Lecture 6 Z Scheme & Photophosphorylation.pdf

Charge separation D Chl A D+ Chl A- D + oxidant A - reductant Reaction centre – light energy converted to chemical energy Kaman: Photosynthesis = “series of processes in which electromagnetic energy is converted to chemical free energy which...

Charge separation D Chl A D+ Chl A- D + oxidant A - reductant Reaction centre – light energy converted to chemical energy Kaman: Photosynthesis = “series of processes in which electromagnetic energy is converted to chemical free energy which can be used for biosynthesis”  Photosynthesis - light Chlorophylls and carotenoids form antenna complexes – Harvest and transfer light energy to chlorophylls at the reaction centre energy is transferred from pigment to pigment by resonance transfer reaction centre chemical (redox) energy Reaction centre complexes = photosystems PSI (P700) PSII (P680) 1st Emerson effect – Red Drop 2nd Emerson effect – Enhancement Chloroplasts given light at 650 and 700 nm simultaneously yield more O2 than the sum of when each is used alone  Proof + He 02 + 4H + NADPH Midpoint potential ↳ Y PSI absorbs maximally at 700 nm (FR) PSII absorbs maximally at 680 nm (R) Hill: Chloroplasts 2H2O O2 + 4H+ + 4e- (water oxidised, donates e-) Ferricyanide – acts as “Hill oxidant” (ie accepts e-) - in vivo, NADP+ (Ochoa) PSI and PSII operate in series - Z scheme – PSI absorbs far-red light (>680 nm) = P700* = strong reductant – PSII absorbs red light (680 nm) = P680 = strong oxidant – Oxidizes water + + e- H20 > 02+ H - S P700 -P700 * "I11111111111 >Oxidizes - 128 Other components of PS electron transport Hill – cytochrome b6 , cyto f (cyto b6/f complex) Duysens – cyto f reduced by red light (PSII) oxidised by FR light (PSI) (for red) Cyto f sits between 2 photosystems Plastocynanin (blue copper protein) - mutants can’t PS illuminate mutants – strong reduction Cyto f - PC after Cyt f in ETC p Photochemistry takes place in thylakoids I Lumen Photosystem II Evolves Oxygen from Water = Photolysis Pheophytin = chl lacking Mg2+ PSI)  Photosynthetic electron transport 4 protein complexes embedded in the thylakoid membrane involved in the light reactions PSII, Cytochrome b6f complex, PSI, ATP synthase PSI) generatioan 19 - Lumen pr  Mechanism of photophosphorylation Mitchell : chemi-osmotic hypothesis Jagendorf Generated pH gradient across TK membrane in ABSENCE of light Equilibration, lumen of TK acidifies Dark – add NaOH (creates pH gradient) at same time as ADP + Pi Dissipation of pH gradient drives ATP synthesis  PS ETC causes alkalisation of stroma, acidification of TK lumen In dark, S3TK are around same put during , run, Stroma is alkalized It as moves to TK , A + P generation brings the spaces back to pH  Generation of proton gradient TK membrane impermeable to H+ PSII, PQ, Cytochrome b6f complex, PSI : net movement of protons in to lumen ATP synthase, protons move from lumen to stroma Cyclic photophosphorylation Involves only PSI reaction centre No O2 evolution Generates ATP (1 per cycle ?) Cyclic electron transport  Photosynthesis - chloroplast CO2 fixation Stroma Stroma lamellae Unstacked Non-appressed Thylakoid Grana lamellae Stacked Appressed Light reactions  Summary, requirements for photophosphorylation PS e- transport establishes proton gradient TK membrane impermeable to protons ATP synthesis, reversal of hydrolysis Dissipation of H+ gradient drives ATP synthesis