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I. Light Reactions Photosynthesis The Nature of Light Outline Light Absorption by pigments Absorption and Action Spectra Photosystems I & II Light-Harvesting Complexes and Reaction Centers Z-Scheme electron carriers Proton Transport and ATP synthesis Repair and regulation of photosynthetic machinery Electromagnetic Spectrum, Wavelength & Frequency C = Speed of a light wave (3.0 X 108 m s-1) (Greek letter lambda) C = λv λ = Wavelength (nm) -1 V = frequency (s ) (Greek letter nu) II. Carbon Reactions Calvin cycle: carboxylation, reduction & regeneration Sucrose and starch synthesis C2 Oxidative Photosynthetic Carbon Cycle CO2 concentrating mechanisms CO2 pumps, CAM, C4 Photosynthesis III. Physiological and Ecological Considerations Concepts and Units in the Measurement of Light Leaf anatomy and light absorption Photosynthetic responses to light, CO2 & Temperature Leaf Energy Budget: Dissipation of Excess Energy Properties of Light 1. Light has a dual nature Light behaves like a wave wavelength = distance between successive crests frequency = number of wave crests in a given time The Nature of Light – Solar Spectrum Solar constant = 1366 watts per square meter = one two-billionth of solar output! Irradiance = Watts m-2 Irradiance = power m-2 Irradiance = energy time-1 area-1 Light behaves like a particle Photon = light particle Quantum = energy in one photon 450 2. Sunlight = photon rain Different wavelengths Different frequencies Different energies Physics of Light Energy E= (h ∗ c) λ E = energy per photon h = Planck’s constant c = speed of light λ = wavelength 3x1017 speed of light (nm/sec) Light Absorption by Chlorophyll Fates of an excited electron 1. Return to Ground State A. Heat B. Fluorescence 1 X 10-8 s 6.63x10-34 Plancks constant 450 wavelength (nm) 4.42x10-19 Energy of light per photon in joules 252 Energy (kJ 2. Resonance Transfer 1 X 10-12 sec mole-1) 3x1017 speed of light (nm/sec) 6.63x10-34 Planck’s constant 3. Photochemistry = Excited Electron Transfer 660 wavelength (nm) 3.01x10-19 Energy of light per photon in joules 181 Energy (kJ mole-1) 1 Photosynthetically Active Radiation PAR Photosynthesis Equation = Photosynthetically Active Radiation = photons of light from 400-700nm = unit is microEinsteins Æ uE m-2 s-1 Einstein = 1 mole of photons microEinstein = 10-6 mole photons microEinstein = 6.02 x 1017 photons Light + 6CO2 + 6H2O Æ C6H12O6 + 6O2 Values for PAR 2000 uE m-2 s-1 Full, direct sunlight high noon summer. <100 uE m-2 s-1 Shade 15 - 85 uE m-2 s-1 = Light Compensation Point 2000 uE m-2 s-1 ≈ 0.5 kJ energy ≈ 0.5kw sec Pigments of Photosynthesis Chlorophylls Carotenoids Photosynthesis Equation Light + CO2 + H2O ÆC6H12O6 + O2 1. Nearly impossible for products to form Keq = 10-500 Porphyrin ring 2. About 26% of absorbed light energy is used: For CO2 + H2O Æ(CH2O) + O2 ΔGo’ = +460 kJ mol-1 Light @ 680nm = 1760 J/mole O2 460/1760 = 26% Energy conversion efficiency ≈ 26% …or 74% photon energy is lost as heat. Chlorophylls a & b Hydrocarbon tail Photosynthesis Overview Chloroplast Structure H20 Thylakoid Inner membrane Outer membrane Granum O2 Light Harvesting Thylakoid Reactions Stroma ATP Sugar Building NADPH Stroma Reactions Carbohydrate CO2 2 7.9 Schematic diagram of the action spectrum measurements by T. W. Engelmann 7.8 Action spectrum compared with an absorption spectrum 7.11 Relationship of oxygen production to flash energy 7.10 Basic