LIGHT REACTION OF PHOTOSYNTHESIS

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LIGHT REACTION OF PHOTOSYNTHESIS
1. Photosynthetic reaction 1) 6 CO2 6 H2O  ?
C6H12O6 6 O2 2) not a reverse of the
degradation pathways - ?G lt 0 - ATP coupling -
alternative pathway 3) reduction - NADPH 4) light
reaction - electron transport from H2O to NADP ?
proton gradient - ATP - NADPH 5) dark reaction -
CO2 H2O (ATP NADPH) ? CH2O O2

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2. Chloroplast 1) stroma - dark reaction 2)
thylakoid membrane - 40 galactolipids - 10
phospholipids - 4 sulfolipids - conversion of
light to chemical energy 3) chlorophyll -
magnesium porphyrin - photoreceptor - chlorophyll
a 400 - 700 nm - visible light 4 - 80 kcal/mole

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4) photosynthetic bacteria - cyanobacteria -
purple bacteria 5) endosymbiosis - chloroplasts
semiautonomous organelles - chloroplast DNA -
ancestor of a cyanobacterium

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3. Electron transfer 1) photoinduced charge
separation - D A ? D A- 2) electron transfer
at the reaction center - donor molecule ?
chlorophyll ? acceptor molecule

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• 4. Bacterial photosynthetic reaction center
• 1) Rhodopseudomonas viridis
• - a purple bacteria
• - homologous to photosystem II
• four polypeptides (L, M, H, C) L and
• M, transmembrane proteins
• 2) atomic resolution
• P960 (special pair) ? bacteriochlorophyll (BChl)
  ?
• bacteriopheophytin b (BP) ? QA (quinone) ? QB ?
  
• cytochrome bc1 ? cytochrome c2 ? P960

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5. Photosystem II 1) two photosystems - lt 680 nm
(photosystem II), lt 700 nm (photosystem I) 2) a
core with a reaction center - D1, D2
transmembrane proteins - P680 chlorophyll
dimer - P680 ? pheophytin (Ph) ? QA
(plastoquinone) ? QB ? QH2 3) an oxygen-evolving
complex - P680 cation a very strong oxidant -
extraction of electrons from water the Mn
center a cluster of four manganese ions, a
charge accumulator - 4 photons for one oxygen
molecule

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• 6. Photosystem I
• 1) electron-tranfer from QH2 to photosystem I
• QH2 ? (cytochrome bf complex) ? plastocyanin
  (PC) ?
• photosystem I
• 2) cytochrome bf complex 4 subunits
• cytochrome f, cytochrome with 2 b-type hemes,
• a Fe-S protein, a 17-Kd polypeptide
• - Q-cytochrome c oxidoreductase in mitochondria
• - electron-transfer from QH2 to plastocyanin Q
  cycle
• - two proton pumping per QH2 oxidation ? ATP
  synthesis
• 3) plastocyanin
• - a water soluble protein
• - a copper ion (Cu1 ? Cu2)

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4) psaI and psaII - P700 - A0 (an acceptor
chlorophyll) - A0 ? A1 (quinone) ? 4Fe-4S
clusters 5) reduction of ferredoxin (Fd) - a
water-soluble protein - a 2Fe-2S cluster -
PC(Cu) ferrodoxinox. ? PC(Cu2)
ferrodoxinred. 6) ferredoxin-NADP reductase -
FAD-FADH2-mediated electron transfer - NADP
2Hstroma 2e-? NADPH

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7. ATP synthesis 1) proton-motive force - 3.5 pH
units -?p - 0.06(?-3.5) 0.20 V 4.8
kcal/mol - the major factor pH gradient -
thylakoid membrane (quite permeable to Mg2 and
Cl-) 2) thylakoid lumen - acidic (pH ?4) 3) ATP
synthase - CF1-CF0 complex - 2 NADPH in stroma
12 H in lumen by 8 photons

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• 8. Cyclic electron flow
• 1) an alternative pathway
• P700 ? ferredoxin (Fd) ? cytochrome bf complex ?
  
• plastocyanin ? P700
• 2) cyclic phosphorylation
• - a high ratio of NADPH to NADP
• - no O2 generation

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9. Accessory pigments 1) accessory pigments -
450 - 650 nm of the visible regions - relative
low density of chlorophyll a molecules at the
active center - additional chlorophyll a
molecules - other classes of molecules
chlorophyll b, lycopene, ß-carotene -
transfer the absorbed energy to a reaction
center 2) light-harvesting complexes - the
26-kd subunit of a light-harvesting complex
(LHC-II) 7 chlorophyll a, 6 chlorophyll b, 2
carotenoid - bacterial light-harvesting
complexes 8 polypetides (3 chlorophyll, 1
carotenoid)

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10. Localization of photosystems, cytochrome bf,
and ATP synthase 1) separation of
photosystem II from photosystem I - direct
electron-transfer from photosystem II to
photosystem I 2) photosystem I in unsacked
regions - direct access to stroma - NADPH 3) ATP
synthase in unsacked regions - direct access to
stroma - a large size of CF1 4) photosystem II in
sacked regions - H2O - plastoquinone (a highly
lipid-soluble electron carrier)

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• 11. Herbicides
• 1) photosystem II
• - diuron (urea derivatives), atrazine (triazine
  derivatives)
• - the QB site of D1
• 2) photosystem I
• paraquat accept electrons from photosystem I to
  become
• a radical 

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