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ATP Formation by Electron-Transport Chains
ATP Formation by Electron-Transport Chains
Mitochondrial Electron-Transport
Components of the Electron-Transport Chain
Oxidative Phosphorylation
Recycling of Cytoplasmic NADH
Photosynthetic Electron-Transport
Synthesis of Carbohydrates by the Calvin Cycle
Mitochondrial Electron-Transport
Components of the Electron-Transport Chain
Oxidative Phosphorylation
Recycling of Cytoplasmic NADH
Photosynthetic Electron-Transport
Synthesis of Carbohydrates by the Calvin Cycle
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IntroductionIntroduction
Up to this point, we have dealt with
• Oxidation of substrates.
• Collection of electrons by cofactors.
Energy from the cofactors is recovered using O2 as the final electron acceptor.
This is accomplished using a series of carriers in the inner mitochondrial membrane .
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Mitochondrial electron transportMitochondrial electron transport
Stage I and II of carbohydrate catabolism converge at the mitochondria.
Stage I
Stage II
citric acid cycle
electron-transport
oxidative phosphorylation
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Mitochondrial electron transportMitochondrial electron transport
• Extensive inner membrane folding in the mitochondria provides a large surface area.
• There are many molecular systems on this membrane for production of ATP.
• Electron-transport chain components are arranged in packages called respiratory respiratory assembliesassemblies.
• There are also knob-like spheres called FF11 particlesparticles.
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Electron-transport andoxidative phosphorylation
Electron-transport andoxidative phosphorylation
Electrons obtained from nutrients and metabolic intermediates are transferred to NAD+ and FAD.
AH2 + NAD+ A + NADH + H+
BH2 + FAD B + FADH2
Since NAD+ and FAD are in limited supply, they must be recycled.
dehydrogenase
dehydrogenase
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Electron-transport andoxidative phosphorylation
Electron-transport andoxidative phosphorylation
Recycling is accomplished by oxidation and transfer of electrons to oxygen.
NADH + H+ + 1/2 O2 NAD+ + H2O
FADH2 + 1/2 O2 FAD + H2O
ADP + Pi ATP
ADP + Pi ATP
NAD+ and FAD are then available for additionaloxidative metabolism. The energy released duringelectron transport is coupled to ATP synthesis.
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Electron-transport chainElectron-transport chain
Composed of four large protein complexes.
• Complex I - NADH-Coenzyme Q reductase
• Complex II - Succinate-Coenzyme Q reductase
• Complex III - Cytochrome c reductase
• Complex IV - Cytochrome c oxidase
Many of the components are integral membrane proteins with prosthetic groups to move electrons.
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Electron-transport chainElectron-transport chain
Two important characteristics of the Two important characteristics of the electron-transport chainelectron-transport chain
• order of electron carriers
• quantity of energy produced
Electron carriers are arranged in order of increasing electron affinity.
This results in the spontaneous flow of electrons from carrier to carrier.
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Flow of electronsFlow of electrons
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Energy producedEnergy produced
The amount of energy can be calculated in terms of Go’ :
NADH + H+ + 1/2 O2 NAD+ + H2O
Go’ = - 220 kJ/mol
FADH2 + 1/2 O2 FAD + H2O
Go’ = - 152 kJ/mol
Note:Note: ADP + Pi ATP Go’ = +31 kJ/mol
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Components of theelectron transport chain
Components of theelectron transport chain
Complex IComplex I
• Electrons flow from NADH to flavin mononucleotide (FMN) - similar to FAD.
• Electrons then flow to a prosthetic group on an iron-sulfur cluster - iron cycles between 3+ and 2+ states.
• Complex I terminates at ubiquinone - also called coenzyme Q or CoQ.
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Components of theelectron transport chain
Components of theelectron transport chain
Complex IComplex I
H+
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Components of theelectron transport chain
Components of theelectron transport chain
flavoprotein
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Iron-sulfur clustersIron-sulfur clusters
Fe
S
S
Fe
Cys
Cys
Cys
Cys
Fe
S
SS
FeFe Fe
S
SS
S
S Cys
CysCys
Cys
protein
S
S
S
S
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Components of theelectron transport chain
Components of theelectron transport chain
CoQ - ubiquinoneCoQ - ubiquinoneHighlighted region serves as an anchor to
inner mitochondrial membrane.
