Class Note for BIOC 460 at UA 2
Class Note for BIOC 460 at UA 2
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Date Created: 02/06/15
Lecture 29 Photophosphorylation Key Concepts Overview of photosynthesis and carbon fixation Chlorophyll molecules convert light energy to redox energy The Z scheme of photosynthetic electron transport Protein complexes in the photosynthetic ETS Key Concept Question in Photosynthesis How do photosynthetic reaction centers convert light energy into redox energy Biochemical Applications of photosynthesis DCMU dichlorophenyl dimethylurea is a broad spectrum herbicide that functions by blocking electron flow through photosystem II and is used to reduce weeds in noncrop areas Another herbicide paraquat prevents reduction of NADP by accepting electrons from intermediate reductants in photosystem I Paraquat was used extensively in the 1980s as an aerial herbicide to destroy illegal fields of marijuana and coca plants in North and South America however its use was discontinued because smoking paraquatcontaminated plants causes lung damage Overview of photosynthesis and carbon fixation Figure 1 shows where the photosynthetic electron transport system and Calvin cycle carbon fixation fit into the metabolic forest Together these two processes convert sunlight energy into chemical energy ATP NADPH triose phosphate and in the process oxidize H20 to form 02 The photosynthetic electron transport system is often referred to as the light reactions of photosynthesis and consists of two photosystems PSI and PS whereas the Calvin cycle has been called the dark reactions However as you will see in lecture 30 the term quotdark reactionsquot can be misleading because the Calvin cycle is most active in the light when ATP and NADPH levels are high In the dark plants actually depend on newly synthesized carbohydrate glycerladehyde3P as a metabolic fuel for mitochondrial respiration just like any other aerobic organism Light excitation of Photosystems l and II results in oxygen evolution from the splitting H20 and the generation of chemical energy in the form of ATP and NADPH Plants use this chemical energy ATP and NADPH to convert C02 into sugars carbon fixation via the Calvin cycle which figure 1 quot Citrate Bioc 460 Dr Miesfeld Fall 2008 El Cl CH N Nf CH H 3 lt H gt a f 0 Cl C 0 7 NCH Paraquat 32 methyl viologen DCMU diuron Dichlorophenyh dimethylurea Paraquat i NADPH PGBOquot DCMU l l GA 03 mm l l l l l l H20 02 P680 Cycle J 39r Carbon l 39 39 SunI h 39 Electron F39Xatmquot 9 Transport Oxidative Ph h Phosphorylation atos vm es39s H 0 oz 2 ATP NADPH takes place in the stroma We will describe the Calvin cycle and carbon fixation in lecture 30 The 1 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 combined reactions of photosynthetic electron transport system and carbon fixation can be written as Figure 2 H20 C02 light energy gt 02 CHZO in which CHZO represents hexose sugars such as starch and sucrose lmportantly even though this is written as a balanced equation 02 generation is the result of H20 oxidation whereas the C02 is used to synthesize carbohydrate As you will see shortly it requires the oxidation of two H20 to generate one 02 Moreover since six C02 molecules are 7quot 39 required for the synthesis of each molecule of glucose by the Calvin cycle this reaction can be rewritten as l L 39 25H 6 H20 6 C02 energygt C6H1206 6 02 The change in standard free energy AG 39 for this reaction is 2868 kJmol which is overcome by the energy potential stored in the products of photosynthetic electron transport namely ATP and NADPH It was discovered over 200 yrs ago that live plants and respiring animals could coexist in a closed system for a limited period of time as long as H20 and light were provided As illustrated in figure 2 Joseph Priestly an Englishman with training in chemistry and theology showed that in the presence of light plants give off a gas that keep a mouse from passing out in a closed container He later discovered that this gas was in fact oxygen A modern version ofthe Priestly experiment was attempted in the early 1990s by a private company called Space Biospheres Ventures which built Biosphere 2 outside of Tucson figure 3 The experiment did not actually work very well because C02 levels built up inside the sealed environment and periodic C02 removal was needed Biosphere 2 is now a research center for the University of Arizona College of Science Figure 3 It is now time to answer the four metabolic pathway questions that relate to the photosynthetic electron transport system and the Calvin cycle Together these two pathways are sometimes collectively called quotphotosynthesisquot 1 What does photosynthetic electron