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Photosynthesis1% sunlight is converted to chemical energy by plants
Fixation of carbon dioxide and evolution of oxygen
Overall reaction: 6CO2+ 6H2O 1 glucose + 6 O2
granum
Light reactions-Light-induced electron transfer-Thylakoid membrane-H2O + 3ADP + 2NADP
+O2+ 3ATP + NADPH
Dark reactions-CO2 fixation in the stroma-Biosynthetic reactions-Requires ATP and NADPH
Two components in photosynthesis:
Ch
loroplast
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Photosynthetic organisms
Green sulfur bacteria
Red tide red algae
Diatoms
Purple
bacteriaCyano-
bacteria
Cyanobacteria
Chlamydomonas
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CHLOROPHYLLS
Light-absorbing pigments in thylakoid membranesGreen pigments: chlorophylls a and bPolycyclic (5-ring system), planar
Similar to hemoglobin
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ACCESSORY PIGMENTS
Absorption spectra of different pigments
other pigments inthylakoid membranes
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Excitation of electrons in pigment molecules
(or other pigment molecules)
a photon
(= single quantum
of light energy)
Reaction
center
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Exciton transfer between neighboring pigment molecules
Reaction center- Photosystem- Light-induced e-flow
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Photosystems in light reactions
- Protein complexes with pigments and electron carriers- Photosystems I and II
(1) Purple bacteriaPS II only
- Cyclic electron flow
Q- quinone
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(2) Green sulfur bacteria PSI only
- H2S as electron donor in non-cyclic flow
Fd: Ferredoxin (an iron-sulfur protein)
FNR: Ferreodoxin reductase
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(3) Cyanobacteria, algae, and plants Both PSI and PSII
Purple bacteria (PSII)
Green sulfur bacteria (PSI)
Cyanobacteria(PS I + II)
Chloroplasts(PS I + II)
Evolution of chloroplasts?
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Cyanobacteria, algae, and plants Both PSI and PSII
OEC oxygen evolving complex
PQ plastoquinone
PC plastocyanin
Fd ferredoxin
FNR ferredoxin reductase
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Photosynthetic electron transfer: Noncyclic vs cyclic pathways
Noncyclic pathway: the Z scheme, unidirectional electron flow
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Distribution of PSI, PSII, and ATPsynthase in thylakoid membranes
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Photophosphorylation: light-driven ATP synthesis
2H2O + 8 photons + 2NADP++ 3ADP + 3Pi O2+ 2NADPH + 2H++ 3ATP
Non-cyclic photophosphorylation: Z scheme of etransfer involving both PSIand PSII
plastoquinone
ATP synthasecomplex
photophosphorylation
Thylakoid membrane sac
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Cyclic Photophosphorylation
(ferredoxin)
Overall reaction:
ADP + Pi ATP + H2O
light
Cyclic electron flow involving PSI onlySimilar to purple bacteriaNo production of NADPH and O2efrom ferredoxin move back throughthe cyt b6fcomplexProton pumping and phosphorylationRegulation of ATP to NADPH ratio
-1.0
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Lab 3: Hill reaction (Friday Nov 14)
- Investigation of light-induced electron transfer in isolated spinach chloroplasts using a dye
(DCPIP)
- DCPIP (blue) accepts electrons from PQBand is reduced to DCPIPH2(colorless)
- DCPIP replaces NADP+as the final electron acceptor
- Requirements for Hill reaction: light, protein complexes (electron carriers)- DCMU: a herbicide blocking PQA to PQB electron flow
- Ammonia: an uncoupler of ATP formation by elimination of proton gradient
Ph h b h d h
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Photosynthetic carbohydrate synthesis
Photosynthesis might be thought of as the reverse of glycolysis and TCA cycle:
6 CO2+ 6 H2O 1 glucose + 6 O2 (G' = +686 kcal/mol)
Obviously this must be coupled to favorable reactions:18 ATP 18 ADP + 18 Pi12 NADPH 12 NAD+
(i.e. reducing power)
Assimilation of carbon dioxide into biomass in plants
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Three stages of carbon dioxide assimilation: The Calvin cycle
Stroma
(Triose-P)
Transketolase &transaldolase
reactions
1 C b d d f
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Stage 1: Carbon dioxide fixation
Ribulose-1,5-bisphosphate + CO2+ H2O 2 3-phosphoglycerate + 2 H+
C3plants: fixation of CO2into 3-C compoundsEnzyme: ribulose bisphosphate carboxylase/oxygenase (rubisco)
Rubisco: found in stroma, most abundant protein (50% of chloroplast soluble proteins)
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Stage 2: Formation of glyceraldehyde-3-P
Two gluconeogenesis reactions in stroma:
St 3 R ti f ib l 1 5 bi h h t
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Stage 3: Regeneration of ribulose 1,5-bisphosphate
Overall: 5 triose-P 3 pentose-P
*
Carbohydrate
synthesis
*
*
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Stages
1 and 2
Stage 3
Outline of different stages in Calvin cycle
Stage 3:
aldolase
transketolase
aldolase
transketolase
Ribulose 1,5-
bisphosphateRibulose 1,5-
bisphosphate
Glyceraldehyde 3-
phosphate
Carbohydrate synthesis
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Net results of three turns of the Calvin cycle
- Consumption of 9ATPs, 6NADPH- Release of 1 G3P for carbohydrate
synthesis
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Carbohydrate synthesis from Glyceraldehyde 3-Phosphate
Two G3P out of 12 G3P from 6 turns of Calvin cycle are used to make a hexose:
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Starch biosynthesis in chloroplast stroma
Fructose 6-phosphate
- Starch synthase: adds glucose units to existing starch chain (-1,4 linkage)- Glucose 1-P can be released by starch phosphorylase
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Sucrose biosynthesis in cytosol
anomericcarbons
1
2
a non-reducing sugar
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Maple syrup Amyloplasts in potato cells
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Photorespiration
Light-driven reactions with no carbon fixationOxygenaseactivity of Rubisco(ribulose bisphosphate carboxylase/oxygenase)Generation of 3-phosphoglycerate and 2-phosphoglycolate2-Phosphoglycolate: a metabolic wasteful product
Calvin cycle
2 x 3-Phosphoglycerate Calvin cycle
CO2+ H2O
2H+
oxygenase
carboxylase
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Glycolate pathway- Salvages the carbons in 2-phosphoglycolate to form 3-phosphoglycolate- Energy consuming (requires ATP)- Releases CO2
CO2
ATP
Glycolate pathway
(multiple steps)
2 glycolate + 2 Pi
X 2
CHO
COO-
ADP
Why is the process also called photorespiration?
Calvin cycle
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Carbon assimilation in C4plants
CO2fixation into a 4-C compound
Tropical grasses/cereals (e.g. maize,sugarcane vs rice a C3 plant)
Minimization of photorespiration
More ATPs are required
(PEP)
(OAA)
CO assimilation in CAM plants
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CO2assimilation in CAM plants
Succulent plants (e.g. cactus)
Hot and dry habitat
Temporal separation of CO2capture and Rubisco activities
Fixation of CO2into OAA by PEPcarboxylase at night (stomata open)
OAA is converted to malate bymalate dehydrogenase for storagein vacuoles
CO2released from malate by
malate enzyme during the day(stomata closed)
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(PEP)(OAA)
vacuole