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
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
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
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
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
H
+
NADP+ NADPH
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
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.
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
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
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
Cooperation between PSI and PSII
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
Cyclic Electron Flow
When NADPH/NADP+ ratio is high …..
Thylakoid lumen
Stroma
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