O
O
CH3H3CO
H3CO CH C
CH3
CH2)10(CH2 H
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Reduction of CoQReduction of CoQ
O
OH
R
CH3H3CO
H3CO
OH
OH
R
CH3H3CO
H3CO
O
O
R
CH3H3CO
H3CO
Oxidized formUbiquinone (CoQ)
Reduced formUbiquinol (CoQH2)
intermediate,semiquinone
e- +H+
e- +H+
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Components of theelectron transport chain
Components of theelectron transport chain
Complex IIComplex II
• Entry point for both FADH2 and Complex I.
• Succinate dehydrogenaseFrom the citric acid cycle. Directs transfer of electrons from succinate to CoQ via FADH2.
• Acyl-CoA dehydrogenaseFrom -oxidation of fatty acids. It also transfers electrons to CoQ via FADH2.
Both enzymes have iron-sulfur clusters as prosthetic groups and are integral proteins.
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Components of theelectron transport chain
Components of theelectron transport chain
All electrons from FADH2 and NADH must pass through CoQ.
Fe-S
FMN
NADH NAD+
I
II
Succinate
FAD
Fe-S
Fatty acylCoA
FAD
matrix
innermembranespace
CoQ
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Components of theelectron transport chain
Components of theelectron transport chain
Complex IIIComplex IIIElectron transfer from ubiquinol to
cytochrome c.
cytochrome c
heme prosthetic group
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Components of theelectron transport chain
Components of theelectron transport chain
Structure of cytochrome c heme group.
Fe
N
N
N N
H3C CHCH3
S
H3C
H3C
H3C CH2CH2COO-
CH2CH2COO-
CH3
Protein
Fe
N
N
N N
H3C CHCH3
S
H3C
H3C
H3C CH2CH2COO-
CH2CH2COO-
CH3
Protein
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Components of theelectron transport chain
Components of theelectron transport chain
Complex IVComplex IV
• Combination of cytochromes a and a3 - cytochrome c oxidasecytochrome c oxidase.
• Consists of 10 protein subunits, 2 types of prosthetic groups - 2 heme and 2 Cu.
• Cytochromes a and a3 are the only species capable of direct transfer of electrons to oxygen.
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Components of theelectron transport chain
Components of theelectron transport chain
matrix
ComplexI
ComplexIII
ComplexIV
CoQcyt b
cytc
cytc1
(Cu)cyta/a3
NADH O2
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Oxidative phosphorylationOxidative phosphorylation
• The electron-transport chain moves electrons from NADH and FADH2 to O2.
• The next step is the phosphorylation of ADP to produce ATP.
Catalyzed by the inner membrane enzyme ATP synthaseATP synthase.
• The steps are coupled - electrons do not flow to oxygen unless ATP is needed.
Each NADH produces 3 ATPEach FADH2 produces 2 ATP
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Coupling of electron-transportwith ATP synthesis
Coupling of electron-transportwith ATP synthesis
Chemiosmotic coupling mechanismChemiosmotic coupling mechanism
• Electron-transport causes unidirectional movement of H+ into the innermembrane space.
• The results in a H+ gradient being produced.
• The gradient then drives the synthesis of ATP.
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Coupling of electron-transportwith ATP synthase
Coupling of electron-transportwith ATP synthase
Inner mitochondrial membrane
Outer mitochondrial membrane
H+ H+
H+
H+
H+H+H+
H+H+
H+
H+
H+ H+
H+
H+
ADP + Pi ATP
Electron Transport
Chain
Electron Transport
Chain
F1-ATPsynthasecomplex
F1-ATPsynthasecomplex
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Components of ATP synthaseComponents of ATP synthase
These are knob-like projections into the matrix side of the inner membrane.
Two unitsTwo units
• F1 contains the catalytic site for ATP synthesis.
• F0 serves as a transmembrane channel for H+ flow.
F1-F0 complex serves as the molecular apparatus for coupling H+ movement to ATP synthase.
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Components of ATP synthaseComponents of ATP synthase
H+
H+H+ H+
H+
H+H+
H+H+
H+
F0
F1
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Regulation of oxidative phosphorylation
Regulation of oxidative phosphorylation
• Electrons do not flow unless ADP is present for phosphorylation
• Increased ADP levels cause an increase in catabolic reactions of various enzymes including:
glycogen phosphorylaseglycogen phosphorylase
phosphofructokinasephosphofructokinase
citrate synthasecitrate synthase
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Uncoupling of electron-transportand oxidative phosphorylation
Uncoupling of electron-transportand oxidative phosphorylation
• In some special cases, the coupling of the two processes can be disrupted.
