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Light Reactions Takes place in the Thylakoids of chloroplasts
in eukaryotes Captures solar energy and converts it to Energy
storage molecules ATP and NADPH along with O2
NADP+=nicotinamide adenine dinucleotide phosphate
NADP+ + H++ 2e- --->NADPH Method of storing high energy electrons
Chlorophyll
Contained in Thylakoids Similar to Heme in structure Two main types chlorophyll a and chlorophyll b
Structurally similar, but slight differences
Absorbs light at red and blue wavelengths Reflects light at green wavelenghts
Spectral absorption of light Absorbed light is used as energy Chlorophyll a and b have slight
differences in absorption Carotenoids can absorb light
energy Red, yellow and orange pigments Sometimes function in
photosynthesis but are usually for “sunscreen”
In fall, chlorophyll production drops and carotnoids remain
Fig. 10-11
(a) Excitation of isolated chlorophyll molecule
Heat
Excitedstate
(b) Fluorescence
Photon Groundstate
Photon(fluorescence)
En
erg
y o
f el
ectr
on
e–
Chlorophyllmolecule
How solar power works
Excites electrons to a higher energy state Electrons remain at excited state for very short periods
of time When electron returns to ground state a photon of light
is given off This photon may excite an electron on another
chlorophyll molecule Process continues
Photosystems Act as “dish antennae”
Many photon collectors Only one reaction center
Reaction center is adjacent to Primary electron acceptor
Electron acceptor captures excited electron from reaction center before it is able to return to ground state
Fig. 10-12
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
STROMA
e–
Pigmentmolecules
Photon
Transferof energy
Special pair ofchlorophyll amolecules
Th
yla
koid
me
mb
ran
e
Photosystem
Primaryelectronacceptor
Reaction-centercomplex
Light-harvestingcomplexes
Photsystems I and II
Photosystem I Discovered first Acts second Has P700 chlorophyll at
reaction center Yields NADPH
Photosystem II Discovered second Acts first in Light Cycle Has P680 chlorophyll at
reaction center Yields ATP + O2
Noncyclic electron flow
Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
4
Pq
Pc
Cytochromecomplex
Electron transport chain
5
ATP
Photosystem I(PS I)
Light
Primaryacceptor
e–
P700
6
Fd
Electron transport chain
NADP+
reductase
NADP+
+ H+
NADPH
8
7
e–
e–
6
Fig. 10-13-5
Photosystem II(PS II)
Noncyclic electron flow
Noncyclic electron flow 2 Photosystem II collects photon and activated
electron is harvested by Primary e acceptor Reaction center chloropyll requires electron
Enzyme degrades H2O to obtain 2e-, 2H+ and O Electron is transferred to reaction center
Electron is transferred through electron transport chain Energy levels decrease while released energy is used to
make ATP Plastoquinone, Cytochrome complex and plastocyanin
Noncyclic electron flow 3 Newly grounded electron is used to fill void in P700
following electron loss Electron is excited and captured by Primary acceptor Transferred through another electron transport chain
Ferredoxin, then NADP+ reductase
NADP+ reductase takes 2H+ produced earlier plus 2 high energy electrons + NADP+ and produces NADPH + H+
NADPH stores activated electrons for later use
Noncyclic electron flow summary
Cyclic electron flow
Ferredoxin transfers electron back to Cytochrome complex Electron is recycled and ATP is produced instead of NADPH
Results in more ATP than NADPH (required in Calvin)
Fig. 10-15
ATPPhotosystem II
Photosystem I
Primary acceptor
Pq
Cytochromecomplex
Fd
Pc
Primaryacceptor
Fd
NADP+
reductaseNADPH
NADP+
+ H+