transport and Calvin cycle accomplish for the cell The photosynthetic electron transport system converts light energy into redox energy which is used to generate ATP by chemiosmosis and reduce NADP to form NADPH Calvin cycle enzymes use energy available from ATP and NADPH to reduce C02 to form glyceraldehyde3 P a three carbon carbohydrate used to synthesize glucose Photosynthetic cells use the carbohydrate produced by the Calvin cycle as a chemical energy source for mitochondrial respiration in the dark Photosynthetic organisms are autotrophs because they derive energy from light rather than from organic materials 2 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 2 What are the net reactions of photosynthetic electron transport system and the Calvin cycle Photosynthetic electron transport system production of A TP and O2 2 H20 8 photons 2 NADP 3 ADP 3 Pl 9 02 2 NADPH 3 ATP Calvin cycle six turns yields glucose 6 C02 12 NADPH 18 ATP 12 H20 9 Glucose 12 NADP 18 ADP 18 Pl 3 What are the enzymes in the photosynthetic electron transport system and the Calvin cycle Protein components of the photosynthetic electron transport system three protein complexes are required for the oxidation of H20 and reduction of NADP photosystem II P680 reaction center cytochrome bf and photosystem I P700 reaction center Chloroplast ATP synthetase enzyme responsible for the process of photophosphorylation which converts protonmotive force energy available from the electrochemical proton gradient into net ATP synthesis through a series of protondriven conformational changes This enzyme is very similar to mitochondrial ATP synthetase in both structure and function Ribulose 15 bisphosphate carboxylaseoxygenase rubisco the most abundant enzyme on planet earth is responsible for CO2 fixation in the first step of the Calvin cycle Rubisco activity is maximal in the light when stromal pH is 8 and Mg levels are elevated due to proton pumping proton influx to the thylakoid lumen is accompanied by MgP efflux to stroma 4 What are examples of photosynthetic electron transport system and Calvin cycle in real life Many types of herbicides block photosynthetic electron transport through photosystem II or function as photosystem I electron acceptors Accidental exposure to herbicides is not generally harmful to humans however paraquat an herbicide used to control coca and marijuana crops in South America can cause lung damage if inhaled The chloroplast ATP synthase is functionally similar to the mitochondrial ATP synthase except that 4 H are required for every ATP and the energy required for establishing the electrochemical proton gradient is derived from photon absorption rather than metabolite oxidation gure 4 Figure 4 21 ZNADP Stroma l llII 39yloehromLbnf Ft ZNJwPHSADTquot 31 JATP p5 m Conl plex hi it Irv It v r l wmpx I H 4 l r es 1wm C1quot ch 2 x l 4 quot Ar Fd NADP 2 ml uuusc 39l hylakuid space l ATP symlmse lzn 9 In 12 pages Bioc 460 Dr Miesfeld Fall 2008 Chlorophyll molecules convert light energy to redox energy The photosynthetic machinery in eukaryotic plant cells is contained within M organelles called chloroplasts which Inner number 10500 per cell Chloroplast membrane like mitochondria contain their own DNA and carry out protein synthesis within the organelle to make proteins required for chloroplast function Both chloroplasts and mitochondria are thought to represent an endosymbiotic relationship which was established when a eukaryotic I cell engulfed a bacterium In the case of lntermembrane 5 plant cells the photosynthetic bacterium Space ramellae was most likely related to a prokaryotic species called cyanobacteria which has a photosynthetic electron transport system resembling that of chloroplasts As shown in gure 5 chloroplasts contain three membranes The outer membrane is permeable to most metabolites whereas the inner 59m Outer Thylakoid membrane Thylakoid space membrane and the thylakoid H E thto membrane are essentially I I 55ng GAP r nggrsseg sucrose Elanttlssues Impermeable and form a barrrer pathway 39 between two compartments ADP ATP within the chloroplast These F39i lt compartments consist of the 7 thylakoid lumen which is the GARVL aqueous phase inside the Cyt 5 g thylakoid membrane and the x Calvin stroma which is the aqueous I phmn Wequot phase outside of the thylakoid 7y H 1 mm membrane Similar to the inner mitochondrial membrane the thylakoid membrane contains all of the protein complexes that constitute the electron transport system and is the location of the ATP synthase complex The thylakoid membrane is the physical barrier giving rise to the electrochemical proton gradient 39 ADP Chlgmplast As discussed in lecture ADP quot innerandouter 28 proton