Light Light Reaction Reaction of of PhotosynthPhotosynthesisesis

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

-occurs in thylakoid

(“stacks of paper plates”)

-thylakoids thylakoids found in found in

chloroplastchloroplast

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

Light enters and is absorbed by photosystem II

Photosystem II = •consist of: pigments, proteins, and organic molecules.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

act as “antennas” that absorb light energy

Photosystem II

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

•Photon energyPhoton energy is transferred from pigment to pigment until it reaches chlorophyll a

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:light-absorbing“head” of molecule; note magnesium atom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes of chloroplasts; H atoms not shown

PP

II

GG

MM

EE

NN

TT

Fills in “lost” e- from H20 being split

to the

Photosystem IIPhotosystem II

•Photo II aka. P680- absorbs wavelengths of light at 680nm

Light Reaction Light Reaction overviewoverview:

1)pigments absorb light photon & the photon is relayed around photo II

2) an e-, from chlorophyll a bounces to the next energy level & caught by the primary e- acceptor

Solar ESolar E. --> Chemical E.:Chemical E.:

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

CALVINCYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2

O2

Light Reaction Light Reaction overviewoverview:

3)meanwhile… an enzyme splits H2O -> H+, e-, O

an e- fills in the e- lost by the P680 molecule

O2 goes up your nose

Solar ESolar E. --> Chemical E.:Chemical E.:

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f el

ectr

on

sO2

2 H+H2O

O21/2

Light Reaction Light Reaction overviewoverview:

4)Each photexcited e- is now passed down an e- transport chain

5) The e- “fall” synthesizes ATP (H+

gradient from H20 splitting)

Solar ESolar E. --> Chemical E.:Chemical E.:

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

ATP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f el

ectr

on

sO2

2 H+

H2O

O2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

H+H+

H+

H+

H+

H+

H+

H+

Light Reaction Light Reaction overviewoverview:

6) chlorophyll in PS I absorbs light photon & the photon is relayed

an e- is bounced to the next energy level & caught by the Primary e- acceptor

Solar ESolar E. --> Chemical E.:Chemical E.:

Photosystem I and Photosystem I and IIII what makes them different?•Photo II aka. P680- absorbs wavelengths of light at 680nm

Photo I aka. P700- absorbs wavelengths of light at 700nm

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

2 H+H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

P700

e–

Primaryacceptor

Photosystem I(PS I)

Light

FYI:

P680 and P700 are identical chlorophyll a molecules.

difference in light absorption comes from association with different proteins in the thylakoid membrane.

Light Reaction Light Reaction overviewoverview:

7) e-’s passed down the second “chain”

8) 2 e-’s hop on an NADP+ bus and form NADPH

Solar ESolar E. --> Chemical E.:Chemical E.:

NADPH

off to the dark side…

Photophosphorylation - Light energy

generates ATPATP

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

+2 H+

2 H+

+2 H+ 2 H+

+2 H+ 2 H+

+2 H+ 2 H+ +2 H+

2 H+

+2 H+

2 H+

Purpose of Light Reactions (revisited):

Generate ATP

Generate NADPH these will provide E. & reducing power to the Calvin Cycle.

Light Light ReactionReaction

ee- - carriercarrier

ee- - carriercarrier

via. ATP via. ATP synthasesynthase

ATP

Photosystem II

e–

e–

e–e–

MillmakesATP

e–

e–

e–

Ph

oto

n

Photosystem I

Ph

oto

n

NADPH

Go to the dark side

CyclicCyclic Electron Electron FlowFlow--

•occurs under condition occurs under condition where there where there is not enoughis not enough

ATP for Calvin Cycle.ATP for Calvin Cycle. •Uses photosystem I but not II.•No production of NADPH

•No production of O2

CyclicCyclic Electron Electron FlowFlow-- •Rise in NADPH (b/c Calvin Cycle consumes more ATP)

•triggers this process, goal to make ATP “catch up” with NADPH.

•Side steps Photo II (P680).

Photosystem IATP

Pc

Fd

Cytochromecomplex

Primaryacceptor

CyclicCyclic Electron Electron FlowFlow--•Excited electrons from Photo I are passed by electron acceptor to Fd (ferrodoxin).

•Then passed to ETC to make ATP . •No NADPH is made and there is no release of O2.