Lecture07 - MCDB321 Plant Physiology MCDB321 Feb 01 2011...

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Unformatted text preview: MCDB321 Plant Physiology MCDB321 Feb. 01, 2011 Lecture 7 Photosynthesis-Light reactions 1. Pigments A. Chlorophylls and carotenoids B. Antenna pigments C. Absorption/action spectra D. 4 different ways to rid of excessive energy D. 2. Two photosystems each with a reaction center 2. A. 2 puzzling experiments: Red-drop effect & the enhancement effect B. The Z-scheme of the two PSs C. Structure of the photosynthetic apparatus 3. Four membrane complexes & three mobile electron carriers to transport e- and H+ three A. distribution pattern B. the photosystem II (OEC, S-clock) C. the Cyt b6f complex and the Q cycle D. the photosystem I (Fd-NADP reductase & the cyclic photophosphorylation) E. the ATP synthase (PMF, chemiosmotic mechanism) Photosynthesis is the chemical process by which green plants use light energy to produce food (Photosynthesis Means: “Synthesis using Light”) Photosynthesis can be simplified into two reactants (carbon dioxide and water) water and two products (glucose and oxygen), represented by the chemical equation: and oxygen 6CO2 (g) + 6H2O (l)-------> C6H12O6 (aq) + 6O2 (g) Chlorophylls are the major photosynthetic pigments that absorb the energy of sunlight that Carotenoids are accessory pigments (the light energy absorbed by carotenoids is transferred to chlorophylls for photosynthesis) 1. Bacteriochlorophyll a 1. 2. Chlorophyll a 3. Chlorophyll b 4. Phycoerythrobilin 5. carotenoid Action Spectra Relate Light Absorption to Photosynthetic Activity Action [The magnitude of a response of a biological system to light as a function of wavelength] Light absorption by chlorophylls mediates Light the oxygen evolution the of the whole chloroplast Many pigments, including chls, serve as an antenna, collecting light and transferring its energy to the collecting reaction center, where real photochemical reaction reactions takes place reactions Each reaction center is associate with 250-300 photosynthetic pigments. The large number of photosynthetic antenna pigment molecules in each photosynthetic unit enables its reaction center to be constantly supplied with quanta of energy the excited Chl has several possible pathways for disposing of its available energy. 1. RE-EMISSION FLUORESCENCE 2. RE-EMISSION AS HEAT 3. RESONANCE TRANSFER 4. PHOTOSYNTHESIS [measure the effectiveness to drive photosynthesis of absorbed photons] photosynthesis absorbed Red Drop Effect with far-red light Red Enhancement Effect with both red and far-red light These experiments led to the discovery These that two photosynthetic complexes, photosystem I and II (PSI and PSII) photosystem II PSII Operate in series to carry out the early Operate Energy storage reactions of photosynthesis Photosystem II, preferentially absorbs red light of 680 nm, produces a very strong oxidant, capable of oxidizing water and a weak reductant strong Photosystem I, preferentially absorbs far-red light of 700 nm, produces a very strong reductant capable of reducing NADP+, and a weak oxidant strong and Four Major Thyroid Membrane Protein Complexes Four Involved in the Light Reaction Involved Where the dark reaction takes place reaction Photosystem I and II are Spatially Separated Photosystem in the Thyroid Membrane PSII is located in the grana lamella While PSI is found in the stroma lamella and at the edge of the grana lamella. D1: binds the reaction center, the majority of the e--transport cofactors, & majority -transport the Mn cluster the D2: helps binding P680 and binds QA CP43/CP47: core light harvesting system, directly/indirectly involved in H2O splitting Light-harvesting Light-harvesting complexes complexes associated with associated the PSII 1. Light excites P680 1. 2. The initial photochemical act results in the formation of 2. P680+ and Pheo-, a charge separation. This step is the P680 charge This actual conversion of light energy to chemical energy actual It is essential that this charge separation be stabilized It Charge stabilization in PSII is achieved in two ways: Charge A. pheophytin quickly passes e- to downstream A. electron acceptors electron B. the P680+ is the strongest oxidant that is capable B. of extracting electrons from water of 2Q + 2H2O ----> 2QH2 +O2 The electrons that reduce P680 + are most immediately supplied by a cluster of 4 Mn2+ ions associated with a small supplied protein complex called oxygen-evolving complex protein oxygen-evolving In order to complete the oxidation of two H 2O molecules and produce a single O2 molecule, the PSII must be excited 4 times times The cytochrome b6f complex is cytochrome a plastoquinone-plastocyanin oxidoreductase, transferring two electrons from QH2 to PC It contains 2 Cyt b, 1 Cyt f, 1 quinone, and 1 Rieske FeS protein Q-cycle: (involving Q and 2 Cytb) Double the number of protons transferred from the stroma transferred side to the lumen side of the side thylakoid membrane In the meanwhile, a similar light driven charge separation occurs at the PSI reaction center, P700, P700--> P700*-->P700+ + eP700--> e- is transferred to A0, a chlorophyll a molecule, is then to A1, phylloquinone (vitamin K1), followed by additional e- transfer through 3 membrane associated (FeSX, FeSA, FeSB) and 1 soluble and (ferredoxin) FeS proteins. (ferredoxin) The generated P700+ receives an electron from PC The The membrane associated Ferrodoxin-NADP The reductase reduces NADP+ to NADPH using the electron donated by the Fd(red). electron PSI units may transport electron independently of PSII, (cyclic electron transport) PSI In this case, the electron from Red-Fd transfers back to the PQ of the Cytbf complex. 4 ...
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