motive force across 39 m lpl memb39anes membranes is all about directionality In the case of photosynthesis light activation of the chloroplast electron transport system located in the thylakoid membrane results in proton pumping into the thylakoid lumen as shown in gure 6 The oxidation of H20 takes place in the thylakoid lumen and generates protons which also contribute to the proton gradient The ApH across the thylakoid membrane pH 5 inside the lumen relative to pH 8 in the stroma causes protons to ow out through the chloroplast ATP 4 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 synthase complex leading to ATP synthesis in the stroma The final electron acceptor in the photosynthetic electron transport system is NADP which accepts a pair of electrons from ferredoxinNADP reductase to produce NADPH in the stroma The Calvin cycle enzymes g located in the stroma use energy from ATP hydrolysis and NADPH oxidation to convert 002 into glyceraldehyde3P GAP As we will describe later this triose phosphate is either used to synthesize starch in the stroma as an energy reserve or transported to the cytosol and converted to hexose sugars for export to other plant tissues GAP can also be metabolized by the glycolytic pathway in the 2 Change buffer stroma or cytosol to produce ATP for cellular l pH 8 processes One of the key biochemical experiments supporting Peter Mitchell39s Chemiosmotic Theory was performed using a chloroplast suspension that was maintained in the dark and treated with buffers at different pH values As illustrated in figure 7 chloroplasts were initially suspended in buffer at pH 4 high H concentration and allowed to equilibrate They were then switched to a buffer at pH 8 low H concentration and ADP and 32P were added to initiate proton flow through the chloroplast ATP synthase complex Within just 15 seconds a significant amount of radioactive ATP was synthesized in the chloroplast stroma These results were consistent with Mitchell39s chemiosmotic theory and demonstrated that an electrochemical proton gradient was sufficient to drive ATP synthesis even in the absence of a functional energy conversion system redox or photon absorption Light energy is absorbed by numerous accessory Figure 8 pigments which can transfer the absorbed energy to nearby Light Harvesting Accessory Pigments reaction centers containing specialized chlorophyll molecules These accessory pigments function as light harvesting antenna as shown in figure 8 Chlorophyll molecules are polycyclic planar structures that resemble hemoglobin and maximally absorb light in the blue 450 nm and red 700 nm range of the visible spectrum which is why W they appear green reflected light Ratherthan having a Fe2 t 39 atom in the middle of the heterocyclic five ring system like 39 hemoglobin chlorophylls have a Mg2 coordinated to nitrogens figure 9 Chlorophylls also have an extended hydrophobic side chain which anchors the chromophore to various pigment proteins The alternating single and double bonds of the chlorophyll polyene structure are essential to its light absorbing properties because it results in delocalized electrons above and below the heterocyclic ring These electrons have energy differences with the next higher orbital that 5 of 12 pages Allow to equilibrate Add ADP and P Quantitate amount of ATP synthesized gt Bioc 460 Dr Miesfeld Fall 2008 corresponclto the g cghlorophylla Chlorophyll b of photons in the Visible X range This is important quotl CH I W because photon we 7 gt inc 39 I absorption is an all or r 39 V none phenomena in which ii the photon energy must 39 HI I H be equal to the energy 39 required to lift the electron o 0 a Y 0 0 to the next higher orbital quot w L m The two chlorophylls found in plants chlorophyll a and chlorophyll b differ only slightly in the structure of the heterocyclic ring CH3 side group in chlorophyll a compared to CH0 side group in chlorophyll b This difference however is sufficient to alter the light absorbing properties of these two chromophores in the blue and red range of the spectrum As shown in figure 10 photon absorption of the appropriate wavelength excites an electron from its ground state and lifts it to a higher orbital where it is said to be in an quotexcitedquot state sometimes denoted with an asterisk Chl In most cases the chlorophyll electron will return to its ground state and through a process called resonance energy transfer or exciton transfer pass the absorbed energy to a nearby chlorophyll molecule This energy transfer process is very important in photosynthesis because it serves to quotharvestquot light energy by g allowing one chlorophyll molecule to absorb the photon and then pass the energy along to a