light reaction of photosynthesis -occurs in thylakoid (“stacks of paper plates”) -thylakoids...
TRANSCRIPT
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
H2O
LIGHTREACTIONS
Chloroplast
Light
ATP
NADPH
O2
Light enters and is absorbed by photosystem II
Photosystem II = •consist of: pigments, proteins, and organic molecules.
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act as “antennas” that absorb light energy
Photosystem II
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•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
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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+
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.