Photosynthesis• General Aspects of Photosynthesis• The Photoreactivity of Chlorophyll• 2 Distinct Photosystems• The Z Scheme of Photosynthesis• Architecture of Photosynthetic Reaction Centers• ATP Synthesis - Photophosphorylation• Carbon Dioxide Fixation• The Calvin-Benson Cycle• Photorespiration• The C-4 Pathway of CO2 FixationThe Sun - Ultimate Energy1.5 x 1022 kJ falls on Earth each day• 1% is absorbed by photosynthetic organisms and transformed intochemical energy• 6 CO2 + 6 H2O → C6H12O6 + 6 O2• 1011 (100 billion) tons of CO2 are fixed globally per year• Formation of sugar from CO2 and water requires energy• Sunlight is the energy source!• So how many MW are available to organisms?• 1% of 1.5 x 1022 kJ in (60 x 60 x 24) seconds• 1.7 x 1012 MW!• Itaipu is the world’s largest power station and produces 12,600 MW ofhydroelectric power• Most power stations are 10 - 1000 MW• Photosynthesis uses 800,000 times more power than the world’s totalelectricity generation!1PhotosynthesisGeneral Aspects• Photosynthesis occurs inthylakoid membranes ofchloroplasts - structures involvingpaired folds (lamellae) that stackto form grana• The soluble portion of thechloroplast is the stroma• The interior of the thylakoidvesicles is the thylakoid space orthylakoid lumen• Chloroplasts possess DNA, RNAand ribosomesLight Reactions and Dark Reactions• The light reactions capture light energy and convert it to chemical energy in theform of reducing potential NADPH and ATP with evolution of O2• Carbohydrate synthesis is not directly driven by light• The dark reactions use NADPH and ATP to drive hexose formation (+ve ΔG)from CO2 in a series of reactions in the stroma• The light reactions take place in the thylakoid membrane and in the lumen (inside)• H2O and light in, ATP, NADPH and O2 produced• The dark reactions take place in the stroma (outside)• CO2, ATP and NADH consumed, carbohydrates produced2Water is the electron donorfor photosynthetic NADP+ reduction• The following equations describe the light and dark reactions in green plants2H2O + 2NADP+ + xADP + xPi → O2 + 2NADPH + 2H+ + xATP + xH2O12NADPH + 2 H+ + 18ATP + 6CO2 + 12H2O → C6H12O6 + 12NADP+ + 18ADP + 18PiChlorophyllphotoreactive, isoprene-based pigment• A planar, conjugated ring system - similar toporphyrins• Mg2+ in place of Fe2+/3+ in the center• Long chain phytyl group confers membrane solubility• Aromaticity makes chlorophyll an efficient absorber oflight3The Photosynthetic Unitmany chlorophylls, channel light energy to asingle reaction center• The "unit" consists of several hundred light-capturing chlorophylls plus a pair of specialchlorophylls in the reaction center• Light is captured by one of the antennachlorophylls and routed from one to theother until it reaches the reaction centerThe Red Drop• Absorption spectrum extends beyond 700 nm• But there is a decrease in quantum yield of NADPH at longer wavelengths (> 680 nm)• Both short and long wavelengths necessary for full efficiency• Implies two photosystems acting together4Eukaryotic PhotosystemsPSI (P700) and PSII (P680)• All chlorophyll is part of either LHC, PSI or PSII• PSI absorbs at 700 nm• PSII absorbs at 680 nm• Chloroplasts given light at 680 and 700 nm simultaneously yield more O2than the sum of amounts when each is used alone.• PSI reduces NADP+• PSII oxidizes water (photolysis)• ATP is generated by establishment of a proton gradient as electrons flow fromPSII to PSI (or from the cytochrome b6/cytochrome f complex to PSI)• PQ = plastoquinone• PC = plastocyanin• F = ferredoxins• Ao = a special chlorophyll a• A1 = a special PSI quinone• Cytochrome b6/cytochrome f complexis a proton pumpThe Z Scheme5Electron Transfer isMediated by Plastoquinone,a Lipid-Soluble 2e- Carrier……and Plastocyanin, aWater-Soluble(Cu+/Cu2+) 1e- Carrier6Photosystem Iuses photoexcitation to generate a powerful oxidant• Photoexcited P700* (strong reductant) Eo -0.6 V• Loss of e- to Ao (chl a) gives P700+ Eo = +0.4 V• eT via plastoquinone A1 to ferredoxins Fd• Soluble Fds transfer electrons to flavoprotein Fp• Fp uses [FADH2] to reduce NADP+ to NADPHCytochrome b6/Cytochrome fa cytochrome bc1 analog• Plastocyanin transfers electrons from cyt b6/cyt f to P700+• Plastoquinol:plastocyanin oxidoreductase• Cyt b6/cyt f is a proton pump driven by a Q-cycle• Just like cyt bc1 complex III• Recharged by reduced plastoquinones from PSIIPhotosystem II and Oxygen Evolutionrequires accumulation of four oxidizing equivalents• PSII (P680) cycles through 5 oxidation states• 1 e- is removed in each of four steps• Fifth step involves H2O oxidized to O2 + 4H+7The Oxygen-Evolving Complex OEC4 manganese, 1 calcium cluster with bridging oxygensHOHCa:BCl2+MnOMn3+OH HH(Mn4+2Ox)HOHCa:BClMn3+OMn3+OH H(Mn4+2Ox)HOHCa:BClMn3+OMn4+OH H(Mn4+2Ox)HOHCa:BClMn4+OMn4+OH(Mn4+2Ox)HOHCa:BClMn4+OMn4+O(Mn4+2Ox)e-; H+e-YZOYZOHYZOYZOHHCaClMn3+OMn4+O(Mn4+2Ox)O2 H2OO2Proposed S-state cycle of the OEC8Photophosphorylationlight-driven ATP synthesis• Electron transfer through the proteins of the Z scheme drives the generation of aproton gradient across the thylakoid membrane• Protons pumped into the lumen of the thylakoids flow back out, driving thesynthesis of ATP• CF1-CFo ATP synthase is similar to the mitochondrial ATP synthaseThe Quantum Yieldamount of O2 evolved per photon• 8 photons: 4 photons per reaction center• 8 protons: 4H+ in lumen from 2H2O, 4H+ pumpedby cyt b6/f (2 PQH2 → 2PQ)• 1 O2 evolved• 2 NADPH from reduction of 2 NADP+• 22/3 ATP phosphorylated (3H+ per ATP)9Cyclic PhotophosphorylationATP without NADPH• The photo-excited electron removed from P700returns to P700• Cyclic photophosphorylation depends only onPSI not on PSII• 1 photon pumps 1 proton• ATP synthesis is 1 ATP for 3 photons (same asfor PSI and PSII)• No NADPH is synthesized• Corrects imbalance - biosynthesis of glucoserequires 3 ATP per 2 NADPH• http://science.nhmccd.edu/biol/bio1int.htm#photo10Carbon Dioxide Fixationa unique ability of plants, algae, etc.• Melvin Calvin at Berkeley in 1945 showed that Chlorella could take up14CO2 and produce 3-phosphoglycerate• What was actually happening was that CO2 was combining with a 5-C sugarto form a 6-C intermediate• This breaks down to two 3-P glyceratesRibulose-1,5-Bisphosphatethe CO2 acceptor• Fixation is accomplished by