concept of energy transfer during photosynthesis Evidence for an antenna complex (Robert Emerson & William Arnold 1932) Light harvesting antenna complex Reaction center complex Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Light Harvesting Complex & Reaction Centers Fig. 10.6a (TEArt) Chlorophyll molecules, Carotenoids & proteins in Light-Absorbing Antenna System Sun Evidence for two photosystems Quantum yield of PS = oxygen produced/light quanta absorbed = 1 oxygen/10 photons Red drop RED DROP at λ > 680nm, i.e. Far red light had little effect on O2 production Thylakoid membrane Chlorophyll still absorbs at 680 nm Reaction Center Chlorophyll P700 in PSI P680 I PSII 3 Z Scheme of Photosynthesis Evidence for two photosystems The Emerson enhancement effect 700nm 680nm 2. Two photosystems operate in series A. Photosystem I = PSI Æ Produces a strong reductant (reducing agent) & weak oxidizing agent B. Photosystem II = PSII Æ Produces a strong oxidant (oxidizing agent) & weaker reducing agent than in PSI Strong reducing agent Redox potential Enhancement effect of photosynthesis 1. Two Photosystems: each chemically & physically distinct Separate antenna pigments Separate photochemical reaction centers Linked by an electron transport chain 680 + 720nm Electron transport Strong oxidizing agent Organization of Four Major Protein Complexes of Thylakoid Membrane Energy Transfer in Light-Absorbing Antenna PSII – in stacked regions of thylakoid PSI – adjacent to stromal regions of thylakoid PSI Æ in Granal Thylakoids PSII Æ in Stromal Thylakoids PSII/PSI ≈ 1.5:1 Evenly distributed Adjacent to stromal regions 7.14 Z scheme of photosynthesis Z-Scheme of Photosynthesis Photosystem II 4 Energy & Electron Flow in Photosystem II Structure of LHCII Antenna Complex Protein and Pigments Energy and electron flow from antenna pigments to Cytb6f complex. Protein 3 α-helical regions 1. Light energy picked up by antenna pigments (carotenoid or chlor b) Carotenoids Four X-shaped molecules 2. FRET energy transfer from carotenoid Æ chlor b Æ chlor a 3. FRET energy transfer from chlor a to Rx Center P700. Chlorophyll a & b 14 bound to protein 4. P700 energy transfer to Pheophytin (chlor a –Mg) 5. One e- transfer from Pheo Æ Plastoquinone A then same e- Æ PQB 6. PQB receives two electrons Æ PQB-2 7. PQB-2 receives 2H+ from stroma Æ PQBH2 8. PQBH2 detaches from the RX center protein Æ enters membrane hydrocarbon layer Electrons enter Cytb6f complex. Protons enter thylakoid lumen. Structure of PSII Reaction Center Protein and Pigments “View from Lumen” “Side View” Antenna Complex Extrinsic Oxygen-evolving complex Reaction Centers PSII Reaction Center & Antenna Complex 1. Multisubunit pigment-protein complex Barrels = Protein Helixes Colors = Different proteins 2. D1 & D2 = Core Reaction Center Proteins P680 = Primary Rx. Center Chlorophyll Carotenoids Pheophytin (chlorophyll without Mg+) Plastoquinones (2 e- acceptors) 3. Antenna complex (LHCII) proteins and chlorophylls ∼200-300 chlorophylls 4. Oxygen Evolution proteins Bound to D1 & D2 Primary donor chlorophyll More chlorophylls Carotenoids Pheophytins (Mg replaced by 2H) Plastoquinones (2e- acceptors) Antenna Complex Electron Transfer from Reaction Center Water Oxidation in PSII 2 H 2O → O2 + 4 H + + 4e − Pheophytin Æ 2 electrons (sequentially) Æ QA Æ QB + 2H+ S0 Æ S4 = different oxidation states of Manganese 1. Oxygen produced PQH2 Plastohydroquinone = Mobile electron & proton carrier + 2 H 2O → O2 + 4 H + 4e − 2. Electrons released Diffuses in core of membrane bilayer Picked up by a tyrosine on D1 protein 3. Protons released 5 Electron Transfer between PSII & PSI Cytochrome b6f Complex 1 Plastoquinone 1. PQ picks up two electrons from Pheo e- from PQ flows linearly to PC PC reduces P700 in PSI 2nd e- from PQ flows in a cyclic process back to PQ This is a proton pump transferring protons from stroma to lumen of thylakoid. 