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Fig. 10-1
Photosynthesis
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• Photosynthesis is the process that feeds the biosphere by converting solar energy into chemical energy
• Directly or indirectly, photosynthesis nourishes almost the entire living world
• The summary equation is the reverse of the cellular respiration equation, but the processes are not simply the opposites or reverse of each other!
• 6 CO2 + 6H2O + Light energy C6H12O6 + 6 O2
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• Photosynthesis can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O
Or
6 CO2 + 6H2O + Light energy C6H12O6 + 6 O2
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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• Algae is responsible for most of the photosynthesis on the planet. Plants are responsible for a large portion of photosynthesis that occurs and some takes place in other protists. A few prokaryotes also perform photosynthesis.
• These organisms feed not only themselves but also most of the living world
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• Autotrophs sustain themselves without eating anything derived from other organisms
• Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules
• Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 10-2a
(a) Plants
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Fig. 10-2b
(b) Multicellular alga
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Fig. 10-2c
(c) Unicellular protist10 µm
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Fig. 10-2d
40 µm(d) Cyanobacteria
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Fig. 10-2e
1.5 µm(e) Purple sulfur bacteria
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• Heterotrophs aka consumers obtain their organic material from other organisms and are dependent on photoautotrophs for food and O2
• Only the anaerobic chemoheterotrophs can live independent of the services of photoautotrophs!
• Chloroplasts are the functional organelle of photosynthesis likely evolved from photosynthetic bacteria. (Endosymbiotic Theory)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 10-3b
1 µm
Thylakoidspace
Chloroplast
GranumIntermembranespace
Innermembrane
Outermembrane
Stroma
Thylakoid
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Fig. 10-3a
5 µm
Mesophyll cell
StomataCO2 O2
Chloroplast
Mesophyll
Vein
Leaf cross section
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Light
Fig. 10-5-1
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
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Light
Fig. 10-5-2
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
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Light
Fig. 10-5-3
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
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Light
Fig. 10-5-4
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
[CH2O]
(sugar)
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Mitochondrion
Substrate-levelphosphorylation
ATP
Cytosol
Glucose Pyruvate
Glycolysis
Electronscarried
via NADH
Substrate-levelphosphorylation
ATP
Electrons carriedvia NADH and
FADH2
Oxidativephosphorylation
ATP
Citricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosis
Compare Cellular Respiration to Photosynthesis!
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Light
Fig. 10-5-4
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
[CH2O]
(sugar)
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Reactants:
Fig. 10-4
6 CO2
Products:
12 H2O
6 O26 H2OC6H12O6
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• Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
• Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Reactants:
Fig. 10-4
6 CO2
Products:
12 H2O
6 O26 H2OC6H12O6
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Fig. 10-7
Reflectedlight
Absorbedlight
Light
Chloroplast
Transmittedlight
Granum
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UV
Fig. 10-6
Visible light
InfraredMicro-waves
RadiowavesX-raysGamma
rays
103 m1 m
(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm
380 450 500 550 600 650 700 750 nm
Longer wavelength
Lower energyHigher energy
Shorter wavelength
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• A spectrophotometer measures a pigment’s ability to absorb various wavelengths
• This machine sends light through pigments and measures the fraction of light transmitted at each wavelength
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Galvanometer
Slit moves topass lightof selectedwavelength
Whitelight
Greenlight
Bluelight
The low transmittance(high absorption)reading indicates thatchlorophyll absorbsmost blue light.
The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.
Refractingprism
Photoelectrictube
Chlorophyllsolution
Spectrophotometer
1
2 3
4
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• An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
• The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
• An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process
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Wavelength of light (nm)
(b) Action spectrum
(a) Absorption spectra
(c) Engelmann’s experiment
Aerobic bacteria
Ra
te o
f p
ho
tos
yn
the
sis
(me
as
ure
d b
y O
2 re
lea
se
)A
bs
orp
tio
n o
f li
gh
t b
yc
hlo
rop
las
t p
igm
en
ts
Filamentof alga
Chloro- phyll a Chlorophyll b
Carotenoids
500400 600 700
700600500400
Absorption Spectrum
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• Chlorophyll a is the main photosynthetic pigment
• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
• Chlorophyll a bright green
• Chlorophyll b dull (khaki) green
• Carotene orange
• Xanthophyll yellow
• Phycoerythrin red
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Fig. 10-10
Porphyrin ring:light-absorbing“head” of molecule;note magnesiumatom at center
in chlorophyll aCH3
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshown
CHO in chlorophyll b
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Fig. 10-11
(a) Excitation of isolated chlorophyll molecule
Heat
Excitedstate
(b) Fluorescence
Photon Groundstate
Photon(fluorescence)
En
erg
y o
f el
ectr
on
e–
Chlorophyllmolecule
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• Photosynthesis occurs in the thylakoid membrane and in the stroma.
