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