Chapter 10 Photosynthesis Notes

Fig. 10-2(a) Plants(c) Unicellular protist10 µm(e) Purple sulfurbacteria(b) Multicellular alga(d) Cyanobacteria1.5 µm40 µmOverview: The Process That Feeds the Biosphere Photosynthesis is the process that convertssolar energy into chemical energy Heterotrophs obtain their organic materialfrom other organisms Directly or indirectly, photosynthesis nourishesalmost the entire living world Heterotrophs are the consumers of thebiosphere Almost all heterotrophs, including humans,depend on photoautotrophs for food and O2Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsConcept 10.1: Photosynthesis converts light energyto the chemical energy of food Autotrophs sustain themselves without eatinganything derived from other organisms Chloroplasts are structurally similar to andlikely evolved from photosynthetic bacteria Autotrophs are the producers of the biosphere,producing organic molecules from CO2 andother inorganic molecules The structural organization of these cells allowsfor the chemical reactions of photosynthesis Almost all plants are photoautotrophs, usingthe energy of sunlight to make organicmolecules from H2O and CO2Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Chloroplasts: The Sites of Photosynthesis in PlantsTracking Atoms Through Photosynthesis: ScientificInquiry Leaves are the major locations ofphotosynthesis Photosynthesis can be summarized as thefollowing equation: Their green color is from chlorophyll, thegreen pigment within chloroplasts6 CO2 12 H2O Light energy C6H12O6 6 O2 6 H2O Light energy absorbed by chlorophyll drives thesynthesis of organic molecules in thechloroplast CO2 enters and O2 exits the leaf throughmicroscopic pores called stomataCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsThe Splitting of Water Chloroplasts are found mainly in cells of themesophyll, the interior tissue of the leaf A typical mesophyll cell has 30–40 chloroplasts Chloroplasts split H2O into hydrogen andoxygen, incorporating the electrons ofhydrogen into sugar molecules The chlorophyll is in the membranes ofthylakoids (connected sacs in the chloroplast);thylakoids may be stacked in columns calledgrana Chloroplasts also contain stroma, a dense fluidCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig. 10-3Fig. 10-4Leaf cross astMesophyll lakoidspaceIntermembranespaceInnermembrane1 µm5 µm6 CO2C6H12O612 H2O6 H2O6 O2

Photosynthesis as a Redox Process Photosynthesis is a redox process in whichH2O is oxidized and CO2 is reducedFig. 10-5-1H2OLightNADP ADP PiLightReactionsChloroplastCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsThe Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions(the photo part) and Calvin cycle (the synthesispart)Fig. 10-5-2H2OLightNADP ADP P The light reactions (in the thylakoids):iLightReactions– Split H2OATP– Release O2NADPH– Reduce NADP to NADPH – Generate ATP from ADP byphotophosphorylationChloroplastO2Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig. 10-5-3CO2H2O The Calvin cycle (in the stroma) forms sugarfrom CO2, using ATP and NADPHLightNADP ADP The Calvin cycle begins with carbon fixation,incorporating CO2 into organic molecules PiLightReactionsATPNADPHChloroplastO2Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCalvinCycle

Fig. 10-5-4CO2H2O The electromagnetic spectrum is the entirerange of electromagnetic energy, or radiationLightNADP ADP PiLightReactionsCalvinCycleATP Visible light consists of wavelengths (includingthose that drive photosynthesis) that producecolors we can see Light also behaves as though it consists ofdiscrete particles, called photonsNADPHChloroplastO2[CH2O](sugar)Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsConcept 10.2: The light reactions convert solarenergy to the chemical energy of ATP and NADPH Chloroplasts are solar-powered chemicalfactoriesFig. 10-610–5 nm 10–3 nm103 nm1 nmGammaX-raysraysUV106 nmInfrared1m(109 nm)Microwaves103 mRadiowaves Their thylakoids transform light energy into thechemical energy of ATP and NADPHVisible light380450500550600Shorter wavelengthHigher energy650700750 nmLonger wavelengthLower energyCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsThe Nature of SunlightPhotosynthetic Pigments: The Light Receptors Light is a form of electromagnetic energy, alsocalled electromagnetic radiation Pigments are substances that absorb visiblelight Like other electromagnetic energy, light travelsin rhythmic waves Different pigments absorb differentwavelengths Wavelength is the distance between crests ofwaves Wavelengths that are not absorbed arereflected or transmitted Wavelength determines the type ofelectromagnetic energy Leaves appear green because chlorophyllreflects and transmits green lightCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 10-7LightReflectedlight Chlorophyll a is the main photosyntheticpigmentChloroplast Accessory pigments, such as chlorophyll b,broaden the spectrum used for photosynthesisAbsorbedlight Accessory pigments called carotenoidsabsorb excessive light that would damagechlorophyllGranumTransmittedlightCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsExcitation of Chlorophyll by Light 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 workbest for photosynthesis An action spectrum profiles the relativeeffectiveness of different wavelengths ofradiation in driving a processCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings When a pigment absorbs light, it goes from aground state to an excited state, which isunstable 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 heatCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig. 10-11Chlorophyll aCarotenoidse–400(a) Absorption spectraChlorophyll b500600700(b) Action spectrumRate of photosynthesis(measured by O2 release)Wavelength of light (nm)Energy of electronRESULTSAbsorption of light bychloroplast pigmentsFig. rophyllmoleculeGroundstateAerobic bacteriaFilamentof alga(c) Engelmann’sexperiment400500(a) Excitation of isolated chlorophyll molecule600700(b) Fluorescence