• Large amounts of O2 are consumed but no ATP is produced.
• Used by newborn animals and hibernating mammals.
• Occurs in ‘brown fat’- dark color due to high levels of mitochondria which contain thermogeninthermogenin (uncoupling protein).
• Thermogenin allows the release of energy as heat instead of ATP.
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Energy production from glucoseEnergy production from glucose
GlycolysisGlycolysis2 ATP 2 ATP2 NADH 3 ATP/NADH 6 ATP*
Citric Acid CycleCitric Acid Cycle2 GTP 1 ATP/GTP 2 ATP6 NADH 3 ATP/NADH 18 ATP2 FADH2 2 ATP/FADH2 4 ATP
38 ATP(in heart)
* 4 ATP in muscle and brain.36 ATP / glucose
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Energy production from glucoseEnergy production from glucose
GlycolysisMitochondria
Glucose 2 PyruvateGlucose 2 Pyruvate
Oxidativephosphorylation
Oxidativephosphorylation
6 NADH+
2 FADH2
6 NADH+
2 FADH2 2 NADH2 NADH2 NADH2 NADH
2 ATP2 ATP 2 ATP2 ATP32-34 ATP32-34 ATP
2 Acetyl CoA2 Acetyl CoA
2 GTP2 GTP
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Recycling of cytoplasmic NADHRecycling of cytoplasmic NADH
Different methods are used to recycle NADH. This accounts for the different energy productions from glucose.
Glycerol-3-phosphate shuttleGlycerol-3-phosphate shuttleUsed by skeletal muscles and the brain
Malate-aspartate shuttleMalate-aspartate shuttleUsed by the heart and liver
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Glucose-3-phosphate shuttleGlucose-3-phosphate shuttle
NAD+ NADH + H+
cytoplasmicglycerol-3-phosphate
dehydrogenase
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Malate-aspartate shuttleMalate-aspartate shuttle
L-aspartate
Matrix
-ketoglutarate
L-glutamate
L-aspartate
-ketoglutarate
L-glutamate
oxaloacetate
mitochondrialaspartate
aminotransferase
L-malate L-malate
oxaloacetate
NAD+
3 ATP
NAD+NADH+ H+
NADH+ H+
Glycolysis
mitochondrialmalate
dehydrogenase
cytoplasmicaspartateaminotransferase
cytoplasmic malate dehydrogenase
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Photosynthetic electron transport
Photosynthetic electron transport
HeterotrophsHeterotrophsObtain energy by ingestion of other plants and animals.
PhototrophsPhototrophsAbsorb solar radiation and divert the energy through the electron transport chain.
They can produce their own carbohydrates from CO2 and H2O
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Photosynthetic electron transport
Photosynthetic electron transport
Two type of reactions.Two type of reactions.
Light reactions - photo phaseLight reactions - photo phaseAbsorb energy using chlorophyll and other pigments.
Dark reactions - synthesis phaseDark reactions - synthesis phaseCarbon metabolism to make carbohydrates. Light is not directly required.
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ChloroplastChloroplast
Outer membrane
Inner membrane
Granum
Stroma
Innermembranespace
Thylakoid
The apparatus for light absorption and carbonfixing in eukaryotic photosynthetic cells.
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ChloroplastChloroplast
StromaStromaGel-like, unstructured matrix within the inner compartment. It contains the enzymes for the dark reactions.
ThylakoidsThylakoidsMembranes folded into sacs that are the sites for light receiving pigments, electron carriers and ATP synthesis. They are arranged into stacks called grana.
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Biomolecules and lightBiomolecules and light
Several types of light absorbing pigments are used.
Green plantsGreen plantsChlorophylls a and b.
BacteriaBacteriaBacteriochlorophyll.
Accessory pigmentsAccessory pigmentsCarotenes and phycobilins - absorb light outside the range of chlorophyll.
