Rutgers University MBB 408 - Lecture 2 - Light Reactions of Photosynthesis (9 pages)

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Lecture 2 - Light Reactions of Photosynthesis



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694 40 8 115 512 M ol ec ul ar Bi olo gy Bi oc he mi str y Lecture 2 Light Reactions of Photosynthesis November 4 2008 1 Overall photosynthetic process light converted into chemical energy for biosphere provides atmospheric O2 fixes atmospheric CO2 into CH2O 1011 tons worldwide yr 2 Photosynthesis consists of light reactions drive O2 evolution generate NADPH and ATP and carbon assimilation reactions NADPH and ATP drive CO2 fixation via ribulose 1 5 bis P carboxylase Calvin cycle to regenerate ribulose 1 5 bis P Improved understanding of light reactions is basis for harnessing solar energy a sustainable and renewable source of clean energy 3 Photosynthesis occurs in chloroplasts in higher phototrophs light reactions in thylakoid membrane stacked grana and unstacked stroma lamellae regions thylakoid lumen accumulates H CO2 fixation occurs in stroma 4 Distinguishing oxygenic photosynthesis H2O as e donor algae plants and anoxygenic H2S organic acids alcohols as e donors green and purple bacteria 5 Photosynthetic pigments associated noncovalently in pigmentprotein complexes a Chlorophyll Chl Mg tetrapyrrole has light harvesting role funnels collected energy to reaction center and photochemical reaction center role catalyzes charge separation b Carotenoids carotene accessory light harvesting and photoprotective roles 6 Fates of photoexcitations resonance energy transfer in lightharvesting proteins e g light harvesting complex II LHC II electron transfer initiated in reaction center loss as fluorescence useful in energy transfer studies and radiationless decay 7 Molecular architecture and role of LHC II major light harvesting complex of higher plants X ray structure reveals how LHC II Chls funnel excitations to PSII 8 Oxygenic phototrophs use two photosystems PSI and PSII multisubunit integral membrane complexes with reaction center and electron transport components surrounded by light harvesting Chls a PS I reaction center P700 absorbs 700 nm photons strong reductant generating NADPH weak oxidant b PS II reaction center P680 absorbs 680 nm photons strong oxidant photolyzing H2O weak reductant c PSII reductant and PSI oxidant connected by the cytochrome b6f complex in Z scheme of non cyclic electron flow from H2O to NADP 9 Oxygen evolution by oxygen evolving complex OEC a Accumulation of 4 oxidizing equivalents by PSII b Cycles through 5 Mn oxidation states 10 Photophosphorylation a Driven by H gradient generated arising from light driven etransport reactions b Catalyzed by CF1 CFo ATPase 11 Structures of the thylakoid membrane complexes have been determined by X ray crystallography a Cyanobacterial PSII structure revealed detailed organization of protein subunits and cofactors and OEC separate core lighthavesting proteins CP43 and CP47 localized next to reaction center D1 and D2 proteins respectively in arrangement similar to PsaA and PsaB of PSI OEC assumes trigonal pyramid like structure b PSI structure determination in cyanobacteria trimeric and pea monomeric reveals how extensive Chl arrays use solar energy to transport electrons both complexes have12 core subunits containing core antenna and reaction center Chls pea complex also contains 4 LHCI light harvesting proteins on one side of core all cofactors were localized in the structures permitting elucidation of pathways of excitation energy and electron transfer c Structure of thylakoid membrane complexes completed by determination of cytochrome b6f complex in a thermophilic cyanobacterium forms a dimer like bc1 complex but has different domain arrangement outside core and additional prosthetic groups Chl a carotene of unknown function and heme x which may mediate cyclic electron flow from FNR Reading Lehninger Nelson Cox 2008 Principles of Biochemistry Chapter 19 pp 742 764 4th Edition Chapter 19 pp 723 743 The Light Reactions of Photosynthesis Convert 1 of the energy of sunlight to chemical energy serving as main energy input for the biosphere Also the main source of O2 in atmosphere Needed for all aerobic metabolism by heterotrophic organisms O2 arising from cyanobacterial photosynthesis drove evolution of animal life 2 billion years ago Provide energy to fix atmospheric CO2 into CH2O at the bottom of the food chain 1011 tons worldwide yr Harnessing of solar energy is essential for meeting future needs for a sustainable and renewable source of clean energy as the world runs out of fossil fuel Requires an improved understanding of the light reactions of photosynthesis Basis for construction of biosolar photoelectric cells for conversion of sunlight into electric current and molecular hydrogen as a fuel source Photosynthetic and Heterotrophic Organisms Exist in a Balanced Steady State in the Biosphere 6CO2 6H2O The light reactions result in O2 evolution and the generation of NADPH and ATP at the expense of solar energy The reducing power of NADPH and the energy of ATP hydrolysis are used to drive the carbon assimilation reactions in which CO2 is fixed into triose P to form starch and sucrose occur both in light and in darkness Ribulose 1 5 bisphosphate carboxylase oxygenase Rubisco catalyzes the first major step of carbon fixation in the Calvin cycle a process in which atmospheric carbon dioxide is converted to energy rich molecules such as sucrose most abundant enzyme on earth and major determinant of global CO2 levels How is solar energy captured in the light reactions and transformed into metabolically useful chemical energy Light C6H12O6 6O 2 The evolved O 2 arises from photolysis of H2O The Chloroplasts of Eukaryotic Oxygenic Phototrophs are the Sites of the Photosynthetic Reactions Photosynthesis can be Separated into Light Dependent and Carbon Assimilation Reactions Photosynthetic organisms serve as the ultimate source of all biological energy They trap solar energy and form NADPH and ATP as energy sources for making carbohydrates and other reduced organic compounds from CO2 and H2O releasing O2 into the atmosphere Aerobic heterotrophs use the O2 as the terminal electron acceptor in the catabolism of energy rich products of photosynthesis to CO2 and H2 O generating ATP The CO2 is recycled back into atmosphere to be fixed into new organic compounds by photosynthesis The overall equation for photosynthesis in higher phototrophs represents a redox reaction with H2 O providing the reducing equivalents for CO2 reduction Stroma lamellae Electron micrograph of chloroplast showing grana stacked thylakoid membranes and the


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