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Light-Dependent Reactions of Photosynthesis Light-dependent reactions of photosynthesis Tom Donald © 2015 American Society of Plant Biologists Lesson outline • Photosynthesis overview • Evolution and diversity of photosynthesis • Light and pigments • The light response curve and quantum efficiency • Plastids and chloroplasts • Structure and function of photosynthetic complexes • Pathways of electron transport • Damage avoidance and repair: Acclimations to light • Monitoring light reactions • Optimizing and improving photosynthesis • Artificial photosynthesis • Photosynthetic fungi and animals © 2015 American Society of Plant Biologists Overview: Photosynthesis captures light energy as reduced carbon “High energy”, Light-dependent reactions reduced The first step is the capture of carbon light energy as ATP and Energy input reducing power, NADPH from sunlight ATP NADPH Oxygen is Light-independent reactions released as The second step is the transfer of energy and reducing power from “Low energy”, a byproduct ATP and NADPH to CO , to produce oxidized carbon 2 high-energy, reduced sugars in carbon dioxide 6 CO2 + 6 H2O C6H12O6 + 6 O2 © 2015 American Society of Plant Biologists Photosynthesis is two sets of connected reactions 2 NADPH e− 2 H+ 2 NADP+ 2 H O O + 2 H+ + 2 e− ADP ATP Chloroplast 2 2 + H The LIGHT reactions take place in the thylakoid membranes The CARBON-FIXING reactions take place in the chloroplast stroma Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70–78. © 2015 American Society of Plant Biologists Light reactions (usually) take place in thylakoid membranes Prokaryotes Eukaryotes Gloeobacter violaceus, Chlamydomonas the only cyanobacterium PLANT without thylakoid reinhardtii, a model green algae membranes Light-induced differentiation Proplastid Chloroplast Synechocystis spp. PCC6803, a model cyanobacterium Reproduced with permission © Annual Reviews of Plant Biology Nickelsen, J. and Rengstl, B. (2013).Photosystem II assembly: From cyanobacteria to plants. Annu. Rev. Plant Biol. 64: 609-635. © 2015 American Society of Plant Biologists Light reactions produce O2, ATP and NADPH 2 NADPH Cytochrome e− 2 H+ The reactions b6f complex 2 NADP+ require several ADP ATP large multi-protein Photosystem complexes: two I (PSI) light harvesting photosystems (PSI 2 H O O + 2 H+ + 2 e− 2 2 + and PSII), the H cytochrome b6f complex, and ATP Photosystem II (PSII) ATP synthase synthase Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70–78. © 2015 American Society of Plant Biologists Chlorophyll captures light energy to initiate the light reactions First step of photochemistry Chl* e− Chlorin ring Photon captures photons H+ H2O + Chlorophyll is Chl Chl e− held in pigment- O2 Photon capture by protein + complexes in a chlorophyll excites the Chl is reduced by highly organized chlorophyll (Chl*). Chl* stripping an electron manner can lose an electron to from water, releasing become oxidized oxygen and protons chlorolphyll (Chl+) Buchanan, B.B., Gruissem, W. and Jones, R.L. (2015) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists. © 2015 American Society of Plant Biologists Excited chlorophyll can release energy in several ways Photochemistry (first step in photosynthesis) Chl* Fluorescence Photon Non-photochemical quenching (e.g., heat) 3Chl* Alternatively, it can convert to a damaging triplet state Chl The fate of captured light energy and photosynthetic efficiency depend on many factors including temperature, water availability, nutrient availability, stress etc. © 2015 American Society of Plant Biologists Heat, drought, & other stresses affect photosynthetic efficiency Light reactions The light reactions and carbon-fixing reactions are linked ADP + ATP NADP NADPH through pools of ATP/ADP and Carbon-fixing reactions NADPH/NADP+ For example…. • High temperature affects protein complex stability and thylakoid membrane fluidity • Low temperatures slow enzyme-catalyzed reactions • Drought causes stomata to close, lowering CO2 uptake and carbon-fixing reactions • Nutrient deficiency or toxicity can affect electron transfer machinery © 2015 American Society of Plant Biologists Oxygenic photosynthesis requires TWO photosystems Strong −1.5 PSII PSI P700* is a very strong reductant reductant – strong Reductants donate P700* electrons to other enough to donate species −1.0 electrons to NADP+ P680* e− −0.5 NADP+ NADPH Energy 0.0 Redox potentialeV + 0.5 P700 4 e− P700 2 H2O Strong P680+ is a very strong oxidant 1.0 4 H+ O oxidant – strong enough to Oxidizers remove 2 P680+ electrons from other pull electrons from H2O species P680 © 2015 American Society of Plant Biologists PSI & PSII are connected by an electron