Photosynthesis
Chapter 19
Plants• Have mitoch
– Nutrient breakdown
– ets
– ATP production
• Also have another ATP prod’n mech
– Solar free energy trapped
– Reduces carriers (NADPH), produces ATP
– Side rxn:
H2O 2 H+ (used in ATP prod’n) + ½ O2
• Overall, light rxns:
– 2 H2O + 3 ADP + 3 Pi + 2 NADP O2 + 3 ATP + 2 NADPH
• Dark rxns
– Prod’s of light rxns + CO2 carbohydrates
•CO2 + H2O O2 + (CH2O)n (w/ light)
– Source of plant CH’s in our diets
Light Rxns Similar to Mitoch ets
• Both involve redox rxns
• Both have membr-bound enz’s and proton gradients
• Both have structures sim to Complex III (mitoch)
Similarities – cont’d
• Both in dbl-membr organelles
– Outer membr semipermeable
– Inner membr impermeable
• Both use ATP synthase complexes
– Sim structures
– Same rxn: ADP + Pi ATP
Light Rxns Differ from Mitoch ets
• e- Transfer
– Mitoch e- from NADH to O2 NAD+ + H2O
– Photosynth e- from H2O to NADP+ NADPH + O2
• Proton gradient– Mitoch incr’d [H+] in intermembr
space
– Photosynth incr’d [H] in lumen of thylakoid (analogous to mitoch matrix)
Differences – cont’d
• Location of ATP synth’d
– Mitoch ATP rel’d to matrix
•Transporter moves ATP out
– Photosynth ATP rel’d to chloroplast stroma (analogous to intermembr space in mitoch)
•So synth’d ATP avail to cell w/out transporter
Chloroplast
• Outer membr semipermeable
• Intermembr space = stroma– Aqueous
• Inner membr folded thylakoids– “Stacks” of thylakoids = grana
• Lumen = space inside thylakoid membr “loops”
Review of Physics of Light
• Light energy = wave of particles
– Particles = photons
= wavelength of light
– Visible range = 400 nm (violet) 700 nm (red)
• Energy of photons inverse to – Energy of 1 “mole” of photons =
170-300 kJ
Chromophores
• Conjugated
– Have “fluid” electrons
– Available for excitation by incident energy
– Rel low energy needed for e- excited state
• Chromophore e- move to higher energy level– All or nothing
– Photon energy level must match prescribed energy levels of chromophore mol electrons
•= e- orbital levels
• At higher energy level– e- excited, unstable
– e- returns to lower level (ground state) for stability
• Energy rel’d when e- falls back to ground state
– = quantum
– May be rel’d as light, heat, or chem energy
– May be transferred to second chromophore
Chromophores in Photosynthesis = Pigments
• Absorb radiant energy
– Extensive conj’d db systems
– Many fluid electrons can move to higher energy levels
– Absorb light energy of visible wavelengths
• 2 Impt pigments: chlorophylls a, b
– Structure sim to porphyrins
• Where did you see porphyrin structure before?
• Chlorophylls a, b – cont’d – Metal ion coordinated w/ structure =
Mg
•What was metal ion in previously studied porphyrins?
– Hydrophobic side chain (called phytol)
•How might this be related to its location? (Hint…)
– In thylakoid membranes
• In light-harvesting complexes (LHC’s)
•Other impt proteins assoc’d
•Arr’d in partic order
• Other pigments – accessory pigments in LHC’s
– Carotenoids (ex: carotene)
– Phycobilins (linear tetrapyrroles)
– Lutein
• Absorb light @ varied
– Match of sunlight reaching earth
– Different absorbance maxima
•Different structures
Phycobilisome – A “Simple”
Photosystem
• Photosystem = light-harvesting pigment arrangement
– Embedded in thylakoid membr
• Phycobilisome in cyanobacteria, red algae
• Phycobilin pigments complex w/ proteins
– Phycoerythrin, phycocyanin, allophycocyanin
– Analogous to accessory pigments, antenna molecules in higher plants
• Final energy acceptor = chlorophyll a
– Analogous to reaction center
• Arranged in ordered complex
• Incident light of 2 ranges supply
energy
– Energy transferred pigment to pigment
•Energy excites electrons of each sequential pigment
•“Exciton transfer”
– Reaches chlorophyll a
• Initiates redox rxn and electron transfer
•Will be used to generate ATP
Photosystems in Higher
Plants
• ~200 chlorophyll molecules
– Some make up Rxn Center
– Some serve as antenna molecules
• ~ 50 accessory pigments
• Arrangement
– Rxn Center
– Surrounded by antenna molecules, accessory pigments
– All embedded in thylakoid membr bilayer
• Two types of rxn center
– PS I
•Mostly chl a’s, some chl b’s
•Other specialized structures
•Abs max = 700 nm
– PS II
•Chl’s a + b + c
•Other specialized structures
•Abs max = 680 nm
Photosystem Energy Transfer
• Light energy strikes antenna molecule
– Mostly chl a’s
– Excites e- of 1st antenna mol to higher energy level
• e- falls back to ground state
– Releases energy
– Energy avail to nearby antenna mol or accessory pigment
• 2nd antenna mol/ accessory pigment accepts energy
– Its e- excited to higher energy level (= exciton transfer)
• e- falls back to ground state
– Releases energy
– Energy avail to nearby antenna mol or accessory pigment
