Photosynthesis: Capturing Energy Photosynthesis: Capturing Energy Chapter 9

Photosynthesis:Capturing EnergyChapter 9

Learning Objective 1What are the physical properties of light?What is the relationship between a wavelength of light and its energy?

Electromagnetic Spectrum

Fig. 9-1, p. 192One wavelengthLonger wavelengthTV and radio wavesRed700 nmMicro- wavesOrangeInfraredColor spectrum of visible light600 nmYellowVisibleX-rays500 nmGreenBlueGamma raysViolet400 nmElectromagnetic spectrumShorter wavelength760 nm380 nmUV

LightConsists of particles (photons) that move as wavesPhotons with shorter wavelengths have more energy than those with longer wavelengths

Sunlight

Fig. 9-2, p. 192SunSunlight is a mixture of many wavelengths

Light and Energy

Fig. 9-3, p. 193PhotonPhoton is absorbed by an excitable electron that moves into a higher energy level.Low energy levelElectronHigh energy levelEitherElectron acceptor moleculeThe electron may return to ground level by emitting a less energetic photon.The electron may be accepted by an electron acceptor molecule.Or

KEY CONCEPTSLight energy powers photosynthesis, which is essential to plants and most life on Earth

Learning Objective 2What is the internal structure of a chloroplast?How do its components interact and facilitate the process of photosynthesis?

StructuresPhotosynthesis in plantsoccurs in chloroplastslocated in mesophyll cellsinside the leaf

Leaf Structure

Fig. 9-4a, p. 194(a) This leaf cross section reveals that the mesophyll is the photosynthetic tissue. CO2 enters the leaf through tiny pores or stomata, and H2O is carried to the mesophyll in veins.Palisade mesophyllVeinAir spaceStomaSpongy mesophyll

Fig. 9-4b, p. 194(b) Notice the numerous chloroplasts in this LM of plant cells.Mesophyll cell10 m

Fig. 9-4c, p. 194Outer membraneInner membraneStroma1 mIntermembrane spaceThylakoid membrane(c) In the chloroplast, pigments necessary for the light-capturing reactions of photosynthesis are part of thylakoid membranes, whereas the enzymes for the synthesis of carbohydrate molecules are in the stroma.Granum (stack of thylakoids)Thylakoid lumen

ChloroplastsEnclosed by a double membrane inner membrane encloses stroma and thylakoidsThylakoids enclose thylakoid lumenarranged in stacks (grana)

Photosynthetic PigmentsIn thylakoid membraneschlorophyll achlorophyll b Carotenoids

Fig. 9-5, p. 195Hydrocarbonside chainPorphyrin ring(absorbs light)in chlorophyll bin chlorophyll a

Learning Objective 3What happens to an electron in a biological molecule such as chlorophyll when a photon of light energy is absorbed?

Energizing ElectronsPhotons excite photosynthetic pigmentschlorophyll Energized electronsmove to electron acceptor compounds

Active WavelengthsCombined absorption spectra of chlorophylls a and b action spectrum for photosynthesis

Absorption Spectra

Fig. 9-6a, p. 195Chlorophyll bChlorophyll aEstimated absorption (%)Wavelength (nm)(a) Chlorophylls a and b absorb light mainly in the blue (422 to 492 nm) and red (647 to 760 nm) regions.

Fig. 9-6b, p. 195Relative rate of photosynthesisWavelength (nm)(b) The action spectrum of photosynthesis indicates the effectiveness of various wavelengths of light in powering photosynthesis. Many plant species have action spectra for photosynthesis that resemble the generalized action spectrum shown here.

Action Spectrum

Fig. 9-7a, p. 196100 m(a)

Fig. 9-7b, p. 196Wavelength of light (nm)

Learning Objective 4Describe photosynthesis as a redox process

PhotosynthesisLight energyis converted to chemical energy (carbohydrates) Hydrogens from waterreduce carbon Oxygen from wateris oxidized, forming molecular oxygen

Learning Objective 5What is the difference between light-dependent reactions and carbon fixation reactions of photosynthesis?

Energy for ReactionsLight-dependent reactionslight energizes electrons that generate ATP and NADPH Carbon fixation reactionsuse energy of ATP and NADPH to form carbohydrate

Photosynthesis

Fig. 9-8, p. 197Light-dependent reactions (in thylakoids)Carbon fixation reactions (in stroma)ChloroplastATPLight reactionsADPCalvin cycleNADPHNADP+H2OO2CO2Carbohydrates

Stepped ArtFig. 9-8, p. 197Light-dependent reactions (in thylakoids)ChloroplastCalvin cycle

Learning Objective 6How do electrons flow through photosystems I and II in the noncyclic electron transport pathway?What products are produced?Contrast this with cyclic electron transport

The PhotosystemsPhotosystems I and II photosynthetic unitsinclude chlorophyll, accessory pigmentsorganized with pigment-binding proteins into antenna complexes