2. PQ picks up 2 protons from Stroma. 3. PQ Shuttles electrons through nonpolar lipid hydrocarbons to Cytochrome b6f releasing H+ into Lumen Plastocyanin 1. small Cu-containing protein 2. H2O soluble & mobile 3. Found in lumen 4. Transfer e- from Cyt b6f to PSI. Z-Scheme of Photosynthesis Photosystem I PSI Reaction Center 2 Core proteins 1. Large multisubunit complex with major core proteins PSaA & PsaB 2Reaction Center Complex Major Proteins: Psa A & Psa B P700 Core antenna of 100 chlorophylls 3. Electron Transfer Cofactors centered inside core antenna pigments Ao = Chlorophyll ? A1 = quinone ? Iron-sulfur proteins FeSx, FeSA, FeSB 4. Ferredoxin (Fd) soluble protein Reduces NADP+ PSI & Cyclic Electron Flow Chemiosmotic ATP Synthesis 1. Protons from: Water oxidation & Pumping at Cyt b6f e- 2. Proton gradient drives ATP Synthase 4 protons ≈ 1 ATP 3. Molecular Rotation of Stalk and CFo drives ATP synthesis 6 Herbicide Effect on Photosynthesis Light Harvesting Light-Harvesting Component Photosystem I Photosystem II Cytochrome b6f Complex ATP Synthase Function(s) Reduce NADP+ to NADPH Cyclic electron flow Æ ATP synthesis Electron Transfer to PSI Water oxidation Protons pumped into thylakoid lumen Transfers e- between PSII & PSI Proton Pumping Chemiosmotic ATP Synthesis O2 superoxides Photosystem II Responses to High Light Intensity Protection of the LightHarvesting Apparatus Defense Mechanisms Antenna System 2) Nonphotochemical quenching by carotenoids = Xanthophyll cycle conversion of energy to heat Controlled by pH of thylakoid lumen in peripheral antenna complex Excess Energy delivery 1) Photochemical quenching by carotenoids that quench excited state of chlorophyll 1. Suppression 2. Scavenge 3. Repair Toxic photoproducts O2 ÆSinglet oxygen formed when excited chlor reacts with oxygen 1. Superoxide dismutase 2. Ascorbate peroxidase Damage to Rx Center = Photoinhibition Damage to D1 protein by excess light Xanthophyll Cycle PSII Reaction Center 1) Remove & resynthesize D1 protein 2) Recycle other Rx center components Diurnal changes in xanthophyll content as a function of irradiance Low Light Energy Light energy increases Zeaxanthin binds to light-harvesting antenna proteins… leading to … quenching … heat dissipation Light energy increases High Light Energy 7 Protection of the Light-Harvesting Apparatus 1. Problem Æ Excited chlorophyll forms highly reactive substances singlet oxygen 1O2* superoxide O2peroxide END Photosynthesis H2O2 2. Defense mechanisms Antenna System A. Photochemical Quenching by chlorophyll excitation energy transfer to carotenoids … carotenoids collect excitation energy … energy reradiated as heat END Light Harve sting B. Nonphotochemical quenching by xanthophylls … Xanthophylls use the Xanthophyll Cycle … Zeaxanthin (Zea) synthesis under high light … Zea binds to antenna proteins & energy dissipated as heat Reaction Center Damage & Repair A. Photoinhibition … Damage to PSII reaction center D1 protein … Removal and synthesis of new D1 protein 8