nearby molecule Ground quot without it being lost in the from of heat or state fluorescence ml The second possibility is that the excited chlorophyll electron will be Excited transferred to a nearby acceptor State Chli molecule of lower reduction potential resulting in oxidation of the chlorophyll molecule loss of electron and reduction Resonance energy transfer Photooxidation l of the acceptor molecule gain of 1 electron This reaction called Chl acm photooxidation or energy Chm Ch 39 GE 9 3 oxidized transduction takes place in the reaction centers and is the key to energy quotquot ene39gy elmm CH3 U H In Light energy transfer transfer conversion in photosynthesis because it results in photoinduced charge separation Once photooxidation occurs Chlz quot Clquot the reduced acceptor molecule now negatively charged donates the electron to another acceptor molecule of higher reduction potential more positive E and thereby activates the photosynthetic electron transport system In the chloroplast PSII reaction center the electron acceptor is a molecule called pheophytin which becomes negatively charge as denoted by 39Pheo39 Together energy transfer and photooxidation permits closely arranged chlorophyll molecules to absorb photons and pass the energy along to reaction center complexes where it can be converted 6 of 12 pages Pheo gt Pheo reduced Bioc 460 Dr Miesfeld Fall 2008 to redox energy through a charge separation reaction Importanty the oxidized chlorophyll molecule now positively charged ChF returns to the ground state by accepting an electron through a coupled redox reaction involving the oxidation of H20 This process of Oz evolution takes place in the manganese center present in the thylakoid membrane and is ultimately the source of electrons needed for the photosynthetic electron transport system In this sense H20 has the same role in photosynthesis as NADH and FADHz have in mitochondrial electron transport As shown in figure 11 the absorption spectra of chlorophylls a and b are slightly different with both absorbing light in the blue 400500 nm and red 600700 nm range Photosynthetic organisms contain a number of other chromophores as well that are capable of EQM absorbing light at wavelengths in the green Sunlight and yellow range of the visible spectrum 450 52222 650 nm These pigments include 3 carotene an accessory pigment in plants and Chlorophwa phycocyanin a phycobilin type chromophore P Y quoty quotquot found in red algae While green plants contain large amounts of chlorophylls a and b and in most cases Bcarotene the phycobilin chromophores phycocyanin and phycoerythrin provide an evolutionary I lt Chorophylla advantage to aquatic microorganisms that are able to absorb light filtering through the upper layers of the ocean Because of their unique chemical properties chlorophyll molecules in photosynthetic reaction center complexes are responsible for photooxidation however most chlorophyll molecules in photosynthetic membranes function as light harvesting antennae and are associated with chromophore proteins that participate in energy transfer reactions rather than photooxidation These chromophore proteins are called light harvesting LH complexes and are the most abundant proteins in the thylakoid membrane of chloroplasts The important role of chlorophyll antennae in capturing light energy for photosynthesis was first suggested by H we 12 experiments done in 1932 by Robert g 39 photon Emerson and V lliam Arnold Using isolated e spinach chloroplasts under conditions of saturating light they found that one Oz was generated for every 2400 chlorophyll molecules Since 8 photons are required to form each 02 and Oz generation only occurs in reaction center complexes this means that roughly 300 electron transfer events 24008 in LH complexes occurfor every one photooxidation reaction in an reaction center complex Figure 12 illustrates how energy transfer reactions between LH II and LH l complexes in the chloroplast thylakoid membrane function as highly efficient quotsolar panelsquot to capture light energy for photooxidation in reaction center complexes Carotene Phycocyanin Absorption 300 400 500 600 700 800 Wavelength nm LH complexes LHl complex 7 of 12 pages Bloc 460 Dr Miesfeld Fall 2008 The Z scheme of photosynthetic electron transport The photosynthetic electron transport system in plants consists of two linked electron circuits each requiring an input ofenergy from light absorption at PSII and PSI reaction center complexes to initiate electron flow Figure 13 depicts an energy diagram showing how photon absorption by the PSII reaction center complex results in electron flow from H2O to plastocyanin providing energy to Figure 13 r nllxl