• The terms light reactions and dark reactions were once used.
• The ‘light dependent’ reactions occur in the thylakoid membrane and the ‘light independent’ reactions occur in the stroma.
• Now there is evidence that the ‘light independent’ reactions don’t occur in the dark!?
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Light
Fd
Cytochromecomplex
ADP +
i H+
ATPP
ATPsynthase
ToCalvinCycle
STROMA(low H+ concentration)
Thylakoidmembrane
THYLAKOID SPACE(high H+ concentration)
STROMA(low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
LightNADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2OO2
e–
e–
1/21
2
3
The ‘Light Reactions’
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Fig. 10-12
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
STROMA
e–
Pigmentmolecules
Photon
Transferof energy
Special pair ofchlorophyll amolecules
Th
yla
koid
me
mb
ran
e
Photosystem
Primaryelectronacceptor
Reaction-centercomplex
Light-harvestingcomplexes
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ChlorophyllIn the
ThylakoidMembrane
of achloroplast
Light
P680
e–2
1
Photosystem II(PS II)
Primaryacceptor
A photosystem (previous slide)consists of a reaction-center complex (protein) surrounded by light-harvesting complexes.
The light-harvesting complexes (pigment molecules bound to proteins as seen on the previous slide) funnel the energy of photons to the reaction center complex
Reaction Center Complex
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• A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a
• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
Fig. 10-13-2
Photosystem II(PS II)
There are two types of photosystems in the thylakoid membrane
Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm
The reaction-center chlorophyll a of PS II is called P680
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• P680+ (P680 that is missing an electron) is a very strong oxidizing agent
• H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680
• O2 is released as a by-product of this reaction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
4
Pq
Pc
Cytochromecomplex
Electron transport chain
5
ATP
Fig. 10-13-3
Photosystem II(PS II)
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Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
4
Pq
Pc
Cytochromecomplex
Electron transport chain
5
ATP
Photosystem I(PS I)
Light
Primaryacceptor
e–
P700
6
Fig. 10-13-4
Photosystem II(PS II)
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• Photosystem I (PS I) is best at absorbing a wavelength of 700 nm
• The reaction-center chlorophyll a of PS I is called P700
• After leaving (PS 1) electrons travel down the second electron transport chain.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
4
Pq
Pc
Cytochromecomplex
Electron transport chain
5
ATP
Photosystem I(PS I)
Light
Primaryacceptor
e–
P700
6
Fd
Electron transport chain
NADP+
reductase
NADP+
+ H+
NADPH
8
7
e–
e–
6
Fig. 10-13-5
Photosystem II(PS II)
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Light
Fd
Cytochromecomplex
ADP +
i H+
ATPP
ATPsynthase
ToCalvinCycle
STROMA(low H+ concentration)
Thylakoidmembrane
THYLAKOID SPACE(high H+ concentration)
STROMA(low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
LightNADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2OO2
e–
e–
1/21
2
3
The ‘Light Reactions’ of Photosynthesis
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Protein complexof electroncarriers
H+
H+H+
Cyt c
Q
V
FADH2 FAD
NAD+NADH
(carrying electronsfrom food)
Electron transport chain
2 H+ + 1/2O2H2O
ADP + P i
Chemiosmosis
Oxidative phosphorylation
H+
H+
ATP synthase
ATP
21
Compare the ETC of cellular respiration to the two ETC’s of photosynthesis!