• 3rd antenna mol accepts energy, etc., etc. Rxn Center
Energy Transfer to Rxn Center • Rxn Centers have special chlorophyll
a
– “Sandwiched” between 2 other rxn center structures
• e- acceptor is “above” chl a
• e- donor is “below” chl a
– W/ energy transfer from antenna mol/ accessory pigment, e- @ special chl a excited
• e- moves (physically transferred) to e- acceptor structure near chl a
– Now acceptor structure has an extra e-
•Takes on formal – charge
– Now special chl a has no electron
•Takes on formal + charge
•Get “electron hole”
• e- donor structure near chl a replaces e- in chl a
– Now donor structure has no electron
•Takes on formal + charge
• Now chl a uncharged; lies between
– e- acceptor structure (now – charged)
– e- donor structure now (+ charged)
• Have generated formal sep’n of charge in Rxn Center– REMEMBER: this is a highly
energetic condition
– Excited e- in rxn center -- good e- donor
– Initiates redox chain among other structures in thylakoid membr
Pheophytin-Quinone – Simplified
Rxn Center• In purple bacteria
• “Special Chl a” = (Chl)2
– Excitons gen’d w/ incident light of 870 nm
• “e- acceptor” = Pheophytin
– Chlorophyll w/out Mg
• e- from pheo radical quinone (Q)
– Sim to Ubiquinone (=CoEnzyme Q) in mitoch
– Can accept one or two reducing equivalents
– Moves through thylakoid bilayer
• Q Cyt bc1 complex
– Sim to Complex III in mitoch
Cyt bc1 complex transfers e- Cyt c2
– Cyt c2 carries e- back to rxn center
– Rxn center returned to neutral state to receive another exciton
• Energy gen’d w/ e- transport
– Can calc G from voltage gen’d w/ e- transfer
– (Chl)2’+ QH2 G ~ -180 kJ/mole
Higher Plants Have 2 Rxn Centers• Sim Rxn Center, e- transport
structures as bacteria
• BUT others also, so more complex
• PSII “first” Center
– Like bacterial model
• Pheophytin
• Quinones (as Plastoquinones)
• Cyt bf Complex (has a cyt f, not cyt c inc’d)
• H+ gen’d, collects in thylakoid lumen
• PSII “first” Center – cont’d
– Not like bacterial model
• Accepts incident light @ 680 nm
• Cyt bf Complex transfers e- Plastocyanin, not cyt c
• Final acceptor transfers e- to rxn center of SECOND photosystem
• So NOT a cycle, w/ rxn center regen’d w/ cycle
• Rather, rxn center regen’d w/ e- from H2O splitting
– 2 H2O 4 H+ + 4 e- + O2
• Catalyzed by water splitting complex (= oxygen-evolving complex)
– Requires 4 light photons
– Cleaves water
– AND transfers 4 e- one at a time to rxn center of PSII to regenerate rxn center
– Mn impt to function
• So light abs’d to:
– Excite rxn center e- to initiate e- transfer, AND
– Energize gen’n e- to regenerate rxn center electronically
PSII Summary
• Light energy accessory pigments, etc. rxn center
• Charge sep’n + excited “special” chl e- e- transferred to pheophytin
Cyanobacterium model
• e- @ pheophytin plastoquinones (2) Cyt bf complex
– Q cycle releases 1 e- at a time to Cyt bf complex
– Generate H+ lumen
• e- @ Cyt bf complex plastocyanin
• Plastocyanin travels to PSI w/ its e-
PSI• Accepts incident light at = 700 nm
accessory pigments, etc. rxn center
• Charge sep’n + excited “special” chl a e- transferred to A0 (special type of chl a; analogous to pheophytin)
• e- @ A0 A1(phylloquinone) Fe-S centers ferredoxin (has Fe-S centers)
• e- @ ferredoxin NADP+– Cat’d by ferridoxin:NADP+
oxidoreductase
– NADP+ + 2 H+ + 2 Fd(red’d) NADPH + H+ 2 Fd(ox’d)
• No H+ generated in lumen, but [H+] decr'd in stroma
• Still need to regenerate rxn center electronically– Through plastocyanin
– Has carried e- from PSII
ATP Synthesis Linked to Electron Transport
• Light energy captured, transformed phosphate bond energy of ATP = photophosphorylation
– Why not oxidative phosphorylation?
• Have generated electrochem gradient during e- transport– [H+] incr'd in lumen, decr'd in stroma
•103 x higher [H+] in lumen than stroma
– How many pH units is that?
– Sep'd by impermeable thylakoid membr
– Large amt chem and electrical energy "stored" in this system
•Approx -200 kJ/water-splitting+PSII+PSI event
•Used to make ATP
– Book: approx 3 ATP/O2 gen'd
• BUT also need 8 light photons
– Nec at both rxn centers + water-splitting complex
ATP Synthase in Plants
• Very similar in structure, function as mitochondrial
• Has Fo region (here CFo)
– Serves as channel
– H+ ions move through
– Causes conform'l change in Fo proteins
• In CFo, H+ moves from lumen stroma
•Opp analogous movement in mitoch
• Has F1 region (here CF1)
– Serves as catalyst of rxn: ADP + Pi ATP
– 6 subunits and alternating
's bind ADP/release ATP alternating
– Release ATP dependent on H+ movement through CFo
• Catalysis subunits produce, release ATP stroma
– No need for transporter proteins through thylakoid membr
– ATP free to move through semipermeable outer membr of chloroplast
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