translocate H across the thylakoid membrane A second photon is absorbed by the PSI reaction center which provides the energy necessary to initiate another series of redox reactions culminating in the reduction of NADP to form NADPH Electron flow through proteinlinked redox reactions involves numerous electron carriers including FeS centers the hydrophobic molecule plastoquinone Q which is which is reduced to form plastoquinol QH2 and is analogous to ubiquinoneubiquinol in the mitochondrial electron transport system Plastocyanin has the same job in photosynthetic electron transport as does Figure 14 cytochrome c in mitochondrial electron transport The electron path on this energy diagram looks like the letter quotNquot but it is often referred to as the quotZ schemequot because it was originally presented as a vertical energy diagram think of the Z scheme as a quotzig zagquot energy diagram lmportantly the oxidation of 2 H20 to generate 1 02 3 ZHZO ATP 2 NADPH requires 8 e e 7 8 Photons photons with 4 photons being absorbed at each of 02 4H the two reaction centers ZHZO 1 002 CH20 t 02 t H20 EM co2 CHZO H20 8 of 12 pages Protein complexes in the photosynthetic ETS Photosystem ll oxidizes H20 to qenerate 02 Photosystem PSII contains chlorophylls a and b and absorbs light at 680nm This is a large protein complex that is located in the thylakoid membrane as shown in figure 15 Excitation of a reaction center chlorophyll molecule in PSII with a photon at a 680 nm results in photooxidation of the excited chlorophyll molecule P680 to create a positivelycharged chlorophyll P680 molecule P680 and a negativelycharged electron acceptor molecule called pheophytin Pheo39 As shown in figure 16 the extra electron in pheophytin is then passed to a tightly bound molecule of plastoquinone PQA which subsequently donates it to a loosely bound plastoquinone PQB molecule Once PQB acquires two electrons from PQA to become fully reduced PQBHz it translocates through the thylakoid membrane where it binds to the cytochrome bf complex and donates electrons one at time using a Q cycle mechanism lmportantly the charge separation event at P680 creates a powerful oxidant in P680 which is able to extract an electron from H20 through a series of reactions involving a manganese cluster Mn4 and thereby regenerate P680 for another round of photooxidation However since chlorophyll P680 requires only one electron to be reduced the four electrons obtained from the oxidation of 2 H20 need to be temporarily stored in an oxygenevolving complex OEC consisting of four manganese Mn4 atoms a calcium atom a chloride atom and a reactive tyrosine residue called Tyrz Each time the chlorophyll P680 loses an electron due to photooxidation an electron from the oxygenevolving complex is donated to Tyrz which donates it to P680 to return it to ground state figure 17 Taken together the combined effect of these redox reactions in PSII is that light energy absorbed by the P680 reaction center extracts electrons from H20 which are used to reduce plastoquinone PQB to form plastoquinol PQBHz Since two H20 molecules need to be oxidized in order to generate molecular oxygen 02 and one photon is required for each photooxidation event the redox reactions of PSII can be summarized as 2PQB 2H20 4 photons gt 2PQBH2 02 Bioc 460 Dr Miesfeld Fall 2008 Figure 15 D2 D1 g g 1 Thylakoid lumen Special 7 pair Figure 16 4 H photons Stroma 2PQB Cytbf 3H2 eh P680 achlorophyll up Thylakoid lumen 2 H20 02 4 H Figure 17 t e Photon 139 Oxygen 39 Evolving 5 Complex 9 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 Cytochrome bf functions as a proton pump Electrons are transported from PSII to the cytochrome bf complex by PQBH2 which has the same functional role of ubiquinol UQHZ in the mitochondrial electron transport system In mitochondria the two electrons in UQHZ are transferred to two molecules of cytochrome 0 whereas in chloroplasts the two electrons in PQBH2 are donated to two molecules of plastocyanin Plastocyanin is a small soluble Cucontaining protein located on the thylakoid lumen side of the membrane that shuttles electrons one at Figure 18 a time from the cytochrome bf complex to the PSI reaction center The Q cycle in the cytochrome bf complex operates in much the same way as we saw in mitochondrial complex Ill namely it sequentially oxidizes two molecules of PQBHZ each time transporting one electron to plastocyanin and using the other electron to regenerate PQBH2 from PQB to replenish the plastoquinoneplastoquinol pool Figure 839 illustrates that the split electron path in the cytochrome bf complex sends one electron from PQBH2 to plastocyanin through the FeS cluster and cytochrome f and the other electron goes