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Light
Fd
Cytochromecomplex
ADP +
i H+
ATPP
ATPsynthase
ToCalvinCycle
STROMA(low H+ concentration)
Thylakoidmembrane
THYLAKOID SPACE(high H+ concentration)
STROMA(low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
LightNADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2OO2
e–
e–
1/21
2
3
The ‘Light Reactions’ of Photosynthesis
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Fig. 10-16
Key
Mitochondrion Chloroplast
CHLOROPLASTSTRUCTURE
MITOCHONDRIONSTRUCTURE
Intermembranespace
Innermembrane
Electrontransport
chain
H+ Diffusion
Matrix
Higher [H+]Lower [H+]
Stroma
ATPsynthase
ADP + P i
H+ATP
Thylakoidspace
Thylakoidmembrane
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Both Chloroplasts and Mitochondria perform chemiosmosis!
• Mitochondria:
transfer chemical energy from food to ATP
protons are pumped to the intermembrane space
diffuse back into the matrix
• Chloroplasts:
– transform light energy into the chemical energy of ATP
– protons are pumped into the thylakoid space
– diffuse back into the stroma
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Light
Fd
Cytochromecomplex
ADP +
i H+
ATPP
ATPsynthase
ToCalvinCycle
STROMA(low H+ concentration)
Thylakoidmembrane
THYLAKOID SPACE(high H+ concentration)
STROMA(low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
LightNADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2OO2
e–
e–
1/21
2
3
The ‘Light Reactions’ of Photosynthesis
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• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I
• Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
• Diffusion of H+ (protons) across the membrane drives ATP synthesis
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• In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor
• P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain
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• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
• The electrons are then transferred to NADP+ and reduce it to NADPH
• The electrons of NADPH are available for the reactions of the Calvin cycle
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Light
Fd
Cytochromecomplex
ADP +
i H+
ATPP
ATPsynthase
ToCalvinCycle
STROMA(low H+ concentration)
Thylakoidmembrane
THYLAKOID SPACE(high H+ concentration)
STROMA(low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
LightNADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2OO2
e–
e–
1/21
2
3
The ‘Light Reactions’ of Photosynthesis
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• Linear electron flow, as previously described is the primary pathway for electrons, it involves both photosystems and produces ATP and NADPH using light energy
• However there is a second possible route for electron flow. The cyclic route.
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Cyclic Electron Flow
• Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH
• Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle
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• Some organisms such as purple sulfur bacteria have PS I but not PS II
• Cyclic electron flow is thought to have evolved before linear electron flow
• Cyclic electron flow may protect cells from light-induced damage
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ATPPhotosystem II
Photosystem I
Primary acceptor
Pq
Cytochromecomplex
Fd
Pc
Primaryacceptor
Fd
NADP+
reductaseNADPH
NADP+
+ H+
The Cyclic Route
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• Regardless of the route… the light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH
• ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place
• The Calvin Cycle occurs in the stroma
after the ‘light reactions’
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Calvin Cycle of Photosynthesis!
Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
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Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
66 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 POutput G3P
(a sugar)
Glucose andother organiccompounds
CalvinCycle
Calvin Cycle of Photosynthesis!
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Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
66 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 POutput G3P
(a sugar)
Glucose andother organiccompounds
CalvinCycle
3
3 ADP
ATP
5 P
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
G3P
Calvin Cycle of Photosynthesis!
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Acetyl CoA
CoA—SH
Citrate
H2O
IsocitrateNAD+
NADH
+ H+
CO2
-Keto-glutarate
CoA—SH
CO2NAD+
NADH
+ H+SuccinylCoA
CoA—SH
P i
GTP GDP
ADP
ATP
Succinate
FAD
FADH2
Fumarate
CitricacidcycleH2O
Malate
Oxaloacetate
NADH
+H+
NAD+
1
2
3
4
5
6
7
8
Comparethe Krebs Cycle of Cellular Respiration to the Calvin Cycle of Photosynthesis
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Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
66 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 POutput G3P
(a sugar)
Glucose andother organiccompounds
CalvinCycle
3
3 ADP
ATP
5 P
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
G3P
Calvin Cycle of Photosynthesis!
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In plants… chloroplasts are concentrated in the leaves but are found in all green portions of the plant including stems.
•Their green color is from chlorophyll, the green pigment within chloroplasts
•Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast
•CO2 enters and O2 exits the leaf through microscopic pores called stomata
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• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
• A typical mesophyll cell has 30–40 chloroplasts
• The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
• Chloroplasts also contain stroma, a dense fluid
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• Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
• The light reactions (in the thylakoids):
– Split H2O
– Release O2
– Reduce NADP+ to NADPH
– Generate ATP from ADP by photophosphorylation
• The Calvin cycle (in the stroma)
– forms sugar from CO2, using ATP and NADPH
– begins with carbon fixation, incorporating CO2 into organic molecules
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Photosynthesis occurs in different forms! All forms of photosynthesis require a hydrogen source but that source varies.
