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BISC 367 - Plant Physiology Lab Spring 2009 Plant Biology Fall 2006 Notices: • • General reading • Taiz and Zeiger: Chapter 7, Photosynthesis: the Light Reactions Photosynthesis - light • Photosynthesis (P/S) uses light energy to build carbon compounds Light energy CO2 + H2O Inputs: CO2 H2O Light Outputs: Carbohydrate (CHO) O2 (CH2O)n + O2 Photosynthesis - light • P/S is divided into 2 stages Light NRG oxidizes H2O Reductant & NRG are used to reduce CO2 Photosynthesis - light P/S takes place in chloroplasts (cp) Stroma lamellae Unstacked membrane Thylakoid Light rxns occur here Grana lamellae Stacked membranes Carbon reduction rxns occur here Stroma Photosynthesis - light Pigments (chlorophylls and carotenoids) exist in antenna complexes – – Harvest and transfer light NRG to chlorophylls at the reaction centre • NRG is transferred from pigment to pigment by resonance transfer At the reaction centre NRG is converted to chemical (redox) NRG Antennal pigment/ reaction centre complexes are also referred to as photosystems There are 2 photosystems • PSI • PSII Photosynthesis - light • Light NRG moves down an energy gradient towards the reaction centre Photosynthesis - light • PSI and PSII operate in series – PSI absorbs far-red light (>680 nm) = P700 • – Produces a very strong reductant PSII absorbs red light (680 nm) = P680 • Produces a strong very strong oxidant – Can oxidize water Ultimate source of e- Z scheme of Photosynthesis Photosynthesis - light • 4 protein complexes embedded in the thylakoid membrane are involved in the light reactions • • • • PSII Cytochrome b6f complex PSI ATP synthase Photosynthesis - light Oxidation of water: • Performed by the oxygen evolving centre (OEC) of PSII 2H2O O2 + 4e- + 4H+ Water is extremely stable - oxidation is not easy! 4H+ are released into thylakoid lumen • OEC is located on lumenal side of membrane Photosynthesis in an intact plant Photosynthesis in isolated cps O2 is evolved as e- are passed to Ferricyanide Some herbicides block e- flow Photosynthesis – blocking e- transport DCMU Electron flow is blocked, O2 evolution is inhibited Photosynthesis – flavin mononucleotide (FMN) DCMU O2 is consumed as FMN passes e- to O2 to generated H2O2 O2 can accept e- from PSI • Mehler reaction (light dependent consumption of O2): O2 + Fdred O2- + Fdox • 5 - 10% of e- generated by PSI react with O2 • Plants have built in systems to “scavange” ROS • SOD (superoxide dismutase): O2- + O2- + 2H+ H2O2 + O2 • H2O2 must be reduced to prevent reaction with O2. to form the highly toxic hydroxyl radical (OH ) O2 can accept e- from PSI • Asada-Halliwell pathway reduces H2O2: H2O 2 H2 O Ascorbate GSSH NADPH Monodehydroascorbate GSH NADP+ PSI
