ring:light-absorbing“head” ofmolecule; notemagnesium atomat centerHydrocarbon tail:interacts withhydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshownCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsExcitation of Chlorophyll by Light• When a pigment absorbs light, it goes from aground state to an excited state, which is unstable• When excited electrons fall back to the groundstate, photons are given off, an afterglow calledfluorescence• If illuminated, an isolated solution of chlorophyllwill fluoresce, giving off light and heatLE 10-11ExcitedstateHeatPhoton(fluorescence)GroundstateChlorophyllmoleculePhotonExcitation of isolated chlorophyll moleculeFluorescenceEnergy of electrone–Copyright ©