Chapter 22 – The Light Reactions
(Problems 1,2,4,5,8-10,13-19)
Energy + 6H2O + 6CO2
Light
C6H12O6 + O2
Glycolysis/CAC
ETC/OX-Phos
Photosynthesis
C6H12O6 + O2
6H2O + 6CO2 + Energy
ATP
• Photosynthesis: a process that converts
atmospheric CO2 and H2O to carbohydrates
• Solar energy is captured in chemical form as
ATP and NADPH
• ATP and NADPH are used to convert CO2 to
hexose phosphates
• Phototrophs: photosynthetic organisms (some
bacteria, algae, higher plants)
Respiratory ETC
Energy + 6H2O + 6CO2
Light
C6H12O6 + O2
Glycolysis/CAC
ETC/OX-Phos
Uphill Energy Process
Downhill Energy Process
Photosynthesis ETC
Photosynthesis
C6H12O6 + O2
6H2O + 6CO2 + Energy
ATP
Light and dark reactions
• Both processes can occur simultaneously
Reactions that require light (light reactions):
H2O + ADP + Pi + NADP+
O2 + ATP + NADPH + H+
Reactions which do not require light (dark reactions):
CO2 + ATP + NADPH + H+
Sum: CO2 + H2O
(CH2O) + ADP + Pi + NADP+
(CH2O) + O2
22.1 Photosynthesis Takes Place in Chloroplasts
Three membranes, and three “spaces”.
H2O + ADP + Pi + NADP+ → O2 + ATP + NADPH + H+
CO2 + ATP + NADPH + H+ → (CH2O) + ADP + Pi + NADP+
22.2 Photosynthesis Transforms Light Energy
into Chemical Energy
I
II
I
II
I
II
I
II
Structures of Chlorophyll and
bacteriochlorophyll
• Chlorophylls - usually most abundant and most important pigments
in light harvesting
• Contain tetrapyrrole ring (chlorin) similar to heme, but contains
Mg2+
• Chlorophylls a (Chl a) and b (Chl b) in plants
• Bacteriochlorophylls a (BChla) and b (BChlb) are major pigments
in bacteria
Light harvesting complexes enhance the
efficiency of photosynthesis
Accessory pigments
Reaction centers of the photosystems
• PSI and PSII each contain a
reaction center (site of the
photochemical reaction)
• Special pair: two chlorophylls
in each reaction center that
are energized by light
• In PSI special pair is: P700
(absorb light maximally at
700nm)
• In PSII the special pair is:
P680 (absorb light maximally
at 680nm)
Some Plants Produce Toxins; e.g. Potatoes
22.3 Two Photosystems Generate a Proton
Gradient and NADPH
Photosystem I: Ferredoxin and NADPH
Production
NADP+
NADPH
H
+
Problem: Fd accepts and
donates electrons one at a
time, but NADP+ accepts a
pair of electrons.
Solution: Fd NADP+
reductase.
Fd NADP+ reductase uses
the co-factor FAD which
can accept electrons one
at a time from Fd and
donate a pair of electrons
to NADP+ to form NADPH
Photosystem I
P700*
NADP+ + H+
P700
Fd
P700+
Fd-NADP+ oxidoreductase
(via FAD/FADH2—why?)
NADPH
From PC
Reduction of NADP+ (Eo’ = -0.32 V) by
Fd (Eo’= -0.43 V) is catalyzed by
ferredoxin-NADP+ oxidoreductase.
Photosystem II: Transfers Electrons to
PSI and Generates Proton Gradient
PSII: Reduction, excitation and oxidation of P680
• P680 special-pair
pigment of PSII
x 2
• P680+ is reduced by
e- derived from
oxidation of H2O
• Light energizes to
P680*, increasing
its reducing power
Pha
e2H+ + e-
PAQH2
2H2O → O2 +
Via OEC
4H+
(bound)
+
4e-
PBQH2
(soluble)
PAQ
Photosystem II: Electrons are Transferred
from Q to Pc via Cytbf, and then from Pc
to PSI
Cytbf
PSI
Transfer of electrons from QH2 to
plastocyanin
QH2 + 2Pc(Cu2+)
Cytbf
→ Q + 2 Pc(Cu+) + 2H+
Thylakoid
lumen
Photosystem II: The
Evolution of O2; the
OEC of PSII
2H2O → O2 + 4H+ + 4eVia OEC
2H2O → 4e- + 4H+ + O2
To P680+
D1-Tyr-OH → D1-Tyr-O. + eD1-Tyr-O. + e- → D1-Tyr-OH
From Mn
The manganese center of PSII
Cooperation between PSI
and PSII
22.4 A Proton Gradient Drives ATP Synthesis
2H2O + 2NADP+ + 10H+stroma → O2 + 2NADPH + 12H+lumen
3ADP3- + 3Pi2- + 3H+ + 12H+lumen → 3ATP4- + 3H2O + H+stroma
2NADP+ + 3ADP3- + 3Pi2- + H+ → O2 + 2NADPH + 3ATP4- + H2O
Cyclic Electron Flow
Stroma
Thylakoid lumen
When NADPH/NADP+ ratio is high …..
Organization of Photosynthesis
Components
Stroma
Thylakoid lumen
Organization optimizes location
PSI for NADPH production in
stroma, PSII for increasing
proton
concentration
in
thylakoid lumen, and ATP
synthase for production of
ATP in stroma
PSII (QH2
Inhibition)
Herbicides
PSI e acceptor