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
Energy of electron
e–
Excited
state
Heat
Photon
(fluorescence)
Photon
Chlorophyll
molecule
Ground
state
(a) Excitation of isolated chlorophyll molecule
(b) Fluorescence
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
Photosystem
STROMA
Light-harvesting Reaction-center
complex
complexes
Primary
electron
acceptor
Thylakoid membrane
Photon
e–
Transfer
of energy
Special pair of
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
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
Fig. 10-13-5
4
Primary
acceptor
2
H+
+
1/ O
2
2
H2O
e–
2
Primary
acceptor
e–
Pq
Cytochrome
complex
7
Fd
e–
e–
8
NADP+
reductase
3
NADPH
Pc
e–
e–
P700
5
P680
Light
1 Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
NADP+
+ H+
Photosystem I
(PS I)
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
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Photosystem I
Photosystem II
ATP
NADP+
+ H+