transport chain Strong −1.5 PSII PSI reductant Reductants donate P700* electrons to other species −1.0 P680* e− −0.5 PQ NADP+ NADPH Cyt b6f Energy 0.0 Redox potentialeV PC + 0.5 P700 4 e− P700 2 H2O Strong oxidant 1.0 The electron transport chain 4 H+ O generates proton-motive force that Oxidizers remove 2 P680+ electrons from other drives ATP production species P680 © 2015 American Society of Plant Biologists This diagram is known as a Z-scheme Strong −1.5 PSII PSI reductant Reductants donate P700* electrons to other species −1.0 P680* e− −0.5 PQ NADP+ NADPH Cyt b6f Energy 0.0 Redox potentialeV PC 0.5 P700+ 4 e− P700 2 H2O Strong oxidant 1.0 The electron transport chain 4 H+ O generates proton-motive force that Oxidizers remove 2 P680+ electrons from other drives ATP production species P680 © 2015 American Society of Plant Biologists PSI can function without PSII, but it doesn’t produce oxygen or NADPH PSI Cyclic electron transport: P700* • Involves PSI • Does not involve PSII • Involves the electron transport chain • Results in ATP production PQ • Does not liberate O2 • Does not produce NADPH Cyt b6f PC P700+ P700 The electron transport chain generates proton-motive force that drives ATP production © 2015 American Society of Plant Biologists The photosystems are embedded in thylakoid membranes Plastid STROMA 2 NADPH LUMEN Cytochrome b6f (Cyt b6f) is a multiprotein e− 2 H+ membrane-embedded + complex 2 NADP STROMA Thylakoid PQ Cyt b6f Membrane − 2 H O 4 e PSI 2 PSII PC Plastoquinone (PQ) is 4 H+ O 2 a small molecule and LUMEN mobile electron carrier Plastocyanin (PC) is a PQ small protein and mobile electron carrier © 2015 American Society of Plant Biologists Electrical and H+ gradients drive ATP synthesis 2 NADPH + − ATP 2 H e ADP + Pi ATP Synthase 2 NADP+ H+ Thylakoid Cyt b6f Membrane − 2 H O 4 e PSI 2 PSII 4 H+ O2 [H+] Proton gradient from high (in) to low (out) © 2015 American Society of Plant Biologists Products of the light-dependent reactions drive carbon-fixation Light-dependent reactions Carbon-fixing reactions Each CO2 fixed requires 3 ATP CO2 and 2 NADPH Rubisco ADP ATP Carboxylation Ribulose-1,5- bisphosphate NADPH + H+ H Calvin- NADP+ Benson Regeneration Cycle ATP Energy Glyceraldehyde 3- phosphate (GAP) input ADP + Pi For every 3 CO fixed, one NADPH Reducing 2 1 x GAP power input GAP is produced for NADP+ + H+ biosynthesis and energy Adapted from: Buchanan, B.B., Gruissem, W. and Jones, R.L. (2000) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists. © 2015 American Society of Plant Biologists Lesson outline • Photosynthesis overview • Evolution and diversity of photosynthesis • Light and pigments • The light response curve and quantum efficiency • Plastids and chloroplasts • Structure and function of photosynthetic complexes • Pathways of electron transport • Damage avoidance and repair: Acclimations to light • Monitoring light reactions • Optimizing and improving photosynthesis • Artificial photosynthesis • Photosynthetic fungi and animals © 2015 American Society of Plant Biologists Evolution and diversity of photosynthesis Most There are two types of reaction centers, Type I & Type II photosynthetic Each type is found in various photosynthetic bacteria prokaryotes use Both types are found in cyanobacteria and chloroplasts only a single Type II are pheophytin- Type I are iron-sulfur type of reaction quinone reaction centers reaction centers center and do not release oxygen Note that oxygenic photosynthesis PSI requires Type I & Type II PSII reaction centers working in series O2 Reprinted from Allen, J.P. and Williams, J.C. (1998). Photosynthetic reaction centers. FEBS Letters. 438: 5-9 with permission from Elsevier. © 2015 American Society of Plant Biologists Type I & Type II reaction centers are broadly distributed in prokaryotes Several bacterial lineages Cyanobacteria, Type I have some photosynthetic chloroplasts members, indicating that lateral gene transfer has Type I + Type II played an important role in the Type II evolution of photosynthesis. Type I Type I Type II Reprinted from Macalady, J.L., Hamilton, T.L., Grettenberger, C.L., Jones, D.S., Tsao, L.E. and Burgos, W.D. (2013). Energy, ecology and the distribution of microbial life. Phil. Trans. Roy. Soc. B: 368: 20120383. by permission of the Royal Society. See also Blankenship, R.E. (2010). Early evolution of photosynthesis. Plant Physiol. 154: 434–438; © 2015 American Society of Plant Biologists Prokaryotic photosynthetic diversity Phylum Discovery Reaction Pigments Colloquial name center Cyanobacteria 1800s Type I + Type II Chl a,b,c,d Proteobacteria 1800s Type II BChl a,b Purple sulfur / nonsulfur bacteria Chlorobi 1906 Type I
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