If an isolated solution of chlorophyll is illuminated

•
If an isolated solution of chlorophyll is
illuminated
–
It will fluoresce, giving off light and heat
Excited
state
Energy of election
Heat
Photon
(fluorescence)
Chlorophyll
molecule
Ground
state
Photon
e
–

How do we capture the energy of that excited
electron?
Excited
state
Energy of election
Heat
Photon
(fluorescence)
Chlorophyll
molecule
Ground
state
Photon
e
–

The Photosystem:
The System of Organized Light Capture
Components: A) Light Harvesting Complexes
B) Reaction Center
Primary election
acceptor
Photon
Thylakoid
Light-harvesting
complexes
Reaction
center
Photosystem
STROMA
Thylakoid membrane
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Figure 10.12
e –
Photosystem
A reaction center surrounded
by light-harvesting complexes
How do we capture the energy of that excited electron?

The Photosystem
A) Light-harvesting complexes (LH)
–
Consist of pigment molecules (chlorophyll a,
chlorophyll b, carotenoids) bound to proteins
–
“Harvest light” and transfer the energy to the
reaction center
B) Reaction Center
–
2 chlorophyll a molecules bound to proteins
–
Primary electron acceptor
•
Captures excited electron from LH complex,
prevents it from plunging back to ground
state

a

2 Types of Photosystems in Thylakoid Membrane
•
Photosystem I-discovered first
•
Photosystem II-discovered second
LIGHT
REACTOR
NADP +
ADP
ATP
NADPH
CALVIN
CYCLE
[CH
2
O] (sugar)
STROMA
(Low H
+
concentration)
Photosystem II
LIGHT
H
2
O CO
2
Cytochrome
complex
O 2
H
2
O
O 2
1
1
⁄
2
2
Photosystem I
Light
THYLAKOID SPACE
(High H
+
concentration)
STROMA
(Low H
+
concentration)
Thylakoid
membrane
ATP
synthase
Pq
Pc
Fd
NADP
+
reductase
NADPH + H
+
NADP
+
+ 2H
+
To
Calvin
cycle
ADP
P
ATP
3
H
+
2 H
+ +2 H
+
2 H
+
Figure 10.17

How does the photosystem use the energy
from the excited electrons?
2 Pathways:
1)Noncyclic Electron Flow
-the primary pathway of energy
transformation in the light reactions
-Makes NADPH (electron carrier),
ATP and O
2
2)Cyclic Electron Flow
-makes ATP only

Non-Cyclic Electron Flow
•
Produces NADPH, ATP, and oxygen
Figure 10.13
Photosystem II
(PS II)
Photosystem-I
(PS I)
ATP
NADPH
NADP +
ADP
CALVIN
CYCLE
CO
2
H
2
O
O
2
[CH
2
O] (sugar)
LIGHT
REACTIONS
Light
Primary
acceptor
Pq
Cytochrome
complex
PC
e
P680
e
–
e
–
O 2
+
H 2 O
2 H
+
Light
ATP
Primary
acceptor
Fd
e
e
–
NADP
+
reductase
Electron
Transport
chain
Electron transport chain
P700
Light
NADPH
NADP
+
+ 2 H
+
+ H
+
1
5
7
2
3
4
6
8

Non-Cyclic Electron Flow
•
A mechanical analogy for the light reactions
Mill
makes
ATP
ATP
e
–
e
–
e
–
e
–
e
–
Photon
Photosystem II
Photosystem I
e
–
e
–
NADPH
Photon
Figure 10.14

Cyclic Electron Flow
•
In cyclic electron flow
–
Only photosystem I is used
–