Chapter 22 – The Light Reactions · 2020. 3. 18. · Chapter 22 – The Light Reactions (Problems...
Transcript of Chapter 22 – The Light Reactions · 2020. 3. 18. · Chapter 22 – The Light Reactions (Problems...
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Chapter 22 – The Light Reactions (Problems 1,2,4,5,8-10,13-19)
• 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)
Energy + 6H2O + 6CO2 C6H12O6 + O2
C6H12O6 + O2 6H2O + 6CO2 + Energy
Photosynthesis
Light
ETC/OX-Phos
ATPGlycolysis/CAC
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Dow
nhill E
nerg
y Pr
oces
s
Uph
ill E
nerg
y Pr
oces
s
Respiratory ETC Photosynthesis ETC
Energy + 6H2O + 6CO2 C6H12O6 + O2
C6H12O6 + O2 6H2O + 6CO2 + Energy
Photosynthesis
Light
ETC/OX-Phos
ATPGlycolysis/CAC
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Light and dark reactions
• Both processes can occur simultaneouslyReactions 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+ (CH2O) + ADP + Pi + NADP+
Sum: CO2 + H2O (CH2O) + O2
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Three membranes, and three “spaces”.
CO2 + ATP + NADPH + H+ → (CH2O) + ADP + Pi + NADP+
H2O + ADP + Pi + NADP+ → O2 + ATP + NADPH + H+
22.1 Photosynthesis Takes Place in Chloroplasts
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22.2 Photosynthesis Transforms Light Energy into Chemical Energy
I II I II I II I II
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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
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Light harvesting complexes enhance the efficiency of photosynthesis
Accessory pigments
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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)
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Some Plants Produce Toxins; e.g. Potatoes
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22.3 Two Photosystems Generate a Proton Gradient and NADPH
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Photosystem I: Ferredoxin and NADPH Production
H
+
NADP+ NADPH
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Problem: Fd accepts and donates electrons one at a time, but NADP+ accepts a pair of electrons.
Solution: Fd NADP+
reductase.
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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
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P700
P700*
P700+
From PC
FdNADP+ + H+
Fd-NADP+ oxidoreductase(via FAD/FADH2—why?)
NADPH
Photosystem I
Reduction of NADP+ (Eo’ = -0.32 V) by Fd (Eo’= -0.43 V) is catalyzed by ferredoxin-NADP+ oxidoreductase.
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Photosystem II: Transfers Electrons to PSI and Generates Proton Gradient
PSII: Reduction, excitation and oxidation of P680
• P680 special-pair pigment of PSII
• P680+ is reduced by e- derived from oxidation of H2O
• Light energizes to P680*, increasing its reducing power
Pha
x 2
2H+ + e-e- PAQ
PAQH2(bound)
PBQH2(soluble)
2H2O → O2 + 4H+ + 4e-
Via OEC
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Cytbf
Transfer of electrons from QH2 to plastocyanin
QH2 + 2Pc(Cu2+) → Q + 2 Pc(Cu+) + 2H+Cytbf
Thylakoid lumen
PSI
Photosystem II: Electrons are Transferred from Q to Pc via Cytbf, and then from Pc
to PSI
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2H2O → O2 + 4H+ + 4e-
Via OEC
Photosystem II: The Evolution of O2; the
OEC of PSII
2H2O → 4e- + 4H+ + O2
The manganese center of PSII
D1-Tyr-OH → D1-Tyr-O. + e-D1-Tyr-O. + e- → D1-Tyr-OH
To P680+
From Mn
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Cooperation between PSI and PSII
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22.4 A Proton Gradient Drives ATP Synthesis
2H2O + 2NADP+ + 10H+stroma → O2 + 2NADPH + 12H+
lumen3ADP3- + 3Pi2- + 3H+ + 12H+
lumen → 3ATP4- + 3H2O + H+stroma
2NADP+ + 3ADP3- + 3Pi2- + H+ → O2 + 2NADPH + 3ATP4- + H2O
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Cyclic Electron Flow
When NADPH/NADP+ ratio is high …..
Thylakoid lumen
Stroma
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Organization of Photosynthesis Components
Stroma
Thylakoid lumen
Organization optimizes locationPSI for NADPH production instroma, PSII for increasingproton concentration inthylakoid lumen, and ATPsynthase for production ofATP in stroma
HerbicidesPSII (QH2Inhibition)
PSI e acceptor