light reaction of photosynthesis -occurs in thylakoid (“stacks of paper plates”) -thylakoids...
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Light Light Reaction Reaction of of PhotosynthPhotosynthesisesis
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-occurs in thylakoid
(“stacks of paper plates”)
-thylakoids thylakoids found in found in
chloroplastchloroplast
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H2O
LIGHTREACTIONS
Chloroplast
Light
ATP
NADPH
O2
Light enters and is absorbed by photosystem II
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Photosystem II = •consist of: pigments, proteins, and organic molecules.
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act as “antennas” that absorb light energy
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Photosystem II
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•Photon energyPhoton energy is transferred from pigment to pigment until it reaches chlorophyll a
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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
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Fills in “lost” e- from H20 being split
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to the
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Photosystem IIPhotosystem II
•Photo II aka. P680- absorbs wavelengths of light at 680nm
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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.:
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LightP680
e–
Photosystem II(PS II)
Primaryacceptor
[CH2O] (sugar)
NADPH
CALVINCYCLELIGHT
REACTIONS
NADP+
Light
H2O CO2
O2
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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.:
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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
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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.:
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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+
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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.:
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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
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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
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FYI:
P680 and P700 are identical chlorophyll a molecules.
difference in light absorption comes from association with different proteins in the thylakoid membrane.
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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.:
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NADPH
off to the dark side…
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Photophosphorylation - Light energy
generates ATPATP
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+2 H+
2 H+
+2 H+ 2 H+
+2 H+ 2 H+
+2 H+ 2 H+ +2 H+
2 H+
+2 H+
2 H+
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Purpose of Light Reactions (revisited):
Generate ATP
Generate NADPH these will provide E. & reducing power to the Calvin Cycle.
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Light Light ReactionReaction
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ee- - carriercarrier
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ee- - carriercarrier
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via. ATP via. ATP synthasesynthase
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ATP
Photosystem II
e–
e–
e–e–
MillmakesATP
e–
e–
e–
Ph
oto
n
Photosystem I
Ph
oto
n
NADPH
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Go to the dark side
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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
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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).
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Photosystem IATP
Pc
Fd
Cytochromecomplex
Primaryacceptor
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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.