A Photosystem: A Reaction-Center ComplexAssociated with Light-Harvesting Complexes A photosystem consists of a reaction-centercomplex (a type of protein complex)surrounded by light-harvesting complexes The light-harvesting complexes (pigmentmolecules bound to proteins) funnel the energyof photons to the reaction center There are two types of photosystems in thethylakoid membrane Photosystem II (PS II) functions first (thenumbers reflect order of discovery) and is best atabsorbing a wavelength of 680 nm The reaction-center chlorophyll a of PS II iscalled P680Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings A primary electron acceptor in the reactioncenter accepts an excited electron fromchlorophyll a Photosystem I (PS I) is best at absorbing awavelength of 700 nm Solar-powered transfer of an electron from achlorophyll a molecule to the primary electronacceptor is the first step of the light reactionsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig. 10-12PhotosystemSTROMALight-harvesting ptorThylakoid membranePhotonSpecial pair ofchlorophyll amoleculesLinear Electron Flow During the light reactions, there are twopossible routes for electron flow: cyclic andlinear Linear electron flow, the primary pathway,involves both photosystems and produces ATPand NADPH using light energye–Transferof energy The reaction-center chlorophyll a of PS I iscalled P700PigmentmoleculesTHYLAKOID SPACE(INTERIOR OF THYLAKOID)Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 10-13-2 A photon hits a pigment and its energy ispassed among pigment molecules until itexcites P680Primaryacceptor22 H /2 O21 An excited electron from P680 is transferred tothe primary electron acceptorH2Oe–3e–e–P6801 LightPigmentmoleculesPhotosystem II(PS II)Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig. 10-13-1 Each electron “falls” down an electron transportchain from the primary electron acceptor of PSII to PS IPrimaryacceptor2e– Energy released by the fall drives the creationof a proton gradient across the thylakoidmembraneP6801 LightPigmentmoleculesPhotosystem II(PS II) Diffusion of H (protons) across the membranedrives ATP synthesisCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig. 10-13-3 P680 (P680 that is missing an electron) is avery strong oxidizing agent H2O is split by enzymes, and the electrons aretransferred from the hydrogen atoms to P680 ,thus reducing it to P680ElePrimaryacceptor22 H 1/2 O2H2Oe–Pqctro4n transportc3Pce–e–5P6801 LightATP O2 is released as a by-product of this reactionPigmentmoleculesPhotosystem II(PS II)Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingshainCytochromecomplex

Fig. 10-13-5 In PS I (like PS II), transferred light energyexcites P700, which loses an electron to anelectron acceptorPq2H2O2 H /2 O2ctro4n transportc–e7Fde–haine–Cytochromecomplex31 P700 (P700 that is missing an electron)accepts an electron passed down from PS IIvia the electron transport chainElePrimaryacceptorEtra lectn roch spo nain rtPrimaryacceptor8e–NADP reductasePce–e–NADP H NADPHP7005P680Light1 Light6ATPPigmentmoleculesPhotosystem I(PS I)Photosystem II(PS II)Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig. 10-13-4Cyclic Electron FlowElePrimaryacceptor22 H 1/2 O2H2Oe–Pqctro4n transportc Cyclic electron flow uses only photosystem Iand produces ATP, but not ��e–P7005P680Light1 Light6 Cyclic electron flow generates surplus ATP,satisfying the higher demand in the CalvincycleATPPigmentmoleculesPhotosystem II(PS II)Photosystem I(PS I)Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig. 10-15 Each electron “falls” down an electron transportchain from the primary electron acceptor of PSI to the protein ferredoxin (Fd)PrimaryacceptorCopyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFdPq The electrons are then transferred to NADP and reduce it to NADPH The electrons of NADPH are available for thereactions of the Calvin cyclePrimaryacceptorFdNADP reductaseCytochromecomplexNADPHPcPhotosystem IPhotosystem IIATPNADP H