through the cytochrome bL and bH carriers to the first PQB molecule that has moved Figure 19 from the OP site to the ON site where 4photons reduction leads to the formation of a semiquinone intermediate After a second PQBH2 molecule enters the OP site and is oxidized three things happen 1 this PQB molecule is released back into the membrane 2 one electron is used to reduce a second molecule of plastocyanin and 3 the other electron is used to reduce the P05 semiquinone intermediate in the ON site leading to formation of the quotpaybackquot PQBH2 molecule figure 19 These reaction steps are thought to be the same as those worked out for the Q cycle in complex III of the mitochondrial electron transport system resulting in the net translocation of 4 H from the stroma to the thylakoid lumen Thylakoid lumen 12H Figure 20 Photosystem I qenerates NADPH for carbohydrate a synthesis 4Fe4s a The PSI reaction center functions to transfer the clusters 37 a electrons from plastocyanin to NADP using the energy Stroma Viv derived photon absorption by a pair of chlorophyll v quot1 f 1 7 P700 molecules contained within the large protein y 39 39 V f complex figure 20 As shown in figure 21 39 V 33 photooxidation of chlorophyll P700 molecules in the d 7 5 PSI reaction center increases their oxidation potential Th I k 39dl resulting in electron transfer through two nearby ya 039 men 5 pecial pair 10 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 chlorophyll molecules the second of which is called chlorophyll Ao which passes the electron to phylloquinone also called QK The source of the P700 electron ultimately used to reduce NADP is from plastocyanin which docks on the luminal side of the thylakoid membrane Similar to the role of H20 oxidation in reducing the P680 cation plastocyanin reduces the P700 cation to return it to the ground state Electrons are transferred one at a time through the PSI complex and passed to an FAD coenzyme within the flavoprotein ferredoxinNADP reductase The FAD group accepts one electron at a time from two sequential reduction reactions involving ferredoxin as Fi ure 21 Fi ure 22 NADP Ferrodoxin NADPHH NADP reductase Stroma r r Ferrodoxin y NADP 39 7 reductase Ferrodoxin a Photons fl 39 39 I lquot 39 Thylakoid lumen L shown in gure 22 The fully reduced FADH2 moiety is then able to pass two electrons to NADP in the form of a hydride ion to generate NADPH and thereby complete the photosynthetic electron transport system Note that since a H from the stromal side is used to reduced NADP this reaction contributes to the ApH across the thylakoid membrane The NADPH produced by ferredoxinNADP reductase is used by the Calvin cycle to generate carbohydrate by 002 xation as described later in the chapter Paraquat is a potent herbicide that kills plants by accepting electrons from PSI and donating them to 02 This illicit electron capture blocks NADPH production and produces superoxide anion 02quot and hydrogen peroxide H202 that are highly toxic to cells gure 23 Paraquat was used in the 1980s as an aerial herbicide to destroy illegal fields of marijuana and coca plants in North and South America The use of paraquat was suspended for this purpose when it was found that growers were harvesting the sprayed plants before they wilted and processing them for distribution Smoking the contaminated marijuana led to numerous cases of paraquat toxicity in humans resulting in 19 severe lung damage due to superoxide mediated destruction of cell membranes Figure 23 f Pa raquatoxidized e ii Superoxide H20 dismutase 11 of12 pages Bloc 460 Dr Miesfeld Fall 2008 ANSWER TO KEY CONCEPT QUESTION IN PHo TOSYNTHESIS How do photosynthetic reaction centers convert light energy into redox energy Photosynthetic reaction centers convertight energy into redox energy through photon absorption by chlorophyll molecules in the PSII and PSI reaction center complexes This provides energy to lift a chlorophyll electron to a higher orbital where it is said to be in an quotexcitedquot state Photooxidation occurs when this excited electron is donated to a nearby acceptor molecule pheophytin in the case of PSII and chlorophyll A0 in PSI leading to a separation of charge The reduced acceptor molecule now negatively charged donates the electron to another acceptor molecule of lower reduction potential thereby activating the photosynthetic electron transport system The reaction center chlorophyll now positively charged in PSI is reduced by an electron derived from the oxidation of H20 which returns the chlorophyll molecule to the ground state whereas the reaction center chlorophyll in PSII is reduced by plastocyanin 12 of 12 pages
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