A Photosystem

Fig. 9-10, p. 198Primary electron acceptore-ChloroplastPhotonThylakoid membranePhotosystem

Reaction CentersReaction center of antenna complexspecial pair of chlorophyll a molecules release energized electrons to acceptorP700reaction center for photosystem IP680reaction center for photosystem II

Noncyclic Electron TransportLight-dependent reactionsform ATP and NADPH

Noncyclic SystemsElectrons in photosystem Ienergized by lightpass through electron transport chain convert NADP+ to NADPH Redox reactions pass energized electrons along ETCfrom photosystem II to photosystem I

Noncyclic SystemsElectrons given up by P700 (photosystem I) replaced by electrons from P680 (photosystem II)Electrons given up by P680 (photosystem II)replaced by electrons from photolysis of H2O (releasing oxygen)

Cyclic Electron TransportElectrons from photosystem Ireturn to photosystem I ATP produced by chemiosmosisNo NADPH or oxygen generated

Table 9-1, p. 200

KEY CONCEPTSPhotosynthesis, which occurs in chloroplasts, is a redox process

Learning Objective 7Explain how a proton (H+) gradient is established across the thylakoid membrane and how this gradient functions in ATP synthesis

Proton Gradient

Fig. 9-12, p. 200StromaThylakoid lumenThylakoid membraneProtons (H+)

PhotophosphorylationPhotophosphorylation synthesis of ATP coupled to transport of electrons energized by photonsElectron energy pumps protons across thylakoid membrane energy gradient generates ATP by chemiosmosis

ATP SynthesisATP synthaseenzyme complex in thylakoid membraneprotons diffuse through enzymephosphorylate ADP to ATP

Electron Transport and Chemiosmosis

Fig. 9-13, p. 201Thylakoid lumenThylakoid membranePhotosystemIIPhotonThylakoidmembranePlastocyaninPlastoquinoneFerredoxinCytochromecomplexPhotosystem IPhotonFerredoxin-NADP+reductaseNADPHNADP+ADPPiATPATPsynthase

KEY CONCEPTSLight-dependent reactions convert light energy to the chemical energy of NADPH and ATP

Learning Objective 8Summarize the three phases of the Calvin cycle, and the roles of ATP and NADPH

Calvin Cycle (C3 pathway)Carbon fixation reactions 3 phasesCO2 uptake phaseCarbon reduction phaseRuBP regeneration phase

CO2 Uptake Phase

Enzyme rubisco (ribulose bisphosphate carboxylase/ oxygenase)combines CO2 with ribulose bisphosphate (RuBP), a five-carbon sugarforms 3-carbon phosphoglycerate (PGA)

Carbon Reduction Phase

Energy of ATP and NADPH convert PGA molecules to glyceraldehyde-3-phosphate (G3P)For each 6 CO2 fixed12 G3P are produced2 G3P leave cycle to produce 1 glucose

RuBP Regeneration Phase

Remaining G3P molecules are modified to regenerate RuBP

The Calvin Cycle

KEY CONCEPTSCarbon fixation reactions incorporate CO2 into organic molecules

Learning Objective 9How does photorespiration reduce photosynthetic efficiency?

PhotorespirationC3 plants use O2 and generate CO2 by degrading Calvin cycle intermediatesbut do not produce ATPOn bright, hot, dry daysplants close stomata, conserving waterprevents passage of CO2 into leaf

Learning Objective 10Compare the C4 and CAM pathways

C4 PathwayTakes place in mesophyll cellsEnzyme PEP carboxylase binds CO2CO2 fixed in oxaloacetateconverted to malate Malate moves into bundle sheath cellCO2 is removed Released CO2 enters Calvin cycle

C4 and C3 Plants

Fig. 9-15a, p. 205Upper epidermisPalisade mesophyllBundle sheath cells of veinsSpongy mesophyllChloroplasts(a) In C3 plants, the Calvin cycle takes place in the mesophyll cells and the bundle sheath cells are nonphotosynthetic.

Fig. 9-15b, p. 205Upper epidermisBundle sheath cells of veinsMesophyllChloroplasts(b) In C4 plants, reactions that fix CO2 into four-carbon compounds take place in the mesophyll cells. The four-carbon compounds are transferred from the mesophyll cells to the photosynthetic bundle sheath cells, where the Calvin cycle takes place.

C4 Pathway

Fig. 9-16, p. 206CO2Mesophyll cellPhosphoenol- pyruvateOxaloacetateADPNADPHATPNADP+PyruvatePyruvateBundle sheath cellNADP+CO2GlucoseNADPHVein(4C)(3C)(3C)Malate(4C)Malate(3C)(4C)

(CAM) PathwayCrassulacean acid metabolism (CAM)similar to C4 pathway PEP carboxylase fixes carbon at nightin mesophyll cellsCalvin cycle occurs during the day

A CAM Plant

Learning Objective 11How do photoautotrophs and chemoheterotrophs differ with respect to their energy and carbon sources?

Energy SourcesPhotoautotrophs use light as energy sourceincorporate atm