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Chlorophyll aChlorophyll a
NMg
N
N
N
O
CH3
CH2CH3
CH3CH2
CH2
H3C
H3C
H
C H2
CH
H
H3CO O
C H2
C
O
OCH3CH3CH3CH3
H3C
I II
IIIIV
NMg
N
N
N
O
CH3
CH2CH3
CH3CH2
CH2
H3C
H3C
H
C H2
CH
H
H3CO O
C H2
C
O
OCH3CH3CH3CH3
H3C
I II
IIIIV
phytol side chain
CH3
CO
in bacteriochlorophyll
COH
in chlorophyll b saturated bond inbacteriochlorophyll
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CarotenesCarotenes
HO
OH
HO
OH
-carotene
lutein
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PhycoerytherinPhycoerytherin
N H
O
H3C CH
CH3
N H
H3C
CH2
COO-
N H
O
CHH3C
N H
CH3
CH2
COO-
CH2
N H
O
H3C CH
CH3
N H
H3C
CH2
COO-
N H
O
CHH3C
N H
CH3
CH2
COO-
CH2
unsaturated bondin phycocyanin
CH2
CH3
in phycocyanin
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Photosynthetic light reactionsPhotosynthetic light reactions
Electrons flow through an electron transport chain from water to an electron acceptor.
NADP+ is the acceptor in green plants.
2 H2O + 2 NADP+
2 H+ + O2 + 2 NADPHlight
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PhotosystemsPhotosystems
Two typesTwo typesEach contain one primary acceptor molecule - usually chlorophyll
A set of accessory molecules help funnel additional light.
Cat
Chl aReactionCenter
Chl Chl
CatCat
Chl Chl
Chl
Chl
Chl
Chl Chl
Chl
Chl
Chl
Chl
Cat
Cat
CatChl
h
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PhotosystemsPhotosystems
Photosystem I - P700Photosystem I - P700• Chlorophyll a and accessory pigments• Absorb in 600-700 nm range
Photosystem II - P680Photosystem II - P680• Chlorophyll a, b and accessory pigments• Absorb light with a maximum at 680 nm
All photosynthetic cells have P700. Both are present in O2 evolving organisms - higher plants, algae and cyanobacteria.
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Linkage of photosystems I and IILinkage of photosystems I and II
In green plants, the two systems are linked.In green plants, the two systems are linked.
• Light is absorbed by Photosystem I.
• Energy is transmitted to the P700 center and an electron is excited.
• Electron is passed via an electron transport chain.
• The ‘electron hole’ is filled by another electron transport chain driven by Photosystem II.
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Photosystem IPhotosystem I
P700
P700*A0
A1
Fe-SComplex Ferredoxin
Ferre doxin-NADPreductase
+NADP+
NADPH + H+ proton gradient
+
Photosystem I
lightRed
uct
ion p
ote
nti
al, V
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Photosystem IIPhotosystem II
2 H O2
O + 4 H +proto n gradient
+2
P680
Water-splittingcomplex
P680*
QA
QB
Photosystem II
light
0.0
+0.5
+1.0
-0.5
-1.0R
ed
uct
ion p
ote
nti
al, V
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Linkage of photosystems I and IILinkage of photosystems I and II
light
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Photosystems I and IIPhotosystems I and II
Net reactionNet reaction
2 H2O + 2 NADP+
O2 + 2 NADPH + 2 H+
Eight photons are required to transfer four electrons.
8 h
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PhotophosphorylationPhotophosphorylation
• Converting light into chemical bonds- very similar to oxidative phosphorylation.
• Photoinduced electron transfer from water to NADP+ pumps H+ through thylkaloid membrane - from stromal side to inner compartment.
• Protein complexes CF0 and CF1 are the ATP synthases of chloroplasts.
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PhotophosphorylationPhotophosphorylation
H+ H+
H+H+ADP
ATP
Stroma
High Mg Low H+2+
Low Mg High H+2+Lumen
thylakoidmembrane
proton pump withinthe light-inducedelectron transportsystem.
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PhotophosphorylationPhotophosphorylation
Process is non-cyclicProcess is non-cyclic
• Starts with H2O and ends with NADPH and O2.
• Products will accumulate as long as there is light.
A cyclic process exists for photosystem I.A cyclic process exists for photosystem I.
• No H2O is consumed and no NADPH or O2 is produced.
• ADP is phosphorylated.
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Cyclic photophosphorylationCyclic photophosphorylation
Cytochromebf complex
proto n gradient
P700
P700*A0
A1Fe-S
Complex
Ferredoxin
light
Plastocyanin
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Synthesis of carbohydratesSynthesis of carbohydrates
The Calvin CycleThe Calvin Cycle• The ‘dark’ reactions - fixation of carbon
from CO2.
• Four stages - fix one carbon at a time.• Six cycles per glucose.