• Familiar: 6CO2 + 6H2O + Light Energy C6H12O6 + 6O2
• Generally: CO2 + 2H2X [CH2O] + H20 + 2X
• Plants: CO2 + 2H2O [CH2O] + H20 + O2
• Sulfur Bacteria: CO2 + 2H2S [CH2O] + H20 + 2S
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• The plants that perform the form of photosynthesis we are most familiar with are called C3 plants.
• An alternate form of photosynthesis which is quite common is called Photorespiration.
• Plants that perform photorespiration are referred to as C4 plants and others as CAM plants!
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• Photorespiration may be an evolutionary relic that evolved at a time when the atmosphere had far less O2 and more CO2
• Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle
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• In most plants (C3 plants), initial fixation of CO2, catalyzed by rubisco, forms a three-carbon compound
• In photorespiration (C4 and CAM plants), rubisco adds O2 instead of CO2 into the Calvin cycle
• Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
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• The first product of C4 plants in the Calvin Cycle is a 4 carbon compound thus the name C4.
• Includes sugarcane, corn and others.
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• CAM plants (Crassulacean acid metabolism) are adapted to arid conditions. Crassulaceae is a family of succulent plants in which the process was first discovered. Includes pineapples, cactus and others.
• Since CO2 is not available during the day because stomata are closed to conserve water…CO2 uptake occurs at night and is stored as organic acids. The next morning when the light reactions are available to supply ATP and NADPH, the organic acids are converted back to CO2 and used in the Calvin Cycle to make carbohydrates.
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C4 Plants
C4 leaf anatomy
Mesophyll cellPhotosyntheticcells of C4
plant leafBundle-sheathcell
Vein(vascular tissue)
Stoma
The C4 pathway
Mesophyllcell CO2PEP carboxylase
Oxaloacetate (4C)
Malate (4C)
PEP (3C)ADP
ATP
Pyruvate (3C)
CO2
Bundle-sheathcell
CalvinCycle
Sugar
Vasculartissue
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C4 Plants
Stoma
C4 leaf anatomy
Photosyntheticcells of C4
plant leaf
Vein(vascular tissue)
Bundle-sheathcell
Mesophyll cell
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C4 Plants
Sugar
CO2
Bundle-sheathcell
ATP
ADP
Oxaloacetate (4C) PEP (3C)
PEP carboxylase
Malate (4C)
Mesophyllcell
CO2
CalvinCycle
Pyruvate (3C)
Vasculartissue
The C4 pathway.
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Fig. 10-20
CO2
Sugarcane
Mesophyllcell
CO2
C4
Bundle-sheathcell
Organic acidsrelease CO2 to Calvin cycle
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Pineapple
Night
Day
CAM
SugarSugar
CalvinCycle
CalvinCycle
Organic acid Organic acid
(a) Spatial separation of steps (b) Temporal separation of steps
CO2 CO2
1
2
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The Big Idea
Review
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Fig. 10-21
LightReactions:
Photosystem II Electron transport chain
Photosystem I Electron transport chain
CO2
NADP+
ADP
P i+
RuBP 3-Phosphoglycerate
CalvinCycle
G3PATP
NADPHStarch(storage)
Sucrose (export)
Chloroplast
Light
H2O
O2
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• Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits
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Fig. 10-UN1
CO2
NADP+
reductase
Photosystem II
H2O
O2
ATP
Pc
Cytochromecomplex
Primaryacceptor
Primaryacceptor
Photosystem I
NADP+
+ H+
Fd
NADPH
Electron transport
chain
Electron transport
chain
O2
H2O Pq
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Fig. 10-UN2
Regeneration ofCO2 acceptor
1 G3P (3C)
Reduction
Carbon fixation
3 CO2
CalvinCycle
6 3C
5 3C
3 5C
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Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
66 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 POutput G3P
(a sugar)
Glucose andother organiccompounds
CalvinCycle
3
3 ADP
ATP
5 P
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
G3P
Calvin Cycle of Photosynthesis!
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• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
• For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2
• The Calvin cycle has three phases:
– Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor (RuBP)
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