Have mitoch

Nutrient breakdown à

ets à

ATP production

Also have another ATP prod’n mech

Solar free energy trapped

Reduces carriers (NADP), produces ATP

Side rxn: H₂O à 2 H+ (used in ATP prod’n) + ½ O₂

Overall, light rxns:

2 H₂O + 3 ADP + 3 Pi + 2 NADP à O₂ + 3 ATP + 2 NADPH

Dark rxns (19-34)

Prod’s of light rxns + CO₂ à carbohydrates

Source of plant CH’s in our diets

Both involved redox rxns

Both have membr-bound enz’s and proton gradients

Both have structures sim to Complex III (mitoch)

Both in dbl-membr organelles

Outer membr semipermeable

Inner membr impermeable

Both use ATP synthase complexes

Sim structures

Same rxn: ADP + Pi à ATP

e- Transfer

Mitoch e- from NADH to O₂ à NAD+ + H₂O

Photosynth e- from H₂O to NADP+ à NADPH + O₂

Proton gradient

Mitoch à incr’d [H+] in intermembr space

Photosynth à incr’d [H] in lumen of thylakoid (analogous to mitoch matrix)

Location of ATP synth’d

Mitoch ATP released to matrix

Transporter moves ATP out of matrix

Photosynth ATP released to chloroplast stroma (analogous to intermembr space in mitoch)

So synth’d ATP avail to cell w/out transporter

Outer membr semipermeable

Intermembr space = stroma

Aqueous

Inner membr folded à thylakoids

"Stacks" of thylakoids = grana

Lumen = space inside thylakoid membr "loops"

Light energy = wave of particles

Particles = photons

l = wavelength of light (19-36)

Visible range = 400 nm (violet) à 700 nm (red)

Energy of photons inverse to l

Energy of 1 "mole" of photons = 170-300 kJ

Chromophores are conjugated

Have "fluid" p electrons

Available for excitation by incident energy

Rel low energy needed for e- à excited state

e- of chromophore mol’s move to higher energy level

All or nothing

Photon energy level must match prescribed energy levels of chromophore mol electrons

These are e- orbital levels

At higher energy level

e- is excited, unstable

e- will return to lower level (ground state) for stability

Energy released when e- falls back to ground state

= quantum

May be released as light, heat, or chemical energy

May be transferred to second chromophore

Absorb radiant energy

Extensive conj’d db systems

Many fluid p electrons can move to higher energy levels

Absorb light energy of visible wavelengths

2 Impt pigments: chlorophylls a, b (19-37)

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 which serve as accessory pigments in LHC’s (19-37)

Carotenoids (ex: b carotene)

Phycobilins (linear tetrapyrroles)

Lutein

Absorb light @ varied l (19-38)

Match l of sunlight reaching earth

Different absorbance maxima

Different structures

Photosystem = light-harvesting pigment arrangement

Embedded in thylakoid membr

Phycobilisome in cyanobacteria, red algae (19-40)

Phycobilin pigments form complexes 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 l ranges supply energy

Energy transferred pigment to pigment

Energy excites p electrons of each sequential pigment

Called exciton transfer

Reaches chlorophyll a

Will initiate redox rxn and electron transfer

Will be used to generate ATP

~200 chlorophyll molecules

Some make up Rxn Center

Some serve as antenna molecules

~ 50 accessory pigments

Arrangement (19-42)

Rxn Center

Surrounded by antenna molecules, accessory pigments

All embedded in thylakoid membr bilayer

Two types

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

Light energy strikes antenna molecule

Mostly chl a’s

Excites e- of 1^st antenna mol to higher energy level

e- falls back to ground state

Releases energy

Energy available to nearby antenna molecule or accessory pigment

2^nd antenna molecule/accessory pigment accepts energy

Its e- excited to higher energy level (= exciton transfer)

e- falls back to ground state

Releases energy

Energy available to nearby antenna molecule or accessory pigment

3^rd antenna mol accepts energy, etc., etc. à 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- here excited

e- moves (is physically transferred) to e- acceptor structure near chl a

Now acceptor structure has an extra electron

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

In purple bacteria (19-44; 19-45)

"Special Chl a" = (Chl)₂

Excitons gen’d w/ incident light of l 870 nm

"e- acceptor" = Pheophytin

Chlorophyll w/out Mg

"e- donor" = Cyt c₂

Its e- will be re-gained following cycle of transfers

(Chl)₂ + 1 exciton à (Chl)₂* (excitation)

(Chl)₂* + Pheo à (Chl)₂’+ + Pheo’- (charge sep’n)

e- Transport in Purple Bacteria

e- from pheo radical à quinone (Q)

Red’d to semiquinone, then dihydroquinone (QH₂)

Similar to Ubiquinone (=CoEnzyme Q) in mitoch

Can accept one or two reducing equivalents

QH₂ moves through thylakoid bilayer à Cyt bc1 complex

Similar to Complex III in mitoch

Quinone "docks"; gives up 2 e- in succession during Q cycle

Protons "consumed" on one side of thylakoid membr, gen’d on other

à Electrochem gradient est’d for ATP synth

Cyt bc1 complex transfers e- à 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 D G from voltage gen’d w/ e- transfer

(Chl)₂’+ à QH2 D G ~ -180 kJ/mole

Similar Rxn Center, e- transport structures as bacteria

BUT others also, so more complex

PSII "first" Center (19-46)

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- à 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 à 4 H+ + 4 e- + O₂

Catalyzed by water splitting complex (= oxygen-evolving complex) (19-51)

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 both:

Excite rxn center e- to initiate e- transfer

Energize gen’n e- to regenerate rxn center electronically

Light energy à accessory pigments, etc. à rxn center

Charge sep’n + excited "special" chl e- à e- transferred to pheophytin

e- @ pheophytin à plastoquinones (2) à Cyt bf complex

Q cycle releases 1 e- at a time to Cyt bf complex

Generate H+ à lumen

e- @ Cyt bf complex à plastocyanin

Plastocyanin travels to PSI w/ its e-

Accepts incident light at l = 700 nm

à accessory pigments, etc. à rxn center

Charge sep’n + excited "special" chl e- à e- transferred to A0 (special type of chl a; analogous to pheophytin)

e- @ A0 à A1(phylloquinone) à Fe-S centers à ferredoxin (has Fe-S centers)

e- @ ferredoxin à NADP+

Cat’d by ferridoxin:NADP+ oxidoreductase

NADP+ + 2 H+ + 2 Fd(red’d) à 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

Light energy captured, transformed à 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

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 à stroma

Opposite analogous movement in mitoch

Has F1 region (here CF1)

Serves as catalyst of rxn: ADP + Pi à ATP

6 subunits

a and b alternating

b's bind ADP/release ATP alternating

Release ATP dependent on H+ movement through CFo

Catalysis subunits produce, release ATP à stroma

No need for transporter proteins through thylakoid membr

ATP free to move through semipermeable outer membr of chloroplast