Overall reaction for one glucoseOverall reaction for one glucose
6 CO2 + 12 NADPH + 12H+ + 18 ATP + 12 H2O
C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi
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Calvin cycleCalvin cycle
Stage 1Stage 1
• Addition of CO2 to an acceptor molecule.
• Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubiscorubisco) catalyzes the addition of CO2
• The ribulose-1,5-bisphosphate that is produced will immediately cleave into two molecules of 3-phosphoglycerate.
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Calvin cycleCalvin cycleStage 1Stage 1
ribulose-1,5-bisphosphate
3-phosphoglycerate
-keto acid intermediate
O
CH2
C
C
C
CH2
O
P
O
O-
O-
O
H
H
OH
OH
P
O
O-
O-
O
CH2
C
C
C
CH2
O
P
O
O-
O-
C
H
O
OH
P
O
O-
O-
O
O-
O
C O-
CH OH
C H2
O P
O
O-
O-
+ CO2
H+
2
H2O
H+
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Calvin cycleCalvin cycle
Stage 2Stage 2• Phosphorylation of the C1 carboxyl group,
producing 1,3-bisphosphoglycerate.• Stromal 1,3-bisphosphoglycerate (1,3-PBG) is
reduced to glyceraldehyde-3-phosphate.
COO-
CH OH
CH2OPO32-
COPO32-
CH OH
CH2OPO32-
O HC
C
O
CH2OPO32-
H OH3-phosphoglycerate
kinaseglyceraldehyde
3-phosphatedehydrogenase
ATP ADP NADPH
+ H+NADP+
+ Pi
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Calvin cycleCalvin cycle
Stage 3Stage 3• Carbohydrates are formed from glyceraldehyde-
3-phosphate. The same gluconeogenesis pathways used earlier are used.
glyceraldehyde-3-phosphate dihydroxyacetone phosphate
DHAP + glyceraldehyde-3-phosphate fructose-1,6-bisphosphate
fructose-1,6-bisphosphate + H2O fructose-6-phosphate + Pi
fructose-6-phosphate glucose-6-phosphate
glucose-6-phosphate glucose-1-phosphate
isomerase
aldolase
phosphatase
isomerase
phosphoglucomutase
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Calvin cycleCalvin cycle
Stage 4Stage 4• Only one of each six cycles results in
carbohydrate production.• The other passes through the cycle are used to
regenerate the ribulose-1,5-bisphosphate.• The first step is the conversion of glyceraldehyde-
3-phosphate to dihydroxyacetone phosphate.
C
C
C
O
OHH
H H
P-O O-
O
C
C
C
O
OH
H H
P-O O-
O
OH
H
H
H O
isomerase
C
C
C
O
OHH
H H
P-O O-
O
C
C
C
O
OH
H H
P-O O-
O
OH
H
H
H O
isomerase
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Ribulose-5-phosphate
Ribulose-1,5-bisphosphate
Glycerate-1,3-bisphosphate
Glyceraldehyde-3phosphate (G3P)
Dihydroxyacetonephosphate (DHAP)
Sucrose, starch,cellulose, etc.
Fructose-1,6-bisphosphate
Fructose-6-phosphate
Glucose-6-phosphateGlucose
X5P
X5P
R5P
G3PDHAP
G3P
DHAP
G3P
FbisPF6P
E4PS7P
ATP
ADP
CO2
H O2ATP
ADP
NADPH
NAD+
Pi
PiPi
Calvin
cycle
Calvin
cycle
3-phospho-glycerate
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PhotorespirationPhotorespiration
Rubisco can act as an oxygenase by substituting O2 for CO2.
CH2OPO32-
C O
CHOH
CHOH
CH2OPO32-
+ O2
CH2OPO32-
C-O O
C
CHOH
-O O
CH2OPO32-
+rubisco
CH2OPO32-
C O
CHOH
CHOH
CH2OPO32-
+ O2
CH2OPO32-
C-O O
C
CHOH
-O O
CH2OPO32-
+rubisco
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PhotorespirationPhotorespiration
• This appears to be a counter productive path - oxygen is consumed.
• Some plants have adapted this process as an optional pathway for carbon fixation.(sugar cane, corn, sorghum, ...)
• This can be described by the Hatch-Slack pathway - C4 pathway
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Hatch-Slack pathwayHatch-Slack pathway
Mesophyllcell
Bundlesheath cell