1 Photosynthesis Katerina Dadakova, Department of Biochemistry Figures adopted from Buchanan et al., Biochemistry & molecular biology of plants Using light energy to synthesize organic compounds from inorganic precursors Photosynthesis CO2 + 2H2O (CH2O) + O2 + H2O hν The free energy change is DG = +2840 kJ per mol of glucose formed • Oxygenic photosynthesis Photosynthesis Plant chloroplast Photosynthesis Two phases of photosynthesis Water oxidation and CO2 reduction are not obligately linked. Photosynthesis Light absorption The energy of a photon is inversely proportional to its wavelength E = hc/λ Energy levels in the molecule of the light-absorbing pigment chlorophyll Photosynthesis Light absorption pigment + acceptor hν pigment* + acceptor pigment+ + acceptor - Mechanisms of energy release: •relaxation •fluorescence •energy transfer •charge separation (photochemistry) Φ = number of products formed photochemically / number of quanta absorbed Photosynthesis Light absorption Chlorophyll a Chlorophyll b Bacteriochlorophyll Photosynthesis Light absorption Carotenoids Violaxanthin Phycobilins Photosynthesis Light absorption Chlorophylls Other pigments Photosynthesis Light harvesting LHC-II structure Monomer Trimer Photosynthesis The reaction center complex •Reaction centers are integral membrane protein complexes involved in conversion of light energy into chemical products • •Plants contain two different reaction center complexes: Photosystem I and Photosystem II • • Reaction centers contain both chlorophyll and electron acceptor molecules Carrier PSI PSII Chl P700 P680 A0 Chlorophyll a Pheophytin a A1 Phylloquinone Plastoquinone (QA) A2 Fe-S center Plastoquinone (QB) Chl AOA1A2 Chl*AOA1A2 Chl+AO-A1A2 Chl+AOA1-A2 Chl+AOA1A2- hν Photosynthesis Electron transport pathways The chloroplast noncyclic electron transport chain produces O2, NADPH, and ATP and involves the cooperation of PSI and PSII Strong oxidant Reductant Oxidant Strong reductant Photosynthesis Electron transport pathways Photosynthesis Oxidation of water 2H2O O2 + 4H+ + 4e- Barber, Nature Plants 3 (2017) Photosynthesis PSII QH2 + 2PCox + 2H+stroma Q + 2PCred + 4H+lumen 1.P680 QA QB 2. 2.P680+ QA- QB 3. 3.P680 QA QB- 4. 4.P680 + QA - QB- QB2- + 2H + QBH2 Photosynthesis Cytochrome b6f Photosynthesis PSI Structure and redox chemistry of flavin coenzymes Photosynthesis Cyclic electron transport chain Photosynthesis ATP synthesis in chloroplasts Chloroplasts synthesize ATP by a chemiosmotic mechanism driven by a proton gradient Photosynthesis Organization of the thylakoid membrane Component (%) Granal thylakoids Stromatal thylakoids PSII 85 15 PSI 10 90 Cytochrome b6f complex 50 50 LHC-II 90 10 ATP synthase 0 100 Plastocyanin 40 60 Photosynthesis Energy distribution between PSI and PSII Balanced excitation of both photosystems is required for maximum electron transfer efficiency Photosynthesis Carbon reactions in C3 plants • C3 plants produce a three-carbon compound as the first stable product. • • In these plants, photosynthetic carbon fixation is catalyzed by a single enzyme, Rubisco. • • Rubisco, probably the most prevalent protein on Earth, constitutes up to half the protein of the chloroplast stroma • Photosynthesis Photosynthesis Carbon reactions (Calvin cycle) Fixation of one molecule of CO2 requires two molecules of NADPH and three of ATP. Photosynthesis The Calvin cycle regulation Activation of Rubisco by carbamylation The ferredoxin-thioredoxin system Photosynthesis C4 fixation mechanism • C4 plants contain two distinct CO2-fixing enzymes • • They have specialized foliar anatomy: • Mesophyll cell Bundle sheat cell •They form four-carbon organic acids as the first products of CO2 fixation Photosynthesis CAM fixation mechanism